Cube maps are an omni-directional layer source that are directly supported by the graphics hardware. The nature of the cube map definition results in higher resolution (in pixels per solid angle) at the corners and edges of the cube and lower resolution at the center of each face. While the cube map does have variability in sample density, the variability is spread symmetrically around the sphere.
Sometimes it is valuable to have an omni-directional format that has a directional bias where quality and sample density is better in a particular direction or over a particular region. If we changed the cube map sampling
from: color = texture( cubeLayerSampler, direction ); to: color = texture( cubeLayerSampler, normalize( direction ) + offset );
we can provide a remapping of the cube map sample distribution such that samples in the "offset" direction map to a smaller region of the cube map (and are thus higher resolution).
A normal high resolution cube map can be resampled using the inverse of this mapping to retain high resolution for one direction while signficantly reducing the required size of the cube map.