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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,
                             GLvoid * 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 call-back callback
       back 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  argu-ment argument
                 ment 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 argu-ment. argument.
                 ment. 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 ver-tex vertex
                 tex  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 func-tion function
                 tion 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  argu-ment. argument.
                 ment. 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  associ-ated associated
                 ated 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  argu-ment. argument.
                 ment. 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 call-back callback
                 back 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 con-stant constant
                 stant GLU_TESS_MAX_COORD in absolute value, and that the value has been  clamped.  (Coordi-nate (Coordinate
                 nate  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 argu-ment. argument.
                 ment. 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)





                                                                                         GLUTESSCALLBACK(3G)