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Bump mapping 26
Bump mapping
Bump mapping is a technique in
computer graphics for simulating
bumps and wrinkles on the surface of
an object. This is achieved by
perturbing the surface normals of the
object and using the perturbed normal
during lighting calculations. The result
is an apparently bumpy surface rather
than a smooth surface although the
surface of the underlying object is not
actually changed. Bump mapping was
introduced by Blinn in 1978. [1]
Normal mapping is the most common variation of bump mapping used [2] .
Bump mapping basics
Bump mapping is a technique in
computer graphics to make a rendered
surface look more realistic by
simulating small displacements of the
surface. However, unlike traditional
displacement mapping, the surface
geometry is not modified. Instead only
the surface normal is modified as if the
surface had been displaced. The
modified surface normal is then used
for lighting calculations as usual,
typically using the Phong reflection
model or similar, giving the
appearance of detail instead of a
smooth surface.
Bump mapping is much faster and
consumes less resources for the same
level of detail compared to
displacement mapping because the geometry remains unchanged.
A sphere without bump mapping (left). A bump map to be applied to the sphere (middle).
The sphere with the bump map applied (right) appears to have a mottled surface
resembling an orange. Bump maps achieve this effect by changing how an illuminated
surface reacts to light without actually modifying the size or shape of the surface
Bump mapping is limited in that it does not actually modify the shape of the underlying
object. On the left, a mathematical function defining a bump map simulates a crumbling
surface on a sphere, but the object's outline and shadow remain those of a perfect sphere.
On the right, the same function is used to modify the surface of a sphere by generating an
isosurface. This actually models a sphere with a bumpy surface with the result that both
its outline and its shadow are rendered realistically.
There are primarily two methods to perform bump mapping. The first uses a height map for simulating the surface
displacement yielding the modified normal. This is the method invented by Blinn [1] and is usually what is referred to
as bump mapping unless specified. The steps of this method is summarized as follows.
Before lighting a calculation is performed for each visible point (or pixel) on the object's surface:
1. Look up the height in the heightmap that corresponds to the position on the surface.
2. Calculate the surface normal of the heightmap, typically using the finite difference method.
3. Combine the surface normal from step two with the true ("geometric") surface normal so that the combined
normal points in a new direction.