Second moment of area

Question: I am having trouble understanding the term. How come that calculating the integral of the product of multiplying the Area differential by (distance)^2 determines the stiffness of the beam or so and its resistance to bending? does it have anything to do with the first moment of area and torque?
Answer: I can easily explain the stress-moment relationship in beams. I am sure you are familiar with it, and the area moment $I$ comes out naturally.
$$ \sigma_{max} = \frac{M y_{max}}{I} $$
Take a section of the beam with internal moment $M$. This moment has to be supported by the stress distribution internally. For straight beams, there is a neutral axis in the middle of the beam, and the stress values vary linearly from the distance $y$ to the neutral axis $$ \sigma(y) = k y $$
At the limit $\sigma_{max} = k y_{max}$ or $$k = \frac{\sigma_{max}}{y_{max}}$$
Take a cross section and integrate slices along the height $y =y_{min} \ldots y_{max}$ each with a thickness $b(y)$.
The area if the section is thus
$$ A = \int {\rm d} A = \int \limits_{y_{min}}^{y_{max}} b(y)\,{\rm d}y $$
The moment is found by the product if the infinitesimal tension ${\rm d}F = \sigma(y) \,{\rm d}A$ and the height $y$.
$$ M =\int y {\rm d}F = \int y \sigma {\rm d} A = \int \limits_{y_{min}}^{y_{max}}y \sigma b(y)\,{\rm d}y = \int \limits_{y_{min}}^{y_{max}}k y^2 b(y)\,{\rm d}y $$
or
$$ k = \frac{M}{\int \limits_{y_{min}}^{y_{max}} y^2 b(y)\,{\rm d}y} $$
The denominator we call the area moment because it contains any information from the geometry of the problem
$$ \boxed{I = \int \limits_{y_{min}}^{y_{max}} y^2 b(y)\,{\rm d}y} $$
To answer your question on why the $y^2$ term in the integral, one of the $y$ comes from the linearity of the stress distribution $\sigma = k y$ and the other $y$ from the moment arm calculation ${\rm d}M = y {\rm d}F$.

Second moment of area


Windows 7 Turkish language pack problem

Question: I have Windows 7 Ultimate RTM (7600) installed on my laptop. I installed the Turkish language pack from Windows Update but some texts like "Welcome" "Shutting Down" and "Starting Windows" are still in English, but the others are translated into Turkish correctly. Did anyone face with this problem?
Answer: Language packs are, by default, enabled on a per-user basis. Your Turkish pack is probably enabled just for your user at the moment.
Navigate to:

Control Panel
Clock, Language, and Region
Region And Language
Administrative (Tab)
Copy Settings (Button)

(If your Control Panel is in Classic View, follow the following:)

Control Panel
Region And Language
Administrative (Tab)
Copy Settings (Button)

Check the two check boxes at the bottom of the window displayed:

Welcome screen and system accounts
New user accounts (Optional, check if you want new users to be Turkish by default)

and reboot your system once instructed. This should apply your language pack to the system accounts and the text during boot up and welcome screen will be localized.
Comment: You are right! But it's different on Windows 7. Control Panel -> Region and Language -> Administrative Tab -> Copy Settings.
Comment: **@A Dwarf:** What you just enumerated is the Control Panel in Classic View... Category view is what I enumerated.
Comment: Yes mate it's my solution at last :)
I don't know how I overlooked it but it's fully (%100) Turkish now including all welcome messages etc...Thanx very much...
Answer: Some language packs are what is known as Partially Localized Language Packs. They don't contain a 100% localization of all resources, although they do contain all the resources. The non localized resources are usually in the English language. See, Understanding Language Packs
I'm unsure as to how and when do Partially Localized MUI Packs evolve into fully localized ones. Being the case Windows does get distributed in Turkey fully localized, I'm sure it is simply a matter of time until the folks at Redmond update the Turkish MUI. (Please remember: Windows 7 is still not officially out)
FYI, just a few months ago, the Portuguese Pack was also only partially localized. From what I can gather with a quick inspection I did recently, the pack I downloaded through my MSDN subscription last week is already 100% localized.
Comment: As I told,on my previous install,it was fully %100 translated into Turkish.When you look at the list of language packs on MS Update of Windows 7 you can see all local language packs in there.So no solutions yet...

Windows 7 Turkish language pack problem


GPL License type that permits museum fees?

Question: Is there a GPL license type that permits non-direct revenue (from the asset), such as the entrance fee for a museum? Or educational tuition?
Comment: The GNU GPL *doesn't* restrict recipients' ability to charge for the work in any way. Or do you mean you want a license that *does* restrict recipients' ability to make money from the work *except* in the specific ways you mention?
Comment: Hire a lawyer to draft this for you
Comment: @apsillers - The latter.
Comment: Note: router manufacturers sell their devices even though they contain GPL code. They must provide the code for free, but they certainly do not have to provide the hardware for free.
Answer: The CC BY-NC 2.0 license regulates commercial exploitation in 4(b):

You may not exercise any of the rights granted to You in Section 3
  above in any manner that is primarily intended for or directed toward
  commercial advantage or private monetary compensation. The exchange of
  the Work for other copyrighted works by means of digital file-sharing
  or otherwise shall not be considered to be intended for or directed
  toward commercial advantage or private monetary compensation, provided
  there is no payment of any monetary compensation in connection with
  the exchange of copyrighted works

The first sentence may allow the kind of use that you're describing, since The Work could be made available for money, but not for commercial advantage (which I take to be code for "profit") or private monetary compensation (code for "personal gain"). The second sentence addresses exchanging The Work for Something Else Copyrighted, so would not appear to be relevant to anything that you could do in a museum. (It does, however, say that there can't be any monetary compensation, so if you put the stuff on a DVD, you can't recoup your materials cost, though you can require an exchange DVD).
In case the work in question is music, (d) and (e) are relevant, the Licensor (creator) asserts the right to collect public performance royalties in case the primary purpose is "commercial advantage or private monetary compensation". This mildly contradicts what 4 restricts. This is just crazy, but required by law, because US Copyright law already has the concept of "compulsory license". The importance of that is that if the law allows X, the license cannot disallow X. 17 USC 115 compels the copyright holder to license nondramatic musical works under certain conditions, so the seemingly contradictory language pertaining to music just mirrors the situation with the law. It would have been a bit clearer if they had included the phrase "The restrictions in section 4 notwithstanding".
Copyright law already contains a limit on the copyright holder's ability to limit the work, with respect to public performances and displays (17 USC 110). The law allows

performance or display of a work by instructors or pupils in the
  course of face-to-face teaching activities of a nonprofit educational
  institution, in a classroom or similar place devoted to instruction

(It is best to read the whole thing to see how much is applicable to the situation in question). So even with a blanket ban on collecting money, there is still statutory permission for non-profit educational performance / display. The combination of what the law says and a generic non-commercial license may accomplish what you want. (Really, figuring out what you're trying to do is most important, and while you maybe should ask a lawyer to draft something for you, don't waste your time and money until you have a clear idea of what you're trying to accomplish).
Comment: That last part is an example of *good "legal advice"* ;)
Comment: This does sound close to what I want. I will investigate the links, thanks!


3.3v pin on a 5v Pro Micro?

Question: I have a arduino pro micro that runs on 5v.
I also have a Max4466 that needs 3.3v, however the pro micro doesn't seem to have a 3.3v VCC.
Is there any way to attach the max4466 to the Board?
Answer: You need to add an external 3.3V voltage regulator. You can get these quite cheap on eBay (etc) as a module - some even come in a breadboard friendly format.
Power it from the 5V from the Arduino, then power your MAX4466.
However, the MAX4466 can run at up to 5.5V, so I fail to see why you think you need 3.3V.
Comment: Thanks for giving a solution, so upvoted. The solution is missing from the link of my deleted answer. For completeness, I repeat your mentioned link here: https://majenko.co.uk/blog/why-you-cant-use-resistors-voltage-regulator

3.3v pin on a 5v Pro Micro?


python adding a title row to csv output file

Question: I have a python script that gets data from a database and outputs a csv file. Now I'm trying to add a header row containing titles for each of the columns. What do I need to add so that it outputs an initial row ahead of the db output (I'm quite a newbie to python, so I got stuck in this part and couldn't find the info I needed)?
Thanks in advance!
Answer: If you're using a DictWriter, you can just call writeheader.


Representing $\frac{\mathbf{r-r'}}{|\mathbf{r-r'}|^3}$ in polar coordinates

Question: In his book introduction to electrodynamics, Griffiths uses derives the identity
$$\nabla \cdot \frac{\mathbf{\hat{r}}}{r^2} = 4\pi\delta^3(\mathbf{r})$$
Using the formula for divergence in polar coordinates. He then states that "more generally"
$$\nabla \cdot \frac{\mathbf{\hat{r_s}}}{r_s^2} = 4\pi\delta^3(\mathbf{r_s})$$
Where $\mathbf{r_s}$ is the seperation vector, $\mathbf{r}-\mathbf{r'}$ and $\mathbf{r'}$ is a constant. I'm trying to find a way to derive this identity using polar coordinates, but am struggling to find a way to represent $\frac{\mathbf{r-r'}}{{|r-r'|^3}}$ in terms of the basis vectors in polar coordinates $(\mathbf{\hat{r}}, \hat{\boldsymbol{\phi}},\hat{\boldsymbol{\theta}})$ as $v_r\mathbf{\hat{r}}+v_\theta\boldsymbol{\hat{\theta}}+v_\phi\boldsymbol{\hat{\phi}}$ for some scalar functions $v_r, v_{\theta}, v_{\phi} $ to use in the formula for divergence in polar coordinates.
Comment: Duplicate : [Divergence of  $\frac{ \hat {\bf r}}{r^2} \equiv \frac{{\bf r}}{r^3}$ , what is the 'paradox'?](https://physics.stackexchange.com/q/488220/).
Comment: Would [math.se] be a better home for this question?
Comment: Hi QED. So you mean polar coordinates with origin ${\bf r}=0$, not polar coordinates with origin ${\bf r}={\bf r}'$?
Comment: Yes, I was trying to avoid the 'translation property' of the divergence, because my understanding of it was somewhat hand-wavy
Answer: By chain rule
$$
\frac{\partial}{\partial x}f(x-x_0)=\frac{\partial}{\partial x}f(x)\vert_{x=x-x_0}
$$
You can do the same with the divergence operator $\nabla$.
Therefore, you only need to prove
$$
\nabla \cdot \frac{\hat{\mathbf{r}}}{r^{2}}=4 \pi \delta^{3}(\mathbf{r})
$$
You could use whatever coordinate to prove this, given that $r\neq0$, since at $r=0$, $\frac{\hat{\mathbf{r}}}{r^{2}}$ blows up. Normal differentiation can't help you determine what happens at the origin, so you have to employ some other tool to do so.
We could imagine a small ball of radius $R$ around the origin, and construct the integral
$$
\int_R d^3r \nabla \cdot \frac{\hat{\mathbf{r}}}{r^{2}}
$$
This will give you a finite result no matter how small $R$ is, and it happens only around the origin. You can argue that the only type of "function" that has such property is a delta function centered at the origin.
By the way, if you really wish to expand something like $1/|\mathbf{r}-\mathbf{r'}|$ with polar coordinates, you will get a series of Lengendre polynomials
$$
\frac{1}{\left|\mathbf{r}-\mathbf{r}^{\prime}\right|} =
\frac{1}{r}\sum_{\ell=0}^{\infty}\left(\frac{r^{\prime}}{r}\right)^{\ell}P_{\ell}(\cos\theta)
$$
Comment: Is there an intuitive reason that the $\frac{\partial}{\partial x}f(x-x_0)=\frac{\partial}{\partial x}f(x)|_{x-x_0}$ justifies this in polar coordinates? I see how it's trivial in cartesian coordinates, but in polar coordinates it seems like there'd be more nuance since the basis vectors change when you change the position.


