GLenum which,
_GLUfuncptr 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 tessella‐
tion 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 speci‐
fied, 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 prop‐
erty 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
verted 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 identi‐
cal 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 call‐
backs. 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 indi‐
cates 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 fea‐
tures. 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
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 er‐
ror 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 in‐
serting the missing call(s). GLU_TESS_COORD_TOO_LARGE indi‐
cates that some vertex coordinate exceeded the predefined con‐
stant 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
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); gluTessBe‐
ginPolygon(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, glEdgeFlag, glVertex, gluNewTess, gluErrorString,
gluTessVertex, gluTessBeginPolygon, gluTessBeginContour,
gluTessProperty, gluTessNormal
GLUTESSCALLBACK(3G)
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