GPL Ghostscript for Debian
==========================

Links against shared JPEG library
---------------------------------

On Debian systems, ghostscript is linked against the shared IJG JPEG
library, instead of using the patched local copy bundled with the
ghostscript source.  The two versions of the library behave identically
except in two respects, both concerning invalid JPEG streams:

  - The bundled libjpeg fakes a valid component id when JPEG streams
    include an SOF or SOS marker whose component identifiers are not all
    distinct (see the description of C_i in B.2.2 of the JPEG spec).
    The shared IJG JPEG library produces artifacts (e.g., stripes) when
    presented with such component ids.

    https://bugs.ghostscript.com/show_bug.cgi?id=686980

  - The bundled libjpeg is configured to accept up to 64 blocks per MCU,
    instead of the limit of 10 blocks per MCU described in the
    Postscript reference manual and JPEG standard. According to lore and
    Adobe's tech note 5116, in 1991 Adobe came across some files in the
    wild exceeding the standard for blocks per MCU, and Adobe's
    implementation would sometimes accept such streams.

    That said, the PRLM3 states quite straightforwardly that the number
    of blocks in the MCU must not exceed 10. The PRLM2 contains the same
    condition, but attributes it to "the JPEG-proposed standard". Thus,
    we believe that any PostScript file containing this kind of JPEG
    stream can safely be considered invalid.

If you come across a file triggering either of these conditions, please
let us know by reporting a bug against the ghostscript package.

See Bug #582522 and Bug #582521 for more detail, particularly
https://bugs.debian.org/582522#46


Links against shared OpenJPEG library
-------------------------------------

The GPL Ghostscript project maintains a fork of OpenJPEG with a set of
patches applied related to color calibration of JPEG2000 data:

    https://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=ef8503

On Debian systems, ghostscript is linked against mainline OpenJPEG which
do not (at least for versions 1.3+dfsg-4.7 and 1.5.1-1) include those
patches.


DroidSansFallback is default fallback for CJK documents
-------------------------------------------------------

GPL Ghostscript uses the DroidSansFallback font from the fonts-droid
package to substitute for CID-keyed fonts that are not available (used
for East Asian character sets in PDFs).  If disk space is scarce, remove
the fonts-droid package and the glyphs from missing fonts will be
rendered as bullets.

See Use.htm#CIDFontSubstitution from the ghostscript-doc package for
details.

 -- Jonas Smedegaard <dr@jones.dk>  Thu, 04 Jul 2013 03:46:38 +0200
