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Multiple Color Keys?


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#1 Paul Bruening

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Posted 01 February 2010 - 07:36 PM

Has anyone here used two or more colors for keying in one shot? After giving Josh Campbell what I now consider to be a dumb answer, I've rethought his question but don't know anyone who has done it. Let's say you wanted to fake a person jumping out a high rise window. Could you build the key for the windows structure (outside of building) like a doorway in a wall (but on a platform so the characters can jump off and down onto mattresses), paint it key-green, then place a floor to wall cyc in key blue behind that. Then use a still of a building exterior over the green and a still of an office interior over the blue? Would that be a go? If so, how many layers of color difference, along with careful costuming, could you employ in a keyed shot?
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#2 Scott Fritzshall

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Posted 01 February 2010 - 08:32 PM

You can do both a blue and a green key, or whatever colors. Keying is always done locally anyway; you only extract the key from the area you need to, and use that to create mattes that get used later on. As long as there is enough of a difference between them, you could even have one in front of the other. You might do this, for instance, if you've got a greenscreen behind the characters, but one of the characters is like a robot with a hole through his stomach, so he's wearing a blue outfit revealed under portions of his wardrobe. Sometimes bluescreen or greenscreen can be used just as a visual reference also, rather than for keying. A little while ago I did a shot where a character was supposed to be missing an arm, so the actress playing her wore a greenscreen sleeve over her whole arm, with the rest of her costume going over that. Just due to the nature of the shot, there really wasn't any use in keying it; I just used the color difference visually so that I could draw a roto that would let me keep the rest of her body.

I don't entirely understand the shot you're describing, though. Could you maybe draw a diagram of what you're imagining?
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#3 Paul Bruening

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Posted 01 February 2010 - 10:35 PM

These are just some stills I pulled off of Google (I did the diagram in Illustrator). Something like the window still goes on the green key. Something like the office still goes on the blue key. The actors jump through and down onto the mattress. If everything is lined up right, it should look like they've jumped out of a perfectly good high rise? Am I barking up the right tree?

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  • window_jump_key_diagram.jpg
  • high_rise_green_key_insert.jpg
  • high_rise_blue_key_insert.jpg

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#4 John Sprung

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Posted 02 February 2010 - 04:11 AM

I don't think it has to be that complicated. You could do it all with one keying color. First shoot the exterior plate with green or blue in the window. Key the interior into that. Then build your live action green or blue set with the window cutout -- or just the sill part if you don't need him to touch the side jambs -- shoot the stunt, and matte it over the previous combination.

The hard part is lining it up just right, so he walks on the floor and jumps over the sill.




-- J.S.
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#5 Paul Bruening

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Posted 03 February 2010 - 09:52 PM

Hey John,

I've been thinking about your perspective of getting simpler, not more complicated. I remember plenty of buildings at the local university where the stairs led straight into a long hallway of offices. If Josh found one where the stairs started without a landing, used that trick of Scotch taping green construction paper all around the open door and door sill of the stairwell, put some mattresses a few steps down and out of frame, he could have the actors run down the hall a little and jump through the open door and out of frame onto the mattresses. Then, he just has to lay the office exterior still onto the green construction paper key. Cheap, disposable, easy. Just my kind of gag.
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#6 Scott Fritzshall

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Posted 04 February 2010 - 02:33 AM

Really you only need greenscreen over the areas of the screen that your subjects are crossing. It might be more necessary to have green over the area where they'll be falling. The big problem with shooting in a stairwell, other than the logistics of actually framing and staging it, is that it's going to be nearly impossible to light it to get it to look like outside.
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