Dear gang,

It appears that \defineactivecharacter does not work in lmtx:

\starttext
\defineactivecharacter Ḥ  {\d{H}}
\defineactivecharacter ḥ  {\d{h}}
\defineactivecharacter Ṣ  {\d{S}}
\defineactivecharacter ṣ  {\d{s}}
\stoptext

This should not produce any typeset output but in lmtx it does.

Background: The following characters are essential for Arabic transliteration (assuming your email client has the chars):

Ṯṯ Ḥḥ Ḫḫ Ḏḏ Šš Ṣṣ Ḍḍ Ṭṭ Ẓẓ Ġġ Āā Īī Ūū ʿ ʾ

But most really good fonts don't have them, so we have to compose them. Now there are other methods, e.g,

1) \definecharacter Hbottomdot   {\buildtextbottomdot H}

but cannot use it without something like \defineactivecharacter (there's no way we're going back to the bad old days of control sequences for accented chars). That is, we need to be able to say something like

\definecharacter Ḥ   {\buildtextbottomdot H}

But this doesn't work.

The other method

2) \definefontfeature[default][default][compose=yes]

doesn't appear to work either (at least not with, e.g., Minion Pro Opticals).

And even when it used to (somewhat) work (often ugly default results) it did not support utf 02BE and 02BF (ʿ ʾ). So we still need something like

\defineactivecharacter ʿ {\high{c}}
\defineactivecharacter ʾ {\kern.07 em \high{\rotate[rotation=180,location=high]{c}}}

or better

\definecharacter ʿ {\high{c}}
\definecharacter ʾ {\kern.07 em \high{\rotate[rotation=180,location=high]{c}}}

The upshot is that we need a general, configurable solution for this kind of situation.

And please don't say, "Use a different font!" -)

Thank you in advance for any guidance and assistance in this matter.

Best wishes
Idris