941: Depth Perception
Explain xkcd: It's 'cause you're dumb.
Comic #941 (August 22, 2011)
Title text: I've looked at clouds from both sides now.
Explanation[edit]
This comic is one of those that is less focused on humour and more focused on a sense of wonder at the world for both Cueball/Randall and the reader.
Cueball discusses how difficult it is to intuitively feel the reality of how vast the things he sees every day and night are - how big the clouds are, and how far away the stars are. Depth perception - seeing things in 3-D rather than as a flat 2-D image - is partly created by having "binocular vision", or two eyes spaced apart. Each eye sees a slightly different angle on a scene, and the brain combines these two views to give a genuinely three-dimensional view of something. 3-D glasses work the same way, by feeding a slightly offset image into each eye. When you look at far away objects, the offset from each eye is undetectable, and so they may look more like flat 2-D images - hence the impression Cueball has of stars being painted onto a dome rather than being extremely large, far away objects at very different distances.
He wonders if he can work around this impression as far as the clouds are concerned. Normally, Cueball's eyes are a few centimetres apart, like everyone else's, and his 3-D perspective is based on that scale. Here, Cueball puts HD webcams on the tops of football uprights, which are 360 feet (~110 m) apart instead of a few centimetres. He uses strong reading glasses to hold up a smartphone, and feeds the far more offset images of the webcam feeds to each eye so that his brain will create a 3-D perspective of the clouds, which would normally be too massive for the offset between two human eyes to grasp their three-dimensional structure in the same way as smaller, closer things.
This technique doesn't give him the view as if he were a giant as in the final panel, but rather as if he were a giant "at the bottom of an abyss" as per the second-last panel, as the clouds are higher than the goalposts on which the cameras are mounted. The final panel is some artistic license to give the reader a real sense of what it feels like for Cueball to carry this out; it shows us that he has finally achieved a more truthful perspective on the size and shapes of the clouds than he had when he started.
The reason for the reversal of the "right camera" and "left camera" panes on the smartphone screen is unclear, this is likely just a mistake.
The title text is a line from the 1969 song "Both Sides Now" by Joni Mitchell; the full chorus runs: "I've looked at clouds from both sides now / From up and down and still somehow / It's cloud illusions I recall / I really don't know clouds at all." Binocular depth perception involves seeing the same object from slightly different angles, from 'both sides', so Randall is taking the song lyric and literalising it. The song itself has a bittersweet tone and relates to how you understand things differently as you mature, but still don't necessarily feel like you understand them at all, so the tone also fits pretty un-ironically into the theme of the comic.
Transcript[edit]
[The entire comic is narrated by Cueball, and never spoken by the Cueball shown in the examples. All dialog is shown in rectangular frames overlaid on the comic panels.]
[In the first panel the background shows a cloudy sky in color, with the clouds all running together and appearing as a blue gray smear. Towards the bottom the horizon and the ground appear dark almost black at the very bottom. Two frames with two lines of text are at the top left and right, similar below except the one to the left has four lines and the one to the right only one line.]
I've always had trouble with the size of clouds.
I know they're huge. I can see their shapes.
But I don't really see them as objects on the same scale as trees and buildings.
They're a backdrop.
[The next panel is split in three parts. The top part is in a single frame. The middle part is frame-less and only has text - the only narrating not inside one of the frames. Then at the bottom there are two frames overlaid over three small panels in a row]
[In the top part of the 2ns panel stands Cueball on a flat disk inside a hemispherical dome with the front half cut away. The dome is about three times as tall as Cueball. Above the dome there is one frame with text. There is also two labeled arrows pointing to the dome and the disk.]
Stars are the same way.
Arrow up: Sky
Arrow down: Ground
[Text in between the top and the bottom panels:]
I know they're scattered through and endless ocean, but my gut insists they're a painting on a domed ceiling.
[The next two frames with text is overlaid above (three lines of text) and below (one line of text) the three panels described first. Those three panels are all inverted with black background and white Cueball:]
[Left panel: Cueball stands on a curved surface, looking up.]
[Middle panel: The perspective of the scene shifts, suddenly the surface Cueball is standing on is in the top left of the panel. Cueball is now looking down, leaning back, and waving his arms trying to regain balance.
[Right panel: The perspective of the scene returns to normal, Cueball is now semi-crouched, staring at the ground with legs spaced apart to help him balance.]
If I try hard enough, I get a glimmer of depth, a dizzying sense of space,
But then everything snaps back.
[An American football field is shown with Cueball drawn very small near the middle. Sections at the left tips of each of the goal posts are highlighted and shown as a zoomed view in an insert box. These insert shows the two webcams mounted on the top of the very tip, one for each goal posts. There are two frames with text above the field, the top one most to the left with one line, the second directly below it with two lines, and below the field there is also one frame with one line in it.]
So one summer afternoon
I set up two HD webcams hundreds of feet apart,
Pointed them at the sky,
[The next two frames with two lines of text each are stretched over the two middle panels in this second row of panels:]
[The first panel shows a pair of glasses and a smartphone with an attachment designed to clip onto the glasses. The smartphone screen is setup to display two images side by side such that one camera is visible in the left half of the screen, and the other camera is visible in the right half of the screen. There are four arrows pointing to the two items and to each of the two parts of the screen. They all have labels which are between the two lines of text, but here shown below for clarity.]
[The next panel shows the completed phone glasses assembly.]
And fed one stream to each of my eyes.
