Re: [Mesa3d-dev] bug in sphere normal

[Brian P. <br...@va...>]

Stephen Tse wrote:
> 
> Brian Paul <br...@va...> writes:
> 
> >
> > How do they compare when you enable smooth shading for the sphere?
> >
> 
> Oddly, they look the same when smooth shaded. Why?? Then, it's not the
> problem of the normal?

It's not odd at all.  The color at each vertex is being computed
correctly, and identically by both OpenGL implementations.

It's just that when you're flat-shading that a different vertex
is being chosen to color each quadrilateral.  This is probably due
to the quad strips being drawn in a different order in the two
gluSphere() implementations.


> > It may be that because of the vertex order for the triangle strips
> > being different, that a different provoking vertex is being used
> > for filling each facet.
> 
> Can that be fixed?

You could modify gluSphere() to compute/draw the vertices in reverse
order (or some other order).  I haven't compared Mesa's gluSphere()
to the SI's version so I can't say exactly.


> > The GLU spec doesn't say exactly how the triangle (or quad) strips
> > are to be oriented in gluSphere so this might be an allowable
> > variation.
> 
> Variation? But this is a huge visible discrepancy, though. I draw the
> box in the light source position. It seems that the specular
> reflection is more "accurate" in sgi glu. I can't say for sure though.

It's an unfortunate variation.  But as I said, I don't think the GLU
spec goes into that much detail.


> > Thanks.  My plan is to replace Mesa's GLU with the SI GLU someday
> > (when I find time).
> 
> You mean SGI open-source reference implementation? Their code style is
> horrible because of over-optimization. Honestly, I like Mesa code much
> better. It is more readable.

But Mesa's GLU tessellator and NURBS code isn't nearly as good as the
SI code.

-Brian