Pre-multiplying alpha for DXT5

Question

I load DDS images (DXT5) with transparent alpha into OpenGL. Because the alpha is not pre-multiplied and can not be pre-multiplied based on DXT5 specs I get thin black halo around the visible part of the texture because I do some blending operations on GPU.My question is: what can be the best workaround for it.My OpenGL renderer updates scene textures on each frame loading those from IO so if I decompress the DDS then pre-muliply , then compress back I get huge overhead.Another option I thought would be to add another render pass and do the pre-multiply in fragment shader and render to texture,then use it for the main pass.This one adds overhead to the whole pipeline because I already have a descent number of passes.What are additional options here?

"My OpenGL renderer updates scene textures on each frame loading those from IO" You're loading and compressing textures every frame? Why? — Nicol Bolas
Can't explain here as it is commercial stuff but that is special purpose renderer.It is supposed to work that way :) — Michael IV
That is ,btw,the reason I use DDS here - to speed up upload time.Ah sorry , I am not compressing the textures in real time.I load ready DDSs — Michael IV

Nicol Bolas Nicol Bolas · Accepted Answer · 2012-11-05T15:58:25

Why not just have your shader do the pre-multiplication of the alpha in situ, then do whatever you were going to do normally?

There's no need to render that to a texture. Just always pre-multiply in your shader, for all textures where you need pre-multiplication.

Pre-multiplying alpha for DXT5

2 Answers