(mPDF >= 6.0)
The control of Kerning is complicated! CSS3 allows for 2 methods of specifying kerning. In mPDF 6, these 2 methods have exactly the same effect:
- font-kerning: normal;
- font-feature-settings: 'kern' on;
TrueType fonts allow for 2 possible ways of including kerning data:
- OTL GPOS tables may contain kerning information
- The font may contain a separate kern table
Most fonts contain both or none, but they may exist independently.
If kerning is set to be active (by either of the CSS methods):
- if the useOTL value means that OTL GPOS tables are applied, then this method will be used;
- if not, then the separate kern table will be used - if it exists.
In Latin script, kerning will only be applied if specified by CSS. The configurable variable
useKerning determines behaviour if
font-kerning: auto is used (the default).
When using OTL tables, kerning is set to be on by default for non-LATIN script; this is because a number of fonts use information in the kern feature to reposition glyphs which are essential for correct display in complex scripts.
Limitation: if useOTL is set, but not for Latin script (e.g. = 0x02), and the text string contains more than one script, then kerning will not be applied to the Latin script text e.g. [Cyrillic text][Latin text][Cyrillic text]. This is because mPDF uses the presence of any repositioning applied to determine if kerning has been applied, otherwise using the alternative kern tables.