Impulse Response of a circuit

Question: The following periodic function, \$x(t)\$, is the input to a linear, time-invariant system  The impulse response of this system is $$h(t) = \frac{\sin t}{t} $$ which I converted to $$H(\mathrm{i}\omega) = \pi\,\mathrm{rect}\bigl(\frac{\omega}{2}\bigr).$$ I want to determine an explicit expression for the output \$y(t)\$. I have determined that the period \$T_0=3\$ and \$\omega_0= 2\pi/3\$. I know the equations for the fourier series transform is
$$x(t) = \sum_{n=-\infty}^\infty C_n \mathrm{e}^{2\mathrm{i}nt}$$ and similarly I have $$y(t) = \sum_{n=-\infty}^\infty D_n \mathrm{e}^{2\mathrm{i}nt}$$ where \$D_n = H(\mathrm{i}n\omega_0)C_n\$ but I don't know how to proceed.
Comment: We all had to do this when we were students, so it's homework which is OK as long as you show us what you tried and explain why you're stuck. You should find the answer or a method to find it in most textbooks about the subject.
Comment: It wasn't there when I saw it ! I'll remove the downvote.
Comment: Wait, i did show my work here, why am I still being downvoted? @FakeMoustache
Answer: Note that sin(t)/t is a function called a "sinc".  Convolving with a sinc yields some very special properties.  These are best to consider in the frequency domain.  Look up something called a "dual".
You should go thru the math once, but a sinc and its dual is something you should remember outright.  Again, go look this up.  These are worth knowing without having to do the calculations each time.
In this case, convolution by a sinc yields a special frequency characteristic.  However, due to how duals work, this other function in the time domain results in a sinc in frequency domain.  Again, this is one pair really worth knowing.
Comment: I don't think that he needs to bother with duals right now. They're very advanced signal processing category, while he (as I can see from this and other his posts) is in layman classes. I say he can solve it simply by multiplication in frequency domain.
Comment: @Rok: A dual is just a concept, which he's at the threshold of now.  It also seems he is doing the Fourier transform math already.
Answer: First, redo your math - FT of sin(t)/t is not π rect(ω/2). It's 1/sqrt(2π)*rect(ω/2π).
Okay, so your LTI system is a low pass system with cut-off frequency 2π rad/s, right? So, all of the frequencies higher than 2π rad/s of your input shall be removed at the output. Your input signal base frequency is 2π/3 rad/s, first harmonic is 4π/3 rad/s and 2nd harmonic 6π/3 rad/s. All higher frequency components shall be removed.
Now, you need to know that for frequencies 2π/3 and 4π/3 rad/s the filter attenuation is 1/sqrt(2π) while for the 6π/3 rad/s it is 0.5/sqrt(2π). What you need to do is to find the Fourier coeffs for the these three components and multiply them with these factors and ofc include phase for each of them. The output is a sum of all of them.
y = 1/sqrt(2π) * C1 sin(2π/3 t + Phi1) + 1/sqrt(2π) * C2 sin(4π/3 t + Phi2) + 0.5/sqrt(2π) * C3 sin(2π/3 t + Phi2).

Impulse Response of a circuit


iMessage Vulnerability With iOS 10 Public Beta 1

Question: According to this CNET article, by sending a certain file in an iMessage an attacker could gain access to stuff like passwords on the victim's iPhone. The good news is that Apple is on top of things and this vulnerability was apparently fixed in iOS 9.3.3, however I (and many others) am running the iOS 10 Public Beta. So where does this leave us? Was this fixed in iOS 10 Public Beta 1, 2, or not at all?
Comment: Why the downvotes?
Answer: I haven't officially verified that the bug is not present in the iOS 10 beta, but iOS 9.3.3 was in developer beta for quite awhile before iOS 10 was even announced, so it wouldn't be reasonable for a newer OS, albeit a beta, to not have this patch as well.
If that's not enough, consider that Apple generally does not discuss security vulnerabilities until they are safely patched. CNET only discovered this because Apple posted it in their security report for iOS 9.3.3.  It's not likely that Apple would publicize it if there was still publicly available software that was still vulnerable.


Object detection - how to annotate negative samples

Question: I am using Tensorflow Object detection API to detect 2 objects. I need negative samples because it sometimes detects something random as one of the images . How should I annotate an image to be a 'negative sample'? I am using LabelImg to annotate the images. Do I draw the bounding box around the whole image and label it as 'none'?
Answer: Short answer:
You don't need negative samples to train your model. Focus on improving your model.
Long answer:
Ideally your detector after being trained on your 2 objects would detect them and place a bounding box around them. When you test it and get wrong results this could be caused by a variety of reasons:

A wrong object is being detected if your detector just misinterpreted as belonging to one of the 2 objects you trained it on.
When an image of your 2 objects is being thrown to the detector but it's not detected, then this also means your detector failed to detect your object.

Both cases could of course happen in the same image and also occur more than once.
As far as I know images with no bounding box in them which would be your "negative examples" cannot be processed by the current model because tfrecord reader just fails.
Negative samples do exist implicitly though. All region of your images that do not correspond to a bounding box is a "negative sample". Defining explicitly "negative samples" by selecting them in a bounding box will create a new class with name 'none'. You will have 3 classes then.
So, to make it simpler focus on your positive examples. If your model fails greatly then something is wrong. Check if:

Your test images differ significantly from the trained ones. If so, try to find images similar to your test images and use them along your previous images to retrain your model.
Examine that you have enough training samples to train your model. Enough is not defined strictly but may mean at least a few hundred images.
Examine that your train model contains enough samples from both objects. If not the misrepresented class will be detected poorly compared to the other one.
Examine that you don't have implicit negative examples of your object that could hinder training process. This means if you detect cars make sure at least all visually qualitative appearances in your image are being labels as cars. If you only label 1 car and also left 2 unlabeled ones then those two cars will hamper your training.
Comment: I agree with you!! but Consider this example!! I have trained an object detection model to detect dogs with bounding box!! If I pass the cat picture it should say that its a dog and put bounding around it!! Instead, it should learn that its a cat (-ve image) and should not put a bounding box around it!!
Comment: If your model is trained to detect dogs then only dogs can be detected. In the best case scenario it will ignore cat pictures being thrown at it and in the worse case it will label them as cats.
Answer: You can simply use the verify feature in LabelImg and it will create a file without annotations you can run through your process. Reading through this article it seems negative images are indeed needed.
Comment: If the link goes dead this is not going to be very helpful can you summarise the key points and also expand on why your answer is different from the current one?
Comment: That article is not about the object detection api so is not addressing op's question. Plus, better to give answer rather than link.
Answer: The OP asked about negative samples in Tensorflow Object detection API. I agree that we do not specific images for negative samples in NNs-based Object Detection. All negatives samples are implicitly available when some areas of the images are not labelled (no bounding box on it).


Vacation Days of Assistant Professor in Canada

Question: I may apply for an Assistant Professor (AP) position at York University in Toronto, Canada. Since I live in Europe, I would like to know how many vacation days are available in this position. For example, what does an AP do in those weeks where no classes take place? Does the AP have to go to the office or can he stay home? Thanks for your answers!
Comment: The answer to question depends upon the specific university you would be working at. A 9-month contract might not have any vacation (you are expected to be working when school is in session). A 12-month contract might come with vacation. However, the specific answer to your question depends upon which school you applied to.
Comment: The York university, as most (but not all) Canadian universities is unionized.
 Here is their webpage with the info: [https://www.yufa.ca/](https://www.yufa.ca/). If you look at the agreement you can see that there is one month of paid vacation (over 12 months, I don't know any Canadian university with 9 months contracts). In general you can do what you want during that time, but the North American system expects you to take care of your students even during that time (depending of the field, disappearing for a month can be detrimental to your lab!).
Comment: Is the 9 month contract a thing in Canada as well? I have a faint impression that TT positions are 'full time' here. However that doesn't change the fact that this question is specific to the university and better asked directly.
Comment: @Zenon indeed, disappearing for a month can be detrimental, but nothing prevent you from cutting these vacations into smaller block.
Answer: How much vacation time an assistant professor gets is answered by the collective agreement in force at York U. According to a comment above, that's 4 weeks. These are days when you can do whatever you please.
Obviously, there are constraints regarding the time when you can take this vacation, as pointed out by some comments.
More importantly your question suggests that there's potentially vacation whenever there are no classes. So for one thing you may need the time between semesters to develop your teaching material, but also an Assistant Professor is not a teaching-only position, it's research/supervision + teaching + typically some admin duties.
Finally, note that in many Canadian Universities there is a summer term with classes going on between may and August. So you could very well end up teaching all year around; there's typically about 3 to 4 weeks between the end of classes for one term and the beginning of the next (and that time includes exams and marking).
Comment: That's a very good answer. Thank you very much!


