Blending/lighting performances ?

It may sound simple but…

  1. Is there a speed difference on nowadays’ hardware between rendering a normal triangle and a blended triangle ?

  2. Is there a speed difference when using GL_MODULATE against GL_DECAL ?


Sorry,I can only answer your first question:
If you have a card which is able to do multitexturing(voodoo2,tnt,…) the texture and the blended one can be drawn in one single pass,which means there is nearly no performance-loss.On cards which aren´t able,you´ll have to do this with two single passes(first the texture,than the other texture to blend).Which means that on these cards you´ll only get half the performance with blending.


  1. You always have higher bandwidth needs with blending. When blending with the background multitexturing hardware doesn’t help much.
  2. No, it shouldn’t be.

Ok, thanks for the answers… but:

  1. If not considering multi-pass, do you know a ratio of speed difference between normal/blend rendering ? I just want to know if it’s a lot slower, or if the difference is hardly noticable.

  2. So, what’s exactly the interest of using GL_DECAL…?


i’d say blended rendering is twice slower than normal, because of the memory accesses involved.
probably mults are hardwired, maybe with a barrel multiplier, don’t know exactly: so they are pratically free.
i suppose memory access is the heavy task.

about decals… effectively i never made use of it, just tested, and found it is much the same as modulate. now i give a look to the red book… wait… no it is not.

by the functions i see GL_DECAL mode isn’t influenced by the previous color fragment, so a decal is… well… a decal

it appears on the surface with it’s color, and is not modulated with primitive lighting results.


[This message has been edited by dmy (edited 05-23-2000).]