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 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.
SEE ALSO
glBegin, glEdgeFlag, glVertex, gluNewTess, gluErrorString,
gluTessVertex, gluTessBeginPolygon, gluTessBeginContour,
gluTessProperty, gluTessNormal