SGI GLU Sample Implementation fixes

OpenGL OpenGL: Advanced Coding
1

I hope this is the appropriate forum for this. I have been working
with and debugging the NURBS portion of SGI’s sample implementation
of GLU and propose the following changes/fixes.

  • The following is a bug fix. The original code only works if the
    stride is 2, which is not the case for homogenous coordinates.
    libnurbs/internals/arctess.cc, Line 338:
    for(i=1, j=2; i<bezierArc->order; i++, j+= bezierArc->stride)
    Change to:
    for(i=1, j=bezierArc->stride; i<bezierArc->order; i++, j+= bezierArc->stride)

  • The function monoTriangulationFun calls
    monoTriangulationRecFun with zeroes for both of the current
    indices to the inc_chain and dec_chain vertex arrays. If both of
    these containers are empty (which can happen–did for me with one
    surface), monoTriangulationRecFun will assert. I don’t know
    if this indicates a flaw in the data, but to avoid the assertion, I
    added the following:
    libnurbs/nurbtess/monoTriangulation.cc, Insert at line 622
    (before call to monoTriangulationRecFun):
    if (!(0 == inc_chain.getNumElements() && 0 == dec_chain.getNumElements()))

  • Initialization of the member variable output_triangles of
    the class [/i]OpenGLCurveEvaluator[/i] was overlooked. NURBS curves
    won’t render in a debug build because of this. Add the following
    initialization in the constructor:
    libnurbs/interface/glcurveval.cc, Insert at line 77:
    output_triangles = 0; //don’t output triangles by default

  • The following change eliminates a compiler warning (using MS VC6).
    Add the following delete operator to class PooledObj:
    libnurbs/internals/bufpool.h, Insert at line 131:
    inline void operator delete( void *, Pool & ) { assert( 0 ); }

Another problem this library has, which I have not yet debugged (and
may not get to), is that it does not render trimmed surfaces
correctly when using GLU_PARAMETRIC_ERROR for the sampling method.
The edges of the trim lines are rendered with far too few segments
(at least with NURBS trim curves–not sure about piecewise trim
curves). If anyone has a fix for this, please post it.

I hope this helps helps other people working with GLU NURBS and that
SGI sees this and incorporates these changes into its sample
implementation.

(I will be posting a copy of this to the comp.graphics.api.opengl
newsgroup also.)

John Shell