Incorrect clipping planes on onboard card

[Snapshot]

I'm drawing a sphere with two clipping planes. On nvidia graphic cards everything works fine, the sphere is drawn and also clipped correctly.
But on a different system using an Intel onboard graphic adapter the whole clipping goes terribly wrong. But only if both clipping planes are enabled. Using only one plane, regardless which one of them, the sphere is drawn correctly.
I've already checked GL_MAX_CLIP_PLANES but the guaranteed six planes are supported.
Any idea what's the problem here? A driver bug?

[Jackis]

Originally posted by Snapshot:
I've already checked GL_MAX_CLIP_PLANES but the guaranteed six planes are supported.
Any idea what's the problem here? A driver bug?

By the way, guaranteed six clipping planes are already enabled - they are your frustum near, far, left, right, up and down planes.
So the implementation is free to decide, how many extra planes to support.
It seems, like your onboard card can have only 1 extra-clipping plane.

[Snapshot]

Are you sure about that? The Redbook says:

All implementations of OpenGL must support at least six additional clipping planes...

I think the keyword here is "additional". Doesn't that mean I have at least six clipping planes beside those used for the bounding rect?

I'v checked again the returned number of max clip planes. In both cases it's six.
So both should fail or both should work, shouldn't they?

[Komat]

Originally posted by Snapshot:
Doesn't that mean I have at least six clipping planes beside those used for the bounding rect?

Yes, you have at least six user defined planes available. The frustum clipping is independent and does not count aginst that limit. If the Intel driver can not handle two user cliping planes, it contains a bug.

[Snapshot]

Okay, thank you!

[Jackis]

Eeeeh, thank you Komat, I didn't know that about additional.