user1710: Shooting videos of Ninjutsu events. Top man insists that there are privacy issues as some of the participants don't want their faces to appear in the resulting video. Would really love to be able to solve this problem because I'm sitting on tons of video that I want to distribute!
Blurring all the faces in the video is not a good solution because first there are the instructors in the the video and second some of the participants would LOVE to see their faces in the video. 
So what I was thinking was to blur the faces of those participants who request that their faces be hidden (i.e. opt-out). This means I'd have to be able to tell the software that "this is the face of Dick, Joe and Harry" and have the software identify and blur all occurrences of those faces in the multiple clips of the event. 
A less desirable alternative would be to somehow pixelate/blurr/remove everyone EXCEPT for the instructors (i.e. opt-in) 
So, my question is:
Do you know a solution to preserve the privacy of some of the participants in a video? 
This is a hobby, so preferably the solution would be inexpensive or even free ;)
I'm totally pessimistic - can you surprise me? :)
Update: I ran a test. I figured Google would have the best technology, because of Google Street View. So I uploaded a sample to YouTube and selected to blur all faces. This is a much easier problem than my problem description but the results were very disappointing. First, it was very disconcerting to see the blurred faces but even worse, the blurring was very "temperamental" - sometimes faces were blurred, sometimes they weren't.
So, I figure that if Google can't do it, then  it isn't doable.
However, and here's the moral of the story: Returned to "Top Man" and mailed him that it can't be done. Return mail was a very surprised "who told you this was an issue"!?! So, the moral of the story is: "Always recheck again and again with the client and re-verify the requirements".
As if we didn't know :)
I'm leaving this question open for a few more days, to solicit answers, and then I'll award the "solution" to the best answer. Thanks for helping!

user4954: I just created a video tutorial about how to blur videos with 4 ways. I think you may pick up any of them to solve your problem. 




user5546: I'd have to see your footage to come up with a perfect solution, but I do have some ideas that might help. (They're time-consuming. Sorry. At least they're free!)
First thing is to decide on a solution for each SHOT or even each section of a shot.  For example, you might have a shot that starts pretty stationary, then follows a particular person as they move around. For the start of the shot, you may be able to create a single image (using free downloadable software like GIMP) that is mostly transparent, but has black circles that would cover the faces of people that need to be hidding. Then you lay that over the top of that section of the footage, and those people are blocked out. Once the camera starts tracking the person as they move around, you could use free editing software (Mac computers all have iMovie; Windows computers all have Windows Movie Maker) to zoom in on the person, blocking out people behind them.
You may also find that you have some sections of some shots with nobody in the background! Or the people in the background all don't mind being on camera. Using your free editing software (iMovie or Movie Maker) you can cut those sections out of the shots and add them to your finished video without any further manipulation.

user5933: I do editing work for a Large Surveillance company in the UK who require faces of all Juveniles and Children to be Blurred out. Please be under no illusion there is no easy way to do this and I spent many many hours looking at ways to do it, don't waste your time on free progarmmes. 
Firstly I am not a professional Video Editor but have picked up a lot of bits and tips as I go along specifically related to what I do. I have tried many programmes (including demos/trials costing 5 - 10K and the only programme I find to do the task with high accuracy is After Effects.  I looked at AE with Mocha and AE on its on on the Motion Tracking( even for the high cost of CS6 it is still the most cost effective programme for the task and like someone mentioned there is a subscription service (which I will switch to soon). 
Motion tracking is great if the subject (face) doesn't move out of view, once it does you loose the tracking and its grief.  The best way I  have found is AE using Motion tracking with keyframing which is simply moving the created Gaussian mask frame by frame. It is a laborious tack but as I have prograssed over the past 6 months I have discovered ways to speed up the process by say moving two or more masks at the same time and to split an adjustment layer which saves time creating a new layer which I found invaluable as it allows me to increase or decrease the size of the mask. 
With CS6 it also has the added benefit of Dynamic Link which is brilliant for cross platform editing with Premier Pro or Encore etc. If you take a look at YouTube tutorials on the subject  you will see what I mean.  What ever the Professionals say Ae and PPro is really quite easy to get to grips with the basics and you will very soon add to your knowledge as you go along, there is always that 'wow' I didn't know I could do that with this programme. 

