green screen keying in avid liquid 7 at DVinfo.net

Charlie Yang · February 17th, 2009, 01:33 PM

Hello,

I was shooting a conversation scene of two people in front a green backdrop. in the editing stage, i tried to impose this scene over a second clip of dark night scene (with moon hanging in the sky).

on the first clip, i keyed out the green by using "green screen keying" clip effect in AL7. it works fine. however, after i shrunk this clip to 50% of its original size (by using "CPU 2D editor" clip effect), the border line of the shunken clip (rectangle box border) became very obvious. i tried to use "blur CPU -> alpha channel" effect to get rid of this border but to no avail. since it only blurs the contour line of the subject (two humans in this case) but not the rectangle box border line.

any insight on how to eliminate (or blur) this border is greatly appreciated. or in general, how to shink a keyed clip without an annoying visible border line of the first clip? Thanks.

Charlie

Bryce Comer · February 17th, 2009, 11:39 PM

Hi Charlie,
Maybe try to crop the image as well when you shrink it. Not too much, just enough that it gets rid of the border. I'm not in front of my editing computer at the moment, so i can't have a play, but it just might work.
Bryce

Charlie Yang · February 18th, 2009, 07:02 AM

thanks, Bryce. I tried the crop and then reduced the size of screen to 50%. still the same old problem. anything else i could try? i would imaging a lot of movie will need this technique to shrink their subjects (like Honey, I shunk myself). what technique do they use?

Robert Martens · February 18th, 2009, 06:42 PM

Many movies do use this effect, and they start with what's known as a "garbage mask"; I've never explored doing this in Liquid, so I don't know if you can do it the same way it's done in the visual effects world, but the crop Bryce suggested is, at least in your situation, the same basic principle. Garbage masks are why your chroma key screen only needs to cover the range of motion of your subjects in any given shot, and doesn't need to fill the entire frame. A rough, simple, easily-animated mask will allow you to rapidly remove large amounts of unnecessary image data, leaving the keyer free to take care of all the important details along the edges of your talent.

I see that has not worked for you, however, and the only answer I can think of is Edge Softness. In the Transparency section of any of the 2D editors, there's an entry field for Edge Softness, which in earlier versions of Liquid had a value by default, for some reason. I can't imagine why anyone would want a thing like that turned on right away, but that's the way it worked. Checking my installation of Liquid 7.2, I see that the CPU 2D Editor no longer does this, and is set to 0 by default; I would think it'd behave the same way on your system, but it's worth checking to be sure. The GPU editor still defaults to 2.5, but as you said you're not using that. Maybe you could try it, though, if nothing else works? Apply the GPU version of the effect instead, then turn off Edge Softness, see if perhaps it produces the proper result.

February 17th, 2009, 01:33 PM	#1
Charlie Yang New Boot Join Date: May 2007 Location: Wayne, NJ Posts: 6	green screen keying in avid liquid 7 Hello, I was shooting a conversation scene of two people in front a green backdrop. in the editing stage, i tried to impose this scene over a second clip of dark night scene (with moon hanging in the sky). on the first clip, i keyed out the green by using "green screen keying" clip effect in AL7. it works fine. however, after i shrunk this clip to 50% of its original size (by using "CPU 2D editor" clip effect), the border line of the shunken clip (rectangle box border) became very obvious. i tried to use "blur CPU -> alpha channel" effect to get rid of this border but to no avail. since it only blurs the contour line of the subject (two humans in this case) but not the rectangle box border line. any insight on how to eliminate (or blur) this border is greatly appreciated. or in general, how to shink a keyed clip without an annoying visible border line of the first clip? Thanks. Charlie

February 17th, 2009, 11:39 PM	#2
Bryce Comer Trustee Join Date: Jun 2007 Location: Rossland, British Columbia Posts: 1,024	Hi Charlie, Maybe try to crop the image as well when you shrink it. Not too much, just enough that it gets rid of the border. I'm not in front of my editing computer at the moment, so i can't have a play, but it just might work. Bryce __________________ There's never enough hours in the day!

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February 18th, 2009, 07:02 AM	#3
Charlie Yang New Boot Join Date: May 2007 Location: Wayne, NJ Posts: 6	thanks, Bryce. I tried the crop and then reduced the size of screen to 50%. still the same old problem. anything else i could try? i would imaging a lot of movie will need this technique to shrink their subjects (like Honey, I shunk myself). what technique do they use?

February 18th, 2009, 06:42 PM	#4
Robert Martens Major Player Join Date: Jun 2003 Location: Miller Place, NY Posts: 820	Many movies do use this effect, and they start with what's known as a "garbage mask"; I've never explored doing this in Liquid, so I don't know if you can do it the same way it's done in the visual effects world, but the crop Bryce suggested is, at least in your situation, the same basic principle. Garbage masks are why your chroma key screen only needs to cover the range of motion of your subjects in any given shot, and doesn't need to fill the entire frame. A rough, simple, easily-animated mask will allow you to rapidly remove large amounts of unnecessary image data, leaving the keyer free to take care of all the important details along the edges of your talent. I see that has not worked for you, however, and the only answer I can think of is Edge Softness. In the Transparency section of any of the 2D editors, there's an entry field for Edge Softness, which in earlier versions of Liquid had a value by default, for some reason. I can't imagine why anyone would want a thing like that turned on right away, but that's the way it worked. Checking my installation of Liquid 7.2, I see that the CPU 2D Editor no longer does this, and is set to 0 by default; I would think it'd behave the same way on your system, but it's worth checking to be sure. The GPU editor still defaults to 2.5, but as you said you're not using that. Maybe you could try it, though, if nothing else works? Apply the GPU version of the effect instead, then turn off Edge Softness, see if perhaps it produces the proper result.