On Wed, 03 Oct 2007 05:26:54 +0200, Graham Evans
<[EMAIL PROTECTED]> wrote:
Cinelerra's Chroma key (hsv) has limitations but I haven't used the old
cinelerra Chroma key plugin since this one was written (about 12 months
back). I do find I need to combine it with other effects to get a
reasonable result and that a good chroma key plug-in could automate some
of this stuff. Here is an example of an effect chain I use for chroma
key.
1 Apply blur plug-in, radius 3, to all color channels, horizontal and
vertical direction. If this initial blur is not done then the next step
with Chroma key (hsv) produces stange horizontal striping and jagged
blocky edges to the mask area. This seems to be something to do with
color sampling. I'm not sure if it is a bug in chroma key hsv.
I'd say it's not. If the internal colour format (the format of what the
original documentation calls "the temporary") is 4:4:4, then the effects
should treat the image data as real 4:4:4.
If the codec returns uncompressed 4:2:0 or 4:1:1 data, then the temporary
shouldn't just repeat the colour pixels in a 2x2 or 4x1 pattern. That's
not
upsampling! But I suspect that this is what Cinelerra does. It is not
visible
to the naked eye, since our eyes are rather poor at resolving small specs
of
colour. This is why chroma subsampling is acceptable in the first place.
But once a mask is derived from this coarse chroma signal, it gets ugly!
That said, for a chroma keyer to be usable outside well-lit studios with
smooth greenscreens, it needs a little more features than chromakey-hsv.
Graham has suggested a few.
--
Herman Robak
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