PS to PNG transparency lost in conversion due to ghostscript-bug - possible workaround

[Hans Aikema]

After various google searches I finally found out why my lilypond 
PNG-files did not contain a transparent background:
A bug in GhostScript introduced in version 8.64 
(http://bugs.ghostscript.com/show_bug.cgi?id=690508)

When GhostScript encounters a 'too large' bitmap it enters clist-mode 
rather than page buffer mode, which causes the tranparency to be 
replaced by solid white, loosing the transparency that lilypond put into 
the original postscript file.

I managed to circumvent this by patching my local lilypond installation 
(ps-to-png.scm) to always call gs with an additional argument 
-dMaxBitmap=2147483647 (max integer, the largest possible value gs will 
accept, which means it will process anything up to 2G using page buffer 
mode).

Given posts in the past on transparency not working without any 
solutions (apart from post-processing with other tools) I decided to 
share this one with the user-list. I think that my patch should not get 
into the default lilypond distribution, as it's a dirty trick to 
circumvent misbehaviour of GhostScript, but if you would like to apply 
this fix, just search for the line with
-dTextAlphaBits=4\
in usr/share/lilypond/current/scm/ps-to-png.scm and add a line after 
that line containing
-dMaxBitmap=2147483647\
to overrule the default GhostScript MaxBitmap with the maximum possible 
value

regards,
Hans Aikema