The DjVu Reference Library does not contain the entire implementation of
the  DjVu system.  While  it  contains the  entire  decoder,  and a  big
piece of  the encoder,  it does not  contain the  sophisticated encoding
strategies necessary for reaching  the highest compression ratios.

Among other  things, The DjVu  Reference Library contains the  full DjVu
decoder,  the  full IW44  wavelet  encoder/decoder  for continuous  tone
images, and  the back-end  of JB2  bilevel image  compression technique.
However, it  does not  contain the code  for separating  document images
into foreground and  background layers, nor does it contain  the code of
the lossy  JB2 scheme  that achieves the  highest compression  ratios on
bitonal images.

WWW: http://www.djvu.com/