glReferencePlaneSGIX(3G) OpenGL Reference glReferencePlaneSGIX(3G)
NAME
glReferencePlaneSGIX - define a reference plane for coplanar geometry
C SPECIFICATION
void glReferencePlaneSGIX( const GLdouble *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 SGIXreferenceplane 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
(GLEQUAL, GLLEQUAL, GLGEQUAL) may be used with impunity.
The process of generating depth values from the reference plane is
enabled by executing glEnable with argument GLREFERENCEPLANESGIX. The
plane equation of the reference plane is specified with
glReferencePlaneSGIX.
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 glReferencePlaneSGIX are in
object coordinates, while the coefficients returned by glGetDoublev with
argument GLREFERENCEPLANEEQUATIONSGIX are in clip coordinates.
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glReferencePlaneSGIX(3G) OpenGL Reference glReferencePlaneSGIX(3G)
glReferencePlaneSGIX is part of the SGIXreferenceplane extension, not
part of the core GL command set. If GLSGIXreferenceplane is included
in the string returned by glGetString when called with argument
GLEXTENSIONS, extension SGIXreferenceplane is supported by the
connection. See glIntro for more information about using extensions.
ERRORS
GLINVALIDOPERATION is generated if glReferencePlaneSGIX is executed
between the execution of glBegin and the corresponding execution of
glEnd.
ASSOCIATED GETS
glGetDoublev with argument GLREFERENCEPLANEEQUATIONSGIX
MACHINE DEPENDENCIES
SGIXreferenceplane is supported only on InfiniteReality systems.
SEE ALSO
glEnable, glPolygonOffsetEXT
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