GLUTESSCALLBACK

Section: Misc. Reference Manual Pages (3G)
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NAME

gluTessCallback - define a callback for a tessellation object

C SPECIFICATION

void gluTessCallback( GLUtesselator* tess,


                        GLenum which,
                        _GLUfuncptr CallBackFunc )

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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.