[HarfBuzz] Font substitution

[Dominik Röttsches]

Hi Simon,

On Thu, Nov 12, 2015 at 12:59 AM, Behdad Esfahbod
<behdad.esfahbod at gmail.com> wrote:
> On 15-11-12 12:19 AM, Simon Cozens wrote:
>> Can anyone point me at some sample code which allows performs font
>> substitution? I'm guessing the way to do this is to detect notdefs,
>> break the buffer around them and then try the notdef'd glyphs in another
>> font.

>> Has anyone got some fairly accessible code which does this already?

The main shaping loop in Chrome is here:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp&sq=package:chromium&l=549&rcl=1447771613

The first stage is run segmentation, which we do by
"needs-rotate-for-vertical?",
"needs-uppercasing-for-synthetic-smallcaps", and script in Chrome.
Once you have the segmented runs, we start shaping, and then extract
the results as you described above. One important aspect here is to
ideally perform fallback on a grapheme cluster basis. In Chrome, we
currently do that by using HarfBuzz' cluster values.

Dominik