user3295: While its technically possible to have a program detect specific faces and apply a blur on that face. That would require a lot of programming work on your side, there is no free tool or low cost tool that does this automatically for you. This is possible with OpenCV but I highly doubt that this is a viable solution for you in any way, it's a quite complicated task and the result will not be 100% reliable.
The general workflow of blurring out faces would be to semi automatically track the motion of the faces with a 2D tracker in each clip an apply a blur mask to them.
The usual go to software for that is Adobe After Effects or other Motion Graphics video applications like Apple Motion.
Both are no free applications and I never heared of free software that implents 2D tracking together with a video editor. Though there are some really cheap amateur video editing suites out there that even implent face recognition but they all work pretty bad and unreliable without giving you much or no control over whats happening.
For example: http://www.wondershare.com/video-editing-tips/change-face.html
I highly discourage using such software for any work that goes beyond your average random youtube video.
While Adobe After Effects was always unaffordable for hobbyists, Adobe now switched to a subscription based model which makes it very affordable for small projects. You can use the program for free for 30 days (no commercial use) or a monthly subscription for ~30$ depending on the country you live in.
I recommend After Effects because there are ton of beginner tutorials out there for exactly what you want to do and the tracking workflow is quite straight forward. There is also Apple Motion like already mentioned and Nuke and quite a few other applications though After Effects is available for both Windows and Mac and is the easiest to grasp in my opinion.
Edit: I just found out that there is also an automatic face tracker in Adobe Premiere since CS6, this might not yield perfect results but should ease the workflow quite a bit.

1710: <p>Shooting videos of Ninjutsu events. Top man insists that there are privacy issues as some of the participants don't want their faces to appear in the resulting video. Would really love to be able to solve this problem because I'm sitting on tons of video that I want to distribute!</p>

<p>Blurring all the faces in the video is not a good solution because first there are the instructors in the the video and second some of the participants would LOVE to see their faces in the video. </p>

<p>So what I was thinking was to blur the faces of those participants who request that their faces be hidden (i.e. opt-out). This means I'd have to be able to tell the software that "this is the face of Dick, Joe and Harry" and have the software identify and blur all occurrences of those faces in the multiple clips of the event. </p>

<p>A less desirable alternative would be to somehow pixelate/blurr/remove everyone EXCEPT for the instructors (i.e. opt-in) </p>

<p>So, my question is:
<strong>Do you know a solution to preserve the privacy of some of the participants in a video?</strong> </p>

<p>This is a hobby, so preferably the solution would be inexpensive or even free ;)</p>

<p>I'm totally pessimistic - can you surprise me? :)</p>

<p><strong>Update</strong>: I ran a test. I figured Google would have the best technology, because of Google Street View. So I uploaded a sample to YouTube and selected to blur all faces. This is a much easier problem than my problem description but the results were very disappointing. First, it was very disconcerting to see the blurred faces but even worse, the blurring was very "temperamental" - sometimes faces were blurred, sometimes they weren't.</p>

<p>So, I figure that if Google can't do it, then  it isn't doable.</p>

<p>However, and here's the moral of the story: Returned to "Top Man" and mailed him that it can't be done. Return mail was a very surprised "who told you this was an issue"!?! So, the moral of the story is: "Always recheck again and again with the client and re-verify the requirements".</p>

<p>As if we didn't know :)</p>

<p>I'm leaving this question open for a few more days, to solicit answers, and then I'll award the "solution" to the best answer. Thanks for helping!</p>
 4954: <p>I just created a video tutorial about how to blur videos with 4 ways. I think you may pick up any of them to solve your problem. 
<div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/JvmdRnHzYW4?start=0"></iframe>
            </div></div></p>
 5546: <p>I'd have to see your footage to come up with a perfect solution, but I do have some ideas that might help. (They're time-consuming. Sorry. At least they're free!)</p>