Logo file size for multiple uses in web app

Question: The web app I work on currently has users upload different logo files for each product use--favicon, site logo, print logo, etc. We want to change this so users upload one file that can be used in all cases. I don't have a strong graphic design or visual design background and am struggling to figure out what size file we should ask users to upload for all these uses. My internet searches have only turned up best dimensions for specific use cases.
Comment: Have you confirmed that this is what your users want? I wonder, because the favicon is very often not the same as the company logo (for example, Google's favicon is a simple "G" instead of the full "Google" logo illegibly squished into a 16x16px square). Could you instead consider an option that says something like "[×] Use this logo file as my favicon also"?
Comment: I think you should reconsider this. How often are people going to change the files? My guess is not very often. So the impact of the effort needed to find/create and upload multiple files is smaller than the impact of having wrong or deformed images and not being able to change that.
Answer: OK, so if your server is based on NodeJS, you can try out the RealFaviconGenerator API. The NodeJS plugin can be found here: RealFaviconGenerator NodeJS.
I also found this really neat NodeJS module called favicons (repo)
Just remember that you'll need to make sure to communicate to the user that there's a minimum image size of 260x260px.
Hopefully, you found this useful.


Completely reinstall everything on Mac mini

Question: My Mac mini behaves in strange ways.  Certain applications open randomly, downloads just don't download sometimes, for a while I could not empty the trash, etc.
I want to cleanly erase everything that my computer is including the kernel, Hardware, SMC Recovery firmware and everything else and just put back fresh operational replacements.  I am not looking to buy a new computer (like other questions here said to do) I do not know which one or couple of things is corrupted but I want to just get rid of everything and start over without bricking my computer.  Any ideas?
Comment: What model year is the Mac? What version of macOS do you have installed?
Answer: Restart the machine, while holding ⌘ Command⌥ Option R. The option key gives you the latest supported version of macOS rather than what it shipped with. Then follow the prompts to reinstall a new macOS.
As long as you have a backup of your needed documents and settings I would also erase the drive with disk utility within the process of reinstalling, that will ensure nothing is left over from a potentially damaged OS install.
Comment: Thanks, @Steve Chambers. That's indeed improved.
Comment: Thanks! But will that also reinstall the kernel? A friend of mine said that based on the behavior the kernel might be corrupted
Comment: Yes, the kernel will be replaced. It's part of the operating system.


How can I limit a user to access just his folder? Not able to view/cd into any other thing?

Question: How do I limit a user to access just his folder? Suppose I don't want the user to be able to cd anywhere else, isit possible? Do I need to chmod the whole system?
Comment: If the user does not have permission to see *any* of the rest of the system, then they also cannot run programs, which might be ... inconvenient!
Answer: If the user is sshing in, then you could set up a chroot jail. Not done it myself, but there are various tutorials that can be found via google
Answer: If it's just for a terminal/ssh login, you may want to follow this tutorial to create a chrooted environment for this user.
Regards
Answer: Yes, I think you would have to chmod the whole system. You could create a group for the users to whom you want to allow access, and forbid execution access to everyone else. The user in question would not be in that group.
Why would you want to do that? Maybe there is a simpler way to do what you want.
A better place to ask this would be http://groups.google.com/group/comp.unix.shell
Answer: You could have a look at rbash - basically starting bash with -r restricts what a user can do, but I think it might be a bit too restrictive, ie it doesn't allow you to cd anywhere , not even into subdirs of your homedir.
The problem with implementing something like this is it's usually possible to break out of unless you get really draconian. I just tried rbash, and simply running "bash" in it breaks me out of the restrictions. So you'd have to ensure they can't run any other shell. But then they could upload one, so you'd have to ensure they can't upload anything or mark anything executable. And so on.


In this problem why do we need to use work done by the spring for the work-energy theorem?

Question:

I understand the problem, but I am unsure of why they use the work done by the spring instead of the work the glider does on the spring in the work-energy theorem, and the book also makes it sound like you cannot use the work the glider does on the spring in the work-energy theorem and I am clueless as to why it is so?
Answer: Because the object you are now analyzing is glider, so you have to consider any work the object obtained or lost.
Answer: The work-energy theorem refers to a system.
The system in your example is the glider.
So it is the work done on the glider by an external force (provided by the spring) and the change in energy of the glider which are the relevant quantities.
You could choose the spring as the system and then the glider will provide the external force on the spring and it will do work on the spring this will result in a decrease in the elastic potential energy stored by the spring.
Note that in this case as the direction of the force exerted on the spring by the glider and the direction of motion of the glider are opposite the work done by the glider on the spring will be negative.
You could choose the system as the spring and the glider (and the Earth) with no external forces acting.
Then no work is done on the system by an external force and the elastic potential energy lost by the spring is equal to the kinetic energy gained by the glider.


How to remove or mask sweat spots on clothing in a photograph?

Question: Here is an example

The image isn't mine, I just grab the firs appeared to be good as an example of the topic issue.
Comment: On party photos, I would not be concerned about this... Unless someone pays you for this or you like wasting hours of retouching, this is not a required step.
Comment: @TFuto I'm absolutely agree with you. This work will not worth the effort will be needed. But the OT was regarding spot removal, not the strategy :)
Comment: OK, see the generic and well working method in my answer below then.
Answer: The solution was pretty straightforward.
Covering the stain would work, but just using curves and applying them with a history brush in two modes, soft light and saturation removed the offending stains.
Comment: That is really funny!!! :)))))
Comment: +1 i really like this kind of humor, where the joke is treated as if it wasnt there at all. :D
Answer: It's not particularly easy to remove entirely because where there are sweat stains there are also generally a lot of texture.  I would probably try masking the area (making a layer that covers the area of the stain) and then use that mask to brighten the image up and make the stain less noticeable.  I'd 'also probably feather the edges a bit to help blend it in as the wet spot tapers off.
It's still not going to be 100% perfect, but should make it less noticeable.  It's also still a reasonably challenging thing to do a good job at without making either a dark ring or a light ring around the outside of the stain.
Answer: What you need is the High Frequency Separation technique. Then you will be able to edit the sweat spot on the low resolution image very easily, basically a color replacement, and keep the textures of the T-shirt. You just have to select the proper filter frequency (or pixel blur, in other words, in the described technique).
Do not forget to increase the color bit depth to 16 bit when you start, otherwise you will quickly see color banding.
The other way is just to paint over. With a bit of skill, you can do it with a mouse (... well, I can do it, so you can, too ;-), although a tablet and pen makes this easier (or harder, depending on your skills...).
Comment: +1 for the freq sep, but there's no good reason to go to 16-bit unless you are adjusting gradients (that is, doing calculations on existing pixel values). In this case, simple cloning or painting with a soft brush on the low frequency layer (or, rather, on a new layer above the LF layer but below the HF layer) will suffice, and there's no reason to worry about banding/posterizing.
Answer: You could try a brush with a little exposure compensation which will lighten up the dark patches because the moisture is only darkening those colours.
You won't get it all out without lots of work but it will make a lot of difference.
Comment: Are you talking about "Lighter color" brush mode? I did not find any "exposure compensation" mode of the brush tool (PS CC)
Answer: Will depend on the size and texture of the Stain, either of dodge and burn, exposure brush or layer mask might work, but we'll take efforts and precision.
In case of smaller spots, even spot healing gives decent results.
Comment: You're also talking about "exposure brush". Is it any kind of Photoshop tool? I did not find such Brush tool mode...
Comment: Adjustment brush, select exposure
Answer: Healing brush may work very well in this case.
Comment: Good suggestion. How would someone inexperienced with this tool go about it?
Answer: You can use the clone tool and set its hardness at a low level to avoid the editing being obvious, but you will have to be very precise. So yes, but depending on the position of sweatspots you may not always be able to use this method.
Answer: maybe the "dodge and burn" technique can help for a quick and dirty solution (especially the dodge part of it :-) ). As you probably wont be able to make the stains disappear entirely, you can save yourself some time and try simple but still (maybe) effective ways.


Why doesn't my cat seem to mind loud music despite stronger hearing?

Question: I am effectively making a colloquial assumption that my cat's hearing is better than mine but... if this truly is the case, why doesn't my cat seem to mind loud music? Surprising noises that come out of nowhere (regardless of volume) always freak him out for a second, but for some reason, he seems unbothered by the volume of consistent music. I say this specifically because there are times I have had the music on in the living room with a big stereo system and he will sit very close to it but not really need to move. I am not sure I could stand sitting that close and his ears are stronger than mine... why isn't he phased?
His hearing in other situations is excellent, so I am not sure that it would be related to any sort of deafness.
Comment: Related: https://pets.stackexchange.com/questions/11869/do-loud-noises-damage-cats-hearing
Answer: He could really like the music that you like because cats have senses that humans will never have and probably won't really understand the first time hearing about this, but cats can sense if we are happy, sad, mad, scared, or sick. A lot of times, cats like things that their owners like. But one thing cats really hate is water, I have only had one cat that would like to take a bath or go swimming. But back to your question here is a link to help you understand https://thecatisinthebox.com/blogs/kitty-contemplations/noises-cats-love-noises-cats-hate.
Comment: Please make sure that all relevant information is *in your post*, not at some linked site. The Internet is notorious for link rot, and each answer on SE should be able to stand alone. Links are fine for additional details, of course.


At which end does polymerization of microtubules occur?

Question: My book says that polymerization and depolumerization of microtubules occurs on the + end however, I've found a note that says that depolymerization occurs on the - end. I need help please :) thank you
Answer: From my Campbell's Biology textbook (Ninth Edition):

Because of the orientation of tubulin dimers, the two ends of a microtubule are slightly different. One end can accumulate or release tubulin dimers at a much higher rate than the other, thus growing and shrinking significantly during cellular activities. (This is called the "plus end", not because it can only add tubulin proteins but because it's the end where both "on" and "off" rates are much higher.)

So in summary: polymerization and depolymerization occur on both the + and - ends. The "+" and "-" merely designate the speed/efficiency of polymerization and depolymerization.
Comment: I think this is also a [good reference](https://en.wikipedia.org/wiki/Microtubule_nucleation) that explains how proteins assist in MT nucleation and constrain the direction of MT polymerization.
Answer: Practically speaking, microtubule nucleation occurs at the plus end. This is well established in vivo and in vitro. Notably, the minus ends within most cells are "capped", so dynamics do not occur (think of the centrosome).
However, recently there is a report of MT polymerization at the minus end in a cell; this is far from the consensus among professionals at the moment, and requires much more study. Nonetheless, the rate of any growth and shrinkage at the minus end is significantly slower than the plus end, as stated above.
So for a test question, plus end is the site of polymerization.
Answer: Both polymerization and shrinkage of microtubules take place at plus end and minus end is anchored within MTOC's..hence is the dynamic end


How to recreate Lightroom clarity in Photoshop?

