Updates to ARB_shader_viewport_layer_array
gitlab/opengl/api/issue/44
Add shading language extension boiler plate language.
Clarify that the new built-ins should be members of
the "out gl_PerVertex" block.
Update the gl_Layer and gl_ViewportIndex built-in language
based on the language on in NV_viewport_array2.
diff --git a/extensions/ARB/ARB_shader_viewport_layer_array.txt b/extensions/ARB/ARB_shader_viewport_layer_array.txt
index b27a44e..986513a 100644
--- a/extensions/ARB/ARB_shader_viewport_layer_array.txt
+++ b/extensions/ARB/ARB_shader_viewport_layer_array.txt
@@ -26,8 +26,8 @@
Version
- Last Modified Date: March 19, 2015
- Revision: 1
+ Last Modified Date: February 12, 2019
+ Revision: 2
Number
@@ -109,7 +109,7 @@
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:
+ Modify section 13.6.1 "Controlling the Viewport" as follows:
Replace the paragraph beginning "Multiple viewports are available ..."
with:
@@ -125,36 +125,102 @@
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:
+Modifications to the OpenGL Shading Language Specification, Version 4.50
- out int gl_ViewportIndex;
+ Including the following line in a shader can be used to control the
+ language features described in this extension:
- Modify the language introducing "gl_Layer" on p.123 as follows:
+ #extension GL_ARB_shader_viewport_layer_array : <behavior>
- Replace all instances of "geometry language" with "vertex, tessellation
- evaluation and geometry languages".
+ where <behavior> is as specified in section 3.3.
- Update numbering of "Layer and Viewport Selection" section to 11.4.
+ New preprocessor #defines are added to the OpenGL Shading Language:
- Modify the paragraph introducing "gl_ViewportIndex" on p.124 as follows:
+ #define GL_ARB_shader_viewport_layer_array 1
- 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.
+ Modify Section 7.1 (Built-In Language Variables), p. 122
+
+ Add to the list of vertex shader built-ins:
+
+ out gl_PerVertex {
+ ...
+ int gl_ViewportIndex;
+ int gl_Layer;
+ };
+
+ Add to the list of tessellation evaluation shader built-ins:
+
+ out gl_PerVertex {
+ ...
+ int gl_ViewportIndex;
+ int gl_Layer;
+ };
+
+ Modify the language introducing "gl_Layer" on p.127 as follows:
+
+ The variable gl_Layer is available as an output variable in the vertex,
+ tessellation evaluation, and geometry (VTG) languages and an input
+ variable in the fragment language. In the VTG languages, it is used to
+ select a specific layer (or face and layer of a cube map) of a
+ multi-layer framebuffer attachment. The actual layer used will come
+ from one of the vertices in the primitive being shaded. Which vertex
+ the layer comes from is discussed in section 11.4 “Layer and
+ Viewport Selection” of the OpenGL Specification. It might be undefined,
+ so it is best to write the same layer value for all vertices of a
+ primitive. If a shader statically assigns a value to gl_Layer, layered
+ rendering mode is enabled. See section 11.3.4.5 “Geometry Shader
+ Outputs” and section 9.4.9 “Layered Framebuffers” of the OpenGL
+ Graphics System Specification for more information. If a shader
+ statically assigns a value to gl_Layer, and there is an execution path
+ through the shader that does not set gl_Layer, then the value of
+ gl_Layer is undefined for executions of the shader that take that path.
+
+ ...
+
+ The input variable gl_Layer in the fragment language will have the same
+ value that was written to the output variable gl_Layer in the VTG
+ languages. If the final VTG stage does not dynamically assign a value to
+ gl_Layer, the value of gl_Layer in the fragment stage will be undefined.
+ If the final VTG stage makes no static assignment to gl_Layer, the input
+ gl_Value in the fragment stage will be zero. Otherwise, the fragment stage
+ will read the same value written by the final VTG stage, even if that value
+ is out of range. If a fragment shader contains a static access to gl_Layer,
+ it will count against the implementation defined limit for the maximum
+ number of inputs to the fragment stage.
+
+ Modify the language introducing "gl_ViewportIndex" on p.128 as follows:
+
+ The variable gl_ViewportIndex is available as an output variable in the
+ VTG languages and an input variable in the fragment language. In the
+ geometry language, it provides the index of the viewport to which the next
+ primitive emitted from the geometry shader should be drawn. In the vertex
+ and tessellation evaluation languages, it provides the index of the viewport
+ associated with the vertex being shaded. Primitives 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
+ shaded. However, which vertex the viewport index comes from is
+ implementation-dependent, so it is best to use the same viewport index for
+ all vertices of the primitive. If the final VTG stage does not assign 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" of the OpenGL Graphics System
+ Specification for more information.
+
+ The input variable gl_ViewportIndex in the fragment stage will have the
+ same value that was written to the output variable gl_ViewportIndex in the
+ final VTG stage. If the final VTG stage does not dynamically assign to
+ gl_ViewportIndex, the value of gl_ViewportIndex in the fragment shader will
+ be undefined. If the final VTG stage makes no static assignment to
+ gl_ViewportIndex, the fragment stage will read zero. Otherwise, the
+ fragment stage will read the same value written by the final VTG stage,
+ even if that value is out of range. If a fragment shader contains a static
+ access to gl_ViewportIndex, it will count against the implementation
+ defined limit for the maximum number of inputs to the fragment stage.
Additions to the AGL/GLX/WGL Specifications
@@ -208,10 +274,23 @@
dedicated hardware resources just complicates things like counting rules and
ultimately ends up pretty inefficient.
+ 5) Are the gl_ViewportIndex and gl_Layer builtins part of the "out gl_PerVertex"
+ block or not?
+
+ RESOLVED: The initial version of this extension didn't specify and many
+ implementations treated them as loose variables (or didn't care). However,
+ the spec language makes it quite clear that there is an instance of these
+ variables and a shader is expected to write the same value for each
+ vertex.
+ In practise, unless an application is trying to redeclare these variables
+ it isn't really possible to tell where they are declared.
+
Revision History
- Rev. Date Author Changes
+ Rev. Date Author Changes
---- -------- -------- -----------------------------------------
1 03/19/2015 gsellers Initial version merging AMD_vertex_shader_viewport_index
and AMD_vertex_shader_layer
+ 2 02/12/2019 dkoch opengl/api/44: clarify that builtins should be
+ part of the "out gl_PerVertex" block.
