   Untitled
  With a fragment shader, is it possible for example to write to the red and green channel and not to the blue ? What I want to do is a fragment shader that renders to a texture, in which, for example, red and green represents normal map coordinates, blue is gloss and alpha is a mask. Sometimes I want to change only the red and green channels and leave the other channels unchanged in the destination texture.    
  I've not tested this myself, but I think glColorMask is what you're looking for (it tells the frame-buffer which color-components to accept/reject).   ;  Right, great, missed that one. Thanks   