[theora] Simple RGB->YUV Stuff...
jeremy at emperorlinux.com
Sun Aug 20 20:14:57 PDT 2006
I've spent the weekend or so trying to get my head around Theora and I'm
just not having any luck. First of all, I really don't have any interest
in immersing myself in video encoding--I just have one simple thing I'd
like to do--so I haven't spent any time reading about the theory or
specs or anything like that.
I'm attempting to write a "render-to-theora" target for OpenGL.
Basically, I have access to a buffer of RGBA data that I want to convert
into YUV and pass to Theora to encode for me.
There are lots of different ways to do the per-pixel conversion from RGB
to YUV that I've found online; I think I have this part down fairly
well. Eventually, what I end up with are three buffers, each one as
large as the other and containing the from-RGB-to-YUV values performed
per-pixel. This is where I get hung up.
To pass the YUV data to the various encode functions I have to create a
yuv_buffer struct and set the widths, heights, and strides correctly.
What I've been trying to use is:
yuv.y_width = Width;
yuv.y_height = Height;
yuv.y_stride = Width;
yuv.uv_width = Width;
yuv.uv_height = Height;
yuv.uv_stride = Width;
yuv.y = YDATA;
yuv.u = UDATA;
yuv.v = VDATA;
There isn't a lot of documentation on how Theora "wants" the pixel data
in theora.h, so I really feel like this should be right (since I have a
1:1:1 ratio of RGB data to YUV data). The Y data is perfect--that is,
the grayscale image in my video is perfect. However, the UV stuff is
very wrong, and I've included a screenshot here:
In that screeny, the "blue" bars should be right on top of the grayscale
beneath it (the real image is just a left-to-right blue line drawn by
If anyone has any advice I would very, VERY much appreciate it. It seems
like a simple enough thing to do, and I imagine lots of other people
would be interested in it. I don't really think it's a question of "how
do I convert from RGB to YUV", but rather "how do I populate the
yuv_buffer struct properly so libtheora interprets my data correctly."
Am I missing something very fundamental here? :)
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