Name ARB_shader_draw_parameters Name Strings GL_ARB_shader_draw_parameters Contact Graham Sellers (graham.sellers 'at' amd.com) Contributors Piers Daniell, NVIDIA Daniel Rakos, AMD Christophe Riccio, OpenGL Garden (!) Notice Copyright (c) 2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Status Complete. Approved by the ARB on June 3, 2013. Ratified by the Khronos Board of Promoters on July 19, 2013. Version Last Modified Date: 5 June 2013 Revision: 3 Number ARB Extension #156 Dependencies OpenGL 3.1 is required. This extension is written against Version 4.3 of the Core Profile OpenGL Specification, dated February 14, 2013 and Version 4.30 of the OpenGL Shading Language Specification, dated February 7, 2013. The presence of the GL_ARB_multi_draw_indirect extension affects the definition of this extension. The GL_ARB_indirect_parameters extension affects the definition of this extension. Overview 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. Meanwhile, gl_InstanceID is the integer index of the current instance being processed, but, even for commands that accept a baseInstance parameter, it does not include the value of this argument. Furthermore, the equivalents to these variables in other graphics APIs do not necessarily follow these conventions. The reason for this inconsistency is that there are legitimate use cases for both inclusion and exclusion of the baseVertex or baseInstance parameters in gl_VertexID and gl_InstanceID, respectively. Rather than change the semantics of either built-in variable, this extension adds two new built-in variables to the GL shading language, gl_BaseVertexARB and gl_BaseInstanceARB, which contain the values passed in the baseVertex and baseInstance parameters, respectively. Shaders provided by the application may use these variables to offset gl_VertexID or gl_InstanceID if desired, or use them for any other purpose. Additionally, this extension adds a further built-in variable, gl_DrawID to the shading language. This variable contains the index of the draw currently being processed by a Multi* variant of a drawing command (such as MultiDrawElements or MultiDrawArraysIndirect). New Procedures and Functions None. New Tokens None. IP Status No known IP claims. Additions to Chapter 10 of the OpenGL Core Profile Specification, "Vertex Specification and Drawing Commands" Add the following after the pseudo-code describing MultiDrawArrays on p.310: The index of the draw ( in the above pseudo-code) may be read by a vertex shader as , as described in Section 11.1.3.9. Add the following after the pseudo-code describing MultiDrawArraysIndirect on p.311: The index of the draw ( in the above pseudo-code) may be read by a vertex shader as , as described in Section 11.1.3.9. Add the following after the pseudo-code describing MultiDrawElementsInstancedBaseInstance on p.312: The index of the draw ( in the above pseudo-code) may be read by a vertex shader as , as described in Section 11.1.3.9. Add the following after the pseudo-code describing MultiDrawElements on p.313: The index of the draw ( in the above pseudo-code) may be read by a vertex shader as , as described in Section 11.1.3.9. Add the following after the pseudo-code describing MultiDrawElementsIndirect on p.316: The index of the draw ( in the above pseudo-code) may be read by a vertex shader as , as described in Section 11.1.3.9. Add the following after the pseudo-code describing MultiDrawElementsBaseVertex on p.316: The index of the draw ( in the above pseudo-code) may be read by a vertex shader as , as described in Section 11.1.3.9. Additions to Chapter 11 of the OpenGL Core Profile Specification, "Programmable Vertex Processing" In Section 11.1.1, "Vertex Attributes", in the description of GetActiveAttrib, state that gl_DrawIDARB is enumerated: For GetActiveAttribute, all active vertex shader input variables are enumerated, including the special built-in inputs gl_VertexID, gl_BaseVertexARB, gl_InstanceID, gl_BaseInstanceARB, and gl_DrawIDARB. Replace the paragraphs describing gl_VertexID and gl_InstanceID in Section 11.1.3.9, "Shader Inputs" with the following: Besides having access to vertex attributes and uniform variables, vertex