Presently, sequential tree coders are the best general purpose bilevel
image coders and the best coders of halftoned images? The current ISO
standard, Joint Bilevel Image Experts Group (JBIG), is a good example
, A sequential tree coder encodes the data by feeding estimates of con
ditional probabilities to an arithmetic coder, The conditional probabi
lities are estimated from co-occurrence statistics of past pixels, the
statistics are stored in a tree, By organizing code length calculatio
ns properly, a vast number of possible models (trees) reflecting diffe
rent pixel orderings can be investigated within reasonable time prior
to generating the code, A number of general-purpose coders are constru
cted according to this principle, Rissanen's one-p:lss algorithm, cont
ext, is presented in two modified versions, The baseline is proven to
be a universal coder, The faster version, which is one order of magnit
ude slower than JBIG, obtains excellent and highly robust compression
performance, A multipass free tree coding scheme produces superior com
pression results for all test images, A multipass free template coding
scheme produces significantly better results than JBIG for difficult
images such as halftones, By utilizing randomized subsampling in the t
emplate selection, the speed becomes acceptable for practical image co
ding.