[Octave-bug-tracker] [bug #38779] patch failure with quadrilateral face

[Michael Goffioul]

Follow-up Comment #6, bug #38779 (project octave):

I've done some more debugging, which gave a few additional clues. I don't have
more time to allocate to this atm.

As part of the tessellation process, as described in [1], the polygon is
projected onto a plane, where the triangulation is done. In this case, it
determines that the projection plane is perpendicular to Y axis. So the GLU
algorithm considers that the first 2 vertices of the polygon create a
degenerated edge and merge them.

What I've noticed is that the problem starts to appear when the Y coordinate
gradient becomes small compared to the other axis. So a possible way to
mitigate the problem can be to prescale/normalize the coordinates before
sending them to OpenGL (which would also solve the problem of limited
precision of floating point numbers). Though I'm not sure this would be a
perfect solution, and you could still find edge cases where this problem
occurs while tessellating non-planar polygons.

[1]
http://stackoverflow.com/questions/12852029/what-is-the-algorithm-behind-the-glutess-functions

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