gluTessCallback
[New
- Windows 95, OEM Service Release 2]
The gluTessCallback function defines a callback
for a tessellation object.
void gluTessCallback(
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GLUtesselator * tess, |
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GLenum which, |
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void (* fn)( ) |
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Parameters
tess
The
tessellation object (created with gluNewTessCIEF84).
which
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.
For more
information on these callbacks, see the following Remarks section.
fn
The function
to be called.
Remarks
Use gluTessCallback to specify 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.
The tessellation object uses these callbacks to
describe how a polygon that you specify is broken into triangles.
There are two versions of each callback, one with
polygon data that you can define and one without. If both versions of a
particular callback are specified, the callback with the polygon data you
specify will be used. The polygon_data parameter of gluTessBeginPolygon is a copy of the pointer
that was specified when gluTessBeginPolygon was called.
The following are valid callbacks:
GLU_TESS_BEGIN
The
GLU_TESS_BEGIN callback is invoked like glBegin to indicate the start of a
(triangle) primitive. The function takes a single argument of type GLenum.
If you set the GLU_TESS_BOUNDARY_ONLY property to GL_FALSE, the argument is set
to either GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, or GL_TRIANGLES. If you
set the GLU_TESS_BOUNDARY_ONLY property to GL_TRUE, the argument is set
to GL_LINE_LOOP. The function prototype for this callback is as follows:
void begin
(GLenum type);
GLU_TESS_BEGIN_DATA
GLU_TESS_BEGIN_DATA
is 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 you call gluTessBeginPolygon. The function prototype for this callback is:
void
beginData (GLenum type,
void * polygon_data);
GLU_TESS_EDGE_FLAG
The
GLU_TESS_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 which 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 GLU_TESS_EDGE_FLAG callback (if
defined) is invoked before the first vertex callback is made.
Because
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 is:
void
edgeFlag (GLboolean flag);
GLU_TESS_EDGE_FLAG_DATA
The
GLU_TESS_EDGE_FLAG_DATA callback is 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 you call gluTessBeginPolygon. The function prototype
for this callback is:
void
edgeFlagData (GLboolean flag,
void * polygon_data);
GLU_TESS_VERTEX
The
GLU_TESS_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 that you provided when you defined the vertex
(see gluTessVertex).
The function prototype for this callback is:
void
vertex (void * vertex_data);
GLU_TESS_VERTEX_DATA
The
GLU_TESS_VERTEX_DATA is 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 you call gluTessBeginPolygon. The function prototype
for this callback is:
void
vertexData (void * vertex_data, void * polygon_data);
GLU_TESS_END
The
GLU_TESS_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
GLU_TESS_END_DATA callback is 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 you call gluTessBeginPolygon. The function prototype for
this callback is:
void
endData (void * polygon_data);
GLU_TESS_COMBINE
Call the
GLU_TESS_COMBINE callback to create a new vertex when the tessellation detects
an intersection, or 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.
A pointer to a pointer.
The function
prototype for this callback 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 sum to 1.0. All vertex pointers are valid even when some of the
weights are zero. The coords parameter gives the location of the new
vertex.
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. Free the memory sometime after calling gluTessEndPolygon.
For example,
if the polygon lies in an arbitrary plane in three-dimensional space, and you
associate a color with each vertex, the GLU_TESS_COMBINE callback might look
like the following:
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;
}
When the
tessellation detects an intersection, the GLU_TESS_COMBINE or
GLU_TESS_COMBINE_DATA callback (see below) must be defined, and 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
GLU_TESS_COMBINE_DATA callback is 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 you call gluTessBeginPolygon. 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
GLU_TESS_ERROR callback is called when an error is encountered. The one
argument is of type GLenum; it indicates the specific error that
occurred and is set to one of the following: 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, or GLU_TESS_NEED_COMBINE_CALLBACK.
Call gluErrorString to retrieve character
strings describing these errors. The function prototype for this callback is as
follows:
void error
(GLenum errno);
The GLU library
recovers from the first four errors by inserting the missing call or calls.
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 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 will be generated.
GLU_TESS_ERROR_DATA
The GLU_TESS_ERROR_DATA
callback is 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 you call gluTessBeginPolygon. The function prototype for this callback is:
void
errorData (GLenum errno, void * polygon_data);
Example
You can
directly render tessallated polygons as follows:
gluTessCallback(tess, GLU_TESS_BEGIN, glBegin);
gluTessCallback(tess, GLU_TESS_VERTEX,
glVertex3dv);
gluTessCallback(tess, GLU_TESS_END, glEnd);
gluTessBeginPolygon(tess, NULL);
gluTessBeginContour(tess);
gluTessVertex(tess, v, v);
.
.
gluTessEndContour(tess);
gluTessEndPolygon(tess);
Store the
tessellated polygon in a display list so that it does not need to be
tessellated every time it is rendered.
See Also