Greenscreen (no color key or rotoscoping) ?

#1

Hey,

I am currently working on a project where we have to change a background. Now the
greenscreen is easily edited out with a color key and if there are more colors I can use
the rotoscope-funtion. The problem is that we now have a scene where we don’t have
a greenscreen that is big enough. So we will shoot that in front of a big wall. But there are
multiple characters in the scene and I don’t have time to use rotoscoping to often.

The question is whether there is another possibility. The background is perfectly still and
not moving. Only the actors are moving. Is there a way to automatically select only moving
parts ? I know some webcams do that for effects, you have to show the programm your background and anything that is different, like someone walking in the foreground is seperated and taken into the new footage. I’d need something like that.

Can you do this in After Effects or is there a similar program that can do it ?

Thanks in advance !

#2

In theory you would be talking about a difference key. Though that can be incredibly finicky. Even perfectly still video has a degree of noise to it, and changes to illumination or shadows will mess it up too.

If the wall is a giant even patch with high contrast from the elements you want to preserve, you can use the paint bucket tool as another method. Simply fill the area with color and then put the effects mode on “Stencil Alpha”. It’s basically turns it into another keying method to add to the repertoire.

#3

You don’t actually have to shoot everything at the same time. You might be able to shoot your action in pieces and stitch it together in the composite.

There are, of course, other possibilities. Depending on your camera and the lights you have available, you could do some luma tricks by overlighting the back wall and then slightly underexposing your actors, allowing a relatively easy luma key of the background. You’ll lose a lot of dynamic range on the actors, though, limiting your options for color correction. That will work best if you can capture 10-bit from the camera. The objective is to get enough luminance difference in at least one channel to be able to threshold it into a matte. The green channel is best since it’s typically the least noisy and skin isn’t usually very green.

You might be able to get a reasonable difference key if you denoise the footage aggressively to get the matte. Obviously you only want the alpha from that process, though, not the RGB, since it will be blurry. And as Iaenic says, any shadows on the wall will be a problem.

#4

Have you tried using Roto Brush Tool to automate the rotoscopy as much as possible? If the wall is plain it should work pretty good on the edges of your characters.

#5

Any contrast will work between background and foreground will work. Luminance for instance - dark persons on light backgrounds or the other way around, blueish persons on a yellow wall, red vs green, dark silouettes against a light or blue sky.
If your screen isn’t big enough, put it in places that are harder to roto(hair, moving trees, bushes, smoke etc).
Shoot in layers, move the screen where you need it, for instance you’d shoot person A on the left in front of the screeen, then person B on the right in front of the screen, then comp both into one shot.

#6

You may be able to use a “Difference” mode between two layers.
That will give you a “matte” with the moving objects much lighter. The only problem is that noise can differ from frame to frame and it won’t be very accurate.

To try it, shoot something without moving subjects and then shoot from the same angle with actors.
Then place one layer over the other and put the top layer in “Difference” mode.
Use that as a pre comp and maybe you can use filters to extract a useful mask.

All the above suggestions should work as well, and remember “high contrast” between moving and still will give you a better and faster result.