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Keying separately edges and body: what if the holes are around the edges?

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Paulo Jan
Keying separately edges and body: what if the holes are around the edges?
on Aug 28, 2012 at 2:46:21 pm

Hi all:

I know about the basic technique of keying separately the body and the edges of a subject:

1) Do first a key of the subject ensuring that there are no "holes" in it, even if it means hard edges with no detail.
2) Choke that matte.
3) Do another matte for the edges of the subject, preserving detail there, even if it means "holes" in the body.
4) Put it above the first layer.

But... what if the holes in my matte are around the edges of the subject? Like for example:





In this case, there aren't really any big holes in the middle of the matte to cover; on the other hand, if I try to preserve the edges of the arm, I unavoidably get some amount of noise, sizzle... due to those non-opaque areas you see on the edge. The only way to get rid of them would be to lower "Clip white" in Keylight, but that would imply making the edges much "crunchier" and uglier, which is what I'm trying to avoid.

Of course, I could roto the lower part of the arm and key it separately... but if I start going down that route, I might as well roto the entire arm. Other than that, how would you apply the general principle of "key each part of the shot separately" in a case like this?


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keith mcgregor
Re: Keying separately edges and body: what if the holes are around the edges?
on Aug 28, 2012 at 4:57:56 pm

That footage is washed out and unsaturated. Pretreat it in a pre comp first, then key that comp. You'll have much better results. Maybe it's just the compression on the file you uploaded, bit it looks like an awful lot of chromatic aberration, like looking at an old crt monitor, which is contaminating your key. Too much red on the green screen.
-Keith


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Paulo Jan
Re: Keying separately edges and body: what if the holes are around the edges?
on Aug 28, 2012 at 6:23:07 pm

The footage is shot using a Canon 550D DSLR with the Technicolor Cinestyle curve, hence the unsaturated look (and probably the chromatic aberration you're seeing: 4:2:0 compression and all that). Despite that, it keys quite well: the screengrab I uploaded was done just with the eyedropper in Keylight, without touching any other control, and as you can see, the background and foreground are well separated (I've certainly seen much worse). The only issue is, as I mentioned, with the lower part of the arm.

I just tried doing as you said, adding the Cinestyle recommended S-Curve before the key (using LUT Buddy) to restore it to a more "normal" look, and the resulting key is practically the same, with the same problems.


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keith mcgregor
Re: Keying separately edges and body: what if the holes are around the edges?
on Aug 28, 2012 at 8:20:44 pm

For this don't follow any recommended settings to bring back the lusture. Use levels to increase the contrast. Change the gamma slider to bring down the lightness. Use a saturation to increase the sat. Use crunchy settings to get ypour core. Place that on top of your original footage and color it to something away from the green. Place that comp into a new one and key from there. It's just seeing the spill and wanting to key it out, with a 'color mask' on what you want to keep you'll get better results. Add some screen softness to it and you should be fine. With this screen grab (it's jpeg huh?) it's blocky but you get the idea.
-Keith
screenshot2012-08-28at2.36.33pm.png


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Dave LaRonde
Re: Keying separately edges and body: what if the holes are around the edges?
on Aug 30, 2012 at 10:13:13 pm

I'd first try to use the Clip White and Clip Black controls in Keylight. There are a couple of Keylight tutorials here on the COW, and you can google them, too. You might get lucky.

I wouldn't use any of those fancy Canon settings in the future. If you want a certain look, you can color grade the finished work.

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


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Paulo Jan
Re: Keying separately edges and body: what if the holes are around the edges?
on Aug 31, 2012 at 2:13:25 pm

Actually Cinestyle isn't made by Canon, but by Technicolor, and its point is precisely to output an image as "flat" as possible, to make it easier to grade later. That's the reason why I was using it:

http://www.technicolor.com/en/hi/theatrical/visual-post-production/digital-...

I haven't had time to try Keith's suggestions yet (other work came up), but I'll see if I can do it this weekend.


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