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

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: March 19, 2015
    Revision: 1

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 "Controling 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.

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:

        out int gl_ViewportIndex;

    Modify the language introducing "gl_Layer" on p.123 as follows:

        Replace all instances of "geometry language" with "vertex, tessellation
    evaluation and geometry languages".

        Update numbering of "Layer and Viewport Selection" section to 11.4.

    Modify the paragraph introducing "gl_ViewportIndex" on p.124 as follows:

        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.

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.

Revision History

    Rev.   Date      Author    Changes
    ----  --------    --------  -----------------------------------------

     1    03/19/2015  gsellers  Initial version merging AMD_vertex_shader_viewport_index
                                and AMD_vertex_shader_layer