Question: Adobe Lightroom have a awesome clarity slider which is missing from Photoshop. How one can recreate the same effect in PS?
Answer: If you're in Adobe CC or newer than Clarity can be found in Adobe Camera Raw filter towards the top of the filter menu, which gives you a version of Adobe Camera Raw that works even on jpgs. I suggest converting to Smart Object first.
If you're not on Adobe CC than you need to look at sharpening method tutorials because there's far too many to give you a run down of on here.


Looking for beginner friendly sources of cross correlation and TDOA

Question: After some research, I got a rough idea of how TDOA can be used by a microphone array to multilaterate the location of a sound source. Specifically, I'm not referring to a clustered array of many microphones that only give direction of arrival, instead I'm interested in a spread-out microphone array, similar to the ShotSpotter system. Many paper mentions "general cross correlation" or "cross correlation" but none really explains the concept. I see that cross correlation is a technique used to compare the similarity between 2 signals, but how exactly does that help with TDOA? A challenge for real world TDOA system is that the microphones must be time synchronized. Does cross-correlation play a role in helping to solve this issue?
Answer:
but how exactly does that help with TDOA?

The basic idea is rather simple: The location of the maximum of the cross correlation  is the delay between the two signals.
So the process would be the following

Pairwise calculate the cross correlation between all microphone signals
Estimate the delays by finding the maxima of the cross correlation
Triangulate the source location from the delays.

In practice this tends to be lot more complicated

Most acoustic recordings contain a lot of noise: background noise, traffic, HVAC, competing sources, electrical hum, etc
The acoustic transfer function is very complicated: it contains  lot of reflections and reverb
It's often hard to distinguish between the direct sound and the early reflections (mostly from floor and ceiling).
The direct path between the source and the microphone may be blocked so you need to identify diffraction or diffusion pattern.
Level and spectral content of the source signal can vary a lot and maybe insufficient to get a decent delay estimate

In many cases you need to carefully optimize around your specific environment, setup and requirements to get decent results. Things get a whole lot easier if the source signal is known (i.e. from a loudspeaker and you know what the loudspeaker is playing).
Comment: But all of that TDOA process requires that each signal is time synchronized right? If we are just given a bunch of random time drifted signals, I don't think cross-correlation can be applied.
Comment: Yes: all microphone signals need a common time base. Otherwise TDOA doesn't work (no matter which algorithm you use). There are different ways to accomplish this, but it's by far the easiest to use the sampling clock for all microphones


Apache uses it's own 404 handler instead of index.php for .txt files

Question: I want to serve all requests for non-existing files via .php
.htaccess configuration:
ErrorDocument 404 /index.php
DirectoryIndex index.php
RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [L]

This does seem to mostly work but some specific file extensions (.txt,.jpg, some more?) get handled by apache instead of getting passed through php.
http://localhost:8000/home -> shows homepage
http://localhost:8000/file.zip (does not exist) -> shows custom styled 404 page served by PHP
http://localhost:8000/exists.txt (existing file) -> serves existing file
http://localhost:8000/doesnotexist.txt (does not exist) -> shows default apache 404 page (same as when no PHP is installed)
Is there some default apache handler for specific file extensions?
How can I set ALL requests to get passed through index.php?
Using docker php 7.4 apache as my base image
Comment: Does the error still happened if you only have `ErrorDocument 404 /index.php` in your .htaccess?
Comment: Q1) Do you have any of those instructions wrapped in a  declaration and the txt files are in a different directory? Q2) Do you do some "handling" in your PHP file? If the PHP file errors it actually redirects you to a "500" page that probably does not exist and this then sends you to a page that looks like the default 404. You can spot this looking at the redirects and networks calls to see if you are getting a 404 or a 500. Note that a 401 permission error will do the same thing - looks like a 404 but actually isn't.
Answer: I copied your experiment with Docker and the php 7.4-apache image. When opening the url http://localhost:8000/doesnotexist.txt I'm transfered to the index.php file as I would expect to be. The bottom three rewrite rules will rewrite everything to index.php.
First off: You have not posted a docker-compose.yml or Dockerfile, so to be sure: you have to enable the rewriting for Apache by either a command in the dockerfile or a custom Dockerfile like this:
# Dockerfile
FROM php:7.4-apache
RUN a2enmod rewrite

If you would not have enabled the rewrite, an error 500 should appear, because there are no if-conditions around the rewrite rule.
If you did enable the rewriting, the behaviour is still strange, because even if the line ErrorDocument 404 /index.php is not there the bottom rewrite should make that the index.php is served anyway. In other words: The bottom three lines in the .htaccess would make sure that no 404 is served, because everything is rewritten to the index.php.
When you say 'the default apache 404 page' are you talking about the 'Not found' page?
Not Found (H1 tag)
The requested URL was not found on this server.

Apache/2.4.52 (Debian) Server at localhost Port 8000

Try to remove and rebuild the container, so that no weird caching issue can give problems.
And what about your index.php. Is there anything in there that could trigger something strange? Could you post it here?
Answer: Try this in your .htaccess file (without the ErrorDocument part):
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ index.php?file=$0 [QSA,L]

Make sure you output 404 if the code does'nt find the file.
Answer:
Is there some default apache handler for specific file extensions?
No,  there is not such a handler by default.

How can I set ALL requests to get passed through index.php?

ErrorDocument 404 /index.php is valid.
DirectoryIndex index.php is valid
RewriteEngine on is valid
RewriteCond %{REQUEST_FILENAME} !-f is valid
RewriteCond %{REQUEST_FILENAME} !-d is valid
RewriteRule ^ index.php [L] is also valid!

Keep in mind that you are not rewriting it to the root /index.php, instead you are rewriting without the slash, and it rewrites to the index.php in the current directory. So if index.php doesn't exist in the current directory, chances it can get into an infinite loop.
Try:
ErrorDocument /index.php

DirectoryIndex index.php

RewriteEngine On

RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} -f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [L] # use '/index.php' if needed to redirect to root index.php

This will redirect that everything exists to index.php, and avoid conflicts with ErrorDocument.
But, I think this actually what you mean to do:
ErrorDocument /index.php

DirectoryIndex index.php

# remove those rewrite rules


How to choose the frequency grid when using DFT with numpy (FFT)

Question: I'm currently trying to learn how to calculate DFT using FFT with Python. However, I have been struggling to find how should I choose the frequency grid when plotting the final calculated DFT values out of the data samples that I have. Any helpful insights will be appreciated!
Comment: you can't choose. The frequencies of the individual DFT coeffients are defined by your sampling rate and DFT length. Look at the DFT formula!
Comment: This will tell you what the frequencies are: [scipy version](https://docs.scipy.org/doc/scipy-0.19.0/reference/generated/scipy.fftpack.fftfreq.html), [numpy version](https://docs.scipy.org/doc/numpy/reference/generated/numpy.fft.fftfreq.html)
Answer: It really depends on your application and what you want to see and the actual FFT call you make.  Typically if your time series is real valued, the DFT will be conjugate symmetric with respect to negative and positive frequencies and the magnitude information is redundant.  Let assume your data consists of N samples and N is even and you used a complex DFT,  also assume you use C style indexes, i.e. X[0] is the first element in your array. X[0] is going to correspond to the "DC" bin or a frequency of zero.
X[N/2]will correspond to the Nyquist sample frequency.  Most People with ignore X[N/2+ 1] to X[N-1] (or often X[N/2] to X[N])  The thing to remember is that each bin has a conjugate valued twin.  X[0] and X[N/2] are pairs of a sort but don't follow this rule.  X[1] is paired with X[N],  X[2] with X[N-1] , X[3] with X[N-2], and so forth.  One is the positive frequency and the other the corresponding negative frequency.
If you recall your sampling theory, the Discrete Frequency Spectra is periodic, so you can think of the values X[N/2] to X[N] as either the negative frequencies of the spectrum of the cycle centered at DC or the negative frequencies of the cycle centered at the twice the sample (+) frequency. I typically will reorder the upper frequencies so that they are displayed below so the DC bin is the center point of my plot and Matlab has a function called FFTSHIFT that does that, and it also works if N is odd.
If your time series is complex, there are different possibilities.  If x is a Hilbert transpose, the negative frequencies are zero, so unless you want to verify that, it doesn't make much sense to display zeros.
If x is a signal such as an I and Q pair out of a demodulator, using a FFTSHIFT ordering is generally preferable because the frequency shifted carrier will be in the center.
The fact that the upper half of the output are actually the negative frequencies was something that confused me many moons ago.


Multiple linear regression with one binary variable

Question: Can I add 3 continuous independent variables and one binary categorical variable (without making dummy variables, as a dummy variable is created for more than 3 categories?) For example: one dependent variable and 3 independent variables, with the effect between 2 age groups in SPSS analyzed. Do I have to create an interaction term with all the independent variables?
Answer: Explanation
I'm not sure where you got information about a dummy code only being used for 3 categories...that isn't true. You can dummy code any number of categorical predictors. The core point behind it is to provide one category as the reference group and the other codes as contrasts to the reference group. So all you need is two codes: one for the first reference group and a code for the comparison group. You also don't have to use an interaction term. It depends on what you are trying to investigate with your analysis.
Example
As an example, I have simulated data below that shows three IVs and one DV. One of the predictors is a binary categorical predictor of gender, with the coding $0 = male$ and $1 = female$). I am using R here, but the idea should be essentially the same for SPSS and I show it like this to show you how dummy coding from scratch will get the result you are looking for anyway. First I create three random, normally distributed variables called y, x1, and x2.
#### Simulate Data ####
set.seed(123)
y <- rnorm(n=1000)
x1 <- rnorm(n=1000)
x2 <- rnorm(n=1000)

Then I create our main variable. It is a factor that can only have values from 0 to 1, and then I label them with "Male" and "Female".
gender <- factor(
  rbinom(n=1000, size = 1, prob = .5),
  labels = c("Male","Female")
  )

Thereafter I just smoosh all this data into a data frame and fit a regression.
#### Create Data Frame ####
df <- data.frame(x1,x2,gender,y)

#### Fit Model ####
fit <- lm(y ~ x1 + x2 + gender, data = df)
summary(fit)

If you run summary(fit) it will give you a long output, so I have only included the relevant section below:
Coefficients:
             Estimate Std. Error t value Pr(>|t|)
(Intercept)   0.08013    0.04360   1.838  0.06640 .
x1            0.08283    0.03095   2.676  0.00758 **
x2           -0.02543    0.03196  -0.796  0.42638
genderFemale -0.13999    0.06256  -2.238  0.02546 *
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Here you can see the gender variable is now named genderFemale. This is because it is being contrasted against the other level genderMale, which is the (Intercept) of the regression output (this is how it is done in SPSS as well). We can see that compared to the male group, the female group, after controlling for all other factors, has a conditional mean that is -.14 less than the males.
SPSS Example
I don't have SPSS installed nor do I have the space to install it again, but this video below shows a direct example of dummy coding with a binary:
https://youtu.be/c79i8qZc7KM
Comment: Thanks a lot. it was helpful. yes, I had this information to create dummy variables only if the categorical variable has more than 2 categories. here I created 2 variables from age_MRA and AGE_ARA from age categorical variable and used AGE_ARA in the independent variable list to make AGE_MRA as my reference category.
Comment: So long as they are used as one variable (and here two levels ARA and MRA), that would be fine. Coding them as separate variables wouldn't be super helpful.
Comment: Thanks Shawn. So instead of creating dummy variables, I can directly add age category variable..
Comment: SPSS requires dummy coding to numerically interpret your categorical variables. So dummy code them and enter them into your regression. Also a couple notes. First, don't write answers like the two below if they are not solutions to your question...they should be reserved as comments. Second, the E-5 value is scientific notation representing a very low value. Please watch the video I linked and try the way that is prescribed there. After your variable is transformed you can enter it into a regression.
Comment: Thanks, I was not able to post screen shot, so I added answer. I am new to this..
Comment: That's okay! Just edit these into your question next time, otherwise it is confusing for other readers here.


