Alternatively, we may wish to premultiply the red, green, and blue components by the alpha:
- It is computationally more efficient. It requires one less vector multiplication.
- It is a closed form. Compositing a premultiplied-alpha color over top a premultiplied-alpha color yields another premultiplied-alpha color. The same cannot be said of non-premultiplied-alpha colors. Compositing two non-premultiplied-alpha colors yields, interestingly, a premultiplied-alpha color.
- When filtering (aka downsampling), it produces more visually accurate results. A picture is worth a thousands words.
And that's it. Premutliplied-alpha colors are nifty.