Keying out a background without a greenscreen

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pete66

Member
Hi,

1) Instead of a greenscreen, you decide that you're going to film your subject in front of a big multicoloured painting (or any background, anywhere).

2) Before you get started, you simply let the camera roll for a while, just recording a still of the painting/background.

3) You tell your actor to act... in front of the big painting/background.

4) You tell those stupid programmer people to write some software that simply keys out all the non-moving pixels (especially the ones that match up with the original background shot from step 2!).

Can anyone tell me why this hasn't been done yet?

Cheers
 
The software would need to analyse the footage into motion vectors so that it could figure out where the outlines of the objects are. I'm sure if it's easy to do it with our eyes, it's feasible to make the software do it!

I think it'd be infinitely rare for a foreground pixel to exactly match the backdrop, especially so for a cluster of moving foreground pixels to absolutely line up.

I can see it now, you open up your tripod footage in AE and click "make alpha channel"... voila, all moving objects isolated.
 
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Aren't you talking about a difference key? There is one in After Effects, but it might be in the pro version only. Keys depending on whether a pixel is the same or different than the first ("let camera roll for a while") shot.

Be aware that video noise can also be interpreted as a difference, so your shot would have to be well lit to make it work.

If you don't have a dedicated difference key effect you could try to do your own, by overlaying the before and after footage using a "Difference" transfer mode, and using the resulting image as the basis of a matte track. However, you won't have the "Tolerance" slider that comes with the real key effect which lets you overcome small differences like noise, so it probably wouldn't work out so well.
 
Final Cut also contains a difference matte under it's video filters, and it works great in a pinch. It has tolerance sliders and stuff. Make sure you shoot your background clean with no actors and even lighting, also make sure the actors in the frame aren't casting really huge shadows and stuff on the background. Shoot some experiments and have fun.

Note: This is NOT a replacement to a proper greenscreen key, I've only ever used it for very quick shots. The longest shot I've ever seen this used in (that I noticed) was in Once Upon a Time in Mexico where Banderas slides down the stairs on his guitar case and knocks a guy out at the bottom of the stairs, though I think there was also some roto help. The advantage is the lighting matches because it was all shot right there. Check the DVD for more on it.
 
I use Vegas which doesn't have a built in difference mask.

This is a tutorial on doing difference masking in Vegas 5--the principles should be easy to adapt to any other NLE.

http://matthew.chaboud.com/vegas5/twocats/

And another tutorial from the VASST site.

http://www.sundancemediagroup.com/articles/dayvids/difference_masking_in_sony_vegas.htm

I've played with difference masking myself, and it's difficult to get really good results unless you pay a lot of attention to the lighting. Be prepared to spend fair amount of time rotoscoping to clean up the masking... Still, if you have absolutly no way to get a greenscreen in your shot, then this can be a lifesaver.

You'll want to try to have the talent wear contrasting colors to the background image--if your background is very multicolored, this can be a problem, but area of low contrast are prone to "ghosting."

You might want to consider getting one of those collapsing/portable green screens.
 
You're going to be hard pressed to pull off a good key using the Difference method.
It just never really works. If you can't get a green screen in the shot then it's something to hand over to the roto department.
 
In the Spelberg remake of the movie "War if the Worlds", there's a tracking shot near the beginning of the movie of Tom Cruis and others running down a street with the Alien machines in the background. Surely this wasn't rotososcoped or difference matted?
 
There was A LOT of work done in that sequence and it wasn't always the same technique used because the camera is so mobile. I'm sure roto was used quite a bit though, what with tripods walking behind cruise and people exploding into dust behind and around him.
 
SOE digital said:
You're going to be hard pressed to pull off a good key using the Difference method.
It just never really works. If you can't get a green screen in the shot then it's something to hand over to the roto department.
Click to expand...

Difference/luma keys are a lot in compositing, more than most realize. The green screen isn't the magic answer to everything and pulling a good diff/luma key isn't hard if you know what you're doing and have good footage. With DV it's harder, but so is greenscreening it.
 
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