[Mesa-dev] [PATCH 01/13] mesa: Document SYSTEM_VALUE_VERTEX_ID and SYSTEM_VALUE_INSTANCE_ID

[Roland Scheidegger]

The mesa parts of the series all look good to me.

We definitely want something like that in gallium too (draw fails the
vertex id tests sort of on purpose right now because we needed d3d10
behavior).

Oh and sort of off-topic but since you're familiar with it do you know
why the gl_PrimitiveIn of the geometry shader isn't a system value
whereas all the vertex_id and friends are?

Roland


Am 08.08.2014 09:31, schrieb Kenneth Graunke:
> From: Ian Romanick <ian.d.romanick at intel.com>
> 
> 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 at intel.com>
> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/main/mtypes.h | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> This series is available as the 'basevertex-v9' branch of ~kwg/mesa
> (not ~idr/mesa).  Ken tested this series against Piglit on Haswell and
> Broadwell, but did not test earlier hardware, nor run the ES3 tests.
> 
> diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
> index ff130da..207be0a 100644
> --- a/src/mesa/main/mtypes.h
> +++ b/src/mesa/main/mtypes.h
> @@ -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,
>     /*@}*/
>  
>