Re: [Mesa3d-dev] bug in sphere normal

[Stephen T. <ste...@sf...>]

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.

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


> 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.