@joreg: sry, i should have done it properly the first time around:
blend modes: Blend modes - Wikipedia Photoshop plugins and filters - - Overview of the mathematics behind the Photoshop mixing mode algorithms..
some GLSL and HLSL for that: Photoshop math with GLSL shaders | Projects, Software, Programming, GLSL - Romz Blog
Gamma correction: Photoshop gamma correction shader | Projects, Software, Programming, GLSL - Romz Blog
Levels: Levels control shader | Projects, Software, Programming, GLSL - Romz Blog