[Open-graphics] RGB/YUV converstion... importance?

[James Richard Tyrer]

Timothy Miller wrote:
> Lance is making progress on the YUV->RGB conversion logic.  My general
> question to the list is with regard to back-conversion.
> 
> OGA is designed to deal exclusively with 32-bit ARGB pixels.  But we
> would like to support YUV formats as well.  It seems to me that the
> best place to put the conversion is in the host interface when the
> data comes into the chip.  As you write (or DMA) YUV data into the
> graphics memory, it gets converted automatically to RGB before being
> stored.  Thus, when you want to scale it into a window, the GPU is
> working only on RGB data, as is the video output.
> 
> My question is:  Is there any need to convert back from RGB to YUV?
> We're not doing video capture.  It seems silly to read back any YUV
> data that you just wrote.  And if you draw to a surface that was
> originally a YUV upload, what are the chances you'll ever care to
> download it back to host memory?
> 
> The way I conceive it, YUV->RGB converstion is ONLY for video
> playback, not textures or drawing surfaces.  You can use it for
> textures, but memory management software needs to understand that a
> conversion was done.
> 
> One thing to note is that we're finding that we can't convert YUV->RGB
> and then RGB->YUV without data loss.  Actually, we can, but the
> hardware required is unjustifiable, and it also requires us to
> actually 'alter' the accuracy of the RGB data just so that we can
> convert it back.  (That is, either we convert it accurately but can't
> convert back losslessly, or we convert it inaccurately so we can read
> it back.  But we don't want to use the transistors for that anyhow.)
> 
> Comments?

You are referring to digital conversion?

I don't see that there would be any need for digital conversion unless 
we needed to do video capture of a *digital* video signal.  If we needed 
to do video capture of an analog video signal, I would do analog 
conversion to RGB and then digitize that since otherwise you are going 
to loose information.

I presume that the TV output chip does analog RGB 2 YUV.

OTOH, can the same hardware with different coefficients do the reverse 
conversion?

-- 
JRT