shaders can access the read-only built-in variables gl_VertexID, gl_InstanceID, gl_BaseVertexARB, gl_BaseInstanceARB and gl_DrawIDARB. holds ... (retain original language) holds the integer value passed to the parameter to the command that resulted in the current shader invocation. In the case where the command has no parameter, the value of is zero. holds ... (retain original language) holds the integer value passed to the parameter to the command that resulted in the current shader invocation. In the case where the command has no parameter, the value of is zero. Add the following paragraph after the above discussion of : holds the integer draw number the current draw being processed by the shader invocation. It is dynamically uniform. In MultiDraw* variants, this is the zero-based index of the draw within the list of draws processed by the command. In non-MultiDraw* commands, the value of is always zero. Errors None. New State None. Modifications to Chapter 7 of the OpenGL Shading Language Specification, Version 4.30. Including the following line in a shader can be used to control the language features described in this extension: #extension GL_ARB_shader_draw_parameters : where is as described in Section 3.3. A new preprocessor #define is added to the OpenGL Shading Language: #define GL_ARB_shader_draw_parameters 1 In Section 7.1, "Built-in Language Variables", add the following to the list of input variables to a vertex shader, p.110: in int gl_DrawIDARB; in int gl_BaseVertexARB; in int gl_BaseInstanceARB; Add the following paragraph after the description of gl_InstanceID, p.114: The variable is a vertex language input variable that holds the integer index of the drawing command to which the current vertex belongs (see "Shader Inputs" in section 11.1.3.9 of the OpenGL Graphics System Specification). If the vertex is not invoked by a Multi* form of a draw command, then the value of gl_DrawIDARB is zero. The variable is a vertex language input variable that holds the integer value passed to the parameter of the command that resulted in the current shader invocation (see "Shader Inputs" in section 11.1.3.9 of the OpenGL Graphics System Specification). The variable is a vertex language input variable that holds the integer value passed to the parameter of the command that resulted in the current shader invocation (see "Shader Inputs" in section 11.1.3.9 of the OpenGL Graphics System Specification). Usage Examples --- Example 1 --- --- Using a different transformation matrix per draw --- --- Some draws can share the same matrix --- layout(binding = INDIRECTION) uniform indirection { int Transform[MAX_DRAW]; } Indirection; layout(binding = TRANSFORM0) uniform transform { mat4 MVP[MAX_DRAW]; } Transform; layout(location = POSITION) in vec3 Position; layout(location = TEXCOORD) in vec3 Texcoord; out gl_PerVertex { vec4 gl_Position; }; out block { vec2 Texcoord; } Out; void main() { Out.Texcoord = Texcoord.st; gl_Position = Transform.MVP[Indirection.Transform[gl_DrawIDARB]] * vec4(Position, 1.0); } Dependencies on GL_ARB_multi_draw_indirect If GL_ARB_multi_draw_indirect is not supported, remove the language added after the pseudo-code describing MultiDrawArraysIndirect and MultiDrawElementsIndirect. Dependencies on GL_ARB_indirect_parameters Whilst not a true dependency, it should be noted that the additional commands introduced by ARB_indirect_parameters affect the value of gl_DrawIDARB. Issues 1) - 2) - 3) What is the value of gl_DrawIDARB for non-Multi* draws? RESOLVED: Zero. 4) Do we need language to clarify that gl_DrawIDARB is a dynamically uniform expression? RESOLVED: Yes, this allows accessing resources per-draw. 5) - 6) - Revision History Rev. Date Author Changes ---- -------- -------- ----------------------------------------- 1 05/16/2013 gsellers Initial draft 2 05/29/2013 gsellers Add descriptions of gl_BaseVertexARB and gl_BaseInstanceARB. Document pseudo-dependency on ARB_indirect_parameters. 3 06/05/2013 gsellers Remove controls over gl_VertexID and gl_InstanceID, leaving only the gl_BaseVertexARB and gl_BaseInstanceARB variables. gl_DrawIDARB is unmodified. Issues 1, 2, 5 and 6 become non-issues as a result.