gluTessCallback man page on RedHat
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GLUTESSCALLBACK(3G) OpenGL Manual GLUTESSCALLBACK(3G)
NAME
gluTessCallback - define a callback for a tessellation object
C SPECIFICATION
void gluTessCallback(GLUtesselator* tess, GLenum which,
_GLUfuncptr 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
GLU_FALSE, then the argument is set to either GLU_TRIANGLE_FAN,
GLU_TRIANGLE_STRIP, or GLU_TRIANGLES. If the GLU_TESS_BOUNDARY_ONLY
property is set to GLU_TRUE, then the argument will be set to
GLU_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 GLU_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 GLU_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 GLU_TRIANGLE_FAN or
GLU_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
gluErrorString(), gluNewTess(), gluTessBeginContour(),
gluTessBeginPolygon(), gluTessNormal(), gluTessProperty(),
gluTessVertex(), glBegin(), glEdgeFlag(), glVertex()
COPYRIGHT
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed
under the SGI Free Software B License. For details, see
http://oss.sgi.com/projects/FreeB/.
AUTHORS
opengl.org
opengl.org 03/16/2013 GLUTESSCALLBACK(3G)
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