fglReferencePlaneSGIX(3G)

NAME
     fglReferencePlaneSGIX - define a reference plane for coplanar geometry

FORTRAN SPECIFICATION
     SUBROUTINE fglReferencePlaneSGIX( CHARACTER*8 equation )


PARAMETERS
     equation  Specifies a four-element array containing the plane equation
               coefficients of a reference plane which will be used to
               generate depth values for pixel fragments.  The coefficients of
               x, y, and z are the first three elements of the array,
               respectively, and the constant term is the fourth.  The plane
               equation is specified in object coordinates.  The default is
               equivalent to (0,0,1,0) in window coordinates.

DESCRIPTION
     The SGIX_reference_plane extension allows a group of coplanar primitives
     to be rendered without depth-buffering artifacts.  This is accomplished
     by generating depth values for all pixel fragments from a single
     ``reference plane'' rather than from the primitives themselves.  This
     ensures that all the primitives in the group will have exactly the same
     depth value at any given sample point, no matter what imprecision may
     exist in the original specification of the primitives or in the GL's
     coordinate transformation process.  As a result, depth tests for equality
     (GL_EQUAL, GL_LEQUAL, GL_GEQUAL) may be used with impunity.

     The process of generating depth values from the reference plane is
     enabled by executing fglEnable with argument GL_REFERENCE_PLANE_SGIX.
     The plane equation of the reference plane is specified with
     fglReferencePlaneSGIX.

     equation must be an array with four elements:  the x coefficient, y
     coefficient, z coefficient, and constant term of the reference plane's
     equation, in that order.  The plane equation is specified in object
     coordinates, and transformed by the transpose-adjoint of the object-to-
     clip transformation in order to derive a plane equation in clip
     coordinates.  The clip coordinates are transformed to window coordinates
     when the reference plane is enabled.  Let the window-coordinate version
     of the reference plane equation be ax+by+cz+d=0.  If the reference plane
     is enabled, and a pixel fragment has window coordinates (x,y), then its z
     coordinate will become -(d+ax+by)/c.

     If the generated z coordinate is outside the valid depth buffer range,
     then the resulting depth value is undefined.  If the reference plane does
     not intersect the viewing frustum, it is as if the default coefficients
     (0,0,1,0) (in window coordinates) are used.

NOTES
     The reference plane coefficients specified by fglReferencePlaneSGIX are
     in object coordinates, while the coefficients returned by fglGetDoublev
     with argument GL_REFERENCE_PLANE_EQUATION_SGIX are in clip coordinates.
     fglReferencePlaneSGIX is part of the SGIX_reference_plane extension, not
     part of the core GL command set.  If GL_SGIX_reference_plane is included
     in the string returned by fglGetString when called with argument
     GL_EXTENSIONS, extension SGIX_reference_plane is supported by the
     connection.  See fglIntro for more information about using extensions.

ERRORS
     GL_INVALID_OPERATION is generated if fglReferencePlaneSGIX is executed
     between the execution of fglBegin and the corresponding execution of
     fglEnd.

ASSOCIATED GETS
     fglGetDoublev with argument GL_REFERENCE_PLANE_EQUATION_SGIX


MACHINE DEPENDENCIES
     SGIX_reference_plane is supported only on InfiniteReality systems.


SEE ALSO
     fglEnable, fglPolygonOffsetEXT