The parallax expanded my depth perception by a thousand times,
Arrow top left: Right camera
Arrow top right: Smartphone
Arrow bottom left: Very strong reading glasses
Arrow bottom right: Left camera
[Cueball stands wearing the phone glasses assembly, one hand held up to the device, staring into the sky. There are two frames one above and one below with two lines of text each:]
And I stood in my living room
At the bottom of an abyss
[Another colored panel with blue sky and clouds below the top part of the panel from left to right. Cueball is now a giant who stands in the middle of the frame on the shore of a coastline with a small island off the coast, only a step away for him. A city is near his right foot and the tallest skyscraper appears ankle high. A mountain range is behind him with mountains that are also only barely ankle high. A river flows past the mountains and joins another coming from them on it's way down towards the coast. Cueball is standing with his head well above cloud level as clouds swim around him. At the top above and left of his head the last frame with one line of text is located:]
Watching mountains drift by.
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Somebody needs to try this. Couldn't be that hard. 71.178.11.180 21:27, 22 October 2012 (UTC)
Those must have been some tall goalposts if his point of view is above the clouds! -- mwburden 70.91.188.49 13:16, 10 December 2012 (UTC)
Also, the cameras should be mounted on servos so that when the phone is moved or tilted the cameras can follow, so your viewpoint isn't fixed in one direction. -- mwburden 70.91.188.49 13:18, 10 December 2012 (UTC)
That wouldn't work. The entire football field would have to swivel, or else he'd get some wicked image shearing... 108.28.72.186 01:42, 7 August 2013 (UTC)
actually, it might be possible to correct for that, using bipolar geometry. Essentially, you can derive a 3d model from 2 images from different view points. Here is a (very geeky) demontration of what can be done. Watch the end, where they construct a fly-around video from two images of the opera house in sidney. -- 141.101.104.22 21:10, 19 January 2015 (UTC)
Interesting link, thanks, but I don't think the video was generated from only 2 images, there isn't enough information. If you select "Download the Opera House sequence" you can download the original 43 photographs used. 141.101.99.9 14:05, 19 March 2015 (UTC)
You're right. But of course you wouldn't need a 90-degree flyby for this. 141.101.104.43 16:56, 8 July 2015 (UTC)
An updated solution would be to put the two stabilised cameras on quadracopters which are coded to remain a set distance apart. When you want to look left/right it would take a while for the pair of drones to rotate around their centre point but not too long..... Then you could also get a perspective from the height of a giant (up to 400ft https://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=22615) and with their degree of parallax (from whatever value of height and eye spacing you choose). 108.162.250.225 (talk) (please sign your comments with ~~~~)
This is a very cool project indeed! Some hardcore image stabilizing software would be required too, since you would get nauseous if the two images weren't perfectly aligned at all times. But this setup is the only one I could think of that would enable you to perceive the view from the last frame. Mumiemonstret (talk) 08:44, 17 December 2014 (UTC)
That idea would probably work but at one point, the quadcoptors would have issues flying any higher which would limit the view.
Look at this in stereo mode: http://www.fourmilab.ch/cgi-bin/Solar and cross your eyes so you see three images, then hold your hands up so you only see the one, then... I forget...
I used Google News BEFORE it was clickbait (talk) 12:44, 23 January 2015 (UTC)
I used to do that all the time at one time ... until I got a l...ot of things different to do..
Stereo aerial pairs of clouds do exist see the Google search: https://www.google.co.uk/search?q=stereoscopic+aerial+photos+clouds 141.101.98.206 07:33, 19 August 2015 (UTC)
Or you could ride in an airplane. Or stand on a mountain. 173.245.50.174 19:58, 5 September 2015 (UTC)
That would defeat the point entirely. The distance between the two viewpoints is what provides the increased perspective, not the height of the observer. 141.101.80.84 21:55, 16 November 2015 (UTC)
using a plane isn't a substitute for this, but there's no reason adding the element of flight has to defeat it entirely: you could put one cam on each wing tip & get maybe the coolest effect of all..
Likely the reason the right image is shown on the left and vise versa is that there are two ways to fuse stereo images. Either Walleyed, right-to-rght, or Crosseyed, right-to-left. Doing it the wrong way may result in concave faces and other aberrations. 162.158.255.125 15:20, 30 March 2016 (UTC)
Clouds are fractal, small ones up close look the same as big ones far away. So I don't think this would look as spectacular as imagined. 162.158.39.209 (talk) (please sign your comments with ~~~~)
The thing is that our brains only associate the binocular/3D effect with items that are relatively close, and tend to judge sizes accordingly. If something appears 3D to us, we judge it to be a certain distance away (a key function of binocular vision) and from that we also get a rough estimate of its size. That's why if you see something like a star destroyer in 3D in the movie theater, it looks like something the size of a bus hanging up in the general vicinity of the screen. It doesn't look like something miles long, because big things look flat when they look that size. I believe this way of looking at clouds would give a similar effect. The clouds might look 3D, but they'd also just seem closer and smaller, rather than giving you a real sense of their size. 162.158.78.10 (talk) (please sign your comments with ~~~~)
One way to impress with size is to make sure there's something on the "back wall" to contrast with the foreground. Here's a crosseye cloud picture I made from several pictures out an airplane window. The distance in time between these was one second, according to the EXIF data. Because the foreground cloud is so large, the faraway cloud at the center looks like Randall's "mountains" should. I should note this is a zoom/crop of a much larger pair of pictures. DuplexFields (talk) 01:26, 4 October 2019 (UTC)
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