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Removing or Keying background or something better?

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Rik Trem
Removing or Keying background or something better?
on May 7, 2018 at 11:32:05 pm

Here under you can see two images that I made of two test videos. Sorry for the bad focus on the green screen. For the final video I will do this better.

I was wondering what is the best way to remove the background. With a black or white background or green screen?
The hand will move and the fire too.

If use a luma matte the hand disappears as well. Is there a solution?

Until now, I get the best result with a green screen. (Even with this bad focus and lighting). But it's not yet sufficient.
I think if I use a green screen, it's best to lighten the whole scene more. So the flame becomes less blown out. The background has to be be evenly lit. Therefore the flame must have a good distance from the wall.

Maybe there is another way, that don't know of. In that case ENLIGHTEN me please!

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Steve Bentley
Re: Removing or Keying background or something better?
on May 8, 2018 at 12:47:04 am

For flames that don't have a high temperature element (the blue near the lighter) we use blue screen or shoot on black. Even then we can add that blue element back in with some color correction, to what was a light yellow flame.
Yellow on a green screen is like pink on a blue screen - its not the best to key.

Distance from the screen is a huge factor for either blue or green screen (you can see the lighter flame is changing the color of the green screen).

Dont be afraid to use two approaches - perhaps a luma matte for the flame and a green/blue screen matte for the hand (from the same source footage). Or even just "screen" the flame over the background footage. A sure sign of a matted flame is the dark fringe around the edges because they used a matte instead of a more appropriate transfer function. And remember that out in the daylight you often can't see a flame, so the sure sign of an effects shot (and not a very good one) is a flame comped in that would normally be invisible - its kind of like a lightsaber where the core is dark and edges are bright - it just never looks right.

While this footage may be "raw" and therefore showing washed out in an 8bit jpg, if they aren't raw, both the black and green are pretty washed out and will not provide a good clean key in either luma or green.

You can also change the chemistry of your fuel in the lighter. Butane burns very clean and therefore has a high-temp blue flame component (the yellow flame indicates a lower temperature and incomplete combustion). Ironically what we're going for is a more incomplete combustion so you get an oranger flame which keys better on blue. But changing the fuel can be dangerous if you don't know what you are doing.

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