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)
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