gluTessCallback -- define a callback for a tessellation object
void gluTessCallback(GLUtriangulatorObj *tobj,
void (*fn)() )
Specifies the tessellation object (created with
Specifies the callback being defined. The following values are valid:
GLU_BEGIN, GLU_EDGE_FLAG, GLU_VERTEX, GLU_END,
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 is erased.
These callbacks are used by the tessellation object to describe how a polygon
specified by the user is broken into triangles.
The legal callbacks are as follows:
The begin callback is invoked like
glBegin to indicate the start of a
(triangle) primitive. The function takes a single argument of type GLenum
that is either GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, or
The edge flag callback is similar to
glEdgeFlag. The function takes a
single Boolean flag that indicates which edges of the created triangles
were part of the original polygon defined by the user, and which were
created by the tessellation process. If the flag is GL_TRUE, then
each vertex that follows begins an edge that was part of the original
polygon. If the flag is GL_FALSE, then each vertex that follows
begins an edge that was generated by the tessellator. The edge flag
callback (if defined) is invoked before the first vertex callback is
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 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
The end callback serves the same purpose as
glEnd. It indicates the end of a
primitive and it takes no arguments.
The error callback is called when an error is encountered. The one argument
is of type GLenum, and it indicates the specific error that occurred.
There are eight errors unique to polygon tessellation, named
GLU_TESS_ERROR1 through GLU_TESS_ERROR8. Character strings
describing these errors can be retrieved with the
Polygons tessellated can be rendered directly like this:
gluTessCallback(tobj, GLU_BEGIN, glBegin);
gluTessCallback(tobj, GLU_VERTEX, glVertex3dv);
gluTessCallback(tobj, GLU_END, glEnd);
gluTessVertex(tobj, v, v);
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.
back to the OpenGL index page
© 1995 Uwe Behrens All rights reserved