[PyOpenGL-Users] GLU with multiple tesselators

[Maciej K. <ma...@dg...>]

[This has also been posted to comp.graphics.api.opengl, as I don't know whether
 this is an OpenGL issue, or pyopengl.]

Hello all.  I'm trying to do some 2D CSG stuff using OpenGL's tesselator and
the winding rules.  I've run into a problem with using multiple tesselators.
According to the redbook, I should be able to feed one tesselator's output
(using its callbacks) to the input of another one [midway on p421 of 2nd
ed... look for "GLU_TESS_BOUNDARY_ONLY"].  I tried to set up something along
those lines.  Here's roughly what occurs at runtime:

(NOTE: "outer" refers to actions taken on the "receiving" tesselator, while
"inner" refers to the "feeding" one)

% ./test.py
outer: gluTessBeginPolygon <GLUtesselator object at 0x816c150>
inner: gluTessBeginPolygon <GLUtesselator object at 0x8121898>
inner: gluTessBeginContour <GLUtesselator object at 0x8121898>
inner: gluTessEndContour <GLUtesselator object at 0x8121898>
inner: gluTessEndPolygon <GLUtesselator object at 0x8121898>
inner: cb_begin <GLUtesselator object at 0x816c150> 2
outer: gluTessEndPolygon <GLUtesselator object at 0x816c150>
Traceback (most recent call last):
  File "./test.py", line 75, in ?
    main()
  File "./test.py", line 70, in main
    gluTessEndPolygon(tobj)
OpenGL.GLU.GLUerror: [Errno 100154] gluTessEndContour() must follow a gluTessBeginContour()

Strangely, gluTessEndPolygon() of the "feeding/inner" tesselator only fires the
GLU_TESS_BEGIN callback, and that's it!  Bizarre.

Here's the Python code for the above.  All it's trying to do is tesselate a
circle using one tesselator, which then feeds the boundary (i.e., the same
circle, it should be) to a second tesselator.

Any ideas?  Did I miss something?  I tried pyopengl 2.0.0.44, as well as
2.0.1.02.


# --8<--

#!/usr/bin/env python
import math
from math import *

from OpenGL.GL import *
from OpenGL.GLU import *

def inner(tobj):
    tobj_inner = gluNewTess()

    def cb_begin(type):
        print 'inner: cb_begin', tobj, type
        gluTessBeginContour(tobj)
        print '(done) inner: cb_begin', tobj
    def cb_end():
        print 'inner: cb_end', tobj
        gluTessEndContour(tobj)
    def cb_vertex(v):
        print 'inner: cb_vertex', tobj, v
        gluTessVertex(tobj, v, v)

    gluTessCallback(tobj_inner, GLU_TESS_BEGIN, cb_begin)
    gluTessCallback(tobj_inner, GLU_TESS_END, cb_end)
    gluTessCallback(tobj_inner, GLU_TESS_VERTEX, cb_vertex)

    gluTessProperty(tobj_inner, GLU_TESS_WINDING_RULE,
                    GLU_TESS_WINDING_POSITIVE)
    gluTessProperty(tobj_inner, GLU_TESS_BOUNDARY_ONLY, GL_TRUE)

    print 'inner: gluTessBeginPolygon', tobj_inner
    gluTessBeginPolygon(tobj_inner, None)

    SLICES=32
    print 'inner: gluTessBeginContour', tobj_inner
    gluTessBeginContour(tobj_inner)
    for i in range(SLICES):
        ang = i/float(SLICES-1) * 2 * math.pi
        v = [cos(ang), sin(ang), 0]
        gluTessVertex(tobj_inner, v, v)

    print 'inner: gluTessEndContour', tobj_inner
    gluTessEndContour(tobj_inner)
    print 'inner: gluTessEndPolygon', tobj_inner
    gluTessEndPolygon(tobj_inner)

    gluDeleteTess(tobj_inner)


def main():
    tobj = gluNewTess()

    def cb_begin(type):
        print 'outer: cb_begin', tobj, type
        glBegin(type)
    def cb_end():
        print 'outer: cb_end', tobj
        glEnd()
    def cb_vertex(v):
        print 'outer: cb_vertex', tobj
        glVertex3dv(v)

    gluTessCallback(tobj, GLU_TESS_BEGIN, cb_begin)
    gluTessCallback(tobj, GLU_TESS_END, cb_end)
    gluTessCallback(tobj, GLU_TESS_VERTEX, cb_vertex)

    print 'outer: gluTessBeginPolygon', tobj
    gluTessBeginPolygon(tobj, None)
    inner(tobj)
    print 'outer: gluTessEndPolygon', tobj
    gluTessEndPolygon(tobj)

    gluDeleteTess(tobj)

main()


# --8<--

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"Television: a medium. So called because it is neither rare nor well done."
				-- Ernie Kovacs.