3D graphics eBook - Course Materials Repository

Cube mapping 35
Skyboxes
Perhaps the most trivial application of cube mapping is to create pre-rendered panoramic sky images which are then
rendered by the graphical engine as faces of a cube at practically infinite distance with the view point located in the
center of the cube. The perspective projection of the cube faces done by the graphics engine undoes the effects of
projecting the environment to create the cube map, so that the observer experiences an illusion of being surrounded
by the scene which was used to generate the skybox. This technique has found a widespread use in video games
since it allows designers to add complex (albeit not explorable) environments to a game at almost no performance
cost.
Skylight Illumination
Cube maps can be useful for modelling outdoor illumination accurately. Simply modelling sunlight as a single
infinite light oversimplifies outdoor illumination and results in unrealistic lighting. Although plenty of light does
come from the sun, the scattering of rays in the atmosphere causes the whole sky to act as a light source (often
referred to as skylight illumination).However, by using a cube map the diffuse contribution from skylight
illumination can be captured. Unlike environment maps where the reflection vector is used, this method accesses the
cube map based on the surface normal vector to provide a fast approximation of the diffuse illumination from the
skylight. The one downside to this method is that computing cube maps to properly represent a skylight is very
complex; one recent process is computing the spherical harmonic basis that best represents the low frequency diffuse
illumination from the cube map. however, a considerable amount of research has been done to effectively model
skylight illumination.
Dynamic Reflection
Basic environment mapping uses a static cube map - although the object can be moved and distorted, the reflected
environment stays consistent. However, a cube map texture can be consistently updated to represent a dynamically
changing environment (for example, trees swaying in the wind). A simple yet costly way to generate dynamic
reflections, involves building the cube maps at runtime for every frame. Although this is far less efficient than static
mapping because of additional rendering steps, it can still be performed at interactive rates.
Unfortunately, this technique does not scale well when multiple reflective objects are present. A unique dynamic
environment map is usually required for each reflective object. Also, further complications are added if reflective
objects can reflect each other - dynamic cube maps can be recursively generated approximating the effects normally
generated using raytracing.
Global Illumination
An algorithm for global illumination computation at interactive rates using a cube-map data structure, was presented
at ICCVG 2002.[5]
Projection textures
Another application which found widespread use in video games, projective texture mapping relies on cube maps to
project images of an environment onto the surrounding scene; for example, a point light source is tied to a cube map
which is a panoramic image shot from inside a lantern cage or a window frame through which the light is filtering.
This enables a game designer to achieve realistic lighting without having to complicate the scene geometry or resort
to expensive real-time shadow volume computations.