In fractal image compression an image is partitioned into
ranges for each of which a similar subimage, called domain, is selected
from a pool of subimages. However, only a fraction of this
large pool is actually used in the fractal code. This subset can be
characterized in two related ways: (1) It contains domains with relatively
large intensity variation. (2) The collection of used domains is
localized in those image regions that show a high degree of structure.
Both observations lead to improvements of fractal image compression.
First, we accelerate the encoding process by a priori discarding
the low variance domains from the pool that are unlikely to
be chosen for the fractal code. Second, the localization of the domains
may be exploited for an improved encoding of the domain
indices, in effect raising the compression ratio. When considering
the performance of a variable rate fractal quadtree encoder we
found that a speedup by a factor of 2–3 does not degrade the ratedistortion
curve ranging from compression ratio 5 up to 30.