Cairo and EPS format - a short detective story

The cairo library is a great tool that allows generation of hi-quality graphics in both bitmap (PNG) and vector (PDF, EPS, SVG) formats. It is multi-platform and has bindings into several scripting languages, including Python. For this reason I use it for export both in OASA and BKChem.
In this post I would like to show one feature of Cairo that recently backfired on me - fallback rendering.

As was reported by several users, the BKChem EPS export via Cairo was not what it was expected to be - it was a bitmap rendering of the drawing embedded into an EPS. I tried to find out what is going on and after much googling I quite unexpectedly found a clue in the source of the EPS file - one magical word "fallback". I googled "fallback" in relation to cairo and found these release notes mentioning that the PostScript export uses fallback rendering in case translucency is used.
This led back to discovery that BKChem cairo export inserts some invisible (completely transparent) rectangles into the drawing (these are used internally in BKChem for text borders) and these are causing the fall back to bitmap rendering (even though they are invisible in the output). By removing these from the export, I was able to make the EPS export work properly. It will be part of the next BKChem release.

As always, I hope that this post would be useful to other people possibly struggling with the same problem.
The lesson is clear - Watch out for invisible rectangles :)