Mavericks in Mac App Store shows install button without text and won't download

Question: I am trying to upgrade to OS X Mavericks from 10.7.5 but every time I click the install button in the Mac App Store, it looks like nothing is happening and now the button is all greyed out and I can't do anything.
Answer: You can enable the Debug menu by typing the following into Terminal:
defaults write com.apple.appstore ShowDebugMenu -bool true

You can then clear the cookies using the Clear Cookies option in the Debug menu.
In addition, you can reset the application using the Reset Application option.

You can disable the Debug menu using the following:
defaults write com.apple.appstore ShowDebugMenu -bool false

Answer: I'm posting this due to having the same issue and being frustrated not being able to find a clear solution online.
I had the bug where the AppStore just said "Install App" or "Upgrade Free" and clicking the button would do absolutely nothing.
The fix was to reboot into safe mode, and then reboot again back to normal mode.
Reboot your mac, right as the startup chime happens, but before the Apple logo shows up, hold down the shift key. This will boot into safe mode and will take about 2-3x longer to boot up than normal.
Once it's finished booting up, just reboot again immediately, back to normal mode.
Now the app store should work correctly again and prompt to download Mavericks, rather than install it.
If you accidentally close Launchpad (which pops up and shows the progress of the download) - use CMD+Space and search for & run Launchpad.
Hope this helps others out there.
Comment: the last mentioned solution by rebooting into save mode helped fine - thanks a lot!


Which filaments actually do need to be stored in a drybox?

Question: As I started to learn about 3D printing, the gist I learned was "it's better to store the filaments in a drybox." As I rechecked these notes, they were to a good degree from an era when PLA was rather new to the market and ABS was the goTo.
Then I learned "PLA is not really hygroscopic and can be stored freely."1
Now, I know some materials are pretty hygroscopic, but not all. So, let's try to pin it down:
Which materials are hygroscopic enough to demand a drybox?
I know that it is good practice to store all filament in a somewhat dehumidified or airtight box, but there are some materials out there that become unprintable and need drying before printing if improperly stored. This question is to point out these "special storage mandatory" filaments only. If a material can't be printed without dry storage it belongs here. If it is a nice to have, it doesn't.
This is a Back It Up question: answers need to provide one of two:

explain personal experience, marked as such.
provide an authoritative source (scientific paper/videos, manufacturer papers, quote from an experienced maker).

1 - For some time (month?). I do store my PLA in a closed but unsealed IKEA container with all the desiccant bags I can find as it is clearly benefitial.
Comment: @Greenonline While Filament Storage asks "Do I need a drybox" This is "which filaments absolutely need a drybox". Its scope is in intent *not* filaments like PLA or ABS that everyone has at home but those that one might want to know about to be hygroscopic before purchase.
Comment: Isn't this the same (or similar to) as [Filament Storage?](https://3dprinting.stackexchange.com/questions/6980/filament-storage)
Answer: PVA
From experience I can tell that PVA filaments need to be stored with silica beads in a plastic bag or in a specific dry-box. PVA is soluble in water and is very hygroscopic. With moisture it gets soft and swells. My Ultimaker came with an open spool of PVA which popped when heated (steam bubbles popping) resulting in very poor quality supports and clogging of the nozzle. A newly bought spool which was properly packed did not have these issues.
Nylon
Some Nylon filament brands require to be stored dry or need to be dried before printing. I have a spool of Nylon that has taken up some moisture although carefully packed, the only time it was out is when it was being printed. This experience applies to Ultimaker Nylon; I've learned now that not all Nylon filaments are behaving the same, it is suggested to look-up the specifics from the manufacturer or from reviews prior to buying.
Comment: Is this an all inclusive list?
Comment: @Pᴀᴜʟsᴛᴇʀ2 Just sharing my experience, we could make it a wiki so other people can add.
Answer: All filaments should be kept in a low-humidity, dry environment.  The OP suggests that PLA doesn't need to be dried.  My experience says otherwise.  I had miserable trouble printing with a PLA spool until I dried it in an oven at about 60 C for a couple of hours.  It then printed well.
A comment was made, and then refuted, about FDM being the same as injection molding.  I agree that, for this purpose, it is.  Both involve heating the plastic so that it softens, and neither process will work well if the filament is outgassing steam.  They both have a confined melt chamber, and steam will be pressurized with the plastic.  When the pressure is released, the expanding steam creates defects in the object.
It is true that some filaments are more hygroscopic than others, but all filaments should be treated as if they are moisture sensitive and should be stored in a dry container.
Answer: Necessary:

PVA otherwise it will become unprintable, because it is VERY hydrophilic and becomes soft, but it is dryable

Better to store boxed:

PLA otherwise it can became brittle, I have some of such PLA, it brakes from time to time if it is just loaded, but luckily it doesn't broke while printing so far.
Nylon is also hydrophilic

Generally you should google for the material you want to use and look for hydrophilic properties or other, e.g. gas releasing, properties and so on.
Answer: PEEK (polyetheretherketone) will print better if has less moisture, and should be used at under 1%.
PPSU (polyphenylsulfone) prints terribly unless it is extremely low humidity (under 0.1%), and is really only reliable when run from a drier to the print head without any exposure to "normal" humidity from the air in the room.
The point at which you have visible cosmetic issues comes at a different point than that at which you have visible structural issues, but even when no cosmetic issues are visible, there are still potential structural issues at a microscopic level.
Answer: You should ask the maker or supplier of the filament which ones really need to be stored in dryboxes and which ones are safe.
Technically, they can all benefit from being stored in dryboxes, but some can handle being left in the open more than others, so it also depends on how fast you go through your rolls of filament. If you use one roll of PLA a month, using a drybox for it is not necessary, if you use it in a year, then a drybox becomes useful.
Though, as a rule of thumb and as was pointed in previous answers, PVA absolutely needs a drybox and Nylon also benefit greatly from being stored in one.
Comment: It should be noted that the length of time will vary depending upon the humidity of the country/environment. A month could be too long in a country located in the tropics, for example.
Answer: All filaments need a drybox. Before any kind of plastic is injection molded, it has to be dry. I'd advocate a vacuum sealer. If they are not free of moisture then the water will turn to steam and cause little steam explosions inside of the nozzle. This blows the molten filament out of the nozzle causing blobs, gaps and other z scaring artifacts. This is the same thing that happens in injection molding machines. Before the plastic can be placed into the hopper the plastic pellets (of any kind) must be thoroughly dry.
I have personally noticed surface condensation on my PETG filament on a rainy day; here in the tropics. A spool of nylon filament that I have would not stop crackling and poping during the printing process. Its moisture absorption was so high that just having it outside while printing was enough to cause printing problems. The resulting part that I printed was also hydroscopic; so much so that it expands if you place it in water and shrinks after is is allowed to dry out. I had tried to make a cover for a water container with the nylon and foolishly turned the container upside down on the counter just to prove that it was a good seal. I came back a few hours later to find water all over my counter and the floor. The cap expanded, let go of the container, and all the water leaked out. It then dryed and shrunk back to the original size. This caused me much confusion as to why the water leaked out, as the cap was still fitting snugly on the top of the container when I came back to it.
The following link from "CNC Kitchen" highlights these issues in more detail.
https://www.youtube.com/watch?v=FAXUjZZER5E
In short the video says that:

Wet filament will have stringing, bubbles and substantial oozing.
Moisture can break up the long chains of the polymer changing the mechanical properties of the plastic, resulting in embrittlement.
Air bubbles end up in the printed part and result in delamination.
The surface finish of wet filament will be rough and inconsistent, requiring post processing (sanding, filling etc).
Comment: As the original comment has now been deleted, you may want to include your comment "All filaments. Before any kind of plastic is injection molded, it has to be dry. I'd advocate a vacuum sealer." in your answer (or maybe just the vacuum sealer part) - I'll leave it up to you, but I thought that I'd point out that the comment has been deleted. :-)
Answer: Some brands of PLA+ and colors of filament seem to have issues if left in the open air for too long a period.  These then need to be dried back out in a dry box.  The filament seems to be softer and the hobb seems to strip it out more easily causing print failures.  The colors I have had the most troubles with are transparent Blue, Dark Green, and silky Copper.


Programming image maps

Question: I am looking for a javascript/jquery solution to build a map with selectable tracks. It will consist of area's map (image) with some river routes for boats that have to be selectable - route have to be made clearer on mouse hover and after user clicks on a route a popup window should emerge. Basically I am just looking for a solution for this type of 'map'. Maybe there are some jquery widgets/plugins that would ease the work ? I don't know where to start actually.. Can anybody give some pointers ?
Thanks in advance,
Patrick
Answer: Maphighlight should do the trick.

Programming image maps


Should I buy a lawn mower with or without a cord?

