Interaction between kern table and GPOS table

[Sairus Patel]

Reason:

Sometimes a font can have both a kern table and a GPOS table, and the GPOS may or may not have a kern feature for some scripts and language systems. How is an OFF layout engine to decide which source of kern data, if any, to apply? This proposal specifies this. Question: It is proposed to be in the Recommendations section; is there a better place in the specification for something like this?

Proposal:

ADD the following paragraphs to the end of the 'kern' Table sub-section in the Recommendations section:

When a kern table and GPOS table are both present in a font, and an OFF layout engine is requested to apply kerning to a run of text of a particular script and language system: (a) If the number of kern feature lookups in the resolved language system in the GPOS table is zero, then the kern table must be applied, followed by any remaining GPOS features requested. (b) If the number of kern feature lookups in the resolved language system in the GPOS table is non-zero, then all GPOS lookups, including the kern lookups, must be applied in the usual way and the kern table data ignored.

If a kern table but no GPOS table is present in the font, then an OFF layout engine must apply the kern table to the text, regardless of the resolved language system of the text.

If compatibility with legacy environments is not a concern, vendors are encouraged to use the GPOS table, and not the kern table, for kerning.

Sairus