gluTessCallback()gluTessCallback()NAMEgluTessCallback - define a callback for a tessellation object
SYNOPSIS
void gluTessCallback(
GLUtesselator* tess,
GLenum which,
GLvoid (*CallBackFunc) );
PARAMETERS
Specifies the tessellation object (created with gluNewTess()). Speci‐
fies 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_COM‐
BINE_DATA, GLU_TESS_ERROR, and GLU_TESS_ERROR_DATA. Specifies the
function to be called.
DESCRIPTIONgluTessCallback() is used to indicate a callback to be used by a tes‐
sellation 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: 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 ); 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 ); The edge option callback is similar to
glEdgeFlag(). The function takes a single boolean option that indicates
which edges lie on the polygon boundary. If the option is GL_TRUE, then
each vertex that follows begins an edge that lies on the polygon bound‐
ary, that is, an edge that separates an interior region from an exte‐
rior one. If the option is GL_FALSE, then each vertex that follows
begins an edge that lies in the polygon interior. The edge option call‐
back (if defined) is invoked before the first vertex callback.
Since triangle fans and triangle strips do not support edge
options, the begin callback is not called with GL_TRIANGLE_FAN
or GL_TRIANGLE_STRIP if a non-NULL edge option callback is pro‐
vided. (If the callback is initialized to NULL, there is no
impact on performance). Instead, the fans and strips are con‐
verted to independent triangles. The function prototype for this
callback is: void edgeFlag(
GLboolean option ); 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 option,
void *polygon_data ); The vertex callback is invoked
between the begin and end callbacks. It is similar to glVer‐
tex(), 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 ); 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_dat,
void *polygon_data ); 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 ); 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 gluTessBeginPoly‐
gon() was called. The function prototype for this callback is:
void endData(
void *polygon_data ); The combine callback is called to
create a new vertex when the tessellation detects an intersec‐
tion, or wishes to merge features. The function takes four argu‐
ments: 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. 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 ); 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_MISS‐
ING_END_POLYGON, GLU_TESS_MISSING_BEGIN_CONTOUR, GLU_TESS_MISS‐
ING_END_CONTOUR, GLU_TESS_COORD_TOO_LARGE, GLU_TESS_NEED_COM‐
BINE_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 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
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 indi‐
cates that there is not enough memory so no output is generated.
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.
SEE ALSOglBegin(3), glEdgeFlag(3), glVertex(3), gluNewTess(3), gluEr‐
rorString(3), gluTessVertex(3), gluTessBeginPolygon(3), gluTessBegin‐
Contour(3), gluTessProperty(3), gluTessNormal(3)gluTessCallback()
