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GLUTESSCALLBACK(3G)			  OpenGL Manual 		      GLUTESSCALLBACK(3G)

NAME
       gluTessCallback - define a callback for a tessellation object

C SPECIFICATION
       void gluTessCallback(GLUtesselator* tess, GLenum which, _GLUfuncptr CallBackFunc);

PARAMETERS
       tess
	   Specifies the tessellation object (created with gluNewTess()).

       which
	   Specifies the callback being defined. The following values are valid: GLU_TESS_BEGIN,
	   GLU_TESS_BEGIN_DATA, GLU_TESS_EDGE_FLAG, GLU_TESS_EDGE_FLAG_DATA, GLU_TESS_VERTEX,
	   GLU_TESS_VERTEX_DATA, GLU_TESS_END, GLU_TESS_END_DATA, GLU_TESS_COMBINE,
	   GLU_TESS_COMBINE_DATA, GLU_TESS_ERROR, and GLU_TESS_ERROR_DATA.

       CallBackFunc
	   Specifies the function to be called.

DESCRIPTION
       gluTessCallback is used to indicate a callback to be used by a tessellation object. If the
       specified callback is already defined, then it is replaced. If CallBackFunc is NULL, then
       the existing callback becomes undefined.

       These callbacks are used by the tessellation object to describe how a polygon specified by
       the user is broken into triangles. Note that there are two versions of each callback: one
       with user-specified polygon data and one without. If both versions of a particular
       callback are specified, then the callback with user-specified polygon data will be used.
       Note that the polygon_data parameter used by some of the functions is a copy of the
       pointer that was specified when gluTessBeginPolygon() was called. The legal callbacks are
       as follows:

       GLU_TESS_BEGIN
	   The begin callback is invoked like glBegin() to indicate the start of a (triangle)
	   primitive. The function takes a single argument of type GLenum. If the
	   GLU_TESS_BOUNDARY_ONLY property is set to GLU_FALSE, then the argument is set to
	   either GLU_TRIANGLE_FAN, GLU_TRIANGLE_STRIP, or GLU_TRIANGLES. If the
	   GLU_TESS_BOUNDARY_ONLY property is set to GLU_TRUE, then the argument will be set to
	   GLU_LINE_LOOP. The function prototype for this callback is:

	       void begin( GLenum type );

       GLU_TESS_BEGIN_DATA
	   The same as the GLU_TESS_BEGIN callback except that it takes an additional pointer
	   argument. This pointer is identical to the opaque pointer provided when
	   gluTessBeginPolygon() was called. The function prototype for this callback is:

	       void beginData( GLenum type, void *polygon_data );

       GLU_TESS_EDGE_FLAG
	   The edge flag callback is similar to glEdgeFlag(). The function takes a single boolean
	   flag that indicates which edges lie on the polygon boundary. If the flag is GLU_TRUE,
	   then each vertex that follows begins an edge that lies on the polygon boundary, that
	   is, an edge that separates an interior region from an exterior one. If the flag is
	   GLU_FALSE, then each vertex that follows begins an edge that lies in the polygon
	   interior. The edge flag callback (if defined) is invoked before the first vertex
	   callback.

	   Since triangle fans and triangle strips do not support edge flags, the begin callback
	   is not called with GLU_TRIANGLE_FAN or GLU_TRIANGLE_STRIP if a non-NULL edge flag
	   callback is provided. (If the callback is initialized to NULL, there is no impact on
	   performance). Instead, the fans and strips are converted to independent triangles. The
	   function prototype for this callback is:

	       void edgeFlag( GLboolean flag );

       GLU_TESS_EDGE_FLAG_DATA
	   The same as the GLU_TESS_EDGE_FLAG callback except that it takes an additional pointer
	   argument. This pointer is identical to the opaque pointer provided when
	   gluTessBeginPolygon() was called. The function prototype for this callback is:

	       void edgeFlagData( GLboolean flag, void *polygon_data );

       GLU_TESS_VERTEX
	   The vertex callback is invoked between the begin and end callbacks. It is similar to
	   glVertex(), and it defines the vertices of the triangles created by the tessellation
	   process. The function takes a pointer as its only argument. This pointer is identical
	   to the opaque pointer provided by the user when the vertex was described (see
	   gluTessVertex()). The function prototype for this callback is:

	       void vertex( void *vertex_data );

       GLU_TESS_VERTEX_DATA
	   The same as the GLU_TESS_VERTEX callback except that it takes an additional pointer
	   argument. This pointer is identical to the opaque pointer provided when
	   gluTessBeginPolygon() was called. The function prototype for this callback is:

	       void vertexData( void *vertex_data, void *polygon_data );

       GLU_TESS_END
	   The end callback serves the same purpose as glEnd(). It indicates the end of a
	   primitive and it takes no arguments. The function prototype for this callback is:

	       void end( void );

       GLU_TESS_END_DATA
	   The same as the GLU_TESS_END callback except that it takes an additional pointer
	   argument. This pointer is identical to the opaque pointer provided when
	   gluTessBeginPolygon() was called. The function prototype for this callback is:

	       void endData( void *polygon_data );

       GLU_TESS_COMBINE
	   The combine callback is called to create a new vertex when the tessellation detects an
	   intersection or wishes to merge features. The function takes four arguments: an array
	   of three elements each of type GLdouble, an array of four pointers, an array of four
	   elements each of type GLfloat, and a pointer to a pointer. The prototype is:

	       void combine( GLdouble coords[3], void *vertex_data[4],
			     GLfloat weight[4], void **outData );

	   The vertex is defined as a linear combination of up to four existing vertices, stored
	   in vertex_data. The coefficients of the linear combination are given by weight; these
	   weights always add up to 1. All vertex pointers are valid even when some of the
	   weights are 0.  coords gives the location of the new vertex.

