Mesa (master): mesa: Document SYSTEM_VALUE_VERTEX_ID and SYSTEM_VALUE_INSTANCE_ID
Kenneth Graunke
kwg at kemper.freedesktop.org
Wed Sep 10 18:32:26 UTC 2014
Module: Mesa
Branch: master
Commit: 9afb5ae8cae4460b87a5c03da3955f2e83434430
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=9afb5ae8cae4460b87a5c03da3955f2e83434430
Author: Ian Romanick <ian.d.romanick at intel.com>
Date: Wed Jun 18 16:59:20 2014 -0700
mesa: Document SYSTEM_VALUE_VERTEX_ID and SYSTEM_VALUE_INSTANCE_ID
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>
Reviewed-by: Tapani Pälli <tapani.palli at intel.com>
---
src/mesa/main/mtypes.h | 57 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 57 insertions(+)
diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index 4fb30ff..58a55d4 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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