Blending/lighting performances ?

OpenGL OpenGL: Basic Coding
1

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 ?

Y.

2

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.

HTH,XBTC!

3
  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.
4

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…?

Y.

5

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.

Dolo//\ightY

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