Green screen without a screen?

[DirectorErik]

I'm sure that sounds like an absurd question, but I've been thinking about an idea like this for a while.

Here's a scenario: A man runs across an empty suburban street while a spacecraft crash lands in the background.
Here's the goal: To have it all in one shot... the man in a layer above the spacecraft, he himself overlaps the spacecraft on frame, and the spacecraft above a blank street layer.

I know you could achieve this by setting up a blue/green screen on location, or by rotoscoping, but is it possible to just use a blank street layer as sort of keyer for the man's running video. I figure the technology should be available for this, and would be helpful in places that are hard to set up screens, but I've never heard of any masking through this method of pixel comparison.

Anything like this exist? Or am I just crazy for wanting it...

 

[David Jimerson]

You're describing a difference mask. They generally have so-so results and do worse when there's a lot of motion. Plus, it's a lot of work.

And if you're dealing with outdoors, lighting conditions constantly change and that will affect the feasibility of the mask.

 

[DirectorErik]

Gotcha. So it's just one of those composite modes, correct? Like Add, Multiply, Darken, etc.? I always keep forgetting those exist, and they're so crucial. I guess I usually just have "Add" on and use crazy alternative methods to get what the others would do in a second. Are there any tutorials or explanation videos/articles you'd recommend that describe the pros and cons to each of those composite modes?

 

[David Jimerson]

http://en.wikipedia.org/wiki/Blend_modes

 

[fde101]

I wonder if the results might improve by using a tracker on the foreground object (actor) and limiting the range of the difference mask to a window around the tracker?

 

[arniepix]

What you're describing is really a rotoscoping job. Adding camera motion, without a robotic motion control rig, will pretty much make using a difference key impossible.

 

[DirectorErik]

Thanks for the quick responses everybody. You all totally answered my question. I guess I'll do some tests with all the options you mentioned (composite modes, trackers, rotoscoping) and see which work best for each of my shots (still, moving, etc.). I figure some mixture of these tools will be the best for me since I'm kind of leery on relying on just one.

But once again, thanks everybody, you've given me a lot of options and things to think about.