Author :
Werner Lemberg
Date :
2025-04-20 06:27:52
Hash :573201be Message :[autofit] Speed up creation of the adjustment database's reverse map. (1/3)
As it turns out, the original implementation using
`hb_ot_shape_glyphs_closure` is extremely slow if a font has a rich set of
OpenType features. For example, this function was called 66954 times while
loading font `arial.ttf` version 7.00, increasing FreeType's startup time by
a factor of 10, which is unacceptable.
The new algorithm uses a completely different, more low-level approach, no
longer working with OpenType features but with OpenType lookups. It relies
on function `hb_ot_layout_lookup_get_glyph_alternates`, also replacing
recursion with a simple loop. In total, this brings the additional startup
time back to an acceptable range of a few percent.
A side effect of the new approach is that it catches more alternate forms:
the old code didn't properly handle script-specific features.
To make the change more readable, this commit only adds new code.
Werner Lemberg