gluTessCallback man page on OpenBSD

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GLUTESSCALLBACK(3G)					   GLUTESSCALLBACK(3G)

NAME
       gluTessCallback - define a callback for a tessellation object

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

       delim $$

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 GL_FALSE, then the argument is set to
		 either GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, or GL_TRIANGLES.
		 If the GLU_TESS_BOUNDARY_ONLY property is set to GL_TRUE,
		 then the argument will be set to GL_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 GL_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 GL_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 GL_TRIANGLE_FAN or
	      GL_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
       glBegin(3G), glEdgeFlag(3G), glVertex(3G), gluNewTess(3G),
       gluErrorString(3G), gluTessVertex(3G), gluTessBeginPolygon(3G),
       gluTessBeginContour(3G), gluTessProperty(3G), gluTessNormal(3G)

								 March 1, 2011
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