Name

    NV_conservative_raster_pre_snap_triangles

Name Strings

    GL_NV_conservative_raster_pre_snap_triangles

Contact

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

Contributors

    Pat Brown, NVIDIA Corporation
    David P Gould, NVIDIA Corporation
    Eric Werness, NVIDIA Corporation

Status

    Shipping in NVIDIA release 367.XX drivers and up.

Version

    Last Modified Date:         February 11, 2016
    Revision:                   1

Number

    OpenGL Extension #487
    OpenGL ES Extension #262

Dependencies

    This extension is written against the NV_conservative_raster extension as
    applied to OpenGL 4.3 specification (Compatibility Profile) but may be
    used with the Core profile or OpenGL ES 2.0 or later.

Overview

    When CONSERVATIVE_RASTERIZATION_NV is enabled, the fragments generated for
    a primitive are conservative with respect to the primitive after snapping
    to sub-pixel grid.  This extension provides a new mode of rasterization
    for triangles where the fragments generated are conservative with respect
    to the primitive at infinite precision before vertex snapping.

    When the conservative raster mode is set to CONSERVATIVE_RASTER_MODE_PRE_-
    SNAP_TRIANGLES, triangles are rasterized more conservatively, and may
    generate fragments not generated when the mode is CONSERVATIVE_RASTER_MODE_-
    POST_SNAP (default). In particular it may generate fragments for pixels
    covered by triangles with zero area, or for pixels that are adjacent to
    but not covered by any triangle. This modified behavior may be useful in
    compensating for rounding errors caused by snapping vertex positions to a
    sub-pixel grid during rasterization.  It's possible that a non-degenerate
    triangle becomes degenerate due to snapping.  It's additionally possible
    that rounding errors in computing the position of a vertex or from
    snapping may cause a primitive that would cover a pixel at infinite
    precision to fail to cover the pixel post-snap.  Rasterizing such
    primitives more conservatively may be useful for "binning" algorithms
    described in NV_conservative_raster.


New Procedures and Functions

    void ConservativeRasterParameteriNV(enum pname, int param);

New Tokens

    Accepted by the <pname> parameter of ConservativeRasterParameteriNV:
        CONSERVATIVE_RASTER_MODE_NV                     0x954D

    Accepted by the <param> parameter of ConservativeRasterParameteriNV:
        CONSERVATIVE_RASTER_MODE_POST_SNAP_NV           0x954E
        CONSERVATIVE_RASTER_MODE_PRE_SNAP_TRIANGLES_NV  0x954F

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetInteger64v, GetFloatv, and GetDoublev:

        CONSERVATIVE_RASTER_MODE_NV                     0x954D


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

    Add the following paragraph to the end of the new subsection 14.6.X
    "Conservative Rasterization" added by NV_conservative_raster

    When CONSERVATIVE_RASTERIZATION_NV is enabled, the rasterization behavior
    may be controlled by calling the following command

        void ConservativeRasterParameteriNV(enum pname, int param);

    with <pname> set to CONSERVATIVE_RASTER_MODE_NV. The <param> parameter
    specifies the conservative raster mode to be used. If the mode is set to
    CONSERVATIVE_RASTER_MODE_POST_SNAP_NV, the generated fragments are
    conservative w.r.t the primitive after it is snapped to sub-pixel grid.
    If the mode is set to CONSERVATIVE_RASTER_MODE_PRE_SNAP_TRIANGLES_NV
    the fragments generated for triangles will be conservative w.r.t the
    triangle at infinite precision. Since non-degenerate triangles may become
    degenerate due to vertex snapping, this mode will generate fragments for
    zero area triangles that are otherwise culled when the mode is
    CONSERVATIVE_RASTER_MODE_POST_SNAP_NV. This mode may also generate fragments for pixels that
    are within half a sub-pixel distance away from a triangle's infinite
    precision boundary.  The default mode is set to CONSERVATIVE_RASTER_MODE_-
    POST_SNAP_NV.

New State

    Get Value                           Get Command    Type  Initial Value          Description             Sec.    Attribute
    ---------                           -----------    ----  -------------          -----------             ----    ---------
    CONSERVATIVE_RASTER_MODE_NV         GetIntegerv    E     CONSERVATIVE_RASTER_-  Control conservative    14.6.X  -
                                                             MODE_POST_SNAP         rasterization mode.


Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Modifications to the OpenGL Shading Language Specification, Version 4.30

    None.

Errors

    INVALID_ENUM is generated by ConservativeRasterParameteriNV if <pname> is
    not CONSERVATIVE_RASTER_MODE_NV, or if <param> is not CONSERVATIVE_RASTER_-
    MODE_POST_SNAP_NV or CONSERVATIVE_RASTER_MODE_PRE_SNAP_TRIANGLES_NV.

Issues

    (1) How does this extension interact with snap control?

    RESOLVED: When the conservative raster mode is set to CONSERVATIVE_RASTER_-
    MODE_PRE_SNAP_TRIANGLES the generated fragments are conservative w.r.t
    triangle at infinite precision i.e. before snapping to the sub-pixel grid.
    To achieve this implementations may augment/dilate the triangle edges by
    an amount (proportional to the sub-pixel precision) to compensate for
    rounding errors caused by snapping vertex positions to a sub-pixel grid.
    The amount of dilation is implementation dependent.

    (2) Which way do the fragments generated for zero-area triangles face?

    RESOLVED: Setting the conservative raster mode to CONSERVATIVE_RASTER_-
    MODE_PRE_SNAP_TRIANGLES generates fragments for zero-area (degenerate)
    triangles that are otherwise culled.  Since degenerate triangles don't have
    a normal and therefore no theoretical facing, all the degenerate triangles
    will be considered back-facing.  Degenerate triangles will be culled when
    back-face culling is enabled in GL.

    (3) How are the attributes computed for the fragments generated for zero-
        area triangles?

    RESOLVED: Since extrapolation/interpolation is not defined for degenerate
    triangles, the provoking vertex's attributes and z are used for all the
    fragments of such triangles.

Revision History

    Revision 1
    - Internal revisions.