gallium: add a cull distance semantic

cull distance is analogous to clip distance. If a register is
given this semantic, then the values in it are assumed to be a
float32 distance to a plane. Primitives will be completely
discarded if the plane distance for all of the vertices in
the primitive are < 0.

Signed-off-by: Zack Rusin <[email protected]>
Reviewed-by: Jose Fonseca <[email protected]>
Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Roland Scheidegger <[email protected]>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
index 926527151c4..f3aebb3b317 100644
--- a/src/gallium/docs/source/tgsi.rst
+++ b/src/gallium/docs/source/tgsi.rst
@@ -2425,6 +2425,17 @@ contains the layer value to use for the color and depth/stencil surfaces.
 Only the X value is used. (Also known as rendertarget array index.)
 
 
+TGSI_SEMANTIC_CULLDIST
+""""""""""""""""""""""
+
+Used as distance to plane for performing application-defined culling
+of individual primitives against a plane. When components of vertex
+elements are given this label, these values are assumed to be a
+float32 signed distance to a plane. Primitives will be completely
+discarded if the plane distance for all of the vertices in the
+primitive are < 0. If a vertex has a cull distance of NaN, that
+vertex counts as "out" (as if its < 0);
+
 
 Declaration Interpolate
 ^^^^^^^^^^^^^^^^^^^^^^^