Bullet time (also known as frozen time, the big freeze, dead time, flow motion, or time slice) Is something I've been wanting to have a go at for sometime but the price of 20 cameras is a bit steep for something I just want to play with.
The bullet time effect was originally achieved photographically by a set of still cameras surrounding the subject. The cameras are fired sequentially, or all at the same time, depending on the desired effect. Single frames from each camera are then arranged and displayed consecutively to produce an orbiting viewpoint of an action frozen in time.
Then I thought that maybe I might be able to make it work with 20 web cams on the laptops in my computer class, the results are a bit rough but lots of fun.
Depending on your internet connection it may take a minute or two for the images to load properly, as they are fairly large gifs.
I would like to put some more gifs on this instructable but I'm finding the more I put on the slower the page loads. If you are having problems viewing the gifs and you are a pro member It may be better to log out and reload the instrcutable so you can view the page one step at a time.

Step 1: Stuff You Will Need

If you have a computer class, it will make this a lot easier.

20 identical laptops with web cams
20 identical photo software as we have macs we use photo booth
Enough tables to fit 20 lap tops on
A brightly coloured marker on a stick
20 people who can follow instructions and are co-operative
Either a network drive or USB drive
Internet access or stop motion program
Masking tape
Good lighting

Step 2: The Set Up

This is really important as it only takes a couple of kids to do one of the steps wrong and it will ruin the whole shoot. The first time i attempted this it was a failure as many of the kids did some unanticipated things, like moving their laptops or tagging the photos wrong or they had previously played with their web cam to give a mirror image. That been said after about 5 failed attempt the kids got the hang of it and we got a few good shoots and they want to improve the each shoot and now have ideas of their own to make it work better.
I show the kids a video like this one
http://www.youtube.com/watch?v=ZlZPmABrLu8l
and show them photos of a bullet time rig, drawed diagrams and give them dot point instructions, But the first few times, some one will stuff it up, but we got there in the end
Here are the instructions I give the kids

I will give you a number from 1-20 that is your camera number write the number on your hand you must not forget it.
close everything down on your computer and open photo booth
move all the tables into a circle with the marker in the centre find your place in the circle
aim your web cam so the marker is in the centre of the screen. I will then remove the marker
turn the volume right up so you can hear the web cam beep three times and then the shutter will fire
everyone place your finger on the enter key and push on my mark (which is ready, set, "clap")
run to the centre where the marker was and on the third beep jump.
we will repeat the shot 5 times
pack up you laptops and tables, and go back to the classroom
Now you must label each of the shot with a letter of the shoot and your camera number. eg if my camera was 15 I would label my photos A15, B15, C 15, D15, E15
You now drop your photos in to a folder that I have set up on the network, and all the A photos can be strung together from 1 -20 to make a gif, stop motion or put onto I movie. Same with all the B photos etc

Step 3: Epic Fail

The first few attempts were a total disaster for quite a number of reasons. First many of the kids were really excited and didn't centre the marker on the screen properly, or they moved their laptop after the fist shot ruining all the shots after the first one. Also the sun was reflecting off the screen making it very hard to to see the marker. We also had a couple of class clowns and their antics ruined some of the shoots.
Another thing was the timing. it was all over the place. I first said everyone go on three, 1,2,3 and some kids would push on three and some would wait and push where 4 would be. So I changed that to ready, set, and then I clapped. I also got 10 kids to push the buttons on two laptops at the same time rather than 20 kids pushing one each which seem to also help.
I also thought that the laptops were too far apart so we made a 1/2 circle and put the laptops much closer together.
another thing that happen was that kids didn't label there photos correctly and they then appeared out of sequence some kids would label a photo "a15" others "A16" or "a 17" so I had to make it clear capital letter, space, camera number. Another problem is that some kids had previously played with their web cam to give a mirror image.
After the first 5 shoots i looked at the photos and only found 2 frames that were useable I was almost going this one into the too hard basket, but I made a gif from the 2 frames, and this really motivated the kids to have another go and do better. Peer pressure can be great some times. We reset in the shade and tried again

