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Chroma keying and noisy, shimmering edges
Hi all:
I know that usually, when you find yourself with the above problem (noisy edges) when keying, it’s due to greenscreen footage shot in DV and HDV, which have a chroma subsampling of 4:1:1 or 4:2:0. But I’m trying right now to key footage shot in DVCPRO HD, which is 4:2:2 (shot with the Panasonic HVX-200, in 720p mode), and I also find myself with shots where the borders have plenty of simmer. (“How was the footage shot? Was it well it?” Well, it isn’t great, but I wouldn’t call it terrible either; I’ve seen worse).
I’ve tried a couple of things: first of all, converting the footage to YUV (Channel combiner), blurring the chrominance channels (channel blur) and turning it back to RGB. This has worked for me in the past, but in this case it didn’t do much. I limited myself to modest amounts of blurring, not more than 1-1.5 pixels.
The other thing I’ve tried was to apply Aharon Rabinowitz’s trick, explained in his tutorial “Super tight junk mattes”, of using Auto-trace to automatically rotoscope the shapes of the people on the foreground. The problem with this is that, since the key edges are noisy, the resulting garbage mask is noisy too, and simmers constantly. I’ve tried adjusting the Autotrace parameters (specially the one about “don’t include details smaller than X pixels” or something like that), but without luck.
Does anyone have other possible tricks for this problem? (Other than “don’t use 4:2:2: shot instead with a Sony Cinealta in RGB 4:4:4” 🙂 ). Forgot to mention: I’m using Keylight for all of this.
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3 Replies
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Yes, no and no. However, I have read Mark Christensen’s “After Effects Studio Techniques”, which I’ve been told is the best book around for AE, and from which I learned the blur chroma channels technique I mentioned above. Since it wasn’t enough to solve my problem, I came here looking for further advice. I’ll now go and have a look at Andrew Kramer’s tutorial.
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when you get to a nice looking lightwrap, you can fudge it and make the lightwrap a few pixels more than usual, and simmer won’t really matter, it will be covered up anyway. Hey, its better than roto right?
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I’ve seen this problem with HVX footage. I had to key out a several dogs that had very wiry fur and some fast motion. Neither Aaron’s not Mark’s techniques were much help.
The solution in my case was to remove as much noise as I could from the footage (using Remove Grain) before preparing a garbage matte and keying with keylight. The resulting key had a softer edge than I would have liked but after hours of experiments it produced the best result.
There was some loss of pixel density that I resolved by compositing several layers of the keyed footage.
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