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GLSL_EXT_demote_to_helper_invocation.txt
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GLSL_EXT_demote_to_helper_invocation.txt
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Name
EXT_demote_to_helper_invocation
Name Strings
GL_EXT_demote_to_helper_invocation
Contact
Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)
Contributors
Status
Draft
Version
Last Modified Date: September 19, 2019
Revision: 2
Number
TBD
Dependencies
This extension can be applied to OpenGL GLSL versions 1.40
(#version 140) and higher.
This extension can be applied to OpenGL ES ESSL versions 3.10
(#version 310) and higher.
This extension is written against the OpenGL Shading Language
Specification, version 4.50 (revision 7), dated May 10, 2017.
Interacts with GL_KHR_vulkan_glsl.
Overview
This extension adds a "demote" keyword that is similar to "discard" but
only suppresses subsequent writes and outputs to the framebuffer, and
does not terminate the execution of the invocation. For the remainder
of the execution, the invocation is "demoted" to act like a helper
invocation.
New Procedures and Functions
None.
New Tokens
None.
Modifications to GL_KHR_vulkan_glsl
Add to the "Mapping to SPIR-V" section
Mapping of demote keyword:
demote -> OpDemoteToHelperInvocation
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_EXT_demote_to_helper_invocation : <behavior>
where <behavior> is as specified in section 3.3
New preprocessor #defines are added to the OpenGL Shading Language:
#define GL_EXT_demote_to_helper_invocation 1
Add to section 3.6 Keywords:
demote
Modify section 7.1 Built-In Language Variables:
Add a new paragraph after the following paragraph:
"The set of helper invocations generated when processing any set of
primitives is implementation dependent."
Non-helper invocations are "demoted" to helper invocations by the
*demote* statement. Demoting an invocation is similar to discarding it,
in that it will no longer write to memory or output to the framebuffer,
but the invocation still participates in computing derivatives. Demoting
an invocation does not cause non-uniform flow control. The implementation
may terminate helper invocations before the end of the shader as an
optimization, but doing so must not affect derivative calculations and
does not make control flow non-uniform. After an invocation is demoted,
the value of the gl_HelperInvocation input variable is undefined.
helperInvocationEXT() can be used to determine whether an invocation is
currently a helper invocation.
Add a new section 8.X Miscellaneous Functions:
bool helperInvocationEXT();
Returns true if the invocation is currently a helper invocation,
otherwise returns false. An invocation is currently a helper invocation
if it was originally invoked as a helper invocation or if it has been
demoted to a helper invocation by the *demote* statement.
Changes to the grammar:
Add the token DEMOTE
Add a new rule:
demote_statement:
DEMOTE
Under the rule for simple_statement, add:
| demote_statement
Errors
None.
Issues
None.
Revision History
Revision 2
- Change dependencies so extension can be used with OpenGL.
Revision 1
- Internal revisions.