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Name
AMD_conservative_depth
Name String
GL_AMD_conservative_depth
Contact
Graham Sellers, AMD (graham.sellers 'at' amd.com)
Contributors
Pierre Boudier, AMD
Graham Sellers, AMD
Status
In Progress
Version
Last Modified Date: 10/25/2010
Author Revision: 4
Number
385
Dependencies
OpenGL 3.0 is required.
This extension is written against the OpenGL Shading Language Version 4.00.
Overview
There is a common optimization for hardware accelerated implementation of
OpenGL which relies on an early depth test to be run before the fragment
shader so that the shader evaluation can be skipped if the fragment ends
up being discarded because it is occluded.
This optimization does not affect the final rendering, and is typically
possible when the fragment does not change the depth programmatically.
(i.e.: it does not write to the built-in gl_FragDepth output). There are,
however a class of operations on the depth in the shader which could
still be performed while allowing the early depth test to operate.
This extension allows the application to pass enough information to the
GL implementation to activate such optimizations safely.
New Procedures and Functions
None.
New Tokens
None.
Additions to the AGL/GLX/WGL Specifications
None.
Modifications to the OpenGL Shading Language Specification version 1.50.9
Additions to Chapter 1 of the OpenGL Shading Language 4.00.08 Specification (Introduction)
None.
Additions to Chapter 2 of the OpenGL Shading Language 4.00.08 Specification (Overview of OpenGL Shading)
None.
Additions to Chapter 3 of the OpenGL Shading Language 4.00.08 Specification (Basics)
Add a new Section 3.3.x, GL_AMD_conservative_depth Extension
Including the following line in a shader can be used to control the language
features described in this extension:
#extension GL_AMD_conservative_depth: <behavior>
where <behavior> is as described in section 3.3.
A new preprocessor #define is added to the OpenGL Shading Language:
#define GL_AMD_conservative_depth 1
Additions to Chapter 4 of the OpenGL Shading Language 4.00.08 Specification (Variables and Types)
Modify Section 4.3.8.2 (Output Layout Qualifiers) page 47
Modify the paragraph beginning: "Fragment shaders allow output layout
qualifiers only..."
Fragment shaders allow output layout qualifiers only on the interface
out, or for the purposes of redeclaring the built-in variable
gl_FragDepth (see Section 7.1, Built-In Language Variables).
Insert the following before the paragraph beginning: "Geometry shaders can
have three types of output layout qualifiers..."
The built-in fragment shader variable gl_FragDepth may be redeclared
using one of the following layout qualifiers.
layout-qualifier-id
depth_any
depth_greater
depth_less
depth_unchanged
For example:
layout (depth_greater) out float gl_FragDepth;
The layout qualifier for gl_FragDepth controls how the GL will interpret
assignments to the gl_FragDepth out variable for the purpose of depth
testing. Any assignment to gl_FragDepth will be honored by the shader
and the updated value of gl_FragDepth will still be written to the
depth buffer if it is determined that the depth write should occur.
However, depending on the layout qualifier of gl_FragDepth and the
current state of depth testing, the GL may perform optimizations or
reordering of operations to depth test and writes to improve performance.
By default, gl_FragDepth assumes the <depth_any> layout qualifier. When
the layout qualifier for gl_FragDepth is <depth_any>, the shader compiler
will note any assignment to gl_FragDepth modifying it in an unknown way,
and depth testing will always be performed after the shader has executed.
When the layout qualifier is <depth_greater>, the GL will assume that
any modification of gl_FragDepth will only increase its value. When the
layout qualifier is <depth_less>, the GL will assume that any
modification of gl_FragDepth will only decrease its value. When the
layout qualifier is <depth_unchanged>, the shader compiler will honor
any modification to gl_FragDepth, but the rest of the GL assume that
gl_FragDepth is not assigned a new value.
If a shader redeclares gl_FragDepth using the <depth_greater>,
<depth_less> or <depth_unchanged> and then violates this contract, the
results of the depth test may be inaccurate and any resulting rendering
will produce undefined results. However, no error is generated.
Redeclarations are performed as follows:
out float gl_FragDepth; // Redeclaration that changes nothing is allowed
layout (depth_any) out float gl_FragDepth; // Assume that gl_FragDepth may be modified in any way
layout (depth_greater) out float gl_FragDepth; // Assume that gl_FragDepth may be modified such that its value will only increase
layout (depth_less) out float gl_FragDepth; // Assume that gl_FragDepth may be modified such that its value will only decrease
layout (depth_unchanged) out float gl_FragDepth; // Assume that gl_FragDepth will not be modified
If gl_FragDepth is redeclared in any fragment shader in a program, it must
be redeclared in all fragment shaders in that program that have static
assignments to gl_FragDepth. All redeclarations of gl_FragDepth in all
fragment shaders in a single program must have the same set of qualifiers.
Within any shader, the first redeclarations of gl_FragDepth must appear
before any use of gl_FragDepth. The built-in gl_FragDepth is only
predeclared in fragment shaders, so redeclaring it in any other shader
language will be illegal.
Revision History
Rev. Date Author Changes
---- -------- -------- -----------------------------------------
4 25/10/2010 gsellers Apply patch from idr.
3 04/01/2010 gsellers Minor cleanup. Update to GLSL 4.00.
2 12/01/2009 gsellers Internal updates
1 11/03/2009 pboudier Initial draft