Step 4: After the Shoot

Its very important to get everyone to label their photos correctly, so they end up in the right sequence and in the right shoot.Because all the photos are very similar it is very confusing to go back and find the photos that are in the wrong place and find the correct place for them. That said some students still had the camera mirror image, which was bad enough going through the photos and flipping the photos that were reversed.
Next we spliced the photos together and there are a few different options to do this, for example put the images into Imovie, or movie maker or a stop motion program. Another idea is to put the images into a powerpoint and run the sides at a high speed. Unfortunately "keynote" which is the mac version of power point will only let you crank up the speed to 500ms which is way to slow. Just another annoyance I also found that gifs don't open properly on some mac programs most notably preview and pages. So if your using a mac open the gif in a web browser or open office or it will be a disappointment.
For this instructable we made the shoot into a gif, so that you don't have to go to youtube to look at the result
I've use an online gif maker
http://www.myspacegens.com/handler.php?gen=animatedimage
which is the best one I found as many gif makers don't let you upload more than 20 images or have other restrictions.
once uploaded you can change the speed, delete images and move them around, and hit the submit button and in a minute or so you have a gif you can download

Step 5: Making a Loop

Because we ended up with a 1/2 circle I wanted to run the gif forward to the end of the clip and then backward so the gif didn't have a big jump in the middle of the of the loop.
This is not too difficult, all that is required is that you copy the 20 images into another folder and rename the camera numbers (this is difficult to explain but not difficult to do) so image number 20 becomes number 21. Image number 19 becomes number 22. image number 18 becomes number 23 and so on till you get to Image 1 which is number 40.
that done copy images 21-40 and put it back in you original folder so you should have images 1-40 in the same place. I will say I found this step long and tedious on a mac computer due to its lack of a "rename" menu and required 4 or so precisely timed and located mouse clicks for each image to be renamed, so I did it on my trusty ubuntu computer which was much quicker.
Now the whole lot can be uploaded to a gif maker or your choice of software.
the 2 gifs on this page use the same images but the second one runs in reverse for 1/2 the clip avoiding the jump in the loop.

Instructables glitch I've discovered that I put 2 gifs on on step they don't work properly so the the reversed loop gif in on the next page

Step 6: The Revesed Loop Gif and Wii and Apple Remotes

Ok the gif on this page should be on the last page... technology is fighting me all the way on this instuctable!

After some thought about how I could set off all the cameras at once, I remembered hacking a Wii remote was popular a few years back, If I could get that to work that would be a great improvement. A quick look on the net and I found a blog with a link to Darwiiin remote which i hope would set off 20 or more computers at once.
http://macapper.com/2007/04/27/how-to-control-your-mac-with-a-wii-remote/
After a bit of fiddling I got it to work on one computer, I had to delete the device from my Bluetooth preferences which is explained on the software set up screen. It comes default with the B key on the remote as the enter key on the computer so it was going well so far time to try it out on a few more computers ..... which of course didn't work, as the Wii remote would only hook up to one computer at a time.
I asked a couple of computer techs and they didn't have any ideas so I thought Id post it in the question on instuctables and see what would happen.
so far haven't come up with an easy solution so stay tuned if i can fix this it should make the bullet time class photo even more awesome!
After a lot playing around I found I could get an apple remote to set of several macs and with a program called remote buddy i could set off the cameras at once.
http://www.iospirit.com/products/remotebuddy/
The apple remote however does have some limitations, the most annoying being that it will not work on the white macbooks, only macbook pros. It also doesn't work very well in full sun, which can be a problem with images blurring due to low light.
I found that it also worked better if the remote was point at a wall opposite all the computers as sometimes I would only get some of the computers taking a photo.
That said the timing of the photos is pretty much perfect using the remote.

Step 7: Other Stuff

Seems that the bullet time lesson with the kids has taken on a life of its own with small groups of kids making their own projects. A group of students produced this Minecraft bullet time which was quite involved, stitching 9 screen shots together on a glass platform that they had made.
The kungfoo gif was a group of 5 students working together, probably needs more cameras and less stuff in the back ground but it came out pretty good.
If you are a teacher or have access to a large number of laptops, be prepared for a bit of frustration but its worth the effort and its a great fun activity to do with kids.