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mesa: Document SYSTEM_VALUE_VERTEX_ID and SYSTEM_VALUE_INSTANCE_ID

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v2: Additions to the documentation for SYSTEM_VALUE_VERTEX_ID.  Quote
the GL_ARB_shader_draw_parameters spec and mention DirectX SV_VertexID.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 9afb5ae8ca)
This commit is contained in:
Ian Romanick 2014-06-18 16:59:20 -07:00
parent 72d8ebb7fb
commit 6bc4331c8e
1 changed files with 57 additions and 0 deletions
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src/mesa/main/mtypes.h
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@ -2055,7 +2055,64 @@ 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."
*
* Additionally, the overview in the GL_ARB_shader_draw_parameters spec
* says:
*
* "In unextended GL, vertex shaders have inputs named gl_VertexID and
* gl_InstanceID, which contain, respectively the index of the vertex
* and instance. The value of gl_VertexID is the implicitly passed
* index of the vertex being processed, which includes the value of
* baseVertex, for those commands that accept it."
*
* gl_VertexID gets basevertex added in. This differs from DirectX where
* SV_VertexID does \b not get basevertex added in.
*/
SYSTEM_VALUE_VERTEX_ID,
/**
* Instanced ID as supplied to gl_InstanceID
*
* Values assigned to gl_InstanceID always begin with zero, regardless of
* the value of baseinstance.
*
* Section 11.1.3.9 (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.
*/
SYSTEM_VALUE_INSTANCE_ID,
/*@}*/