Need help with Arabic contextual positioning

yanone · December 2024

I'm trying to work around tashkeel crashing into each other on top of tightly spaced letters. In particular, I want to move the fatha (red in FontGoggles screenshot) upwards in the given context.

All letters are separate glyphs, not a ligature.

My code is:

lookup tashkeel_crash {
lookupflag RightToLeft;
position @tashkeel_crash_hah @topMarks' @conn_mf_center_target @topMarks <0 200 0 0>;
} tashkeel_crash;

But not matter what I try, it won't work.

I've confirmed the backtrack/input/lookahead sequences to be correct in OTMaster, and more than double-checked the correct OT classes and found no mistakes.

The lookup is called as part of the rclt feature, run last in the GPOS stack, after mark/mkmk.

Has anyone struggled with this before?

ps. the font is generated with fontmake, but given that it appears correct in OTMaster, I don't think this matters.

John Hudson · December 2024

Try putting the lookups into the mark or mkmk features as appropriate.

In theory, any feature is supposed to be able to use either/or/and GSUB/GPOS lookups, but it has long been an issue that some environments presume features to be either GSUB or GPOS based on their registry descriptions. The rclt feature is most often a GSUB implementation.

yanone · December 2024

Thank you.

Putting it mark was half the solution. The other half was putting my adjustment after the automatically generated mark positioning code, which in ufo2ft world is done through a # Automatic Code comment, as in https://github.com/googlefonts/ufo2ft/blob/5bbc79d653e95842dba8f2752aef27cf2d26cf37/tests/featureWriters/markFeatureWriter_test.py#L276

Need help with Arabic contextual positioning

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