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

[Rik]

Update of bug #38779 (project octave):

                  Status:               Postponed => Confirmed              
                 Release:                   3.6.2 => dev                    

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Follow-up Comment #2:

I've tried to do some more debugging on this, but have not had much luck. 
I've added Michael Goffioul, who wrote the original GL renderer code, to the
CC list in case he has an idea about what is happening.

The vertices are correctly added by Octave to a patch_tesselator object in the
draw_patch routine in the file gl-render.cc.  I checked by adding some printf
statements.  I also checked the patch tesselator object by putting a printf
routine in the vertex() callback routine.  Here there seems to be a problem
because I only see 3 vertices printed.  If I change the initial x-coordinate
in the patch pts to 10.5 then I definitely get 4 vertices in the callback
routine.  So, it would appear that OpenGL itself is taking out one of the
vertices.  However, this might be because we haven't set the right options to
OpenGL and it is filtering polygons.  I don't really know.

Separately, it's a different part of the draw_patch routine which puts the
markers on the vertices, but that code at least recognizes the fourth vertex
as shown by the code below.


pts = [    10.6946   -4.0000    3.0253;
   10.6946   -3.7071    3.0253;
   13.1352   -3.7071    2.9571;
   13.1352   -4.0000    2.5862 ]
facet = [ 1, 2, 3, 4 ];
patch('Faces',facet,'Vertices',pts, 'marker', 'x', 'markeredgecolor', 'r');


The patch face (polygon) is still a triangle connecting only 3 vertices, but
the 4 vertices are marked with red 'x'.

So, I'm stumped at this point.  


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