CODEWITHSUNDEEP
pdfcolorspdf-generationghostscript
Miljenko Barbir
Miljenko Barbir

Reputation: 1193

Replace colors in PDF using ghostscript

How can I convert a single color from a PDF document into another color, for example convert all instances of #ff0000 (red) to #ffffff (white)?

I've seen a number of ghostscript commands doing something similar (using setcolor, setcolortransfer), but I can't find a solution for this exact problem.

For example, the follwing will create an image-negative of the input PDF:

gs -o output.pdf -sDEVICE=pdfwrite -c "{1 exch sub}{1 exch sub}{1 exch sub}{1 exch sub} setcolortransfer" -f input.pdf

I'd move past this with a higher level of control, focusing on a single color being replaced with a different (not it's negative) color.

Upvotes: 2

Views: 3405

Answers (1)

KenS
KenS

Reputation: 31199

Essentially, you can't (or at least not using Ghostscript).

Firstly you seem to be assuming that the colours will be specified in RGB when in fact they could be specified in CMYK, ICC, CalRGB or Lab. You also need to consider Indexed colour spaces.

Secondly Ghostscript does not 'edit' PDF files, when you send a PDF file as input to Ghostscript it is fully interpreted to graphics primitives and the primitives are processed.

When the output is PDF the primitives are reassembled into a new PDF file. The goal of this process is that the visual appearance of the new PDF file should match the original. It is NOT the same PDF file, its internals will likely be completely different.

Finally, how do you plan to handle images ? Will you process those byte by byte to massage the colours ? Or do you plan to ignore them ? Same goes for shadings, where the colours aren't even present in the PDF file directly, but are generated from functions.

Without knowing why you want to do this I can't even offer a different approach other than; decompress the PDF file, read it and replace the colours manually.

Upvotes: 2

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