Pango’s CoreText backend now supports font fallbacks

Back in 2010 I started working on a CoreText backend for Pango (blog post 1, blog post 2). I got into trouble with implementing fontsets properly, so the code bitrot on my disk for a full year until I decided to commit a barebones version of the backend without fontset and font fallback support in April 2011 (blog post).

In the second half of 2011 Xamarin and Lanedo teamed up to improve GTK+ on Mac OS X in order to enhance the experience of the MonoDevelop IDE on OS X. Part of this work was to implement proper font fallbacks. This means that when characters are to be rendered which are not in the current font (for example Japanese characters which are not present in a Latin font when rendering mainly Latin text), we should fallback to another font containing these characters instead of drawing ugly boxes with hexadecimal numbers.

This allowed me to dust off the fontset patches I started working on back in 2010 and to investigate how to make fontsets work in the CoreText backend for real (nasty details are available in bug 647969, especially comment 3). We ended up with something that works nicely on both Snow Leopard and Lion. The reliance on a CoreText symbol that’s not in the header files (not nice, I know) has not shown to be a problem so far.

In conjunction with this, we also improved the shaping engine of Pango’s CoreText backend. The shaping engine can now better deal with the output from CoreText’s typesetter which we use to obtain the list of glyphs to render. Especially dealing with zero-width spaces, in both rendering and cursor handling, now works properly and was completely broken before. For some other corner cases (in particular with regard to some non-Latin scripts), we have ensured these are dealt with such that no crashes occur elsewhere in Pango. To fully support these cases, the shaping engine could use further improvement; see the source code comments.


All of this work is available in Pango’s master branch. It is great to see this upstream, together with the many other improvements we have made together with Xamarin so far!