Name

    NV_conservative_raster_underestimation

Name Strings

    GL_NV_conservative_raster_underestimation

Contact

    Kedarnath Thangudu, NVIDIA Corporation (kthangudu 'at' nvidia.com)

Contributors

    Mark Kilgard, NVIDIA

Status

    Shipping in NVIDIA release 388.XX drivers and up

Version

    Last Modified Date:         November 15, 2017
    Revision:                   1

Number

    OpenGL Extension #518

Dependencies

    This extension is written against NV_conservative_raster as applied to
    OpenGL 4.3 specification (Core Profile).

    This extension interacts with NV_gpu_program4 and NV_gpu_program5.
    
Overview
    
    The extension NV_conservative_raster provides a new rasterization mode 
    known as "Overestimated Conservative Rasterization", where any pixel that 
    is partially covered, even if no sample location is covered, is treated as 
    fully covered and a corresponding fragment will be shaded.  There is also 
    an "Underestimated Conservative Rasterization" variant, where only the 
    pixels that are completely covered by the primitive are rasterized. 

    This extension provides the underestimated conservative rasterization 
    information for each fragment in the fragment shader through a new
    built-in gl_FragFullyCoveredNV.

New Procedures and Functions

    None.

New Tokens

    None.

Additions to Chapter 14 of the OpenGL 4.3 (Compatibility Profile) Specification
(Fixed-Function Primitive Assembly and Rasterization)

    Add a new paragraph at the end of subsection 14.6.X "Conservative 
    Rasterization" (added by NV_conservative_raster)

    When CONSERVATIVE_RASTERIZATION_NV is enabled, the fragment shader built-in
    input boolean variable gl_FragFullyCoveredNV will indicate if the fragment 
    being shaded is completely covered by the primitive.

Additions to Chapter 15 of the OpenGL 4.3 (Compatibility Profile) Specification
(Programmable Fragment Processing)

    Add to the Section 15.2.2 (Shader Inputs)

    When CONSERVATIVE_RASTERIZATION_NV is enabled, the built-in read-only 
    variable gl_FragFullyCoveredNV is set to TRUE if the fragment is fully 
    covered by the generating primitive, and FALSE otherwise.

New State

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.
    
Modifications to the OpenGL Shading Language Specification, Version 4.30

    Including the following line in a shader can be used to control the 
    language features described in this extension:

        #extension GL_NV_conservative_raster_underestimation : <behavior>

    where <behavior> is as specified in section 3.3.

    New preprocessor #defines are added to the OpenGL Shading Language:

        #define GL_NV_conservative_raster_underestimation     1

    Modify Section 7.1 (Built-In Language Variables), p. 113

    Add to the list of fragment shader built-ins:

        in bool gl_FragFullyCoveredNV;

    Add the following descriptions for gl_FragFullyCoveredNV:

    The built-in variable gl_FragFullyCoveredNV is a fragment-shader input 
    variable which is set to TRUE, when CONSERVATIVE_RASTERIZATION_NV is 
    enabled and the fragment is fully covered by the generating primitive, and 
    FALSE otherwise. 

Errors

    None.

Interactions with NV_gpu_program4 and NV_gpu_program5

    If NV_gpu_program4 or NV_gpu_program5 is supported and the 
    "NV_conservative_raster_underestimation" program option is specified, 
    fragment programs can read the "fragment.fullycovered" attribute to 
    determine if the fragment is fully covered by the primitive.

    (Add the following rule to the NV_fragment_program4 grammar)

    <attribBasic>   ::= ...
                      | <fragPrefix> "fullycovered"

    (Add the following to the Table X.X Fragment Attribute Bindings)

    Fragment Attribute Binding  Components  Underlying State
    --------------------------  ----------  ----------------------------
    * fragment.fullycovered     (c,-,-,-)   fragment fully covered.

    (Add the following to the Section 2.X.2, Program Grammar)

    If a fragment attribute binding matches "fragment.fullycovered", the "x"
    component of the fragment attribute variable is set to either 0 or 1,
    depending on how the fragment is covered by the primitive.  If 
    CONSERVATIVE_RASTERIZATION is enabled and the fragment is completely 
    covered by the primitive, the fullycovered attribute is set to 1; 
    otherwise, it is set to 0.  The "y", "z", and "w" coordinates are 
    undefined.

    If the "NV_conservative_raster_underestimation" program option is not 
    specified, "fragment.fullycovered" binding is unavailable.

Issues

    (1) Do you need a new rasterization mode for underestimated conservative 
    rasterization?

    RESOLVED: Enabling CONSERVATIVE_RASTERIZATION will compute both 
    overestimated and underestimated coverage for a primitive. A fragment will 
    be shaded for each pixel that is covered under overestimation and the 
    underestimation information is provided as a fragment shader input.  By 
    simply discarding fragments that are not fully covered in the fragment 
    shader, an application can achieve underestimated rasterization of a 
    primitive in the final image.

    (2) Should the y, z, and w components of the fragment.fullycovered
    state for NV_gpu_program4 and NV_gpu_program5 be undefined?

    RESOLVED: Yes.

    The (c,-,-,-) is consistent with all the other scalar attributes in
    NV_gpu_program*.

    This undefined behavior is limited to the assembly interface and
    not exposed in GLSL.

    (3) How does having CONSERVATIVE_RASTERIZATION enabled affect the sample 
    mask (gl_SampleMask) visible in the fragment shader?

    RESOLVED: When you have CONSERVATIVE_RASTERIZATION enabled on, the 
    gl_SampleMask will unconditionally show all 1's for valid samples.

    Even with all 1's in gl_SampleMask doesn't imply that a fragment is
    fully covered.
    
Revision History

    Revision 1
      - Internal revisions.