[GEM-dev] pix_chroma_key not acting as I expect?

chris clepper cgc at humboldtblvd.com
Mon Apr 24 20:53:41 CEST 2006


If this is on a Mac the key values are in YUV.  The green value is the
lowest in the range (16,16,16) that's why the screen is green to begin with.

On 4/24/06, B. Bogart <ben at ekran.org> wrote:
>
> Hey all,
>
> I'm CCing a local artist, Teresa, who asked me this question.
>
> In the process of playing with some keying footage she provided I seem
> unable to make pix_chroma_key actually work in this case...
>
> We have two videos, a figure shot on a green screen (reading value 0, 240,
> 0), and a background to layer the figure into.
>
> I'm using "value 0 240 0" to select the exact green of the background.
>
> Now my understanding means that I should be able to see the green knocked
> out with a very low range, since it is more or less all the same exact
> color/shade, but instead I need to provide a very very high "range" to get
> a horrid result. I can start to see little bits of the figure with a range
> of "104 104 104", but going all the way up to "255 255 255" I don't see
> the whole figure (blacks parts are missing), and I get an approximate 1
> pixel green halo around the figure. Some of this is due to compression,
> but that does not explain how bad this really looks. See attached
> screenshot of "value 0 240 0" "range 255 255 255"
>
> Note I have sent colorspace RGB to both pix_film's so that I'm using RGB
> ranges and values.
>
> It looks to me like the "range" I'm providing is trying to match not the
> green background, but the figure itself...
>
> Since its a good green-screen, and pretty good footage, this should be a
> cinch to do, but I can't figure it out, after playing with the all the
> variables, direction, and which video goes to which pix_chroma_key inlet.
>
> Please CC Teresa as it is her really trying to figure this out...
>
> Thanks!
>
> .b.
>
>
>
>
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