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Dimensions of B/G for Green Screen

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Michael Barnes
Dimensions of B/G for Green Screen
on Aug 7, 2008 at 9:47:06 pm


I am shooting a webcast for a local newspaper. I am using the Sony HDV Z1 camera. I am a little rusty on green screens. The graphic designer is designing the background for a 8 x 12 green screen. He wants to know what size in pixels to make the file. I'm clueless. Can anyone help?

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Dave LaRonde
Re: Dimensions of B/G for Green Screen
on Aug 7, 2008 at 11:51:27 pm

You need to establish a couple of things before you can give the graphic designer any specifics:
• How big will the webcast video be in terms of horizontal & vertical resolution?
• Will the screen's aspect ratio be 4x3 or 16x9?

If I were doing the job, I would would say that the background graphic should be 320x240 in square pixels, a 4x3 screen aspect ratio. It's a common size for webcast video.

Now get this: when shooting with the Z1, I'd shoot HD, and THEN I would edit at 320x240, square pixels.

What? Why do that?

Because, sad to say, the color resolution of HDV sucks. It plain ol' stinks on ice. Oh, it's good enough to fool the human eye, but not chroma keying software! However, if you scale the footage way down as you'd have to do for a 320x240 project, you'll hide a multitude of chroma-keying sins.

Depending on your editing application, this may or may not be an easy thing to accomplish. Believe me, you'll want to run a few tests first.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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Mark Suszko
Re: Dimensions of B/G for Green Screen
on Aug 11, 2008 at 1:53:52 pm

More pixels is always better. In this case, I say you want as big a file as you can handle, because when you shoot green screen and want to cut to a closeup from a wide shot, it is nice to have the replacement footage be big enough that you can zoom it in a little without creating visible pixelization.

This sells the viewer that the background is real, because the subconscious expects to see a shift in the width/size of the background when you cut to a closeup. Chromakey backdrops that remain at one size, while the talent in the shots changes, are a dead give-away.

The larger image also allows for control in x and y of the relative positions for best composition.

As to how big to make a green screen backdrop, you really only need it to be very slightly wider and taller than the subject you want to show: the rest you remove in post with mattes and garbage masking.
Don't forget to make a green floor if you want real full-body shots.

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