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gluTessCallback()
NAME
gluTessCallback - define a callback for a tessellation object
SYNOPSIS
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
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 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 boundary, that is, an edge that
separates an interior region from an exterior one. If the option is
GL_FALSE, then each vertex that follows begins an edge that lies in the
polygon interior. The edge option callback (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 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 option );
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 option,
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_dat,
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.
SEE ALSO
glBegin(3), glEdgeFlag(3), glVertex(3), gluNewTess(3), gluErrorString(3),
gluTessVertex(3), gluTessBeginPolygon(3), gluTessBeginContour(3),
gluTessProperty(3), gluTessNormal(3)
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