<p>First thing is to decide on a solution for each SHOT or even each section of a shot.  For example, you might have a shot that starts pretty stationary, then follows a particular person as they move around. For the start of the shot, you may be able to create a single image (using free downloadable software like GIMP) that is mostly transparent, but has black circles that would cover the faces of people that need to be hidding. Then you lay that over the top of that section of the footage, and those people are blocked out. Once the camera starts tracking the person as they move around, you could use free editing software (Mac computers all have iMovie; Windows computers all have Windows Movie Maker) to zoom in on the person, blocking out people behind them.</p>

<p>You may also find that you have some sections of some shots with nobody in the background! Or the people in the background all don't mind being on camera. Using your free editing software (iMovie or Movie Maker) you can cut those sections out of the shots and add them to your finished video without any further manipulation.</p>
 5933: <p>I do editing work for a Large Surveillance company in the UK who require faces of all Juveniles and Children to be Blurred out. Please be under no illusion there is no easy way to do this and I spent many many hours looking at ways to do it, don't waste your time on free progarmmes. </p>

<p>Firstly I am not a professional Video Editor but have picked up a lot of bits and tips as I go along specifically related to what I do. I have tried many programmes (including demos/trials costing 5 - 10K and the only programme I find to do the task with high accuracy is After Effects.  I looked at AE with Mocha and AE on its on on the Motion Tracking( even for the high cost of CS6 it is still the most cost effective programme for the task and like someone mentioned there is a subscription service (which I will switch to soon). </p>

<p>Motion tracking is great if the subject (face) doesn't move out of view, once it does you loose the tracking and its grief.  The best way I  have found is AE using Motion tracking with keyframing which is simply moving the created Gaussian mask frame by frame. It is a laborious tack but as I have prograssed over the past 6 months I have discovered ways to speed up the process by say moving two or more masks at the same time and to split an adjustment layer which saves time creating a new layer which I found invaluable as it allows me to increase or decrease the size of the mask. </p>

<p>With CS6 it also has the added benefit of Dynamic Link which is brilliant for cross platform editing with Premier Pro or Encore etc. If you take a look at YouTube tutorials on the subject  you will see what I mean.  What ever the Professionals say Ae and PPro is really quite easy to get to grips with the basics and you will very soon add to your knowledge as you go along, there is always that 'wow' I didn't know I could do that with this programme. </p>
 3295: <p>While its technically possible to have a program detect specific faces and apply a blur on that face. That would require a lot of programming work on your side, there is no free tool or low cost tool that does this automatically for you. This is possible with OpenCV but I highly doubt that this is a viable solution for you in any way, it's a quite complicated task and the result will not be 100% reliable.</p>

<p>The general workflow of blurring out faces would be to semi automatically track the motion of the faces with a 2D tracker in each clip an apply a blur mask to them.
The usual go to software for that is Adobe After Effects or other Motion Graphics video applications like Apple Motion.</p>

<p>Both are no free applications and I never heared of free software that implents 2D tracking together with a video editor. Though there are some really cheap amateur video editing suites out there that even implent face recognition but they all work pretty bad and unreliable without giving you much or no control over whats happening.</p>

<p>For example: <a href="http://www.wondershare.com/video-editing-tips/change-face.html" rel="nofollow">http://www.wondershare.com/video-editing-tips/change-face.html</a>
I highly discourage using such software for any work that goes beyond your average random youtube video.</p>

<p>While Adobe After Effects was always unaffordable for hobbyists, Adobe now switched to a subscription based model which makes it very affordable for small projects. You can use the program for free for 30 days (no commercial use) or a monthly subscription for ~30$ depending on the country you live in.</p>

<p>I recommend After Effects because there are ton of beginner tutorials out there for exactly what you want to do and the tracking workflow is quite straight forward. There is also Apple Motion like already mentioned and Nuke and quite a few other applications though After Effects is available for both Windows and Mac and is the easiest to grasp in my opinion.</p>

<p>Edit: I just found out that there is also an automatic face tracker in Adobe Premiere since CS6, this might not yield perfect results but should ease the workflow quite a bit.</p>
