Hello,
This has been an issue in Blender for many versions now, and I'm finally annoyed enough with it to submit a report. ^_^
Essentially, any smoothed, sufficiently-complex mesh model will, randomly, display certain faces without specularity and others with. It causes a flicker while moving around a scene, and does not seem to have any connection to face normals or materials, etc.
Attached is an image. This is not a huge bug, but like I said it's a bit annoying, and has happened on all even remotely organic objects I've modeled over the years.
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Here you go. This is the latest version of the file I was working on from which I took that screen shot.
Now, things that might make a difference which I forgot to mention: I'm on a Powerbook G4 (NVidia GeForce2Go 5200 with 64 Meg of ram; not the best of cards), using Mac OS X 10.4.9 and an official build of Blender 2.43. But this has been happening -- again, not in any manner I can replicate effectively. Sometimes it doesn't happen for thirty minutes at at stretch. But it seems to happen most when rotating or panning around a scene, rotating or panning after exiting mesh edit mode, or when switching away from Blender, doing something else, then switching back.
Anyway, thanks for looking into it. It's not a mission critical bug but it does get to me from time to time.
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I've rotated the model around, zoomed in out, put it even on higher subdiv levels, but it keeps looking good here (OSX 10.3 ATI, 128 MB gfx mem).
So, you got an opengl driver problem. Macs use opengl for a lot of tasks, and unfortunately there is no good protection for you run out of memory. Try for example a smaller screen resolution, and you should definitely use Blender with the framebuffer in "thousands of colors" (16 bits) mode.
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Yeah, I know that the compositing system eats video ram like nothing. ^_^; I ran into serious bottlenecks in Blender before with that with higher res textures. But this model doesn't have any textures on her, just colors. And in the past when running out of video ram, the machine has generally gotten really slow at rendering the screen.
I tried in 16 bit color mode -- I get a higher framerate, but the flicker is still there. (Although, with the higher framerate it's less apparent, as it goes by much more quickly.) Normally I'd agree with you that this is a driver problem, but Maya, the C4D demo, Modo, and Cheetah 3D all run without this graphical peculiarity. The card in these powerbooks doesn't properly support some extensions (or so I've heard), so it could be that it's crapping out on trying to draw something Blender's asking for that a real card wouldn't have any trouble with. (Side note: Blender does *not* work in 8 bit color mode. ^_^)
Anyway, thanks for checking it out on your end. At least if anyone else has the problem they can see that it's been looked at.
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There is really nothing special in Blender's drawing of 3d models... we use plain simple opengl here. Your screenshot suggests the specular disappears sometimes. Try to check if the problem is also there using the default startup .B.blend (move away the one from your home dir).
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Well, I've basically done everything I can to try to change this issue without success. Thanks for listening, but since I'm only one machine (and I'm switching to PC for my main work) I guess you can close this bug report.
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This is a generic request to test your bug report and see if it is still an issue in 2.5alpha2 if so please let me know by making a comment in this report ie 'also in 2.5alpha2' and I will add it to the 2.5 bug list.