GLUTESSCALLBACK(3gl)GLUTESSCALLBACK(3gl)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 gluNewT‐
ess).
which Specifies the callback being defined. The following val‐
ues 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.
CallBackFunc Specifies the function to be called.
DESCRIPTION
gluTessCallback is used to indicate a callback to be used by a tessel‐
lation 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
takes a single boolean flag that indicates which edges lie on
the polygon boundary. If the flag is GL_TRUE, then each ver‐
tex 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 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 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
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 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
features. The function takes four arguments: an array of
three elements each of type GLdouble, an array of four point‐
ers, 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 identi‐
cal 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_MISS‐
ING_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 over‐
flow.) 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_COM‐
BINE_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, gluTessVer‐
tex, gluTessBeginPolygon, gluTessBeginContour, gluTessProperty,
gluTessNormal
15 Mar 97 GLUTESSCALLBACK(3gl)
