Keying Conundrum in Composition
by Derek Schachter
on
Feb 13, 2008 at 9:53:04 am
Hello,
I've been given a task for a music video and asked to perform an effect that I've never tried before. I don't know if this question has been asked in the forum before, but it is tricky to qualify in a search. So here goes:
The effect is based on the composition of 3 shots with a mounted camera, all with the exact same framing (the camera has not moved between any of the shots):
1. A girl walking left to right on a sidewalk, entering and leaving the frame.
2. A group of people behind the sidewalk (where the girl in the first shot would have crossed in front of).
3. An empty frame with no girl or group of people.
Now the effect that the director wants is for the girl to "wipe out" the people as she passes by, as though she is erasing them. My problem is figuring out how to isolate either the girl's image or the group of people. None of the shots are done with a green screen.
The director mentioned that the empty frame could be used to key out and isolate the people in their frame, so that they can then be composited onto the girl's frame and have the effect applied to their image accordingly. He said that is what his last editor was using, but he could not tell me the precise name of the effect used.
From what I see, the effect wouldn't work unless either the people or the girl can be isolated, because since neither was done in front of a green screen, putting their full frames together wouldn't just overlap, and any opacity would make the opposing backgrounds bleed into the people.
If anybody knows the way to key out the background using a matching background, as my director has described, or any other way to make the effect work, I would appreciate it greatly. If not, then no worries, I can work around it.
And if my director is full of it and no such effect exists, or at least not with the capabilities of After Effects 6.0, then by all means let me know!
Re: Keying Conundrum in Composition by Thorsten Miess on Feb 13, 2008 at 2:38:17 pm
hmm i think you can first time the layers then
choose the girl layer and change the blending mode to difference (dont use your group layer only the girl and the BG)
so you see what happen then precomp this and nest it in your final comp.
Apply threshold to the precomp (you can blur it out before doing this, i think this would help) then use this black and white layer as an luma matte for your girl. now you can do the same thing with your gropu layer. i think this should work (if not u maybe need to get rid of the gray in the blending and use alpha matte)
im not sure but i think this would work. im just guessing but one answer is better than none.
Re: Keying Conundrum in Composition by Dave LaRonde on Feb 13, 2008 at 3:39:41 pm
[Derek Schachter]"The director mentioned that the empty frame could be used to key out and isolate the people in their frame, so that they can then be composited onto the girl's frame and have the effect applied to their image accordingly. He said that is what his last editor was using, but he could not tell me the precise name of the effect used."
The effect is called a Difference Matte. It can be difficult to get satisfactory results. You'll need to generate a still image of the clean background plate, which the effect uses for reference to isolate foreground elements.
"Now the effect that the director wants is for the girl to "wipe out" the people as she passes by, as though she is erasing them. My problem is figuring out how to isolate either the girl's image or the group of people. None of the shots are done with a green screen."
Depending on the length of the shot, it may be easier to rotoscope out the leading edges of the girl as she walks through the shot, replacing the empty portion with the shot of the people in the background.
Re: Keying Conundrum in Composition by Darby Edelen on Feb 13, 2008 at 7:56:15 pm
[Derek Schachter]"If anybody knows the way to key out the background using a matching background, as my director has described, or any other way to make the effect work, I would appreciate it greatly. If not, then no worries, I can work around it. "
It's called difference keying in which anything that is the same in both scenes is made transparent, leaving those things that changed. The problems with this approach are changing conditions during shooting and video noise. For example, if the light changes then the clean plate is no longer the same as the other shots. There's no magic bullet for this, but you should be able to get fairly decent results if you use a combination of difference keying, rotoscoping, and maybe some color correction or noise reduction.
The effect you would use for the difference key is Keying > Difference Matte:
Re: Keying Conundrum in Composition by Joe Moya on Feb 14, 2008 at 12:53:41 am
Well...if I read your post correctly... I have done something similar.
The easy way will be to create a difference matte... but, the problem with come when you have two moving objects in the same frame AND the background of each shot is not exact.
The idea of keying will work if the background is blue/green screen... but, that is not the case... so, the directors suggestion will not be as easy as he seemed to assume.
So... it seems, the solution is most likely rotoscoping (which is what I ended up doing). And, this solution could be a major pain if you have moving legs in the composite... leg movements are a bear to rotoscope. If this is a rotoscoping project... I would strongly suggest you use Mocha if you can afford it.
Exactly how hard this will be to pull off depends upon how it was film.
Re: Keying Conundrum in Composition by Derek Schachter on Feb 14, 2008 at 1:01:36 am
Thanks for the tip! She is indeed moving her legs in the frame. My sample difference matte so far seems to be producing fair results (with slight opacity).
Re: Keying Conundrum in Composition by Darby Edelen on Feb 14, 2008 at 2:13:56 am
[Derek Schachter]"My sample difference matte so far seems to be producing fair results (with slight opacity). "
Basic keying concepts still apply with the difference matte, so feel free to use garbage mattes as well as edge mattes and core mattes. The process will vary from the normal chroma keying process, but the resulting key will benefit.
Darby Edelen Designer Left Coast Digital Santa Cruz, CA
