>> Yes, you interpolate the surface normal between the corners to fake a 
>> curved surface. A bit like normal maps allow you to fake ripples, 
>> cracks, bumps and so forth.
> 
> Yes, nowadays the *normal* is calculated by interpolating the normal 
> from each corner of the triangle, then lighting is worked out per 
> pixel.  This means a small specular highlight will show up perfectly on 
> a huge single triangle, in the old days this was impossible to achieve 
> just by interpolating colours from the corners.  The result is that 
> lighting (and in particular specular highlights) look much more 
> realistic without the need for huge numbers of triangles, and as 
> Invisible said it allows you to modify the normal per-pixel to achieve 
> bumps and rough surfaces etc.

Normal trickery is very good for creating surface roughness. The 
problems start when people try to use it to fake large structures such 
as waves. (Yeah, now go look obliquely across the surface. Look wavey? 
No, I thought not.)

The other problem is that usually the bumpmap (and texture map, BTW) is 
far too low-res. (Certainly in a game setting anyway.)

Looking at HL1, and then HL2, the increase in texture resultion seems 
substantial. I wonder when textures will reach the point that you can 
*read* the papers laying on the person's desk? ;-)

>> - If you smooth out all the surface normals, you now can't have sharp 
>> edges where they're supposed to be sharp.
> 
> Well don't smooth out all the normals then, select some to leave sharp!

I'd be surprised if you can actually do that.

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