The begin callback indicates the start of a primitive. The function takes a single argument of type GLenum, which can be one of GLU_LINES, GLU_LINE_STRIP, GLU_TRIANGLE_FAN, GLU_TRIANGLE_STRIP, GLU_TRIANGLES, or GLU_QUAD_STRIP. The default begin callback function is NULL. The function prototype for this callback looks like:

void begin( GLenum type );


                        

The same as the GLU_NURBS_BEGIN callback except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to gluNurbsCallbackData. The default callback function is NULL. The function prototype for this callback function looks like:

void beginData(GLenum type, void *userData);


                        

The vertex callback indicates a vertex of the primitive. The coordinates of the vertex are stored in the parameter ``vertex''. All the generated vertices have dimension 3; that is, homogeneous coordinates have been transformed into affine coordinates. The default vertex callback function is NULL. The function prototype for this callback function looks like:

void vertex( GLfloat *vertex );


                        

This is the same as the GLU_NURBS_VERTEX callback, except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to gluNurbsCallbackData. The default callback function is NULL. The function prototype for this callback function looks like:

void vertexData( GLfloat *vertex, void *userData );


                        

The normal callback is invoked as the vertex normal is generated. The components of the normal are stored in the parameter ``normal.'' In the case of a NURBS curve, the callback function is effective only when the user provides a normal map (GLU_MAP1_NORMAL). In the case of a NURBS surface, if a normal map (GLU_MAP2_NORMAL) is provided, then the generated normal is computed from the normal map. If a normal map is not provided, then a surface normal is computed in a manner similar to that described for evaluators when GLU_AUTO_NORMAL is enabled. The default normal callback function is NULL. The function prototype for this callback function looks like:

void normal( GLfloat *normal );


                        

The same as the GLU_NURBS_NORMAL callback except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to gluNurbsCallbackData. The default callback function is NULL. The function prototype for this callback function looks like:

void normalData( GLfloat *normal, void *userData );


                        

The color callback is invoked as the color of a vertex is generated. The components of the color are stored in the parameter ``color.'' This callback is effective only when the user provides a color map (GLU_MAP1_COLOR_4 or GLU_MAP2_COLOR_4). ``color'' contains four components: R, G, B, A. The default color callback function is NULL. The prototype for this callback function looks like:

void color( GLfloat *color );


                        

The same as the GLU_NURBS_COLOR callback except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to gluNurbsCallbackData. The default callback function is NULL. The function prototype for this callback function looks like:

void colorData( GLfloat *color, void *userData );


                        

The texture callback is invoked as the texture coordinates of a vertex are generated. These coordinates are stored in the parameter ``texCoord.'' The number of texture coordinates can be 1, 2, 3, or 4 depending on which type of texture map is specified (GLU_MAP1_TEXTURE_COORD_1, GLU_MAP1_TEXTURE_COORD_2, GLU_MAP1_TEXTURE_COORD_3, GLU_MAP1_TEXTURE_COORD_4, GLU_MAP2_TEXTURE_COORD_1, GLU_MAP2_TEXTURE_COORD_2, GLU_MAP2_TEXTURE_COORD_3, GLU_MAP2_TEXTURE_COORD_4). If no texture map is specified, this callback function will not be called. The default texture callback function is NULL. The function prototype for this callback function looks like:

void texCoord( GLfloat *texCoord );


                        

This is the same as the GLU_NURBS_TEXTURE_COORD callback, except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to gluNurbsCallbackData. The default callback function is NULL. The function prototype for this callback function looks like:

void texCoordData( GLfloat *texCoord, void *userData );


                        

The end callback is invoked at the end of a primitive. The default end callback function is NULL. The function prototype for this callback function looks like:

void end( void );


                        

This is the same as the GLU_NURBS_END callback, except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to gluNurbsCallbackData. The default callback function is NULL. The function prototype for this callback function looks like:

void endData( void  *userData );


                        

The error function is called when an error is encountered. Its single argument is of type GLenum, and it indicates the specific error that occurred. There are 37 errors unique to NURBS, named GLU_NURBS_ERROR1 through GLU_NURBS_ERROR37. Character strings describing these errors can be retrieved with gluErrorString().