Embedding PDF graphics in PDF output file programmatically

Question

I am looking for a rough overview of how one would go about embedding graphics (coming from a PDF file) into another PDF file when writing a C++ document processor.

Background: I work on the LilyPond music typesetter, and recently added Cairo output to the system. Now I would like to support adding externally provided graphics to the PDF files that we generate (eg. adding a logo onto page laid out). This is trivial with EPS for PS output.

I can see how you could hook up Poppler to read the PDF, and render the PDF contents onto a Cairo surface, but I wonder if there is a simpler shortcut (eg. embed the PDF file as a binary stream, and then point directly to that stream).

Answer 1

If you need to go via an external route, like reading the PDF and writing it into an existing PDF using Cairo, that would be simpler. To do it manually:

A PDF page consists of a stream of operators for drawing it, and a dictionary of external resources (fonts, images etc.). To stamp one PDF page onto another, you would need to:

a) Find all objects for external resources in the stamp which are needed, and add them to the destination PDF.

b) Convert the page to a "Form Xobject", which is a sort of reusable piece of content. Add this to the /XObjects entry in the destination page, making sure to pick a fresh name.

c) Add some operators to the page content in the destination page to invoke the new xobject

To see how this might work, you could play with -stamp-as-xobject and -postpend-content "/XObjName Do" from section 8.4 of the cpdf manual.

Making this work for arbitrary PDFs is really not for the faint of heart, I'm afraid.