Question: The land I have to mow on is rough and rather large. I must decide if I should buy a corded lawnmower or not and I am looking for help making the right purchasing decision. I was hoping someone with first hand experience would care to weigh in on the matter? In truth, any help would be appreciated.
Comment: I once owned a 3/4 acre lawn as part of a 2 1/2 acre parcel in the country. We mowed our huge front lawn (which was on the side of a fairly steep hill) for a couple of years with a gasoline-powered mower and then we asked ourselves "Why are we doing this?". It took over an hour to mow, neither my wife nor I liked doing it, and there was literally no one around to enjoy it but us. And we basically never walked it. So we stopped & mowed two much, much smaller lawns and a path around the formerly mown lawn for maintenance & to keep the ticks at bay. So - is there a reason you're mowing this lawn?
Comment: The significance of the cord itself tends to be overlooked.  If you need 200 ft the cord will  probably be heavier than the mower and substantial cost .  The longer ,the heavier the gage . You will probably need 14 gage; price that ! I use 100 ft of 16 gage , enough for a blower or a weedeater ; not a mower . It is a serious pain in the as* to handle  just for blowing or weedeating much less mowing. Recently had to remove a small tree with a baby electric chain saw ; had to compromise with 50 ft of 14ga + 50 ft of 16 ga.  It is an amperage X resistance problem.
Answer: The first criterion is a mower that is big and powerful enough to do the job efficiently. Corded mowers tend to be small and not very powerful (the maximum power is limited by what is available from a standard household power socket, and/or the maximum safe current on the cable). On the other hand, a mower with a battery too small to finish the job in one go is also annoying and time wasting, or more expensive if you fix the problem by getting an extra battery pack.
If those criteria still leave you a choice between corded and cordless, handling the cord while mowing is an extra task you have to do, and is more extra effort if you need a longer cord for a bigger lawn.
Comment: But small and light could be an advantage, if you have storage issue.
Answer: We don't know the size of you lawn, so it is difficult to give you a precise answer, but we can help you to decide.
I have all kind of mowers.

electric mowers (so corded): very handy for small surface: small, light and quick to do regular cuts. I find it need less maintenance. Possibly it requires to have a second person to help with cable (especially if you have complex geometries and nice flowers). On the other hand, they are small, so it is easier to move it between flowers.
cordless mower: usually much more powerful, so you can use also less regularly and for large surfaces. They requires some more maintenance. You may have them with traction (moving wheels), which help on large surfaces (and on not flat gardens). Some new models (like Grin) mulch grass, so you doesn't have to collect (which is annoying on large gardens [collection and compost]). If you want to be more "green", you may use Aspen gas (for 4 stroke engines).
On larger gardens, a lawn law-mower, you may get a ride-on-version: more maintenance, it needs more space to turn. Depending on models you may have cutter behind wheel, or under the mower. The first are larger (for storage), but you can cut better the borders, and the higher grass (on the second type, your wheel will step on.
I have also a  sickle-bar mower, for larger surfaces, which I cut less. I use the hay. Only for larger surfaces, but you can keep nice flowering field, which is nice for nature (wild flowers and insects).
and string cutter. This is mostly for borders and places with many rocks. Result is not so good (and regular) as the other, but for some places you may need it.

There are also non-motored lawn mowers (which I would not recommend, but if you want to make a lot of exercise), and Scythe/Sickle (now is again fashionable, but I would use them only to replace the sickle-bar mower. Probably not.).
So you see, there are many option. It is difficult to find the best one. I would go to corded lawn mower if you have a small lawn. Non corded lawn mower on the other cases. You will know it when you will need an additional machine.
Answer: I would always choose a cordless lawnmower regardless of the size of the property. Corded lawn mowers, in my experience, are more of a hassle than they are worth. That being said, if your property is large and rough than you have all the more reason to go cordless.
Comment: Some people will argue the opposite: starting a cordless mower first time on spring could be difficult.
Answer: There are also petrol (gas) mowers. In the UK they are commonly used for larger lawns. You may also want to look at the direction of cutting, depending on whether you want stripes in your lawn or not.


How did 'rip' + 'snort' compound to signify 'something of exceptional strength' (ripsnorter)?

Question: These answers don't answer the question, as they no more than repeat the etymology and don't expound Etymonline. So I tried OED, but it also doesn't expound the etymology.
Comment: 'Ripping' is first noted in the meaning 'excellent or splendid' in 1776 _The Battle of Brooklyn_ "ripping work, my lord !" [OED](http://www.oed.com/view/Entry/166237?rskey=iuosTx&result=4&isAdvanced=false#eid).
Comment: Yes there appears to be a dearth of etymological evidence for ripsnorter!
Answer: Snorter is first attested by Green's Dictionary of Slang in 1818 meaning "a blow to the nose."  By 1828, it is cited in reference to "exceptionally large, strong, [or] violent weather," and eventually also objects and people.
The short answer to this question is the latter sense of "snorter" is, as the OED cross-references, the best etymological clue as to where "rip snort" derives, where rip is referred to by GDoS and the OED as a variant of its use in standard English.
Whether those two senses are connected etymologically as they are somewhat conceptually is an interesting question.
In 1824, "snorter" can be found referring to mortar fire in a rhyming dialectical poem, which predates GDoS's reference to extraordinary weather or objects, and could be seen as a bridge between the notion of a blow to the nose and something extraordinary and large and violent.

There was one thing, I swow! 'twas a snorter,
'Twas raly a curious concern.
They told me, I think, 'twas a "morter"--
There is always something to larn.

1824 - Poughkeepsie Journal (Poughkeepsie, NY) 18 Feb. 1/5 (paywall)

If this and other similar citations are a clue, you can trace back "rip-snort" to the notion of "snort" as a sound made by the nose, extended by slang to refer to a strike to the nose, then figuratively extended to the notion of a powerful strike, and onward to simply anything powerful and "of exceptional strength."
Answer: A snort is the sound some people make when they are either displeased or laugh loudly at something that they didn't expect was going to be funny. It's an involuntary response, not pretty to hear as it can sound like a pig snorting,  but at the same time, it may also provoke hilarity from bystanders or other members of the audience as it is ugly to hear but one or more snorts are often signs of genuine mirth.
To be able to rip a snort from someone who makes snorting sounds, i.e. a snorter, requires great (metaphorical) strength and skill. Thus a ripsnorter is a person, or thing that seizes and pulls (rip) with strength a laugh or indignation from someone.
I'm only kidding.
From Dictionary.com

noun snorter
  1. a person or thing that snorts.
  2. Informal. something extraordinary of its kind:
a real snorter of a storm.

From Vocabulary.com

snorter
1. n. someone who expresses contempt or indignation by uttering a snorting sound
n. something outstandingly difficult
n. something that is extraordinary or remarkable or prominent
"a snorter of a sermon"
"the storm wasn't long but it was a snorter"
Comment: Some people have no sense of humor...
Comment: we usually just see your intelligence!
Answer:
Its first appearance was attributed to Davy Crockett ~1840 ("Of all the
  ripsnorters I ever tutched upon, thar never war one that could pull
  her boat alongside of Grace Peabody").  world wide words

Nice short read ... Of Davey AND etymology.


which feature is the strongest predictor of classification algorithm?

Question: For any classification/recommendation algorithm how should I approach to decide from a given list of features which is the strongest predictor? Is this known as feature engineering? what analysis should one do on the data like plotting graph etc. to determine the good set of features?
Comment: This is a very broad question and the subject of an entire subset of statistical analysis. One answer will for sure not cover all relevant factors. Feature engineering is creating new variables from existing ones, you are instead referring to feature selection. If you would like a quick and dirty tip, I would say consider using Lasso regularization. You get good predictions and as a side effect, feature selection through shrinkage.
Comment: This is algorithm dependent, and as such it's too broad to conform to StackExchange format, imo.
Answer: What you are asking about is called either feature selection or feature importance depending on the context.  Feature selection would be if you want to subset your features either before or during modeling to improve performance.  Feature importance would be if after you create your model, you want to look back and see which features were the model helpful in your final classification.  Either way, it is highly dependent on the method your are using for your classifier.  For instance, a feature with a parabolic relationship with the classes (ie very high or very low values are one class but values in the middle are another) won't be a very strong predictor in a linear based model (ex. GLM or regularized GLM), but could be highly predictive for models that allow for non linear relationships (ex. SVMs).
If you are interested in feature importance, you may be able to determine this from the model itself.  For example, in a GLM model, if you normalized your features before modeling, the feature with the largest coefficient will be the strongest predictor.  Random forest can also give feature importance based on the number of trees and the location of the feature in those trees, but it is more complicated than in GLM, though most implementations will offer a method to calculate feature importance for you.
If you are interested in feature selection, there are a plethora of methods available.  Using regularization in your model (ex. lasso) is one option where the model itself has an internal penalty on the coefficient values so it will self select variables by bringing their coefficient to zero to escape the penalty.  Methods for selection before modeling are even more diverse.  A simple one is to look at the correlation between the feature and the class and just choose the variables with the highest correlation.  Another option, called Recursive Feature Elimination, uses set aside training data to first make a model with all the features and then uses variable importance as described above to rank features.  Features are then removed in order of their importance and new models are made to see what is the optimal subset of features to use.  Getting even more complicated, you can use things like genetic algorithms to select the features that optimize a particular model.  These are just a sampling of examples, there are many more methods available.  All of these methods have pros and cons in terms of time to run (correlations are quick, genetic algorithms are not) and flexibility (correlations only look at linear relations ships, genetic algorithms are tuned to the model).
In short, there is no one answer here.  You need to think about why exactly you need to find the strongest predictor, what kinds of methods you are using for modeling, what are the relationships between the predictors and the outcome, and what are the relationships between the features and each other (ie if two features are highly correlated, something like lasso may throw one out even if it is highly predictive because it is redundant with another feature).  When you get a better idea of what you want, post again and I am sure you will get a more specific solution.
Comment: (+1) It might be useful to provide some links to the concepts/methods you discuss (e.g. Q&A's or [tags](http://stats.stackexchange.com/tags/feature-selection/info) on this site).


Why did French have such a lasting influence on the English legal lexicon?

