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

[Ian Romanick]

On 08/08/2014 07:55 AM, Roland Scheidegger wrote:
> The mesa parts of the series all look good to me.

So.. can we put your R-b on patches 1 through 7? :)

> 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?

I have come vague recollection that Paul had a good reason for doing
that, but no recollection of what that reason was.

> 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,
>>     /*@}*/
>>  
>>
> 
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