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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
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 26, 2015.
Ratified by the Khronos Board of Promoters on August 7, 2015.
Version
Last Modified Date: February 12, 2019
Revision: 2
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 "Controlling 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.
Modifications to the OpenGL Shading Language Specification, Version 4.50
Including the following line in a shader can be used to control the
language features described in this extension:
#extension GL_ARB_shader_viewport_layer_array : <behavior>
where <behavior> is as specified in section 3.3.
New preprocessor #defines are added to the OpenGL Shading Language:
#define GL_ARB_shader_viewport_layer_array 1
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
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.
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
---- -------- -------- -----------------------------------------
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.