Name ARB_shader_viewport_layer_array Name Strings GL_ARB_shader_viewport_layer_array Contact Graham Sellers, AMD (graham.sellers 'at' amd.com) Contributors Graham Sellers Notice Copyright (c) 2015 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Status Complete. Approved by the ARB on June 26, 2015. Ratified by the Khronos Board of Promoters on August 7, 2015. Version Last Modified Date: March 19, 2015 Revision: 1 Number ARB Extension #185 Dependencies OpenGL 4.1 is required. The extension is written against the OpenGL 4.5 Specification, Core Profile, February 2, 2015 and the OpenGL Shading Language Specification, version 4.50.5. Overview The gl_ViewportIndex and gl_Layer built-in variables were introduced by the in OpenGL 4.1. These variables are available in un-extended OpenGL only to the geometry shader. When written in the geometry shader, they cause geometry to be directed to one of an array of several independent viewport rectangles or framebuffer attachment layers, respectively. In order to use any viewport or attachment layer other than zero, a geometry shader must be present. Geometry shaders introduce processing overhead and potential performance issues. The AMD_vertex_shader_layer and AMD_vertex_shader_viewport_index extensions allowed the gl_Layer and gl_ViewportIndex outputs to be written directly from the vertex shader with no geometry shader present. This extension effectively merges the AMD_vertex_shader_layer and AMD_vertex_shader_viewport_index extensions together and extends them further to allow both outputs to be written from tessellation evaluation shaders. New Procedures and Functions None. New Tokens None. Additions to Chapter 9 of the OpenGL 4.5 (Core) Specification (Framebuffers and Framebuffer Objects) Modify section 9.8, "Layered Framebuffers" as follows: Remove the bullet list explaining when layer number zero is used and replace the language with: The layer number of a fragment is zero if no vertex processing shader (vertex, tessellation evaluation or geometry) statically assigns a value to the built-in output gl_Layer. Replace the following paragraph with: Otherwise, the layer for each point, line, or triangle is taken from the gl_Layer output of the last active vertex processing stage to one of the vertices of the primitive. The vertex used is implementation-dependent. To get defined results, all vertices of each primitive should set the same value for gl_Layer. The layer number written by a vertex procesing shader has no effect if the framebuffer is not layered. Additions to Chapter 11 of the OpenGL 4.5 (Core) Specification (Programmable Vertex Processing) Add the following to Subsection 11.1.3.10, "Shader Outputs" The built-in output gl_Layer is used in layered rendering, and is discussed further in the next section. The built-in output gl_ViewportIndex is used to direct rendering to one of several viewports and is discussed further in the next section. Move Subsection 11.3.4.6, "Layer and Viewport Selection" into a new section 11.4, "Layer and Viewport Selection", renumber surrounding sections appropriately. Replace all instances of "Geometry shaders" in Section 11.4 with "Vertex, tessellation and geometry shaders". Additions to Chapter 13 of the OpenGL 4.5 (Core) Specification (Fixed-Function Vertex Post-Processing) Modify section 13.6.1 "Controling the Viewport" as follows: Replace the paragraph beginning "Multiple viewports are available ..." with: Multiple viewports are available and are numbered zero through the value of MAX_VIEWPORTS minus one. If an active vertex processing shader is active and writes to gl_ViewportIndex, the viewport transformation uses the viewport corresponding to the value assigned to gl_ViewportIndex taken from an implementation-dependent primitive vertex. If the value of the viewport index is outside the range zero to the value of MAX_VIEWPORTS minus one, the results of the viewport transformation are undefined. If the active vertex, tessellation or geometry shaders (if present) do not write to gl_ViewportIndex, the viewport numbered zero is used by the viewport transformation. Additions to Chapter 7 of the OpenGL Shading Language Specification, Version 4.20 Add to the list of vertex shader built-in variables, Section 7.1, p. 97: out int gl_ViewportIndex; Modify the language introducing "gl_Layer" on p.123 as follows: Replace all instances of "geometry language" with "vertex, tessellation evaluation and geometry languages". Update numbering of "Layer and Viewport Selection" section to 11.4. Modify the paragraph introducing "gl_ViewportIndex" on p.124 as follows: The output variable gl_ViewportIndex is available only in the vertex, tessellation evaluation and geometry languages and provides the index of the viewport to which the next primitive assembled from the resulting vertices or emitted from the geometry shader (if present) should be drawn. Primitives generated during primitive assembly will undergo viewport transformation and scissor testing using the viewport transformation and scissor rectangle selected by the value of gl_ViewportIndex. The viewport index used will come from one of the vertices in the primitive being processed. However, which vertex the viewport index comes from is implementation-dependent. If no shader assigns a value to gl_ViewportIndex, viewport transform and scissor rectangle zero will be used. If a shader statically assigns a value to gl_ViewportIndex and there is a path through the shader that does not assign a value to gl_ViewportIndex, the value of gl_ViewportIndex is undefined for executions of the shader that take that path. See section 11.4, "Layer and Viewport Selection" in the OpenGL Graphics System Specification (Core Profile) for more information. Additions to the AGL/GLX/WGL Specifications None. GLX Protocol None. Errors None. New State None. New Implementation Dependent State None. Issues 1) Do we want to allow writes to these built-in variables from tessellation shaders? RESOLVED: Yes, this is allowed in tessellation evaluation shaders. Note that it is not possible to write either variable in the tessellation control shader. 2) What happens if gl_ViewportIndex or gl_Layer is written in the vertex shader and a geometry shader is present? RESOLVED: The value written by the last vertex processing stage is used. If the last vertex processing stage (vertex, tessellation evaluation or geometry) does not statically assign to gl_ViewportIndex or gl_Layer, index or layer zero is assumed. 3) Are provoking vertex semantics honored for the purposes of writes to gl_ViewportIndex in the VS? RESOLVED: Yes, they are. Query VIEWPORT_INDEX_PROVOKING_VERTEX to determine the 'provokingness' of gl_ViewportIndex. In general, though, it's best practice to ensure that all vertices of a single primitive (including strips, fans and loops) have the same value for gl_ViewportIndex. 4) Do we want gl_ViewportIndexIn or gl_LayerIn inputs to shaders to allow automatic passing of these variables along the pipe? DISCUSSION: Rather not. This is just emualating things that the application could and should do itself. Pushing built-ins down the pipe that aren't consuming dedicated hardware resources just complicates things like counting rules and ultimately ends up pretty inefficient. Revision History Rev. Date Author Changes ---- -------- -------- ----------------------------------------- 1 03/19/2015 gsellers Initial version merging AMD_vertex_shader_viewport_index and AMD_vertex_shader_layer