From 9afb5ae8cae4460b87a5c03da3955f2e83434430 Mon Sep 17 00:00:00 2001 From: Ian Romanick Date: Wed, 18 Jun 2014 16:59:20 -0700 Subject: [PATCH] mesa: Document SYSTEM_VALUE_VERTEX_ID and SYSTEM_VALUE_INSTANCE_ID MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Kenneth Graunke Reviewed-by: Tapani Pälli --- 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 4fb30ffbabe..58a55d45404 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, /*@}*/ -- 2.30.2