Re: clipping planes bypassed in CG vertex shaders

Well, I guess it pays to RTFM.

According to the nVidia vertex_program extension:

    "What part of OpenGL do vertex programs specifically bypass?

       Vertex programs bypass the following OpenGL functionality:

         o  Normal transformation and normalization
         o  Color material
         o  Per-vertex lighting
         o  Texture coordinate generation
         o  The texture matrix
         o  The normalization of AUTO_NORMAL evaluated normals
         o  The modelview and projection matrix transforms
         o  The per-vertex processing in EXT_point_parameters
         o  The per-vertex processing in NV_fog_distance
         o  Raster position transformation
===>    o  Client-defined clip planes"

...and it's right there on the last line.

A further question clarifies the reasoning:

    "Should client-defined clip planes operate when vertex program mode is
     enabled?

       RESOLUTION.  No.

       OpenGL's client-defined clip planes are specified in eye-space.
       Vertex programs generate homogeneous clip space positions.
       Unlike the conventional OpenGL vertex transformation mode, vertex
       program mode requires no semantic equivalent to eye-space.

       Applications that require client-defined clip planes can simulate
       OpenGL-style client-defined clip planes by generating texture
       coordinates and using alpha testing or other per-fragment tests
       such as NV_texture_shader's CULL_FRAGMENT_NV program to discard
       fragments.  In many ways, these schemes provide a more flexible
       mechanism for clipping than client-defined clip planes.

       Unfortunately, vertex programs used in conjunction with selection
       or feedback will not have a means to support client-defined clip
       planes because the per-fragment culling mechanisms described in the
       previous paragraph are not available in the selection or feedback
       render modes.  Oh well.

       Finally, as a practical concern, client-defined clip planes
       greatly complicate clipping for various hardware rasterization
       architectures."

...so the recommendation is to use the texture hack that older nVidia
hardware uses (urgh!).

I'm horrified.

-----------------------------------------------------------------------
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.
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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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