	   The user must allocate another vertex, interpolate parameters using vertex_data and
	   weight, and return the new vertex pointer in outData. This handle is supplied during
	   rendering callbacks. The user is responsible for freeing the memory some time after
	   gluTessEndPolygon() is called.

	   For example, if the polygon lies in an arbitrary plane in 3-space, and a color is
	   associated with each vertex, the GLU_TESS_COMBINE callback might look like this:

	       void myCombine( GLdouble coords[3], VERTEX *d[4],
			       GLfloat w[4], VERTEX **dataOut )
	       {
		  VERTEX *new = new_vertex();

		  new->x = coords[0];
		  new->y = coords[1];
		  new->z = coords[2];
		  new->r = w[0]*d[0]->r + w[1]*d[1]->r + w[2]*d[2]->r + w[3]*d[3]->r;
		  new->g = w[0]*d[0]->g + w[1]*d[1]->g + w[2]*d[2]->g + w[3]*d[3]->g;
		  new->b = w[0]*d[0]->b + w[1]*d[1]->b + w[2]*d[2]->b + w[3]*d[3]->b;
		  new->a = w[0]*d[0]->a + w[1]*d[1]->a + w[2]*d[2]->a + w[3]*d[3]->a;
		  *dataOut = new;
	       }

	   If the tessellation detects an intersection, then the GLU_TESS_COMBINE or
	   GLU_TESS_COMBINE_DATA callback (see below) must be defined, and it must write a
	   non-NULL pointer into dataOut. Otherwise the GLU_TESS_NEED_COMBINE_CALLBACK error
	   occurs, and no output is generated.

       GLU_TESS_COMBINE_DATA
	   The same as the GLU_TESS_COMBINE callback except that it takes an additional pointer
	   argument. This pointer is identical to the opaque pointer provided when
	   gluTessBeginPolygon() was called. The function prototype for this callback is:

	       void combineData( GLdouble coords[3], void *vertex_data[4],
				 GLfloat weight[4], void **outData,
				 void *polygon_data );

       GLU_TESS_ERROR
	   The error callback is called when an error is encountered. The one argument is of type
	   GLenum; it indicates the specific error that occurred and will be set to one of
	   GLU_TESS_MISSING_BEGIN_POLYGON, GLU_TESS_MISSING_END_POLYGON,
	   GLU_TESS_MISSING_BEGIN_CONTOUR, GLU_TESS_MISSING_END_CONTOUR,
	   GLU_TESS_COORD_TOO_LARGE, GLU_TESS_NEED_COMBINE_CALLBACK, or GLU_OUT_OF_MEMORY.
	   Character strings describing these errors can be retrieved with the gluErrorString()
	   call. The function prototype for this callback is:

	       void error( GLenum errno );

	   The GLU library will recover from the first four errors by inserting the missing
	   call(s).  GLU_TESS_COORD_TOO_LARGE indicates that some vertex coordinate exceeded the
	   predefined constant GLU_TESS_MAX_COORD in absolute value, and that the value has been
	   clamped. (Coordinate values must be small enough so that two can be multiplied
	   together without overflow.)	GLU_TESS_NEED_COMBINE_CALLBACK indicates that the
	   tessellation detected an intersection between two edges in the input data, and the
	   GLU_TESS_COMBINE or GLU_TESS_COMBINE_DATA callback was not provided. No output is
	   generated.  GLU_OUT_OF_MEMORY indicates that there is not enough memory so no output
	   is generated.

       GLU_TESS_ERROR_DATA
	   The same as the GLU_TESS_ERROR callback except that it takes an additional pointer
	   argument. This pointer is identical to the opaque pointer provided when
	   gluTessBeginPolygon() was called. The function prototype for this callback is:

	       void errorData( GLenum errno, void *polygon_data );

EXAMPLE
       Polygons tessellated can be rendered directly like this:

	   gluTessCallback(tobj, GLU_TESS_BEGIN, glBegin);
	   gluTessCallback(tobj, GLU_TESS_VERTEX, glVertex3dv);
	   gluTessCallback(tobj, GLU_TESS_END, glEnd);
	   gluTessCallback(tobj, GLU_TESS_COMBINE, myCombine);
	   gluTessBeginPolygon(tobj, NULL);
	      gluTessBeginContour(tobj);
		 gluTessVertex(tobj, v, v);
		 ...
	      gluTessEndContour(tobj);
	   gluTessEndPolygon(tobj);

       Typically, the tessellated polygon should be stored in a display list so that it does not
       need to be retessellated every time it is rendered.

SEE ALSO
       gluErrorString(), gluNewTess(), gluTessBeginContour(), gluTessBeginPolygon(),
       gluTessNormal(), gluTessProperty(), gluTessVertex(), glBegin(), glEdgeFlag(), glVertex()

COPYRIGHT
       Copyright (C) 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI
       Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.

AUTHORS
       opengl.org

opengl.org				    06/10/2014			      GLUTESSCALLBACK(3G)
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