Exactly as forceflow said, parallax mapping is essential a special form
of bump/normal mapping that gives surfaces added depth. I don't know
exact details so if people know better please do correct me, but I'll
attempt to give some sort of explanation.
Bump mapping gives each texel a depth value. A texel is essentially a
pixel on the texture - therefore if you put a 256x256 texture onto a
256x256 surface at a 1x1 scale, there are 256x256 texels on the
surface, whereas at 2x2 scale there are 128x128 texels. Normal mapping
(a further development of bump mapping) gives each texel not only a
depth value, but a normal vector. This normal vector stores the
direction that each texel is facing, which allows for the surface to
react to different light directions as if it featured far more
geometric detail than it actually does. Parallax mapping, as far as I
am aware, lets the surface not only react to the way each texel is
facing, but react to the other texels as well. For example, imagine a
texture has a single block that protrudes outwards from the rest of the
surface the texture represents. This texture could be applied to a
single face on a brush, and even with normal mapping if viewed from the
side it would appear to be flat (even if the texels recieved realistic
lighting due to how they are portrayed to "face" with normal mapping).
Parallax mapping lets each texel be rendered based on its depth and the
direction it faces. This bump in the texture would now block out texels
that would realistically be behind it from the current viewpoint of the
camera, as well as being lit realistically. This lets bumps be rendered
over texels that would actually be hidden behind them, giving surfaces
actual depth even if they are made of very few polygons.
I'm mildly drunk and not 100% on the theory behind the technique, so
this may not make much sense the way I have explained it. I guess the
best idea would be to read up on the link Edge provided - I'm sure it
covers things both better and more accurately than I have!
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