Hi Johannes,
Good catch! Thank you very much. You support in fixing the LZW code is very
appreciated. I commited the change to SVN (will be in the 0.8.3 release).
May I asked, what you are using PoDoFo for and why you need especially the LZW
functionality?
Cheers,
Dominik
On Wednesday 25 August 2010 11:03:22 Dr. Johannes Bitterling wrote:
> Hi,
>
> just found one more bug in LZW decompression. With fix suggestion, of
> course :-)
>
> File: PdfFiltersPrivate.cpp
>
> Bug: class PdfLZWFilter is unable to create 0xff bytes in decoded LZW
> streams. This is obviously not relevant for compressed text streams (99.9%
> of all cases), but is important when image data is LZW-encoded. (No one
> should use LZW on inlined image data nowadays, but some strange workflow
> software still does.)
>
> Explanation: The current default decoding dictionary (created in
> PdfLZWFilter::InitTable) maps code:0xff to cleartext:<two bytes derived
> from an PdfLZWFilter constant>. Thus, any (0*)-1111-1111 bit sequence in
> the LZW stream is decoded as two(!) bytes, which is not correct: it's
> impossible to create 0xff bytes in the output stream, and the output data
> even may become longer as expected. Any n-bit-sequence s for which
> s==(s&0xff) holds, should be an identity. Just the sequences where
> s!=(s&0xff) - where bit 9 (or any higher bit) is set - are codes for
> multibyte sequences.
>
> Suggested fix: the loop counter should run to (including!) 255 to form the
> completed identity function. To keep the table size after "InitTable()"
> exactly the same, just one "dummy" entry is added (no content needed here:
> 0x100 is magic for "clear dictionary" - the encoder will never use this
> code as an abbreviation, so this table entry value is never used: The entry
> is just for correct counting of the dictionary size).
>
> Changed code:
>
> -----X-----
> void PdfLZWFilter::InitTable()
> {
> int i;
> TLzwItem item;
>
> m_table.clear();
> m_table.reserve( LZW_TABLE_SIZE );
>
> for( i=0;i<=255;i++ )
> {
> item.value.clear();
> item.value.push_back( static_cast<unsigned char>(i) );
> m_table.push_back( item );
> }
>
> item.value.clear();
> m_table.push_back( item );
> }
> -----X-----
>
> Regards,
> Johannes
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