remove background with similar color to moving foreground footage?

[Tudor “ted” jelescu]

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
http://www.ennstudio.ro

[Jeff Garrett]

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.

[Todd Kopriva]

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.

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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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[Larry S. evans ii]

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