GS 9.51 and transparency

[Zdenek Wagner]

Hi all,

I do not understand why transparency should be understand unsafe. As
unsafe I consider access to the file system. However, transparency has
consequences. It is explicitely forbidden in PDF/X. If you send
transparent PDF to a phototypesetter or a digital printing machine,
the area with transparency will be rendered as black. You cannot see
it in advance on screen or on your office printer, hence you have to
cerify it by software tools.

Zdeněk Wagner
http://ttsm.icpf.cas.cz/team/wagner.shtml
http://icebearsoft.euweb.cz

pá 13. 3. 2020 v 15:41 odesílatel Bruno Voisin via tex-live
<tex-live at tug.org> napsal:
>
> >From the MacTeX team, answering Uwe Siart's message on transparency in gs 9.51: I'm not sure that's the same issue, but the consequences of the new implementation of -dSAFER in gs 9.50 on transparency in PSTricks was discussed on the MacTeX list and directly with Chris Liddell (of the Ghostscript dev team) in late October last year.
>
> As a result he implemented a new -dALLOWPSTRANSPARENCY switch, bringing back transparency while still in -dSAFER mode:
>
> http://git.ghostscript.com/?p=ghostpdl.git;a=commit;h=d1eac80695b2c6f2a85942a82518715549541969
>
> The problem seems to be that PSTricks implements transparency for distiller with "/SetTransparency pdfmark" which I imagine is standard (I'm no PS/PDF expert), but for gs it uses .setopacityalpha which is documented by gs as non-standard:
>
> https://www.ghostscript.com/doc/current/Language.htm#Additional_operators
>
> This link also describes the new -dALLOWPSTRANSPARENCY switch.
>
> (The following based on my recollection of correspondence with Karl back then; I'll let Karl correct anything that needs correcting.)
>
> We then evoked with Karl the idea to modify epspdf, epstopdf and simpdftex such that -dALLOWTRANSPARENCY is called by default.
>
> Karl wasn't fond of the idea, for two reasons mostly: first, if -dALLOWTRANSPARENCY were really safe, it would be the default; and second, that would only solve the problem for gs 9.51 and above.
>
> So the conclusion was to leave things as they were and instead to warn the user that gs needed to be called with -dALLOWPSTRANSPARENCY if PSTricks transparency was looked for.
>
> Back then I had ended my last message on the topic telling I would contact Herbert Voss about the matter, but then forgot and never did. Sorry!
>
> Is there a test file that could be used to test this (I'm no pstricks user myself)?
>
> Bruno Voisin