NS_ComposeColors(nscolor aBG, nscolor aFG)
{
// This function uses colors that are non premultiplied alpha.
PRIntn r, g, b, a;
PRIntn bgAlpha = NS_GET_A(aBG);
PRIntn fgAlpha = NS_GET_A(aFG);
// Compute the final alpha of the blended color
// a = fgAlpha + bgAlpha*(255 - fgAlpha)/255;
FAST_DIVIDE_BY_255(a, bgAlpha*(255-fgAlpha));
a = fgAlpha + a;
PRIntn blendAlpha;
if (a == 0) {
// In this case the blended color is totally trasparent,
// we preserve the color information of the foreground color.
blendAlpha = 255;
} else {
blendAlpha = (fgAlpha*255)/a;
}
MOZ_BLEND(r, NS_GET_R(aBG), NS_GET_R(aFG), blendAlpha);
MOZ_BLEND(g, NS_GET_G(aBG), NS_GET_G(aFG), blendAlpha);
MOZ_BLEND(b, NS_GET_B(aBG), NS_GET_B(aFG), blendAlpha);
return NS_RGBA(r, g, b, a);
}
NS_ComposeColors(nscolor aBG, nscolor aFG)
{
PRIntn bgAlpha = NS_GET_A(aBG);
PRIntn r, g, b, a;
// First compute what we get drawing aBG onto RGBA(0,0,0,0)
MOZ_BLEND(r, 0, NS_GET_R(aBG), bgAlpha);
MOZ_BLEND(g, 0, NS_GET_G(aBG), bgAlpha);
MOZ_BLEND(b, 0, NS_GET_B(aBG), bgAlpha);
a = bgAlpha;
// Now draw aFG on top of that
PRIntn fgAlpha = NS_GET_A(aFG);
MOZ_BLEND(r, r, NS_GET_R(aFG), fgAlpha);
MOZ_BLEND(g, g, NS_GET_G(aFG), fgAlpha);
MOZ_BLEND(b, b, NS_GET_B(aFG), fgAlpha);
MOZ_BLEND(a, a, 255, fgAlpha);
return NS_RGBA(r, g, b, a);
}