[Mesa3d-dev] bug in sphere normal

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

Hi Brian and all!


There seems to be a bug in calculating normals of a sphere in
src-glu/quadric.c. I cannot track down the exact problem or find a
fix, but the following screenshots (one from mesa 3.4, one from sgi
opengl of xfree86 4.0) obviously show something. Note the highlights
of the spheres.

http://www.sfu.ca/~stephent/tmp/mesa-sphere.gif
http://www.sfu.ca/~stephent/tmp/sgi-sphere.gif


The code is accanti.c from redbook. I can easily reproduce the bug
with another simplified code. Normals of spheres at origin (no
translation or rotation, quadric.c from redbook) *seem* to be okay.

http://trant.sgi.com/opengl/examples/redbook/redbook.html


Also, a (very) minor patch is to remove `TXTR_COORD' calls because of
`!qobj->TextureFlag' condition in the branches.


#define TXTR_COORD(x,y)    if (qobj->TextureFlag) glTexCoord2f(x,y);


Around line 404 in src-glu/quadric.c,

      if (!qobj->TextureFlag) {
	 /* draw +Z end as a triangle fan */
	 glBegin(GL_TRIANGLE_FAN);
	 glNormal3f(0.0, 0.0, 1.0);
	 TXTR_COORD(0.5, 1.0);


Around line 461 in src-glu/quadric.c,

      if (!qobj->TextureFlag) {
	 /* draw -Z end as a triangle fan */
	 glBegin(GL_TRIANGLE_FAN);
	 glNormal3f(0.0, 0.0, -1.0);
	 TXTR_COORD(0.5, 0.0);


Cheers,
Stephen