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Comment by scoofy | Hacker News Reader

I desperately need to be able to display .SVG files with gradients on .PDFs, but no library currently exist in python as far as I know.

I would be willing to help make this happen, but I do not know much about the PDF format.

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It's actually not so bad: it's mostly ASCII, even though some parts of it really need to be treated as binary. If you open up a simple/old PDF in your favorite text editor, you can begin to grok the basic structures quite easy.

One trick for getting started: PDFs are read from the bottom. The first thing that is read is actually an offset pointing back to the xref table, at the end of the file. Then, the xref table itself points to the latest version of all of the objects.

The part you're most likely interested in is the content streams, which contain postscript-like drawing commands. To get a feel for it, following the official spec when reading a simple-looking document can help.

edit: I didn't link any actually useful resources, in part because I actually just have a corpus of files in various file formats that I keep handy as a reference for some weird reason. However, Googling for simple PDF files yielded this, which I feel is very readable in a text editor. https://www.africau.edu/images/default/sample.pdf

If you look for "pdf inspector" apps, there's also lots of those that will let you poke around the parsed tree.

You can include binary data in PDF files, so it's not necessarily ASCII.

The structure however is still largely ASCII text. It needs to be treated as binary of course, due to the use of offsets everywhere and the fact that the xref table is hardcoded to have a specific length per xref in bytes. But if you look at a lot of simple or old PDFs, it's not hard to find some that don't use any binary encodings.

Sounds a lot like the tar file format.

The PDF spec is quite readable. Use the “Adobe equivalent” of PDF 1.7 here: https://www.pdfa.org/resource/pdf-specification-index/

ReportLab can render gradients, but it's poorly documented. See eg https://stackoverflow.com/questions/452074/creating-a-gradie...

I use the method with `canvas.clipPath(path, stroke=False, fill=True)` on a path I've parsed manually from SVG then `canvas.linearGradient`.

This issue?

https://github.com/Kozea/WeasyPrint/issues/1719

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