View Full Version : nvidia register combiners and multipass
03-24-2002, 01:03 AM
I want to set up this equation:
(A dot B) * C + D
where A and B are vector maps, C and D are texture maps, so all 4 need their own texture units.
I'm using a geforce2, so I only have 2 texture units to play with. Is it possible to perform the equation in 2 passes? One pass to perform (A dot B) and another pass to perform the rest?
If so, how do I store the results of (A dot B) without taking up another texture unit?
03-24-2002, 03:15 AM
You can use the alpha buffer to store the result of (A dot B). Fore more information see Ron Frazier's great tutorial:
I don't think that it is possible to do that operation in 2 passes. I think you will need 3 passes. Store (A dot B) in the alpha buffer. Multiply the alpha buffer with C and add the result to the color buffer. Then add D to the color buffer. If your equation is for lighting calculations you could even add several lights using this alpha buffer technique.
Hope that helps a bit
03-24-2002, 03:23 AM
framebuffer = AdotB
framebuffer *= C
framebuffer += D
the only ways i can see use 3 passes, sorry..
only if C or D would be alpha-only, not color, then i would see a way..
03-24-2002, 04:16 AM
****in smilies http://www.opengl.org/discussion_boards/ubb/wink.gif
03-24-2002, 11:55 AM
You should be more explicit with your equations.
Using BODMAS ordering I assume you mean;
((A dot B) * C) + D
You can do (A dot B) * (C + D) in two passes, with 1 blend easily.
Simply in register combiners do
A Dot B, store in framebuffer.
C * D, blend with framebuffer with DST_COLOR, ZERO.
Previous equation will require 3, as Dave said.
03-24-2002, 11:19 PM
That should've been GL_ONE, GL_ONE for the blend.
03-25-2002, 05:48 AM
Yup I meant the previous eqn
((A dot B) * C) + D
The link to Ron Frazier's paper helped a lot.
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