Re: clipping planes bypassed in CG vertex shaders

Hi Stephen

I agree. It must not be implemented in vertex shader. But, it can be 
implemented after or before vertex shader. In my own fx5200 and fx6800 user 
clipping planes do not work (disable) in programmable pipeline. I have tried 
in several ways, and it does not clip anything when vertex shader is enable. 
I found in internet that user clipping planes are disable for programmable 
pipeline. Only frustum clipping planes work after vertex shader...

So, I get troubles because I need both things (shader+clipping).

Best Regards!
Rhadamés

----- Original Message ----- 
To: "Multiple recipients of list OPENGL-GAMEDEV-L" 
<[email protected]>
Sent: Monday, May 23, 2005 3:28 PM


> Rhadamés Carmona wrote:
>
>> I tested the plane [1,0,0,0] and it clips in the middle (non vertex 
>> shader version), but still it does not work in CG vertex shader. I think 
>> user clipping planes are removed in programmable pipeline.
>
> Might this only be on particular hardware?
>
> Some of the older nVidia chips use a 1D alpha texture to implement user
> clip planes without custom clipping hardware.  I could easily believe that
> this would be hard for them to implement correctly with the standard 
> pipeline
> being subsumed by shaders.  Newer nVidia chips do user-clipping in 
> hardware
> and ought not to have this problem.
>
> However, it makes no sense for the OpenGL specification to remove this
> ability.  Clipping happens after the vertex shader - and there is no
> way to implement user clip planes using the shader itself because the
> shader only sees one vertex at a time.  User clip planes are a
> widely-used feature that would be exceedingly painful to do at the
> application level and impossible to implement efficiently in shader-land.
>
> -----------------------------------------------------------------------
> The second law of Frisbee throwing states: "Never precede any maneuver
> by a comment more predictive than "Watch this!"...it turns out that
> this also applies to writing Fragment Shaders.
> -----------------------------------------------------------------------
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: [email protected]           http://www.link.com
> Home: [email protected]       http://www.sjbaker.org
>
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