Question: It seems so pervasive but, still confusing that we don't simply use French altogether for legal purposes, which I suppose was done at one time. But why were certain terms kept from French, while the rest of the language used reverted to English? It would make a bit of sense if there had been no terms available in English to refer to e.g. corporate bodies prior to the Normans' arrivals, but that doesn't seem terribly plausible.
Comment: The language spoken before the Normans arrived was Old English which is unintelligible to current English speakers: The first verse of Beowulf is "Hƿæt! ƿē Gār-Dena in ġeār-dagum, þēod-cyninga, þrym ġefrūnon, hū ðā æþelingas ellen fremedon.
Oft Scyld Scēfing sceaþena þrēatum,"  So the question is whether there was a term in Middle English to refer to corporate bodies that was not adopted from Norman-French.
Comment: English words derived from French have persisted from the 11th century to the present and are often about class. Animals in a field are called: cow, sheep, pig and animal parts on a plate are called: beef, mutton, pork. When William the Bastard conquered England he brought a lot of French rulers and French laws. Their words persist everywhere.
Comment: You may wish to consider asking this over at [EnglishSE](https://english.stackexchange.com/), with either or both [history](https://english.stackexchange.com/questions/tagged/history) and [historical-change](https://english.stackexchange.com/questions/tagged/historical-change) tags. If you do, it's best to check if it's on-topic first.
Comment: @bdb484 they are a little more noticeable in legal parlance, because legal parlance changed less than the common man's language. Parlance by the way? **French**
Comment: An interesting question, but I'm doubtful of its premise. How much French really persists in the English legal lexicon? I've certainly never done any kind of analsis, but I'd guess that French influences are actually less common in the English legal lexicon than in the English lexicon more broadly.
Comment: Yes, I think as well because there is a definite class bias: elite correlated with more likely to speak French, and also more likely to be part of the legal world.

Also, at first I thought @bdb484's comment was going to compare things rather to  the situation of English terms used in French law nowadays, given that English now has an influence on virtually every world language and in virtually every field. That's an interesting juxtaposition, as well.
Comment: But *parlance* isn't legal lexicon....
Answer: Because the Normans spoke French.
The English Nobility didn't speak English till the 14th century at all. Why? Because the Norman Nobility spoke French till the whole debacle of the Hundred Years' War was halfway through.
William The Bastard left only very few nobles from before in power when he landed in 1066 and delivered a smackdown on the previous rulers. He established a whole new nobility and based on his orders (in French), this also formalized most of the Common law between 1066 and the 100 years war. It was the Norman nobility talking, and they had the text written in French and Latin, the civilized languages, not the gibberish Old English/Anglo-Saxon and Norse that was used by the peasants. Any English you and me would understand wouldn't even exist for another 200-300 years. Or to cite the Britannica:

The Normans spoke French and had developed a customary law in Normandy. They had no professional lawyers or judges; instead, literate clergymen acted as administrators. Some of the clergy were familiar with Roman law and the canon law of the Christian church, which was developed in the universities of the 12th century. Canon law was applied in the English church courts, but the revived Roman law was less influential in England than elsewhere, despite Norman dominance in government. This was due largely to the early sophistication of the Anglo-Norman system. Norman custom was not simply transplanted to England; upon its arrival, a new body of rules, based on local conditions, emerged.

English changed.
Would you understand Beowulf from about 700-1000 AD, which is Old English?

Hwæt. We Gardena in geardagum,
þeodcyninga, þrym gefrunon,
hu ða æþelingas ellen fremedon.

Between this and modern English is a very heavily French-influenced English called "Middle English". This still is mostly illegible to a modern reader, but you find this from about 1066 to the 15th century. This example is from The Canterbury Tales:

Our Hoste gan to swere as he were wood,
'Harrow!' quod he, 'by nayles and by blood!.
This was a fals cherl and a fals Iustyse!
As shamful deeth as herte may devyse
Come to thise Iuges and hir advocats!
Algate this sely mayde is slayn, allas!

Only after the Hundred Years war ended, active steps were taken to remove French from the courts and language, which would result in modern English - yet the influence was extreme in the legal world then.
To come back to the example from Beowulf: I bet you do understand the Francis B Gummere (died 1919) translation into modern English, even if it feels a little archaic to us in 2023:

LO, praise of the prowess of people-kings
of spear-armed Danes, in days long sped,
we have heard, and what honor the athelings won!
Comment: More generally, main difference between Old English and Middle English, is that Middle English reflects the influence of Norman French on Old English. The transition from Middle English to modern English, in turn, reflects the discontinuation of continued Norman French linguistic influence.
Answer: At the beginning of the Norman occupation, regardless of how the noble may have talked casually (in French), the language of law and government was Latin. It was not until the movement to centralized "common law" and the onset of French-language statutes around 1300 that French gained a place in the English legal scene. An example is the
Pleading in English Act 1362, written in English and French, where it is decreed that pleadings (the stuff that happens in court) shall be in English because that is what the people understand, but records shall be in Latin. English became the official language of government during the reign of Henry V at the beginning of the 15th century. The point to be noticed here is that there has long been an accepted split between the language of the rulers and the language of the masses. Amongst the rulers, the two dominant languages were Latin and French, but French was "how people talked", and did not have the majestic status of Latin.
During this period, important legal doctrine were developed. Essential to any effective doctrine is a name. Legal doctrines were of course created by upper-class folks who spoke French, English and Latin, with a decline in knowledge of Latin and French over the years. Once a legal concept is named, it is difficult though not impossible to change the name, and thus "fee simple" survives to this day because no Germanic expression was developed to exactly identify this legal concept.
The practice of copying standard forms was a major contributor to the preservation of French in legal language. The underlying idea is that if you can write a legal document that works for a certain purpose in court, you would be well-advised to re-use the expressions of those documents the next time (befitting the status of "precedent" in English law – follow the law as previously identified). These documents would contain French expressions because "tort" and "guarantee", for instance, were well understood concepts, as identified in earlier legal writings (which might be in French – there are also plenty of Latin terms do preserved in legal language). These word were etymologically derived from French, but became part of English, just as many other terms (war, cabbage, pork, poultry) are etymologically from French but they are simple ordinary English words now.
There were no corporations in the modern legal sense in Anglo-Saxon England, so there was no Old English term for "articles of incorporation". The legal concept has to first exist, then we can worry about the name for it, and which language we use to refer to it.
Answer: As explained in the other answers, there are deep historical reasons for the extensive influence of French on modern English generally. However, the peculiar history of Law French in the 16th and 17th centuries is also an important reason for the continued prominence of French terms in English law.
The aristocracy in England stopped speaking French in the 14th century, and spoken French was abolished in the royal courts by the Pleading in English Act 1362. However, English lawyers continued to use French terms in writing, and by the 16th century, these lawyers generally spoke no French at all. Consequently, the law reports were filled with a strange and ungrammatical blend of French and English, which influenced the development of English law for centuries after French had disappeared as a spoken language.
The Wikipedia article gives the following example of Law French in a 1688 law report, which demonstrates that it's not quite French or English:

Richardson Chief Justice de Common Banc al assises de Salisbury in Summer 1631 fuit assault per prisoner la condemne pur felony, que puis son condemnation ject un brickbat a le dit justice, que narrowly mist, et pur ceo immediately fuit indictment drawn per Noy envers le prisoner et son dexter manus ampute et fix al gibbet, sur que luy mesme immediatement change in presence de Court.
Comment: "Brickbat"! Wonderful! :)


Why does Armory rescan the block chain every time it opens?

Question: The title says it all: every time I open Armory, it appears to rescan the entire block chain (after synchronizing). Why can't it just keep a file that says if any blocks contain transactions for my wallets, or something similar?
Answer: We just release version 0.90-beta, which finally solves this issue.  Armory does not need to rescan the blockchain anymore unless you import new private keys, or Armory experiences an unclean shutdown.
With the exception of the first-time startup (to build the new databases), Armory should get into online mode within 10 seconds of Bitcoin-Qt/bitcoind finishing synchronization.
Comment: hm, I'm using 0.90 and it's still scanning the whole transaction history :/
Answer: There is no technical limitation, the developer just hasn't gotten around to it yet. This is part of a larger rework of the Armory block chain database.
See this GitHub issue for more.


Which is bigger: $9^{9^{9^{9^{9^{9^{9^{9^{9^{9}}}}}}}}}$ or $9!!!!!!!!!$?

