JONATHAN DINERSTEIN wrote:
>
> As an example of the kind of rendering errors I want to get rid of,
> check out:
> http://www.npl.com/~jon/
>
> It is a jpeg so some amount of loss has happened, but you can still easily see
> the problem. All I did was have GLUT draw a sphere of about 20 slices by 10
> stacks. There is no specular or ambient, and I used the default settings for
> LIGHT0.
>
> As you can see, except where the lighting is really bright, the seam between
> polygons is brighter than the interior of polygons.
This is called "Mach banding" and is a side effect of the way our eyes
perceive intensity changes.
> So, even when doing no
> specular, there's still some rather poor rendering. I never got this kind of
> error in my own Gouraud engine, so my guess is that this is caused primarily by
> using fixed point math. Am I wrong here???
The use of fixed point math in itself shouldn't be a problem. But
one thing that occurs to me is this: after Mesa computes vertex
colors from the lighting parameters, the color values are stored
as GLubytes. Perhaps we should store fixed-point values instead
so that the fractional bits aren't lost before rasterization.
I'm not sure this will really help with the Mach banding though.
> If it helps at all, I uploaded my source file (VERY brief). Also, I got
> virtually identical results between OpenGL under Windows and Mesa on my Linux
> box.
Thanks. I don't think I'll have any time to investigate this
for a while though.
-Brian
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