The specification for the format (which is what is patented) describes the mathematical relationship between the binary payload and the uncompressed picture data. The actual implementation of the encoder / decoder is copyrighted and may also be patented, (which seems to be what you are alluding to), but with patent encumbered formats, the real issue is that the specification itself is patented.
Practically this means that every encoder / decoder for patent encumbered formats has to have some form of licensing just to interact with the format even if they somehow reverse engineered the specification and built a cleanroom implementation (at least as far as I understand it, IANAL)
Practically this means that every encoder / decoder for patent encumbered formats has to have some form of licensing just to interact with the format even if they somehow reverse engineered the specification and built a cleanroom implementation (at least as far as I understand it, IANAL)