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Re: Green Screen Dancing Edge Pixels (After Effects CS6)

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Steve Bentley
Re: Green Screen Dancing Edge Pixels (After Effects CS6)
on Mar 21, 2018 at 11:21:23 pm

So here's a couple of things:
This is a green screen image with H264 compression (and pretty mightily compressed at that) and you can see everything is sharper but more than that, the visible blocks that make up the H264 compression are away from the hairline, out in the green area where subtle shades have to be decided on. All blocks change every keyframe but with the obvious ones out in the green you won't see the key dance every 15 frames or so at the hairline.



In your footage there is no where near the clarity of detail and there are all these little smudges where the codec (or the camera) has tried to invent what it's seen or average a bunch of data into a few blocks. The more noisy a shot is (by noise I mean lots of different textures and colors and details - in other words the wrinkles in the fabric) the higher the Mb/s it needs to make sure all of those details come out sharp and at 3.5 mb/s sec its just not enough, given the content of this shot. Any codec is going to start to sacrifice things (usually color first) in order to keep details it finds important. One of my clients loves h264 because at a low through put rate all her wrinkles disappear.



If you zoom in on your footage to this level and step through it frame by frame (no key or effects) you can see the edge of the hairline boiling and little areas of distortion going on where chunks of hair literally move over 16 pixels for a few frames and then move back or sections get averaged to keep the bandwidth. This may be function of the camera or the codec, I'm not sure, but this is also going on at the edge of the hair so you are keying a moving target. It's almost has a shower glass effect.
Still playing but I thought I would share.



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