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Live Chroma Keying idea.

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chris Stack
Live Chroma Keying idea.
on Feb 10, 2011 at 8:10:45 pm

Hello, apologies that this is completely the wrong forum, but I know there are a lot of all-rounders in here who could come up with creative solutions to this problem. Basically what I need to do is:

Set up a green screen outside. Yeah I know right? But it'll be in some sort of covered stage area, so not so mental.
Set up 3 cameras.
Save that footage to disk (about 8 hours worth for each cam)
At the same time save that footage to another location, keyed.

The key does not have to be great, I'm well aware that it'll look a bit shitty, it just has to be not bad.
I know I can run the footage live through a video mixer to achieve a rough key, my main obstacle is how to export that footage on the fly with an alpha channel.



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Jeremy Garchow
Re: Live Chroma Keying idea.
on Feb 10, 2011 at 8:45:58 pm

[chris Stack] "my main obstacle is how to export that footage on the fly with an alpha channel."

To what and why? Please give a bit more info as to the why.


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Dave LaRonde
Re: Live Chroma Keying idea.
on Feb 10, 2011 at 10:38:53 pm

[chris Stack] "I know I can run the footage live through a video mixer to achieve a rough key, my main obstacle is how to export that footage on the fly with an alpha channel."

I'm with Jeremy: I have no clue why you'd want to do this; if the key's going to be bad, why do it at all? It just doesn't make sense.

Since a video switcher is designed to chroma key a foreground over a background, there aren't any ways to put the keyed shot over an alpha channel; any switcher I've ever seen has no "transparent background" button. Furthermore, I know of no live video signal that can support a chroma key.

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


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Bouke Vahl
Re: Live Chroma Keying idea.
on Feb 11, 2011 at 9:28:33 am

well, my mixer (panny) can output the matte generated by the chroma key as well as a keyed and source signal.
(all outputs are routable)
So if you tape the source and the matte on two decks, you can generate QT with alpha afterwards.

Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Walter Soyka
Re: Live Chroma Keying idea.
on Feb 11, 2011 at 2:13:53 am

[chris Stack] "my main obstacle is how to export that footage on the fly with an alpha channel."

You can't -- there's no video standard that supports it.

The best I'd think you can do is see if your outboard chroma keyer can output a luma matte, record that output, and use it to cut the hole in the fill footage.

For 3 cameras, you should definitely sync timecode across all 6 decks (matte and fill for each) to make editorial easier.

One other challenge I see doing a multicam green screen shoot -- matching the perspective of the background to the shots during the composite. It's not too hard to rough in by hand with only static shots, but if the cameras will ever move or zoom live, you'll want loads of tracking markers to help you solve the camera move in post.

If you need to do a 3D solve, you'll need markers distributed in space, not just on the screen, which means you'll need to do some garbage matting to remove them, which means you may as well not bother with a dodgy live key in the first place.

I must say, I'm very curious about your project after hearing these requirements. Good luck with it!

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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chris Stack
Re: Live Chroma Keying idea.
on Feb 11, 2011 at 10:27:31 am

Thanks very much.
A bit more info: The final video will be built in flash, so my final format would be .flv or more likely a PNG image sequence. The key doesn't need to be perfect because it's a silly internet promotion playing on the fact that it's clearly a green screen shot. The 3 cameras will all be static. The best option seems to be to record footage and luma matte separately, bring both into FCP, punch it out there and re-export.



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Jeremy Garchow
Re: Live Chroma Keying idea.
on Feb 11, 2011 at 4:00:00 pm

[chris Stack] "he key doesn't need to be perfect because it's a silly internet promotion playing on the fact that it's clearly a green screen shot. The 3 cameras will all be static. The best option seems to be to record footage and luma matte separately, bring both into FCP, punch it out there and re-export."

Yes, that is certainly one way, but if going this route you can also add a BG with a switcher and record that. That will truly be a live key. But your BGs must be set.


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