FRC2 is a quadtree-partitioned iterated function system (PIFS) image format. Every decoded sample is a sample of the PIFS attractor, computed by fixed-point iteration from a constant start image. The only post-processing is an optional deterministic edge filter (§8) that reads nothing but the bitstream. All per-region syntax is coded with one adaptive binary range coder (§5).
The reference decoder is frc.py (decode, read_plane, build_maps, decode_iter, deblock) together with rc.py. Wherever this text and the source differ, the source is normative.
[header: 18 + L bytes] [range-coder payload: rest of file]