3D graphics eBook - Course Materials Repository

Reflection 156
Glossy Reflection
Fully glossy reflection, shows
highlights from light sources, but does
not show a clear reflection from
objects.
Reflection mapping
In computer graphics, environment mapping, or reflection mapping,
is an efficient Image-based lighting technique for approximating the
appearance of a reflective surface by means of a precomputed texture
image. The texture is used to store the image of the distant
environment surrounding the rendered object.
Several ways of storing the surrounding environment are employed.
The first technique was sphere mapping, in which a single texture
contains the image of the surroundings as reflected on a mirror ball. It
has been almost entirely surpassed by cube mapping, in which the
environment is projected onto the six faces of a cube and stored as six
square textures or unfolded into six square regions of a single texture.
The sphere on the left has normal, metallic reflection. The sphere on the right has the
same parameters, except that the reflection is marked as "glossy".
An example of reflection mapping.
Other projections that have some superior mathematical or computational properties include the paraboloid
mapping, the pyramid mapping, the octahedron mapping, and the HEALPix mapping.
The reflection mapping approach is more efficient than the classical ray tracing approach of computing the exact
reflection by tracing a ray and following its optical path. The reflection color used in the shading computation at a
pixel is determined by calculating the reflection vector at the point on the object and mapping it to the texel in the
environment map. This technique often produces results that are superficially similar to those generated by
raytracing, but is less computationally expensive since the radiance value of the reflection comes from calculating
the angles of incidence and reflection, followed by a texture lookup, rather than followed by tracing a ray against the
scene geometry and computing the radiance of the ray, simplifying the GPU workload.
However in most circumstances a mapped reflection is only an approximation of the real reflection. Environment
mapping relies on two assumptions that are seldom satisfied: