[cairo] Blend modes take 3

[Soeren Sandmann]

"Benjamin Otte" <otte at gnome.org> writes:

> On Wed, Nov 19, 2008 at 7:00 PM, Soeren Sandmann <sandmann at daimi.au.dk> wrote:
> > Hi Benjamin,
> >
> > Overall, the separable blend mode stuff looks good, except for the
> > component alpha versions.
> >
> New update pushed to my git branches of pixman/cairo.

Comments on separable blend modes:

- In both fbCombineMultiplyC and fbCombine ## name ## C, 

    - When mask is 0, there is no reason to read the source.

    - The continue is incorrect; it needs to write out 0 in that case,
      since the intermediate buffer is not zero-filled.

- In fbCombineMultiplyC,

    - There is no reason to read the destination if the inverse
      combined src/mask is 0

- In FlashSubtractC, the m = ~m is not used. And fbCombineMaskC()
  seems like it could just be fbCombineMaskValueC.

I still don't completely understand the non-separable blend modes, and
how they interact with premultiplied alpha and with component
alpha. But here are some questions about the code:

- In SetLum and SetSat you have, respectively:

>   if (Max (tmp) > sa) while (1);
>   if (Max (dest) > 255 * 255) while (1);      

Are these supposed to never happen?

- In SetLum:

>  if (min < 0) {
>    tmp[0] = l + (tmp[0] - l) / 4 * l / (l - min) * 4;
>    tmp[1] = l + (tmp[1] - l) / 4 * l / (l - min) * 4;
>    tmp[2] = l + (tmp[2] - l) / 4 * l / (l - min) * 4;
>  }
>  if (max > a) {
>    tmp[0] = l + (tmp[0] - l) / 4 * (a - l) / (max - l) * 4;
>    tmp[1] = l + (tmp[1] - l) / 4 * (a - l) / (max - l) * 4;
>    tmp[2] = l + (tmp[2] - l) / 4 * (a - l) / (max - l) * 4;
>  }

Where do those 4s come from?


Soren