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GSUB/GPOS header field NULL offset issue #79

Open mikeday opened 6 years ago

mikeday commented 6 years ago

Hi, is this a suitable place for raising OpenType spec issues?

For example, in the GSUB header struct the spec implies that scriptListOffset, featureListOffset, and lookupListOffset cannot be NULL, as offsets that can be NULL have a comment saying "may be NULL" (such as the featureVariationsOffset field).

However, existing fonts such as Noto Glagolitic do have NULL values for these offset fields and I haven't found any implementation that actually requires them not to be NULL or emits any errors or warnings during font validation if NULL offsets are encountered.

Since it would be helpful for the spec to describe how to create an interoperable implementation, perhaps these fields could be changed to "may be NULL"? A similar issue affects the GPOS header struct.

PeterCon commented 6 years ago

Thanks. This can be considered for the next version of the spec.