To render Arabic text correctly we need more sophisticated font technologies, what is so called smart fonts, like OpenType, AAT from Apple and.

Basically, the ligature is a special glyph that is composed of two or more glyphs.

Take lam-alef as an example, it is a glyph that is composed of lam (initial or medial form) with alef (final form).

However, if we looked carefully, we will see that lam-alef is no exception, just a special form of lam that it only takes when followed by alef, the same for the alef.

In OpenType, lam-alef and like can be addressed either by using ligatures, or more complex contextual substitution.

As Arabic diacritics may be stacked above each other in relation to one base glyph, it is important to address mark to mark relations.

Arabic block, U0600 - U06FF, which contain almost all Arabic code points, and Arabic supplement, U0750 - U077F, which contains some extra characters.

There also Arabic presentation forms A and B, UFB50 - UFDFF and UFE70 - UFEFF, Arabic presentation forms code different forms of Arabic letters and some ligatures, however, they are included for compatibility with legacy encodings and should never be used to directly encode the text.

Presentation forms can be used when defining substitution tables, but that is not mandatory as you can added the contextual forms as unencoded glyphs to your font, and tell OpenType engine which glyph to use through the lookup tables.

Note that, including the presentation forms in your font doesnt mean that they will be used, you must add lookup tables.

FontForge is capable for both editing glyph outlines, and OpenType table, and scriptable (using either python or its legacy scripting language).

FontForge doesnt encode the new font as Unicode by default, go to Encoding - Reencode and choose ISO 10646-1 (Unicode, BMP), this way Arabic Unicode points will be available for us to edit.

Dual-joining characters has initial, medial, final and isolated forms, while right-joining dont have a medial form, other Arabic characters has only one form.

However, if you want your font to be usable for legacy applications that rely on Arabic presentation forms for rendering contextual forms, you might consider adding those glyphs there.

There is basically two ways to support lam-alef in your font; ligature substitution.

Many OpenType implementations doesnt support ligature caret table, giving us no control on where to insert the cursor between ligature components, which makes the experience of editing text with many ligatures not so pleasant.

Here we will need simple ligature substitution table, liga, FIXME.

Arabic Calligraphy Unicode Font