Premultiplied alpha handle for driver

[Jonathan Morton]

On Tue, 2010-06-01 at 18:55 +0800, Huang, FrankR wrote:
> So I just want to know if picture_1x1 has the alpha value and mask has
> the alpha value. How the driver handle this? If I know the answer, I
> can fix this bug. Hope your reply soon:)

Here's a couple of code fragments from a reference library I wrote a
while ago.  These would be called as follows:

if(msk) {
	PixelsDoMask(src, msk, tmp, count);
	PixelsDoOpOver(tmp, dst, count);
} else {
	PixelsDoOpOver(src, dst, count);
}

void PixelsDoMask (
        PixelsColour * src,
        const u8 * const msk,
        PixelsColour * tmp,
        const u32 count )
{
        for(u32 c=0; c < count; c++) {
                u32 mask = msk[c];

                if(!mask) {
                        tmp[c] = nil;
                } else if(mask == 255) {
                        tmp[c] = src[c];
                } else {
                        tmp[c].a = divBy255(src[c].a * mask);
                        tmp[c].r = divBy255(src[c].r * mask);
                        tmp[c].g = divBy255(src[c].g * mask);
                        tmp[c].b = divBy255(src[c].b * mask);
                }
        }
}

void PixelsDoOpOver (
        const PixelsColour * const src,
              PixelsColour *       dst,
        const u32 count )
{
        for(u32 c=0; c < count; c++) {
                u32 a = 255 - src[c].a;

                if(!a) {
                        dst[c] = src[c];
                } else if(a != 255) {
                        dst[c].a = sat255(divBy255(dst[c].a * a) + src[c].a);
                        dst[c].r = sat255(divBy255(dst[c].r * a) + src[c].r);
                        dst[c].g = sat255(divBy255(dst[c].g * a) + src[c].g);
                        dst[c].b = sat255(divBy255(dst[c].b * a) + src[c].b);
                }
        }
}

-- 
------
From: Jonathan Morton
      jonathan.morton at movial.com