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Old March 25th, 2007, 08:10 PM   #1
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Keying through a window

I searched for but couldn't find any information at all on this.

How would one pull a matte from a green screen that is behind a window or windshield while maintaining a sense of the presence of the window (like reflections, fingerprints, dirt, etc).

I don't have a specific project in mind that required this, but it's something I've always wondered in case it comes up.

I've had some luck doing this a way I concocted. I shot the talent looking out her apartment window, which was essentially black outside, but with a backlight shining in from the outside of the window, out of frame (kind of a hair light, to reveal the marks on the glass).

I than matted out the window manually (by drawing the matte for each frame), and than overlayed a copy of the window on top of itself with the opacity lowered enough so that you could see the window, and what I had put outside it, too.

It worked pretty well, but there obviously must be an easier way. Could someone explain?

Here's a frame to show what I tried. Remember, I didn't use a green/blue screen, so this was extremely time consuming (and not perfect, though the attached frame is one of the best ones from the clip)
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Keying through a window-morning-window.jpg  
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Old March 28th, 2007, 12:29 AM   #2
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Actually, depending on how long the shot is, if you can roto it, do that instead of shooting on a greenscreen.

The greenscreen will add a green or blue (depending on the color screen you used) hue to your shot, and what you did is actually a better way to do it.

Read Stu Maschwitz's book called "The DV Rebel's Guide" and you'll learn a lot from one of the guys who does this for a living.

Hope it helps,

Jim
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Old March 28th, 2007, 01:50 AM   #3
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I would actually go about green screening it. If you can get the green outside the window near perfectly lit (not to hard on a set) then it should be a snap. Keylight in After Effects will take out the green and leave the other colors.

Usually this causes a problem cause it will remove all the green but leave shadows in your green screen if there are any, but in this case its the solution.

As far as having a green hue, just add a spill suppressor. Problem solved.

Its all about getting a good color green outside and thats all.
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Old March 28th, 2007, 08:31 AM   #4
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Like Alan says, Keylight will do a good job of holding reflections if your greenscreen it.

Another technique, that is a bit trickier, but can yield good results is removing the window. Set up a greenscreen outside, and a mirror-image camera (same distance from the window as your main camera, same angle from the window pane, just flipped, so it's outside). Shoot, then do your greenscreen and composite the mirror-image over the background.

This is limited, but can help you get good results. It usually means you can't do big camera moves, and you have to be very careful to line up the two cameras, and to get the compositing done right.
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Old April 9th, 2007, 04:57 PM   #5
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Thank you everyone for the responses -- I was hoping there was one sure-fire method I was just missing. I guess I will have to experiment.
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