[Mesa-dev] Fwd: [PATCH 5/6] st/mesa: implement blit-based ReadPixels

[Michel Dänzer]

On Don, 2013-03-14 at 23:35 +0100, Martin Andersson wrote: 
> On Thu, Mar 14, 2013 at 7:45 PM, Marek Olšák <maraeo at gmail.com> wrote:
> >
> > +   /* See if the texture format already matches the format and type,
> > +    * in which case the memcpy-based fast path will likely be used and
> > +    * we don't have to blit. */
> > +   if (_mesa_format_matches_format_and_type(rb->Format, format,
> > +                                            type, pack->SwapBytes)) {
> > +      goto fallback;
> > +   }
> 
> On my system (Intel core i7 and AMD 6950) the memcpy fast-path takes
> around 210 milliseconds and the blit path takes around 9 milliseconds, for a
> 1920x1200 image. So it is much faster, at least on my system, to use the
> blit path even when the mesa format matches format and type.
> 
> To test this I forced the use of the memcpy fast-path (the mesa format was
> MESA_FORMAT_XRGB8888, the format was GL_BGRA and the type was
> GL_UNSIGNED_BYTE). I visually inspected the resulting image and I could
> not see anything wrong with it.
> 
> But perhaps forcing the use of the memcpy fast-path invalidates the results.
> Or there might be some other issues that I am missing, but I just wanted to
> say that on my system it is better to remove this check.

If the read buffer is in VRAM, reading from it with the CPU directly
will be very slow.


-- 
Earthling Michel Dänzer           |                   http://www.amd.com
Libre software enthusiast         |          Debian, X and DRI developer