OpenGL man pages
gluTessCallback - define a callback for a tessellation object
void gluTessCallback( GLUtesselator *tess, GLenum which, void (*fn)() )
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. fn Specifies the function to be called.
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 fn 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
"polygon_data" 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 looks
like:
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 looks like:
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 which lies on the polygon boundary -- that
is, an edge which separates an interior region from an exterior
one. If the flag is GL_FALSE, then each vertex that follows
begins an edge which lies in the polygon interior. The edge flag
callback (if defined) is invoked before the first vertex callback
is made.
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 an edge flag callback is provided. Instead,
the fans and strips are converted to independent triangles. The
function prototype for this callback looks like:
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 looks like:
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 looks like:
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 looks like:
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 looks like:
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 looks like:
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 looks like:
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 4 existing
vertices, stored in vertex_data. The coefficients of the linear
combination are given by weight; these weights always sum to 1.0.
All vertex pointers are valid even when some of the weights are
zero. 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 sometime after
gluTessEndPolygon is called.
For example, if the polygon lies in an arbitrary plane in 3-space,
and we associate a color 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. (This is the only error that
can occur during tessellation and rendering.)
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 looks like:
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.
Character strings describing these errors can be retrieved with
the gluErrorString call. The function prototype for this callback
looks like:
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 says
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 says 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 will be 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 looks like:
void errorData ( GLenum errno, void *polygon_data );
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.
glBegin, glEdgeFlag, glVertex, gluNewTess, gluErrorString, gluTessVertex, gluTessBeginPolygon, gluTessBeginContour, gluTessProperty, gluTessNormal
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Last Edited:
Fri Dec 6 11:18:03 EST 1996
by
AFV
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