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 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 glEnable with	argument GL_REFERENCE_PLANE_SGIX.  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 GL_REFERENCE_PLANE_EQUATION_SGIX are in clip coordinates.



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glReferencePlaneSGIX(3G)       OpenGL Reference	      glReferencePlaneSGIX(3G)



     glReferencePlaneSGIX 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 glGetString when	called with argument
     GL_EXTENSIONS, extension SGIX_reference_plane is supported	by the
     connection.  See glIntro for more information about using extensions.

ERRORS
     GL_INVALID_OPERATION is generated if glReferencePlaneSGIX is executed
     between the execution of glBegin and the corresponding execution of
     glEnd.

ASSOCIATED GETS
     glGetDoublev with argument	GL_REFERENCE_PLANE_EQUATION_SGIX


MACHINE	DEPENDENCIES
     SGIX_reference_plane is supported only on InfiniteReality systems.


SEE ALSO
     glEnable, glPolygonOffsetEXT


































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