Question: In my classes I sometimes have a contest concerning who can write the largest number in ten symbols. It almost never comes up, but I'm torn between two "best" answers: a stack of ten 9's (exponents) or a 9 followed by nine factorial symbols. Both are undoubtedly huge, but I haven't been able to produce an argument that one is larger (surely they aren't equal). Any insight into which of these two numbers is bigger would be greatly appreciated.
Comment: @Fdart17: $9^{999999999}$ is enormously smaller than the exponential tower of ten 9s, which is the number being considered here.
Comment: "(surely they aren't equal)": One way to see that the exponential tower is not equal to 9!!!!!!!!! is to note that the first is odd and the second is even.
Comment: Depending on what you're willing to allow without bracketing, neither of these is the best you can do with these symbols. $9!!!!!!!!!$ is smaller than $9^9!!!!!!!!$, while (tower of ten 9s) is smaller than (tower of nine 9s)!.
Comment: Try taking the logarithm several times, estimating the results using Stirling's formula: http://en.wikipedia.org/wiki/Stirling's_approximation
Comment: Is $9!!!!!!!!$ even standard notation? Aren't brackets necessary there? For example, $n!!\ne (n!)!$. That wouldn't allow you to use so many factorials. (Although your question still stands open but I'm clueless.)
Comment: You can always define new symbols which give you higher number than before. You want perhaps to limit your alphabet to digits, addition, multiplication, exponentiation, factorial.
Comment: It seems $9!!!!!!!!!$ is bigger.  If you input both into wolfram alpha, then the factorial computation times out, while the $9^{999999999}$ yields a number with $954 242 509$ digits.  This is not perfect since the way factorials are calculated may make this computation time out earlier, however.
Comment: The mathematicians version of "Mine's bigger than yours"!
Answer: $n!$ grows more rapidly than $9^n$ (it grows approximately with $n^n$), so eventually it wins out; in fact, a few moments with Wolfram Alpha suggests that for all $n\gt 21$, $n! \gt 9^n$.  This means that the best solution is mixed: after the first exponentiation (since $9^9$ is greater than $9!$), you're better off using factorials, giving the answer $9^9!!!!!!!!$.  (Also, note that we're all assuming that the correct parentheses here are implicit, since there's a big difference between $\left(9^9\right)^9$ and $9^{\left(9^9\right)}$.)
On the other hand, if you're allowed to use more-or-less stock mathematical notation, you may want to have a look at Knuth's arrow notation; $9\uparrow\uparrow\uparrow\uparrow\uparrow\uparrow\uparrow\uparrow9$ is bigger by far than the rest of these.  (And numbers even larger than this have come up in mathematics - have a look, for instance, at Graham's Number, which most conveniently uses the arrow notation in its definition.)
EDIT: In fact it's pretty straightforward to prove that the answer in the first paragraph, $9^9!!!!!!!!$, is the best that can be done (with these operations).  First, as long as $a$ is at least two symbols (for this group of symbols), then $a^9 \lt 9^a\lt a!$, so adding a factorial symbol is always going to be better than adding a $9$ onto your tower.  But what about structures like $9!^{9!}$?  Well, if both $a$ and $b$ are at least two symbols long, then $a^b\lt \mathrm{max}(a,b)!!$; assuming for the moment that $b$ is larger (because if $a$ were larger, then $b^a\gt a^b$), then $b! \approx b^b$, and while this isn't enough to ensure that it's greater than $a^b$ (consider the case where $a$ = $b$ = $9!$), it's close enough that we can be certain taking a second factorial will be larger - so those extra symbols you spent on exponentiation should retroactively have just gone into more exclamation points, and this is enough to guarantee that $9^9!!!!!!!!$ is the largest possible combination of these particular operations.
Comment: What do you mean by "so eventually it wins out" ? Of course I agree that $9^9!!!!...$ is best possible, but what about the original structures the OP is asking about? I think you can you prove that for any number of characters $n$, the power tower of nines will be larger than the iterated factorials.
Comment: +1 for mentioning Grahams Number.  I always have little chuckle when I see it's definition.
Comment: Sorry - by 'eventually it wins out' I just meant that for some n, n! will be greater than 9^n - but as Jonas points out, that doesn't imply that the iterated-factorial will be larger than the iterated-exponential with one additional iteration.
Answer: Wolfram Alpha shows $$9^{9^9}\approx 10^{10^{8.56}}$$ while $$(9!)! \approx 10^{10^{6.26}}$$  Adding another character adds another 10 to the bottom of the stack without changing the upper exponent much at all.  It will even do the full stack of 10.  The exponents have 8.5678 atop the tower of 10's, while the factorials have 6.26949.  So the exponents win.
Added:  In Douglas Hofstadter's May, 1982 column "On Number Numbness" he declares these essentially equal.  For numbers of this size, the first thing you should look at is how many times you have to take a log to make it reasonable, which is 9 for both of them.  Then look at the number on top of the stack, which is the only one that matters.  So it is like 9.85678 compared with 9.626949, which are very close.
Comment: +1 for being the only one to answer the OP's original question, namely that the tower of nines is larger than the factorials.  In fact, I think you can you prove it is true no matter how many times we iterate.  That is for any number of characters, say $n$, the power tower of nines will be larger than the iterated factorials. (I find that shocking since factorials is basically $n^n$)
Comment: I have to admit, I find this really surprising too, but no less impressive for it!
Comment: For comparison $(9^9)! \approx 10^{10^9.5}$
Comment: @Eric:  thanks for noticing.  Sometimes these questions take on a life of their own, but there was an original question that prompted it.
Comment: $9!!!$ is roughly $(10^{10^{6.26}})^{10^{10^{6.26}}}$, much greater than $9^{10^{10^{8.56}}}$ (most likely), so adding a character (a factorial in one case and an exponent to the tower in the other), seems to favor factorials here by quite a bit.
Comment: @Mitch:  but adding a new number to the bottom of the stack is quite effective.  As I said, Alpha shows the stack of exponentials wins over factorials for the same number of characters all the way up.  But repeated factorials of 9^9 are better yet
Comment: Just to clarify: the method of comparing numbers in the last paragraph does not originate with Hofstadter. It was discussed by Littlewood in his Miscellanies, and probably goes back even further historically.
Answer: A thorough study of this question is given in the article "Exponential vs. Factorial" by Velleman (American Mathematical Monthly Vol. 113, No. 8, Oct. 2006, pp. 689-704).  In the motivating example there are 5 characters instead of 10.  Of course, the conclusion is the same as in Steven Stadnicki's answer, that $9^9!!!!!!!!$ (or $9^9!!!$ in the case of 5 characters) is the largest possible.
The article also addresses your original question, proving that the exponential tower of $n$ $9$s is always larger than a $9$ with $n-1$ factorials applied (and many more general statements).
Comment: Excellent answer! +1!  No need to say more.
Comment: @Eric: Thanks.  I just knew where to look because I remembered skimming the article when that Monthly was new.
Answer: I find it quite sad that none of the answers has actually formally shown that

$$9^{9^{9^{9^{9^{9^{9^{9^{9^9}}}}}}}} > 9!!!!!!!!!$$

The proces is a bit long, but it it worth it in my opinion. First, a simple calculation shows $9! < 9^8$. Then $9!! < (9^8)^{9^8} = 9^{8\cdot9^8}$. Then $$9!!!< \left(9^{8\cdot9^8}\right)^{9^{8\cdot9^8}} < \left(9^{9^9}\right)^{9^{8\cdot9^8}}=9^{9^{8\cdot9^8}\cdot9^9}=9^{9^{8\cdot9^8+9}}$$
$$9!!!! < \left(9^{9^{9^9}}\right)^{9^{9^{8\cdot9^8+9}}}=9^{9^{9^{8\cdot9^8+9}}\cdot 9^{9^9}}=9^{9^{9^{8\cdot9^8+9}+9^9}}<9^{9^{9^{8\cdot9^8+10}}}$$
Now, some notation. Let us use $f(x)$ for $9^x$ and $f^k(x)=f(f(... f(f(x))...))$. Let $x!^k$ denote $x!!!...!!!$ with $k$ factorials.
Clearly, $f(x)+1<f(x+1)$. By induction, $f^t(x)+1<f^t(x+1)$ for any $t$.
Now we proceed by induction to show that $9!^k < f^{k-1}(8\cdot9^8+k+6)$. The base case has been given above. Assume $4 \leq k \leq 9^8-7$.
$$9!^{k+1} < \left(f^{k-1}(8\cdot9^8+k+6)\right)^{f^{k-1}(8\cdot9^8+k+6)}=
9^{f^{k-2}(8\cdot9^8+k+6)f^{k-1}(8\cdot9^8+k+6)}<9^{9^{f^{k-3}(9^9)+f^{k-2}(8\cdot9^8+k+6)}}<9^{9^{9\cdot f^{k-2}(8\cdot9^8+k+6)}}<9^{9^{9^{f^{k-3}(8\cdot9^8+k+6)+1}}}<9^{9^{9^{f^{k-3}(8\cdot9^8+k+7)}}}=f^k(8\cdot9^8+k+7)$$
This completes the induction hypothesis. Hence we have shown the asked.

Now for some fun: The largest number using ten symbols. We can also use shortened Knuth's up arrow notation, $9 \uparrow^{9 \uparrow^{9 \uparrow^{9} 9} 9} 9$ has ten symbols.
Conway chained arrow notation, $9 \rightarrow 9 \rightarrow 9 \rightarrow 9 \rightarrow 9^9$.
Rado's sigma function, $\Sigma(\Sigma(\Sigma(9)))$.


Subtracting a CSV List from Another?

Question: I have a huge list of broken links I generated with Screaming Frog and started to fix a lot of them.. I ran the csv file back through screaming frog to see which broken links I had left so now I have 2 CSV files.  How do I subtract the newer list from the old list so I can see which links I already fixed?
Answer: The following method assumes:
A:  All broken links are in oldfile.
B:  Some broken links are in newfile.
C:  The shared lines are exact duplicates.
sort newfile oldfile | uniq -d > filesThatAreStillBroken

or
sort  newfile oldfile | uniq -u > filesThatAreFixed

Sort merges the files together into a single sorted list.  It doesn't matter if newfile or oldfile is first.
uniq -d puts out only lines that occur more than once.  Since they were in both lists, they are still broken.
uniq -u puts out only lines are are unique.
Note:  This won't catch new errors that you have introduced while fixing the old errors.  New errors will only be in newfiles and so will be falsely reported as fixed in the second invocation, and not reported at all in the first invocation.
Type
man sort
man uniq

for more details about these two command line utilities.
If you are on a windows box, you can install the cygwin environment, or possibly windows now has a posix command set.
Comment: I love Linux, I love bash!
Answer: Import both CSV's into excel.
Add the formula into cells in column B of the larger list: =COUNTIF(Sheet2!A:A,A1)
This will give you a count of how many times that cell appears in the other list.
Now you just have to delete any that have a count > 0.
Tip: To delete the rows easily: Add a header row, turn on auto filters, deselect count of 0, delete the rows, turn off auto filters.  (Alternatively, you could sort your list if you don't mind the order getting messed up)
Answer: Try this function in Excel:
=IF(COUNTIFS($B$1:$B$6, A1), "Borked", "Fixed")

Just make sure that the A value points to something in the smaller list (still broken) and the B range covers the original set of broken links
Comment: Just note that this function will need to be run on every item in your list of still brokens links. (just drag the little black box in the corner of the cell, the $'s keep the B range from changing with the A value)

Subtracting a CSV List from Another?


Exporting map models for games

Question: I am creating a map for a 3d game. How do I do it?
I can create a huge model of the map and export as collada. Is that the right way? Do I have to export in a certain way or is it ok to export it like normal model?
Comment: It depends on the game engine, but exporting as .obj, .fbx or collada will probably work. The only way to know for sure is to communicate this with the person developing the game. There may be certain requirements in their game engine or with their workflow.
Answer: Well, game environment is a very complex process. So you won't find the answer in a forum, is like asking "How to use blender?".
But I can give you a starting point.
Game environment could be divided in two main parts "Terrain" and "Assets". Terrain is the process of creating the soil, mountains, rivers, the topology (in landscape language not mesh language). Commonly it isn't created as a .obj, .fbx, etc... file. To do the landscaping usually you use heightmaps (Unreal Engine 4 https://www.youtube.com/watch?v=E5pHoWfpfYw , Unity https://www.youtube.com/watch?v=qpEMhWgJTZ4 ).
Assets use the .obj, .fbx, etc... file format. They are used to make buildings, add rocks, trees, improve the landscape (because heightmaps are only the foundation of the landscape). They are usually separated by object, a car is one file, a building is one file, a entire city isn't usually a single file because it cost a lot of computer resources, you usually create 5 buildings and repeat them.
Nevertherless there are modular games, but thats a whole new way of creating maps.
Also, usually the maps aren't builded straight away, first you draw like pics of the map (but this isn't necessary), a top view map of it with the main objects (buildings, mountains... Important trees (not all)). Then you design the heighmap and then put boxes and simple meshes to the map as a concept map and then start creating the assets.
In conclusion, maps are created in the game engine editor with the assets that you create in a modeling application (like blender).

Exporting map models for games