The CCITT Group 3 and Group 4 compression
algorithms can only be used with bilevel data.
Group 3 compression can be specified together with
several T.4-specific options: 1d for 1-dimensional
encoding, 2d for 2-dimensional encoding, and
fill to force each encoded scanline to be zero-filled
so that the terminating EOL code lies on a byte boundary.
Group 3-specific options are specified by appending a
‘‘:’’-separated list to the
‘‘g3’’ option; e.g. −c
g3:2d:fill to get 2D-encoded data with byte-aligned EOL
codes.
LZW compression can be specified together
with a predictor value. A predictor value of 2 causes
each scanline of the output image to undergo horizontal
differencing before it is encoded; a value of 1 forces each
scanline to be encoded without differencing. LZW-specific
options are specified by appending a
‘‘:’’-separated list to the
‘‘lzw’’ option; e.g. −c
lzw:2 for LZW compression with horizontal
differencing.
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