Re: [Mesa3d-dev] bug in sphere normal

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

Stephen Tse wrote:
> 
> Brian Paul <br...@va...> writes:
> 
> > 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.
> 
> ... I see...
> 
> > > 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.
> 
> You're totoally right. I simply modify it to draw slices in reverse
> order and get the identical result as sgi glu produces. Great!
> 
> > It's an unfortunate variation.  But as I said, I don't think the GLU
> > spec goes into that much detail.
> 
> I beg to differ. The spec does not say anything but it does not mean
> the discrepancy is allowable in reality. I setup and rotate/move the
> light position around and find sgi's more "accurate". But, again, I am
> not an expert enough to say anything definite.

Well, when I first wrote gluSphere() I didn't have the SI code to
compare to.  And, I don't recall comparing SGI's gluSphere to Mesa's
gluSphere at that level of attention.

If you rotate/transform the sphere in the right way, you may find
that Mesa's gluSphere looks "more accurate".


> BTW, does the conformance test (which covers GL part for sure) cover
> GLU and GLUT parts as well?

GLU, but not in too much detail.  GLUT has its own set of self-test
programs.


> > But Mesa's GLU tessellator and NURBS code isn't nearly as good as the
> > SI code.
> 
> Can you be more specific? As a matter of fact, I am studying closely
> the codes of mesa's glu for personal interests. SGI's codes are not
> meant for easy human reading.

Mesa's polygon tessellator doesn't have the GLU 1.2 features.
Gareth Hughes worked on this for a while but didn't have time to
complete the work.  I'm sure he'd agree that it's a big job.
Also Mesa's GLU NURBS tessellator doesn't handle trimming or a
few other features.

-Brian