[Mesa-dev] [PATCH 05/13] mesa: Document SYSTEM_VALUE_VERTEX_ID and SYSTEM_VALUE_INSTANCE_ID
idr at freedesktop.org
Fri Jun 20 18:00:55 PDT 2014
From: Ian Romanick <ian.d.romanick at intel.com>
Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
Cc: "10.2" <mesa-stable at lists.freedesktop.org>
src/mesa/main/mtypes.h | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 48 insertions(+)
diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index 3899e7f..34e66c9 100644
@@ -2039,7 +2039,55 @@ typedef enum
* \name Vertex shader system values
+ * OpenGL-style vertex ID.
+ * Section 2.11.7 (Shader Execution), subsection Shader Inputs, of the
+ * OpenGL 3.3 core profile spec says:
+ * "gl_VertexID holds the integer index i implicitly passed by
+ * DrawArrays or one of the other drawing commands defined in section
+ * 2.8.3."
+ * Section 2.8.3 (Drawing Commands) of the same spec says:
+ * "The commands....are equivalent to the commands with the same base
+ * name (without the BaseVertex suffix), except that the ith element
+ * transferred by the corresponding draw call will be taken from
+ * element indices[i] + basevertex of each enabled array."
+ * gl_VertexID gets basevertex added in. This differs from DirectX where
+ * it's equivalent to gl_VertexID does \b not get basevertex added in.
+ * Instanced ID as supplied to gl_InstanceID
+ * Values assigned to gl_InstanceID always begin with zero, regardless of
+ * the value of baseinstance.
+ * Section 18.104.22.168 (Shader Inputs) of the OpenGL 4.4 core profile spec
+ * says:
+ * "gl_InstanceID holds the integer instance number of the current
+ * primitive in an instanced draw call (see section 10.5)."
+ * Through a big chain of pseudocode, section 10.5 describes that
+ * baseinstance is not counted by gl_InstanceID. In that section, notice
+ * "If an enabled vertex attribute array is instanced (it has a
+ * non-zero divisor as specified by VertexAttribDivisor), the element
+ * index that is transferred to the GL, for all vertices, is given by
+ * floor(instance/divisor) + baseinstance
+ * If an array corresponding to an attribute required by a vertex
+ * shader is not enabled, then the corresponding element is taken from
+ * the current attribute state (see section 10.2)."
+ * Note that baseinstance is \b not included in the value of instance.
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