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