Convert PDF files to PDF/A via Ghostscript

问题

I'd like to convert arbitrary PDF files to PDF/A with Ghostscript 9.15.

1. Is Ghostscript able to create PDF/A-3b conformant PDFs? There is no parameter which represents a PDF/A conformance level, so I assume there is no possibility. Or is there anything I have overlooked?

2. I was following a blog post where a Windows batch file is used to convert from PDF to PDF/A (see http://www.mcbsys.com/techblog/2013/04/batch-convert-pdf-to-pdfa/). The gs invokation in the batch is:

   "%gs_path%\gswin64c" ^
    -dPDFA ^
    -dNOOUTERSAVE ^
    -sProcessColorModel=DeviceRGB ^
    -sDEVICE=pdfwrite ^
    -o "GS_%file1%" ^
    -dPDFACompatibilityPolicy=1 ^
     "%currentdir%\PDFA_def.ps" ^
     %inputfilelist%
   

The PDFA_def.ps is an adjusted version of the official one:

%!
% This prefix file for creating a PDF/A document is derived from
% the sample included with Ghostscript 9.07, released under the
% GNU Affero General Public License. 
% Modified 4/15/2013 by MCB Systems.

% Feel free to modify entries marked with "Customize".

% This assumes an ICC profile to reside in the file (AdobeRGB1998.icc),
% unless the user modifies the corresponding line below.

% The color space described by the ICC profile must correspond to the
% ProcessColorModel specified when using this prefix file (GRAY with
% DeviceGray, RGB with DeviceRGB, and CMYK with DeviceCMYK).

% Define entries in the document Info dictionary :

/ICCProfile (... PATH TO ... AdobeRGB1998.icc)   % Customize.
def

[ /Title (Title)                  % Customize.
  /DOCINFO pdfmark

% Define an ICC profile :

[/_objdef {icc_PDFA} /type /stream /OBJ pdfmark
[{icc_PDFA} <</N systemdict /ProcessColorModel get /DeviceGray eq {1} {systemdict /ProcessColorModel get /DeviceRGB eq {3} {4} ifelse} ifelse >> /PUT pdfmark
[{icc_PDFA} ICCProfile (r) file /PUT pdfmark

% Define the output intent dictionary :

[/_objdef {OutputIntent_PDFA} /type /dict /OBJ pdfmark
[{OutputIntent_PDFA} <<
  /Type /OutputIntent             % Must be so (the standard requires).
  /S /GTS_PDFA1                   % Must be so (the standard requires).
  /DestOutputProfile {icc_PDFA}            % Must be so (see above).
  /OutputConditionIdentifier (AdobeRGB1998)      % Customize
>> /PUT pdfmark
[{Catalog} <</OutputIntents [ {OutputIntent_PDFA} ]>> /PUT pdfmark

So, I use AdobeRGB1998.icc which is obviously useable for PDF files with RGB color space. Depending on the -sProcessColorModel value (DEVICERGB) a correct value is printed out.

The conversion works for all files. But when I validate the created PDF file against PDF/A-1b, I get different results depending whether the input file has RGB color space or not (e.g. CMYK). So, when I have an input PDF file which uses CMYK color space, the file gets converted by the script, but the validator says something like this:

input.pdf", 1, 38, 0x03418614, "A device-specific color space (DeviceCMYK) without an appropriate output intent is used.", 1
"output.pdf", 20, 0, 0x83410612, "The document does not conform to the requested standard.", 1

My question: Is there a way to get the conversion done for arbitrary files (i.e. independent of the used color space in the input file)?

---

Update

@KenS Thanks for your answer. I've updated my initial post to clarify what I want to achieve.

