PDF transparency groups and composition

The PDF specification has the following image as an example of how to do transparent graphics composition.

This seems simple but actually requires quite a lot of functionality:

• Specifying CMYK gradients
• Setting the blend mode for paint operations
• Specifying transparency group xobjects
• Specifying layer composition parameters

I had to implement a bunch of new functionality to get this working, but here is the same image reproduced with CapyPDF. (output PDF here)

The only difference here is that out of laziness I used a simple two color linear gradient rather than a rainbow one.

Nonportability

This is again one of those features that only Acrobat Reader can handle. I tried Okular, Ghostscript, Firefox, Chromium and Apple Preview and they all rendered the file incorrectly. There was no consistency, each one was broken in a different way.