extensions/SGIX/SGIX_reference_plane.txt

Name

    SGIX_reference_plane

Name String

    GL_SGIX_reference_plane

Version

    $Date: 1996/03/08 00:11:51 $ $Revision: 1.5 $

Number

    60

Dependencies

    None

Overview

    This extension allows a group of coplanar primitives to be rendered
    without depth-buffering artifacts.  This is accomplished by generating
    the depth values for all the primitives from a single ``reference plane''
    rather than from the primitives themselves.  This ensures that all the
    primitives in the group have exactly the same depth value at any given
    sample point, no matter what imprecision may exist in the original
    specifications of the primitives or in the GL's coordinate transformation
    process.

    The reference plane is defined by a four-component plane equation.
    When glReferencePlaneSGIX is called, equation is transformed by the
    transpose-adjoint of a matrix that is the complete object-coordinate
    to clip-coordinate transformation.  The resulting clip-coordinate
    coefficients are transformed by the current viewport when the reference
    plane is enabled.

    The reference plane is enabled and disabled with glEnable and glDisable.

    If the reference plane is enabled, a fragment (xf,yf,zf) will have a
    new z coordinate generated from (xf,yf) by giving it the same z value
    that the reference plane would have at (xf,yf).

Issues

    * What should the default reference plane be?
       - (0,0,1,0) in window coords seems good 

    * What happens if the reference plane does not intersect the view frustum?
       - can disallow enabling of the plane, or substitute the default plane. 

    * Should it be possible to query the validity of the reference plane just
      like RasterPos?
       - could be useful

New Procedures and Functions

    void glReferencePlaneSGIX(const GLdouble *equation);

New Tokens

    Accepted by the <pname> parameter of glGet, and the <cap> parameter
    of glEnable, glDisable, and glIsEnabled:

        REFERENCE_PLANE_SGIX

    Accepted by the <pname> parameter of glGet:

        REFERENCE_PLANE_EQUATION_SGIX

Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    None

additions to Chapter 3 of the 1.0 Specification (Rasterization)

    If enabled, the reference plane is used to calculate the fragment's z
    value based on the fragment's x and y values and the reference plane
    itself.

    Let the reference plane be defined by a four-component plane equation
    in window coordinates.

        a*x + b*y + c*z + d = 0.0

    then if the incoming fragment has an x value of xf and a y value of yf,
    the fragment's z value zf will be:

        zf = -(d + a*xf + b*yf) / c;

    The generated zf is undefined if it falls outside the valid zbuffer range.
    (That is what KONA does).

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None

Additions to the GLX Specification

    A new GL rendering command is needed to support
    ReferencePlaneSGIX().  The following command is sent to the
    server as part of a glXRender request:

    ReferencePlaneSGIX
            2       20              rendering command length
            2       TBD             rendering command opcode
            4       FLOAT32         a (x coefficient) of plane equation
            4       FLOAT32         b (y coefficient) of plane equation
            4       FLOAT32         c (z coefficient) of plane equation
            4       FLOAT32         d (constant term) of plane equation

    No new protocol is required to handle state retrieval.

Errors

    INVALID_OPERATION is generated if ReferencePlaneSGIX is called between
    execution of Begin and the corresponding execution of End.

New State

    Get Value                       Get Command  Type  Initial Value     Attrib
    ---------                       -----------  ----  -------------     ------
    REFERENCE_PLANE_SGIX             IsEnabled     B      FALSE       transform/enable
    REFERENCE_PLANE_EQUATION_SGIX    GetFloatv     V      ???         transform

    The plane coefficiants returned by Get() are in clip coordinates.