3D graphics eBook - Course Materials Repository

Reflection mapping 157
1) All radiance incident upon the object being shaded comes from an infinite distance. When this is not the case the
reflection of nearby geometry appears in the wrong place on the reflected object. When this is the case, no parallax is
seen in the reflection.
2) The object being shaded is convex, such that it contains no self-interreflections. When this is not the case the
object does not appear in the reflection; only the environment does.
Reflection mapping is also a traditional Image-based lighting technique for creating reflections of real-world
backgrounds on synthetic objects.
Environment mapping is generally the fastest method of rendering a reflective surface. To further increase the speed
of rendering, the renderer may calculate the position of the reflected ray at each vertex. Then, the position is
interpolated across polygons to which the vertex is attached. This eliminates the need for recalculating every pixel's
reflection direction.
If normal mapping is used, each polygon has many face normals (the direction a given point on a polygon is facing),
which can be used in tandem with an environment map to produce a more realistic reflection. In this case, the angle
of reflection at a given point on a polygon will take the normal map into consideration. This technique is used to
make an otherwise flat surface appear textured, for example corrugated metal, or brushed aluminium.
Types of reflection mapping
Sphere mapping
Sphere mapping represents the sphere of incident illumination as though it were seen in the reflection of a reflective
sphere through an orthographic camera. The texture image can be created by approximating this ideal setup, or using
a fisheye lens or via prerendering a scene with a spherical mapping.
The spherical mapping suffers from limitations that detract from the realism of resulting renderings. Because
spherical maps are stored as azimuthal projections of the environments they represent, an abrupt point of singularity
(a “black hole” effect) is visible in the reflection on the object where texel colors at or near the edge of the map are
distorted due to inadequate resolution to represent the points accurately. The spherical mapping also wastes pixels
that are in the square but not in the sphere.
The artifacts of the spherical mapping are so severe that it is effective only for viewpoints near that of the virtual
orthographic camera.