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CS4 After Effects - Creating Masks Based on Motion?

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Brian SitesCS4 After Effects - Creating Masks Based on Motion?
by on Jun 16, 2011 at 4:22:22 am

I saw a tutorial once that somehow identified the parts of the video image that were moving from frame to frame and transformed the video based on that identification. I think the moving areas of the image were then masked, which made it really easy to (for example) apply various color changing/etc. effects to only those moving areas.

I would love to know how this is done - just the part about identifying areas that are moving without having to build custom masks frame by frame.

If it helps, the original tutorial I think was a (randomly) Human Torch tutorial, where the person used the motion identification to make a better image base for motion tracking. He was sitting down with his hands in front of him, moving his hands around, and he used the motion identification to change the image to basically all black and white, with only the moving parts visible, and then he could use that high contrast base to motion track his fingers and hands and attach various flame effects he then built over it. Then he restored the video to it's normal appearance (so not black and white weird motion identification mode) and still had the motion tracking on the hands preserved.

Again, in case it helps, one of the kinds of things I'm thinking of doing with this is say I have a camera on a tripod aimed at some guys rocking out at a live performance and I want to turn them, but only them, into Cousin It equivalents via CC Hair. Only the rocking band members are moving, and I want to cover them in CC Hair as they move about but keep the rest of the stage untransformed/unhairy. Weird, of course! But it's an example of what I mean.

Any help is appreciated - I am an amateur hobbyist with no formal training, so please keep the explanations slow! Thanks!


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Shahin GhahremaniRe: CS4 After Effects - Creating Masks Based on Motion?
by on Jun 16, 2011 at 2:47:14 pm

Brian, I really would like to help you about this issue, but the thing is, I still haven't get exactly what you mean! As what I understand from your post, you might want to extract some objects from your video without masking frame by frame (Rotoscoping)! Well practically it is somehow not possible, I mean you probably need to do some frame by frame work on your video, even though there are some new features available on AE CS5 which makes the rotoscoping way easier!
Or else, you might need to have a very good quality video with a clear Green screen background, so you can simply key out the green color and keep the rest of the objects of your scene untouched for applying other stuff on top!
I think you may need to search about Rotoscoping and Green screen masking to figure out how do they work.
I hope this was an answer to your question, if it is not, then I guess you need to provide some links to the tutorial video that you were talking about, so we could have a better view of what you are saying!

Good luck!

Shahin

--------------------------------------------------------
Please visit my YouTube Channel:
http://www.youtube.com/user/Animationstudies


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Brian SitesRe: CS4 After Effects - Creating Masks Based on Motion?
by on Jun 19, 2011 at 1:56:53 am

Thanks for your reply. I'll try to explain more clearly what I'm looking for.

Boiled down to its basic details, I'm looking for a way to convert video of things moving around into a sort of black and white video that displays things based on movement. In other words, the black and white video is an interpretation of the original video and looks very little like it - it's just a visual representation of where there was movement in the original shot. In the tutorial I saw long ago there was a person sitting at a desk facing the camera moving only his hands back and forth. When he did this trick, whatever it is, all you could see was (I think) black lines that would increase or decrease in intensity around the location of the moving hands. There was no longer a person in the background, he was converted to white plain background since he wasn't moving. Where his hands were you saw instead black shapes resembling hands that increased and decreased in intensity as they moved.

How do you do that? Thanks!


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Shahin GhahremaniRe: CS4 After Effects - Creating Masks Based on Motion?
by on Jun 19, 2011 at 4:42:42 am

OK, now I got what you mean, if I am not mistaken you have a video with some moving objects and you need to cover up those objects with a solid white color and make the rest of the shot with solid black or vise versa!
if that's the case, I would say, yes you can but you still need to do frame by frame masking!!! you know why cause this is you who must define the border between the edges of the moving object and its background, and if you use any plugin or filter, I don't think it will be accurate enough in these cases! so I suggest you, to use masking tool and draw a path around the parts you want to cover up them with white color and change the nodes' positions frame by frame and make key frames for the path shape for each frame, if you are a CS4 user, indeed! Obviously, that's a very time consuming and honestly painful job, but this is what we call it rotoscoping, and when it comes to roto there is no way out! However, as I said in the older post, AE CS5 has a very powerful ability for this kind of masking which is called "Rotobrush" which is really making things easier, I have no idea how the programming algorithm works, but the result is awesome, and your computer can amazingly define the moving pixels and your moving objects so you can easily mask it out, but be aware that even this new feature might have some weaknesses, so everything is depending on your video quality and the contrast, and how well the pixels and edge-lines are recognizable!!! here is a link to a tutorial that shows how to use Rotobrush Tool, so you'll get what I am talking about better:







There are a lot on YouTube, just search for Rotobrush on AE CS5!
OK, after you done with the rotoscoping part, you can simply copy the masking layer of what Rotobrush has made for you and paste it into a white or black solid object that you have made before (I assume like you've already made a black or white solid layer, cause you really need it if you want to cover up the masked area with only 1 color!)
Wow! That's a long post already, sorry about that, but I wish this time it works for you.
Good luck!

Shahin

--------------------------------------------------------
Please visit my YouTube Channel:
http://www.youtube.com/user/Animationstudies


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Brian SitesRe: CS4 After Effects - Creating Masks Based on Motion?
by on Jun 20, 2011 at 6:17:10 pm

Thanks for your response, and for linking the video tutorial. I remember that I saw them do some other method that involved almost no frame by frame masking (and it was in CS3 or CS4), but maybe I'm just remembering it wrong. I'll go look a fourth time to try to find the old tutorial that I saw, maybe it's still on YouTube somewhere and I just didn't find it the first several times looking. Thanks again for your advice!


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