GLUTESSCALLBACK() MachTen Programmer’s Manual GLUTESSCALLBACK()

NAME
gluTessCallback - define a callback for a tessellation
object

C SPECIFICATION
void gluTessCallback( GLUtesselator* tess,
GLenum which,
GLvoid (*CallBackFunc)( )

delim $$

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 proto-
type 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 bound-
ary. 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 sepa-
rates 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 trian-
gles. 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 argu-
ment. 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 *poly-
gon_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 func-
tion takes a pointer as its only argument. This
pointer is identical to the opaque pointer pro-
vided 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 *poly-
gon_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 inter-
section, or wishes to merge features. The func-
tion 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 proto-
type is:
void combine( GLdouble coords[3], void *ver-
tex_data[4],
GLfloat weight[4], void **outData
);

The vertex is defined as a linear combination of
up to four existing vertices, stored in ver-
tex_data. The coefficients of the linear combi-
nation 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, interpo-
late parameters using vertex_data and weight,
and return the new vertex pointer in outData.
This handle is supplied during rendering call-
backs. 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 gluEr-
rorString 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 con-
stant 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 call-
back 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 *poly-
gon_data );

EXAMPLE
Polygons tessellated can be rendered directly like this:

gluTessCallback(tobj, GLU_TESS_BEGIN, glBegin); gluTess-
Callback(tobj, GLU_TESS_VERTEX, glVertex3dv); gluTessCall-
back(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

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