extensions/NV/NV_fill_rectangle.txt

Name

    NV_fill_rectangle

Name Strings

    GL_NV_fill_rectangle

Contact

    Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)

Contributors

    Jeff Bolz, NVIDIA Corporation
    James Helferty, NVIDIA Corporation

Status

    Shipping

Version

    Last Modified Date:         March 27, 2015
    Revision:                   2

Number

    OpenGL Extension #466
    OpenGL ES Extension #232

Dependencies

    This extension is written against the OpenGL 4.3 Specification
    (Compatibility Profile).

    This extension interacts with OpenGL ES 3.1 (June 4, 2014)

    If implemented in OpenGL ES, NV_polygon_mode is required.

Overview

    This extension adds a new PolygonMode setting where a triangle is
    rasterized by computing and filling its axis-aligned screen-space bounding
    box, disregarding the actual triangle edges. This can be useful for
    drawing a rectangle without being split into two triangles with an
    internal edge.  It is also useful to minimize the number of primitives
    that need to be drawn, particularly for a user-interface.

New Procedures and Functions

    None.

New Tokens

        FILL_RECTANGLE_NV                            0x933C

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

    Modify Section 14.6.4 (Options Controlling Polygon Rasterization)

    The interpretation of polygons for rasterization is controlled using

        void PolygonMode(enum face, enum mode);

    <face> is one of FRONT, BACK, or FRONT_AND_BACK, indicating that the 
    rasterizing method described by mode respectively replaces the rasterizing 
    method for front-facing polygons, back-facing polygons, or both front- and 
    back-facing polygons. <mode> is one of the symbolic constants POINT, LINE, 
    FILL, or FILL_RECTANGLE_NV. Calling PolygonMode with POINT causes certain 
    vertices of a polygon to be treated, for rasterization purposes, as if they 
    had been drawn with mode POINTS. The vertices selected for this treatment 
    are those that have been tagged as having a polygon boundary edge beginning 
    on them (see section 10.1.17). LINE causes edges that are tagged as 
    boundary to be rasterized as line segments. (The line stipple counter is 
    reset at the beginning of the first rasterized edge of the polygon, but not
    for subsequent edges.) FILL is the default mode of polygon rasterization, 
    corresponding to the description in sections 14.6.1, 14.6.2, and 14.6.3. 
    Note that these modes affect only the final rasterization of polygons: in 
    particular, a polygon's vertices are lit, and the polygon is clipped and 
    possibly culled before these modes are applied.

    FILL_RECTANGLE_NV mode modifies triangle rasterization to consider a sample
    within the primitive if the sample location is inside the window-aligned 
    bounding box of the triangle after projection. Note that the barycentric 
    weights used in attribute interpolation may be outside the range [0,1]
    when these primitives are shaded. Special treatment is given to a sample 
    position on the boundary edge of the bounding box. In such a case we 
    require that if two rectangles lie on either side of a common edge (with 
    identical endpoints) on which a sample position lies, then exactly one of 
    the triangles results in the production of the fragment during 
    rasterization. 
    
    Polygons rendered with FILL_RECTANGLE_NV mode may be clipped by the 
    frustum or by user clip planes, in an implementation-dependent manner. 
    If clipping is applied, the triangle is culled rather than clipped. When 
    quads and polygons are rendered in this mode, they will be tessellated 
    and rasterized as independent triangles (the order of tessellation within 
    a primitive is undefined).

    Polygon antialiasing applies only to the FILL state of PolygonMode. Polygon 
    antialiasing does not apply to the FILL_RECTANGLE_NV mode. For POINT or 
    LINE, point antialiasing or line segment antialiasing, respectively, apply.

    An INVALID_OPERATION error is generated by Begin or any Draw command if 
    only one of the front and back polygon mode is FILL_RECTANGLE_NV.


New Implementation Dependent State

    None.

New State

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

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

    None.

Errors

    An INVALID_OPERATION error is generated by Begin or any Draw command if 
    only one of the front and back polygon mode is FILL_RECTANGLE_NV.

Interactions with OpenGL 4.3 (Core Profile) and OpenGL ES 3.1

    If the OpenGL core profile or OpenGL ES is used, references to Begin, line
    stipple, user clip planes, separate front and back polygon modes, polygon
    primitives and quad primitives should be removed.

Interactions with OpenGL ES 3.1

    Using this extension in OpenGL ES 3.1, the tokens from OpenGL should be
    considered to refer to equivalent tokens from the NV_polygon_mode
    extension, with an "_NV" suffix:  POINT, LINE, FILL, POLYGON_OFFSET_POINT,
    and POLYGON_OFFSET_LINE.  References to PolygonMode should be considered
    to refer to PolygonModeNV.  References to point, line, and polygon
    antialiasing should be removed.

Issues

    (1) Where in the pipeline does the conversion from triangle to screen-
    aligned bbox occur?

    RESOLVED: The new rasterization mode applies during rasterization, not
    earlier. So transform feedback would write out the triangle as if it were
    being FILLed normally, and the triangle may have started life as a 
    different type of primitive (e.g. point or line) before being converted to
    a triangle by a geometry shader.

    (2) What happens if such a triangle is clipped?

    RESOLVED: Clipping will cause the triangle to be discarded.

    (3) How are quads and polygons handled? Are they decomposed into triangles?

    RESOLVED:  When using FILL_RECTANGLE_NV, quads and polygons are decomposed
    into triangles in an implementation-dependent manner, similar to the
    behavior used in transform feedback.

    (4) Do these triangles still have a facingness?

    RESOLVED: Yes. Face culling is still applied, and the orientation of such
    a primitive is still observable in the fragment shader (gl_FrontFacing).

    (5) How is polygon offset applied?

    RESOLVED:  Polygon offset is computed and applied normally to the fill
    rectangle, using the same techniques used for triangle rasterization.

    If the Z values for the vertices of the fill primitive are not constant,
    the slope-dependent portion of the polygon offset will be computed using
    the three vertices specified.  That seems reasonable, since per-pixel Z
    will be interpolated using the same slopes.

    However, when using floating-point depth buffers, the meaning of the
    constant portion of polygon offset will scaled according to the Z values
    of the three vertices specified for the fill rectangle.  Since the
    interpolated Z values for the fill rectangle may fall outside the range of
    Z values in the primitive, it's possible that this rescaled constant
    offset is too small for portions of the fill rectangle where
    "extrapolated" Z values are larger than any of the Z values of the
    original triangle.  This issue will require a workaround in application
    code in the unlikely event that it applies.

Revision History

    Revision 2, 2015/03/27
      - Add ES interactions

    Revision 1
      - Internal revisions.