4/ SILE is digital typesetting system imagined by @simoncozens about a decade ago, and currently maintained by @alerque

It's a nice idea, bridging custom Lua modules with standard libraries such as HarfBuzz and ICU to produce PDF output.

My work on gradients does require a small patch to the C bindings, so it will need a future SILE release before users can benefit from it.

Still, the extensibility and flexibility of SILE made it possible to implement the feature. That was a fun ride 🐱

3/ Then came the details.
Full-page gradients must account for bleed vs sheet size when they differ from the target page size.
Nothing terribly difficult, but I had initially overlooked it.

And then came the work of integrating the feature into other components, such as framed boxes and book cover pages...

2/ SILE's PDF engine is derived from #TeXLaTeX's dvipdfmx but does not expose gradients at the typesetter level.

That led me to implement basic linear gradients from color stops.

The hardest part was the PDF spec itself: not badly written, but full of subtle interactions between coordinate systems (matrix transforms, pattern space, content stream units…). It eventually worked (at least for my intended set of features), but only after quite a bit of trial and error...