- Better quality for textures with sharp alpha cutouts.
- Removes some blending state changes (which can be important if You are coding for a PC).
- Can mix additive blending with alpha blending without a state/shader/texture change.
What are the hidden cons, which aren’t mentioned by Tom or Shawn?
- Fixed pipeline fog doesn’t work with it.
- Worse quality for smooth alpha with DXT5 compression.
Why premultiplied alpha doesn’t work very well with DXT5 for textures with smooth alpha gradients? In DXT5 texture is divided into the 4×4 pixel blocks. For every block, colors (and alphas) are approximated with equidistant points on a line between two end points (using some index table). Color end points are quantized (5:6:5 bits) and alpha end points are saved with 8 bit precision. This means that for the most textures we get better precision for the alpha channel than for the color channel.
Furthermore compressing values of a broader range gives us better precision. For example if we have alpha filled with 1/255 and RGB values in range [0; 1] then premultiplied texture RGB channels will contain only two different numbers – 0 or 1/125. This means that by using standard alpha blending we get better precision in case we would like to multiply RGB by a factor greater than 1 or tonemap final results.
Standard alpha blending:
srcAlpha = Compress_color( src.rgb ) * Compress_alpha( src.a )
Premultiplied alpha blending:
srcAlpha = Compress_color( src.rgb * src.a )
Let’s see how does it look in practice. I created a sample texture with a “lighting” gradient in RGB channels and smoke puff in alpha (from left to right: RGB, alpha and premultiplied RGB by alpha):
Now let’s alpha blend two compressed textures (DXT5), zoom and compare results (left image – standard alpha blending, right – premultiplied alpha blending):
This looks like a small difference, but can be quite visible in a real game – especially when smoke particles are big or their color is multiplied by a factor greater than 1 or when using some kind of tone-mapping.
BTW there is interesting feature/bug in NVIDIA Photoshop texture tools. You can’t save DXT5 with full black alpha (it just creates a DDS without alpha channel).