accurately computing the x-height of a font

[hrj]

The problem:

• CSS21 tests use ex units with Ahem font.
• Ahem font's x glyph is a full-height square, but it defines x-height to 0.8em in the OS/2 table of the font file.
• Reading the OS/2 table is not possible because:
  • The OpenType interface is not available.
  • Getting the underlying file / inputstream of a font is not possible.
• Per CSS Spec, if the x-height can't be correctly calculated, it should default to 0.5em. However, 0.5em is too low for many fonts (causes problems with vote counts on reddit for example).

[hrj]

Interesting links:

• sfntly-java
• js code to read os/2 table data
• Open question: how to find x-height of a font in java