Just as a texture map can vary
a parameter such as the diffuse color over every point on a
surface, a **bump map** can
be associated with a material,
perturbing the normal at every point on a surface which uses the
material, simply by applying an appropriate shader. This will
affect the shading, though not the geometry, giving the illusion of
a pattern being embossed on the surface.

Bump maps, like texture maps,
require a texture space. In addition, bump maps require a pair of
*basis vectors* to define the coordinate system in which the
normal is displaced. A bump map
defines a scalar *x* and a scalar *y* displacement over
the texture space. These components are used together with the
respective basis vectors in order to calculate a perturbed surface
normal. The basis vectors can be automatically defined for
free-form surfaces in a way which conforms
to the texture space; for polygons and
subdivision surfaces, the basis vectors
must be explicitly given along with the texture
coordinates for every polygon
vertex. However, most commonly basis vectors are computed
procedurally by shaders during rendering.

**displacement map** is a
**scalar map** which is used to displace a
free-form surface,
polygon, or
subdivision surface at each point in
the direction of the local normal. Like texture, bump and
**reflection map**s,
displacement maps are controlled by
shaders that are used by mental ray to determine the displacement
of points on the surface, in order to decide how to deform the
surface and introduce geometric detail.

The surface must be triangulated finely enough to reveal the details of the displacement map. In general, the triangles must be smaller than the smallest feature of the displacement map which is to be resolved.

Displacement mapped polygons are at first triangulated as ordinary polygons. The initial triangulation is then further subdivided according to the specified approximation criteria. The parametric technique subdivides each triangle a given number of times. All the other techniques take the displacement into account. The length criterion, for example, limits the size of the edges of the triangles of the displaced polygons and ensures that at least all features of this size are resolved. As the displaced surface is not known analytically, the distance criterion compares the displacements of the vertices of a triangle with each other. The criterion is fulfilled only if they differ by less than the given threshold. Subdivision is finest in areas where the displacement changes. The angle criterion limits the angle under which two triangles meet in an edge contained in the triangulation. Subdivision stops as soon as the given criterion or combination of them is satisfied or the maximum subdivision level is reached. This does not preclude the possibility that at an even finer scale new details may show up which would again violate the approximation criteria.

mental ray supports rendering subdivision surfaces natively, and handles displacement mapping on subdivision surface as part of the tessellation to the limit surface. If detailed displacement on a smooth polygon mesh is the primary goal then the ccmesh primitive should be used for its superior performance.

For displacement mapped free-form surfaces approximation techniques can be specified either on the underlying geometric surface or for the surface resulting from the displacement. If one wants to control the approximation from the geometric surface, the most suitable technique for use with displacement mapping on free-from surfaces is usually the view dependent uniform spatial subdivision technique, which allows specification of triangle size in raster space.

In addition to or even instead of specifying the subdivision criteria for the base surface, they can be given for the displaced surface itself. This approximation statement works exactly the same way as for polygons, i.e. an initial tessellation is subdivided until the criteria on the displaced surface are met.

The final type of map is an **environment map**.
This is a color-mapped virtual
sphere of usually infinite radius which surrounds any object
referencing the given material.
This sphere is also seen by refracted rays; the environment seen by
first-generation (primary) rays can also be specified but is part
of the `camera`, not of any particular material. In general,
if a ray does not intersect any objects, or if casting such a ray
would exceed the trace depth,
the ray is considered to strike the sphere of the
environment map of the last material
visited, or the `camera` environment map in the case of
first-generation rays that did not hit any material. An
environment shader is used to determine
the color of each point of the sphere.

The environment map always covers the entire sphere exactly once. Rotations, translations, and repetitions are supported by special shader parameters or remapping shader nodes such as the ones found in the base shader library. There are also environment shaders that implement cubical instead of spherical environment maps, and cubical maps of finite instead of infinite size.

As described before, textures can be used for any purpose whatever to control shader behavior, in addition to the examples in this section.

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