Name ARB_draw_instanced Name Strings GL_ARB_draw_instanced Contributors Michael Gold, NVIDIA James Helferty, TransGaming Inc. Daniel Koch, TransGaming Inc. Contact James Helferty, TransGaming Inc. (james 'at' transgaming.com) Daniel Koch, TransGaming Inc. (daniel 'at' transgaming.com) Notice Copyright (c) 2008-2013 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 IP Status Unknown Status Approved by the ARB on July 11, 2008 Version Last Modified Date: April 8, 2011 Author Revision: 5 Number ARB Extension #44 Dependencies OpenGL 2.0 is required. The extension is written against the OpenGL 2.1 Specification. EXT_gpu_shader4 or NV_vertex_program4 or OpenGL 3.0 is required. Overview A common use case in GL for some applications is to be able to draw the same object, or groups of similar objects that share vertex data, primitive count and type, multiple times. This extension provides a means of accelerating such use cases while restricting the number of API calls, and keeping the amount of duplicate data to a minimum. This extension introduces two draw calls which are conceptually equivalent to a series of draw calls. Each conceptual call in this series is considered an "instance" of the actual draw call. This extension also introduces a read-only built-in variable to GLSL which contains the "instance ID." This variable initially contains 0, but increases by one after each conceptual draw call. By using the instance ID or multiples thereof as an index into a uniform array containing transform data, vertex shaders can draw multiple instances of an object with a single draw call. New Tokens None New Procedures and Functions void DrawArraysInstancedARB(enum mode, int first, sizei count, sizei primcount); void DrawElementsInstancedARB(enum mode, sizei count, enum type, const void *indices, sizei primcount); Additions to Chapter 2 of the OpenGL 2.1 Specification (OpenGL Operation) Modify section 2.8 (Vertex Arrays), p. 23 (insert before the final paragraph, p. 30) The internal counter is a 32-bit integer value which may be read by a vertex program as , as described in section 2.X.3.2, or vertex shader as , as described in section 2.15.4.2. The value of this counter is always zero, except as noted below. The command void DrawArraysInstancedARB(enum mode, int first, sizei count, sizei primcount); behaves identically to DrawArrays except that instances of the range of elements are executed and the value of advances for each iteration. It has the same effect as: if (mode or count is invalid) generate appropriate error else { for (i = 0; i < primcount; i++) { instanceID = i; DrawArrays(mode, first, count, i); } instanceID = 0; } The command void DrawElementsInstancedARB(enum mode, sizei count, enum type, const void *indices, sizei primcount); behaves identically to DrawElements except that instances of the set of elements are executed, and the value of advances for each iteration. It has the same effect as: if (mode, count, or type is invalid ) generate appropriate error else { for (int i = 0; i < primcount; i++) { instanceID = i; DrawElements(mode, count, type, indices, i); } instanceID = 0; } Add a new section "2.15.4.2 Shader Inputs" before "Position Invariance" Besides having access to vertex attributes and uniform variables, vertex shaders can access the read-only built-in variable gl_InstanceIDARB or gl_InstanceID. The variables gl_InstanceIDARB and gl_InstanceID hold the integer index of the current primitive in an instanced draw call. See also section 7.1 of the OpenGL Shading Language Specification. Additions to Chapter 5 of the OpenGL 2.1 Specification (Special Functions) The error INVALID_OPERATION is generated if DrawArraysInstancedARB or DrawElementsInstancedARB is called during display list compilation. Dependencies on NV_vertex_program4 If NV_vertex_program4 is not supported, all references to vertex.instance are deleted. Errors INVALID_ENUM is generated by DrawElementsInstancedARB if is not one of UNSIGNED_BYTE, UNSIGNED_SHORT or UNSIGNED_INT. INVALID_VALUE is generated by DrawArraysInstancedARB if is less than zero. Modifications to The OpenGL Shading Language Specification, Version 1.10.59 Including the following line in a shader can be used to control the language features described in this extension: #extension GL_ARB_draw_instanced : where is as specified in section 3.3. A new preprocessor #define is added to the OpenGL Shading Language: #define GL_ARB_draw_instanced 1 Change Section 7.1 "Vertex Shader Special Variables" Add the following definitions to the list of built-in variable definitions: int gl_InstanceIDARB; // read-only int gl_InstanceID; // read-only Add the following paragraph at the end of the section: The variables gl_InstanceIDARB and gl_InstanceID are available as a read-only variables from within vertex shaders and hold the integer index of the current primitive in an instanced draw call (DrawArraysInstancedARB, DrawElementsInstancedARB). If the current primitive does not come from an instanced draw call, the values of gl_InstanceIDARB and gl_InstanceID are zero. Issues (1) Should instanceID be provided by this extension, or should it be provided by the core OpenGL 3.0 (similarly to how it was done with EXT_gpu_shader4)? Resolved: Provide both instanceID and instanceIDARB. The original intention was that instanceID and other language changes should be provided by this extension and properly decorated with the ARB suffix. However, many vendors shipped implementations that supported both, and some applications came to depend on this out-of-spec behavior. It appears that the EXT_gpu_shader4 incorrectly neglected to specify the suffix EXT suffix for gl_InstanceID. (2) Should MultiDrawArrays and MultiDrawElements affect the value of instanceID? Resolved: No, this may cause implementation difficulties and is considered unlikely to provide any real benefit. (3) Should DrawArraysInstanced and DrawElementsInstanced be compiled into display lists? Resolved: No, calling these during display list compilation generate INVALID_OPERATION. Revision History #5 April 8, 2011, Ian Romanick - Fix typo "holds holds" - Make both gl_InstanceID and gl_InstanceIDARB available to reflect what vendors actually ship. - Existence of gl_InstanceID and gl_InstanceIDARB do not depend on OpenGL 3.0 or GL_EXT_gpu_shader4. #4 July 8, 2008, jhelferty - corrected dependency typo - expanded Overview #3 June 4, 2008, dgkoch - merged instanceID and relevant language from the EXT_gpu_shader4 extension revision 13. - renamed to InstanceID_ARB to properly follow conventions - merged issues 1 & 4 #2 May 14 2008, jlheferty - added contact, contributer information. - bumped OpenGL spec written against to 2.1 - added issue 4 #1 May 12 2008, dgkoch - initial conversion from EXT to ARB