compress/

README

Note

• This is an improved version of compress/lzw.
• Please refer to the golang proposal for details.

Problem

• PDF's LZWDecode filter comes with the optional parameter EarlyChange.
• The type of this parameter is int and the defined values are 0 and 1.
• The default value is 1.

Without going into details this parameter implies a slight variation of lzw. (See the PDF spec).

compress/lzw:

• the algorithm implied by EarlyChange value 1
• provides both Reader and Writer.

golang.org/x/image/tiff/lzw (mirrored at):

• the algorithm implied by EarlyChange value 0
• provides a Reader, lacks a Writer

Further PDF expects a leading clear_table marker right at the beginning which is not smth the stdlib compress/lzw does.

There are numerous PDF Writers out there and the following can be observed on arbitrary PDF files that use the LZWDecode filter:

• Some PDF writers do not write the EOD (end of data) marker.
• Some PDF writers do not write the final bits after the EOD marker.