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Cube mapping 34
Advantages
Cube mapping is preferred over other methods of environment mapping because of its relative simplicity. Also, cube
mapping produces results that are similar to those obtained by ray tracing, but is much more computationally
efficient – the moderate reduction in quality is compensated for by large gains in efficiency.
Predating cube mapping, sphere mapping has many inherent flaws that made it impractical for most applications.
Sphere mapping is view dependent meaning that a different texture is necessary for each viewpoint. Therefore, in
applications where the viewpoint is mobile, it would be necessary to dynamically generate a new sphere mapping for
each new viewpoint (or, to pre-generate a mapping for every viewpoint). Also, a texture mapped onto a sphere's
surface must be stretched and compressed, and warping and distortion (particularly along the edge of the sphere) are
a direct consequence of this. Although these image flaws can be reduced using certain tricks and techniques like
“pre-stretching”, this just adds another layer of complexity to sphere mapping.
Paraboloid mapping provides some improvement on the limitations of sphere mapping, however it requires two
rendering passes in addition to special image warping operations and more involved computation.
Conversely, cube mapping requires only a single render pass, and due to its simple nature, is very easy for
developers to comprehend and generate. Also, cube mapping uses the entire resolution of the texture image,
compared to sphere and paraboloid mappings, which also allows it to use lower resolution images to achieve the
same quality. Although handling the seams of the cube map is a problem, algorithms have been developed to handle
seam behavior and result in a seamless reflection.
Disadvantages
If a new object or new lighting is introduced into scene or if some object that is reflected in it is moving or changing
in some manner, then the reflection (cube map) does not change and the cube map must be re-rendered. When the
cube map is affixed to an object that moves through the scene then the cube map must also be re-rendered from that
new position.
Applications
Stable Specular Highlights
Computer-aided design (CAD) programs use specular highlights as visual cues to convey a sense of surface
curvature when rendering 3D objects. However, many CAD programs exhibit problems in sampling specular
highlights because the specular lighting computations are only performed at the vertices of the mesh used to
represent the object, and interpolation is used to estimate lighting across the surface of the object. Problems occur
when the mesh vertices are not dense enough, resulting in insufficient sampling of the specular lighting. This in turn
results in highlights with brightness proportionate to the distance from mesh vertices, ultimately compromising the
visual cues that indicate curvature. Unfortunately, this problem cannot be solved simply by creating a denser mesh,
as this can greatly reduce the efficiency of object rendering.
Cube maps provide a fairly straightforward and efficient solution to rendering stable specular highlights. Multiple
specular highlights can be encoded into a cube map texture, which can then be accessed by interpolating across the
surface's reflection vector to supply coordinates. Relative to computing lighting at individual vertices, this method
provides cleaner results that more accurately represent curvature. Another advantage to this method is that it scales
well, as additional specular highlights can be encoded into the texture at no increase in the cost of rendering.
However, this approach is limited in that the light sources must be either distant or infinite lights, although
fortunately this is usually the case in CAD programs. [4]