Optimizing Alpha blending

Question

I have two colors and I use this method to do a simple alpha blending:

int Color::blend(int col1, int col2)
{
    float a1  = ((col1 & 0x000000FF) / 255.0);
    return ((int)((((col1 & 0xFF000000) >> 24) * a1) + (((col2 & 0xFF000000) >> 24) * (1.0 - a1)))) << 24 | 
           ((int)((((col1 & 0x00FF0000) >> 16) * a1) + (((col2 & 0x00FF0000) >> 16) * (1.0 - a1)))) << 16 |
           ((int)((((col1 & 0x0000FF00) >> 8 ) * a1) + (((col2 & 0x0000FF00) >> 8 ) * (1.0 - a1)))) << 8  | 255;
}

(The colors are in RGBA8888 format)

This works, but i was wondering: is this the fastest way, or is there a more efficient one?

Clearly, the fastest way is to let some hardware do it... ;) — Mats Petersson, Apr 13 '14 at 21:58

score 1 · Accepted Answer · answered Apr 13 '14 at 21:58

You might be able to eke out a little more performance by representing a1*(2^24) as an integer, doing the arithmetic in integers, then shifting the result down by 24 bits. On modern architectures I doubt it would gain you much, though. If you want better performance, you'll really need to go for SIMD operations.

Oh, one thing: You should express the calculation of a1 as a1 = ((col1 & 0x000000FF) * (1.0 / 255.0)). That'll avoid an expensive FP division. (Compilers won't usually do that on their own, due to the potential loss of precision.)

Thanks...this pretty much answers my question! – Sylar Apr 13 '14 at 22:23 — Sylar, Apr 13 '14 at 22:23

Optimizing Alpha blending

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