Re: [libdv-dev] XvImage support, YUV value range

[Stefan L. <lu...@be...>]

On Son, 30 Jul 2000, "Charles 'Buck' Krasic" wrote:
> The thing is, I just did some similar stuff this week :-}.  I apologize
> to anyone else who might be listening who's done the same.  It's
> unfortunate when effort is duplicated.

But that happens. So we have two possibilities of improvements.
I'll check yours later on.

> > I just did some tests with XvImages and hardware accelerated YUV to RGB
> > conversion. One problem was the value range used by the dv decoder.
> > (Y: [-223,+223]; UV: [~-63,~+63]). So adjusting the offset for 'ylut' by
> > 128 is not enough.
> 
> I'm not sure what you mean by this exactly.   
> 
> >From the smpte spec, section 5.1.1:
> 
> "Each pixel has a value from -127 to +126 which is obtained by the
>  subtraction of 128 from the input video signal level."
> 
> So the +128 offset is just the exact inverse what is done to the video
> signal level.
That means: I saw Y (*Ytmp) values of -223. UV (cr/cb) values are ok.
So indexing in a table which has only
a base offset of 128 will result in out of bounds addressing (perhaps
seg faults).
 
...
> What does the dga column mean?

dga is a program that ships with X11 and does a performance test of
framebuffer access (You'll need root permission to do this).
DGA is an X11 extension (DirectGraphikAccess ?).

-- 

mfg

  Stefan Lucke (lu...@be...)