To make it more explicit, I will use an example. There are two files: input1.pdf (seems to use RGB) and input2.pdf (seems to use CMYK). I want to convert both of them to PDF/A-1. Thanks to your hint, I've let go of the above mentioned batch script and instead tested the command directly in the command line. After reading Ps2pdf.htm#PDFA, I have adjusted the (official) PDFA_def.ps so that AdobeRGB1998.icc is used. Then I invoked the following command on both input files (replaced output1.pdf by output2.pdf and input1.pdf by input2.pdf for the second file):

gswin64c.exe -dPDFA=1 -dBATCH -dNOPAUSE -dNOOUTERSAVE \
             -sColorConversionStrategy=/RGB \
             -sOutputICCProfile=AdobeRGB1998.icc -sDEVICE=pdfwrite \
             -sOutputFile=output1.pdf -dPDFACompatibilityPolicy=1 \
              "PATH/TO/OFFICIAL/PDFA_def.ps" input1.pdf

The conversion was done without any errors. The output1.pdf seems to be valid, but the output2.pdf is still invalid (tested with 3heights Validator):

"output2.pdf", 1, 40, 0x03418614, "A device-specific color space (DeviceCMYK) without an appropriate output intent is used.", 1
"output2.pdf", 20, 0, 0x83410612, "The document does not conform to the requested standard.", 1

So when I understand your answer correctly, the above command should produce a pdf file which uses the RGB color space - independent of the color space of the input file. If the input file uses CMYK, than the colors have to be translated into RGB with the above command.

When I interpret the first error message correctly, the used color space in the output2.pdf is still CMYK (although the command parameters like ColorConversionStrategy=/RGB). Since I used AdobeRGB1998.icc, the validation error appears.

What am I missing in the above command?

Going back to my original question (which is one step further): Instead of always converting to RGB (or CMYK), I wanted to somehow detect which color space is used in the input file and then dynamically switch to a RGB or CMYK icc file. Is it possible to achieve that?

回答1:

Ghostscript does not support PDF/A-3. The conformance parameter you are looking for is -dPDFA= where valid values are nothing (defaults to 1), 1 or 2. You can find this documented in ghostpdl/gs/doc/ps2pdf/htm#PDFA

I'm not sure what you are asking for here though. You must either create a PDF/A file (in level 1 or 2 anyway, I haven't read the revision 3 spec yet) which is RGB or CMYK, because you aren't allowed to use both (you can convert everything to device independent colour of course). The colour space used in the input isn't relevant, other than to decide whether it needs to be converted.

This is something you need to decide, we can't decide it for you. One important reason is that the OutputIntent must be consistent with either RGB or CMYK, and the pdfwrite device doesn't check it, it assumes you chose one which matches the device space you are using for the PDF file (by the way, don't set the ProcessColorModel, use ColorConversionStrategy instead) In your case you have set OutputIntent to AdobeRGB1988 so your colours must be specified either in device independent colour, or RGB.

Given the errors you quote, I would suggest the problem is that you haven't specified -sColorConversionStrategy, so the input colours are not being converted to the required device space. I would further guess that the script you copied this from set -dUseCIEColor, and you didn't copy that bit. DO NOT set -dUseCIEColor, its a horrbile ancient piece of PostScript hackery. Instead set ColorConversionStrategy, which will convert colours in a much better way, as required.

Updated answer as this started getting too long for a comment:

I can't immediately see any problems with your command line, can you share an example PDF file ? Its much easier to investigate these things with a solid example. I know from our customers and other free users that pdfwrite is capable of producing conforming PDF/A-1b files.

Regarding the second question; its not possible to do that because currently you need to set the OutputIntentProfile to either a CMYK one or an RGB one before you start. You can't just run through the input PDF file until you come to a colour operation and then decide. If you feel like some programming it could be done by modifying pdfwrite, because the profile isn't actually used till the output is closed.

One problem is that, in order to do the colour conversion, you need to set the underlying ProcessColorModel (this is done for you automatically by ColorConversionStategy). The only way to change ProcessColorModel is to execute a setpagedevice, which causes an erasepage. Now I think that's actually fixable with pdfwrite, all it does is write a white rectangle over the page, so you should be able to intercept that and not emit it. Otherwise any marks you made before you encountered an RGB or CMYK operation would be underneath the white rectangle.....

So essentially no, you can't do it right now, if its important to you then you could probably modify the code to do so (don't forget you will also need to supply 2 OutputIntent profiles to choose between as well). We've never had a customer request to do this, so we won't likely take it on as a project. Of course if you did get this working we might very well incorporate it into the code base if you were to offer it back to us.

来源：https://stackoverflow.com/questions/26040997/convert-pdf-files-to-pdf-a-via-ghostscript