[Pixman] RGB24 -> ARGB32 performance

Tristan Schmelcher tschmelcher at google.com
Fri Jul 15 14:50:37 PDT 2011

On 15 July 2011 14:08, Siarhei Siamashka <siarhei.siamashka at gmail.com>wrote:

> On Fri, Jul 15, 2011 at 9:53 PM, Tristan Schmelcher
> <tschmelcher at google.com> wrote:
> > I have a Cairo application that is using Pixman to composite a
> > semantically opaque image by scaling a bunch of source images in RGB24
> > format. If the target surface is also RGB24 format, then it is very
> > fast and uses fast_composite_scaled_nearest_8888_8888_cover_SRC. But
> > if the target surface is the ARGB32 format, then it takes almost three
> > times as long and uses just fast_composite_scaled_nearest.
> >
> > If I had a choice, I would always make the destination surface in
> > RGB24 format, but unfortunately it is being created as ARGB32 by
> > another library which I don't control, so my application is taking the
> > slow path. I want to improve it so that the performance in the RGB24
> > -> ARGB32 case is approximately as good as the RGB24 -> RGB24 case, if
> > possible.
> Have you tried alternatively converting your opaque source images to
> ARGB32 format? This approach may have some advantages.

The source images come from the output of a video decoder. Converting them
would mean touching all the memory an extra time for every frame, so also
not very efficient.

> > My idea for how to go about this is to add a x888_8888 fast-path to
> > Pixman for RGB24 -> ARGB32 that does the same thing as the 8888_8888
> > path but sets all the destination alpha bytes to 0xFF. But before I
> > try coding it I wanted to ask here if anyone has any other suggestions
> > or knows of any "gotchas" with this approach.
> >
> > Comments?
> Yes, this is how pixman development is done at the moment. One finds
> something that is slow and adds new special fast paths for it. I have
> also attached a patch which should add the needed entries to the fast
> path tables. Probably this is what you are intending to get.

Heh, I coded up nearly the same thing moments ago. :)

I tested your patch and it works great. The performance is indistinguishable
from RGB24 -> RGB24.


> --
> Best regards,
> Siarhei Siamashka
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