[Pixman] [cairo] pixman: New ARM NEON optimizations

Soeren Sandmann sandmann at cs.au.dk
Fri Feb 11 02:30:33 PST 2011

Chris Wilson <chris at chris-wilson.co.uk> writes:

> But at the moment, about the only thing I truly want to add to the API
> is a LERP operator.

> Cairo has a slightly different definition for [clip-] masked operators
> where the operation is defined as:
>   ((src IN mask) OP dst) LERP_clip dst
> and for SOURCE and CLEAR:
>   (src OP dst) LERP_(clip IN mask) dst
> Cairo currently implements LERP as:
>   a LERP_t b := (a IN t) ADD (b OUT t)
> So for example, for the typical unbounded operation (such as IN) this
> translates to:
>   result = (((src IN mask) OP dst) IN clip) ADD (dst OUT clip)
> which currently requires 3 composite operations:
>   dst' = dst; /* may require a blt using SRC */
>   pixman_image_composite (OP, src, mask, dst');
>   pixman_image_composite (OUT_REVERSE, clip, NULL, dst);
>   pixman_image_composite (ADD, dst', clip, dst);
> By introducing the LERP operator we can reduce this down to 2:
>   dst' = dst; /* may require a blt using SRC */
>   pixman_image_composite (OP, src, mask, dst');
>   pixman_image_composite (LERP, dst', clip, dst);
> Obviously, we could introduce a quaternary operator and do this in a
> single pass - though the complexity would seem to outweigh this rarely
> used slow path.

So, I realized I never actually responded to this, sorry about that.

If I had a time machine, I would go back and make - among others -
these two changes to Render:

(1) The equation would be 

        (src OP dst) LERP_mask dst

    and not 

        (src IN mask) OP dst

(2) The RGB channels of Alpha-only images would be considered to be
    the same as the alpha channel, and not 0 as they are now. For
    example, a 0xb9 pixel in an a8 image would be considered
    equivalent to 0xb9b9b9b9 and not to 0xb9000000. That is, they
    would be considered a translucent white rather than a translucent

These two changes together would have the effect that (a) the equation
would be much easier to understand visually (composite src and dst,
then clip to the mask and write back), and (b) component alpha would
become completely regular with no need for the "component_alpha" bit
in pictures.

Given the lack of a time machine, a possible direction might be to add
a new set of operators CLEAR_LERP, SRC_LERP, DST_LERP, ..., that would
follow the equation above, and also a new format type
PIXMAN_FORMAT_TYPE_W where W stands for 'white', and the missing color
channels would be defined to be copies of the W channel. 

Even better might be to add a new entry-point:

   pixman_image_composite_lerp (...):

that would use the LERP equation and treat alpha-only formats as
described above.

With either, your LERP operator would simply be SRC_LERP, but the
other other _LERP operators might be useful for cairo too. For
example, the

  ((src IN mask) OP dst) LERP_clip dst

equation could become

         composite (SRC, src, mask, tmp);
         composite (OP_LERP, tmp, clip, dst);

instead of 

         composite (SRC, dst, NULL, tmp);
         composite (OP, src, mask, tmp);
         composite (SRC_LERP, tmp, clip, dst);

Adding all those operators is obviously a lot of work, so I can see
why it would be tempting to just add the LERP operator. However, it
would annoy me to have one single weird operator that would behave
according to a totally different equation than all the other

If it were a huge speedup, then maybe, but it seems to be that it's
only useful in relatively few cases. Or alternatively, if we have to
bite the bullet and add special-cased support for this, then maybe it
would be better to just add the full quaternary operator as a new
entry point, and not pretend it's just another regular operator.


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