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remove background with similar color to moving foreground footage?

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Jeff Moatesremove background with similar color to moving foreground footage?
by on Apr 27, 2010 at 7:37:33 am

Hi, I have worked a bit with PPro CS4 for a few months now but AE is quite new to me.

First off let me say that I know in a perfect world I SHOULD have shot this on a green screen and used color keys, but it was a live performance and thus not possible.

Here is the footage in question.

Basically I would like to isolate the 5 moving actors from the background and change the background to pure black or perhaps overlay the moving actors to another scene or image. (I know, that's what a green screen is for, doh)

Is there any way to do this? Does AE CS4 have some sort of auto mask to motion tool that would be easier then going frame by frame and masking?

If there is no better way I have another question:

I attempted to do the frame by frame but for some reason I set my first frame and drew it perfectly and set a key then skipped ahead and did another frame and keyed it, but I didn't use the exact same points for each body and deleted some on the last frame and apparently that messed up the first frame to.. so I am really confused as to why deleting a pin point mask on the end keyframe would remove it from the first frame as well since they were on the same main mask path and i didn't delete a keyframe. I thought a key locked that mask at that particular time and you could do whatever you wanted to those points in the next keyframe and it wouldn't bother the original.

Here was the end result of the first frame I did which was perfect initially:

And here was the last frame I did which looks good but screwed up the first.

Any help would be greatly appreciated as this is my first paying job and I would like to blow their minds by removing the non-historical objects in the show. Most of it is shot on stage and easy enough to remove (exit signs and such) but this coming down the aisle scene is a tough one


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Tudor "Ted" JelescuRe: remove background with similar color to moving foreground footage?
by on Apr 27, 2010 at 9:09:33 am

Ok- first a word of advice:
don't accept a paying job unless you know at least the basics of the software you're going to use and you can do what is required. There's no quick fix that will replace lack of experience. Read the manual and do tutorials first.
adobe and the cow forums are what you need.

Now on to your questions:
If you have a clean plate (a frame, or a few, of the background that do not have the actors in, and you can paint out in PSD a wider image of the location) you may try to track the move of the camera, aplly the data to the clean plate and try to see if a difference matte will do the trick for ya, although I doubt it for your footage.
So that leaves you with roto.
In AE you can set keyframes to a path, but if you delete a point from that path at any frame, then you delete that point from all frames.
ROTO 101- keep the path point as much as possible in the exact points you put them in the beginning (eg. a point on the tip of a helmet should always stay on that tip)

Tudor "Ted" Jelescu
Senior VFX Artist
Bucharest, Romania

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Jeff MoatesRe: remove background with similar color to moving foreground footage?
by on Apr 27, 2010 at 10:25:29 am

Thanks I will try that out. Good advice about the paying gigs, though I should say it is more of a potential dvd sale thing that I volunteered to do for a share of the profits, so I can't think of a better way to motivate myself and get experience. If I fail, worst case scenario is there is no dvd, but still an opportunity I couldn't let pass after buying my first camera - HMC150. Could do a fine job in PPro but why not walk the extra mile and dabble in AE.

Thanks for the help.

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Todd KoprivaRe: remove background with similar color to moving foreground footage?
by on Apr 27, 2010 at 1:00:41 pm

Tudor's advice is good. Here's are some other pieces:

- Use as few points as possible with your masks when rotoscoping.
- Don't use one mask per person. Use one mask per feature of that person (one per forearm, one per torso, et cetera) and use motion tracking to make the mask follow the motion.
- Don't draw the masks directly on the layer. Draw them on a white solid-color layer and use that layer as a track matte.

All this advice and much more is here:
"Rotoscoping introduction and resources".

Also, don't refer to this sort of work as "keying". Keying is defining transparency according to pixel color values, like with a green screen.

Oh, by the way, this all gets much, much, much easier with Roto Brush in After Effects CS5.

Todd Kopriva, Adobe Systems Incorporated
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Larry S. Evans IIRe: remove background with similar color to moving foreground footage?
by on Apr 27, 2010 at 7:00:46 pm

My general approach is perform at least basic color and lighting correction to any clip before I start addressing any kind of roto work.

Even if the director wants this sequence to look like it's occurring in the night, it's easier to use After Effects to take it back to that look once you've pulled the background out. In fact, if you are later substituting a CG or photo background, you will probably have to do a great deal of manipulation of the finished images to get them to "stick", so there's no harm in raising the levels here at least.

Once you've brightened the image up, you may be able to see specific areas for cleaner roto, and if you have a clean plate (i.e a frame of the space with no-one in it) you can often use that as a difference matte to allow in only the changed pixels. That can sometimes get you very close, particularly in shots where a greenscreen is impossible or impractical. Just rolling a few seconds of the space from the same angle can save hours in roto. (It's not as direct if you have a moving camera, but it can still be done). -R

Larry S. Evans II

Executive Producer

Digital I Productions

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