| 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 (!) |
| Daniel Koch, NVIDIA |
| |
| Notice |
| |
| Copyright (c) 2013 The Khronos Group Inc. Copyright terms at |
| http://www.khronos.org/registry/speccopyright.html |
| |
| Specification Update Policy |
| |
| Khronos-approved extension specifications are updated in response to |
| issues and bugs prioritized by the Khronos OpenGL Working Group. For |
| extensions which have been promoted to a core Specification, fixes will |
| first appear in the latest version of that core Specification, and will |
| eventually be backported to the extension document. This policy is |
| described in more detail at |
| https://www.khronos.org/registry/OpenGL/docs/update_policy.php |
| |
| 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: 29 May 2017 |
| Revision: 4 |
| |
| 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 (<i> in the above pseudo-code) may be read by |
| a vertex shader as <gl_DrawIDARB>, 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 (<i> in the above pseudo-code) may be read by |
| a vertex shader as <gl_DrawIDARB>, 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 (<i> in the above pseudo-code) may be read by |
| a vertex shader as <gl_DrawIDARB>, 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 (<i> in the above pseudo-code) may be read by |
| a vertex shader as <gl_DrawIDARB>, 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 (<i> in the above pseudo-code) may be read by |
| a vertex shader as <gl_DrawIDARB>, 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 GetActiveAttrib, 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. |
| |
| <gl_VertexID> holds ... (retain original language) |
| |
| <gl_BaseVertexARB> holds the integer value passed to the <baseVertex> |
| parameter to the command that resulted in the current shader invocation. |
| In the case where the command has no <baseVertex> parameter, the value |
| of <gl_BaseVertexARB> is zero. |
| |
| <gl_InstanceID> holds ... (retain original language) |
| |
| <gl_BaseInstanceARB> holds the integer value passed to the |
| <baseInstance> parameter to the command that resulted in the current |
| shader invocation. In the case where the command has no <baseInstance> |
| parameter, the value of <gl_BaseInstanceARB> is zero. |
| |
| Add the following paragraph after the above discussion of <gl_InstanceID>: |
| |
| <gl_DrawIDARB> 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 <gl_DrawIDARB> 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 : <behavior> |
| |
| where <behavior> 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 <gl_DrawIDARB> 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 <gl_BaseVertexARB> is a vertex language input variable |
| that holds the integer value passed to the <baseVertex> 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 <gl_BaseInstanceARB> is a vertex language input variable |
| that holds the integer value passed to the <baseInstance> 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. |
| 4 05/29/2017 dgkoch remove edit reference to non-existant |
| MultiDrawElementsInstancedBaseInstance function. |
| Fix typos. |
| |
