[Theora-dev] Problems with Theora DirectShow filters
Timothy B. Terriberry
tterribe at vt.edu
Wed Sep 15 08:15:35 PDT 2004
> For the moment... you'll need to find a colour space converter that can
> do the conversion properly. In the next release i'll include the RGB to
> YUV conversions. I know there are some proprietary filters out there...
> i haven't had much luck finding a good free colour space converter though.
I'd warn against blindly applying someone else's Y'CbCr to R'G'B'
converter. There are multiple Y'CbCr color spaces (and in fact, there
are multiple R'G'B' spaces, though on a PC you can perhaps assume sRGB
support). The available Y'CbCr spaces used by Theora are well-specified;
there is a well-defined conversion into device-independent CIE XYZ space
(and from there to any R'G'B' space you want). See Secton 4.1 of the
spec. However, I don't know what support the Windows APIs have for
describing the different spaces.
The traditional FOURCC codes like I420, IYUV, and YV12 are not even
sufficient to pass Y'CbCr data to Theora directly, since even without
conversion, the sampling locations of the sub-sampled chroma components
can vary from standard to standard. Theora, in general, uses
JPEG-compatible sampling locations.
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