I wrote:
>But aren't the smooth height field and the bicubic patch essentially the
>same?  i.e. they are both internally subdivided into meshes of smooth
>triangles.  This means that, in official POV at least, they both use double
>illumination, which is why you get the translucent effect.

to which Peter Popov <pet### [at] usanet> replied:
> I know, it's just that I don't recall anyone reporting double
> illumination troubles with a height field. Internally the only
> difference between a hf and a bicubic patch is the bounding scheme of
> the hf, but afaik it's not related to illumination at all.

A very quick test (solid colour height map, height field translated to fill
camera view, zero ambient, light source behind height field) reveals that
smooth height fields are indeed doubly illuminated.  I guess in the most
common usages of height fields this isn't ever an issue, as the camera only
sees the "top" surface of the field, lit as expected.

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