Have you studied:
http://www.unicode.org/versions/Unicode6.1.0/ch08.pdf
and
http://www.microsoft.com/typography/OpenType%20Dev/arabic/intro.mspx

Thanks Bob H,

I'm aware of these documentations.

I'm struggling with encoding of the font. I've created all Isolated glyphs with their corresponding Unicode IDs, but for some weird reason the font is not showing those glyphs in MS Word.
e.g. Alef_Isol has its proper Unicode ID (0627) applied in FontLab. When I Generate Font and install it and type Alef in MS Word it is not displayed. (the font changes to Times New Roman).

Hope this explains my problem.

Thanks and regards.

Perhaps Word automatically attempts to use the correct initial, medial, and final forms, and ignores your Isolated forms.

I don't know about Word, but InDesign allows you to manually select the forms. IIRC it also defaults to 'Automatic', but in the Character panel menu ► OpenType ► Positional Forms you can select "Isolated Form".

If this works and you are not planning to add the other glyphs, you could simply copy your glyphs into the positions for the others.

First, drag your generated font back into FontLab and check that the encodings have been correctly applied. This will rule out the possibility that the bug is in the font cmap table.

As I recall, Windows apps expect any Arabic font to contain the OpenType GSUB features for init, medi and fina form substitutions, and may presume any Arabic font without them to be broken.

John

>>As I recall, Windows apps expect any Arabic font to contain the OpenType GSUB features for init, medi and fina form substitutions, and may presume any Arabic font without them to be broken.<<

That may explain why MS Word apparently doesn't accept to disable Open Type font formatting for Arabic ligatures as questioned by me here and here, and so far without a satisfactory answer.

You can't turn of basic script shaping features via OpenType Layout UI controls. Such controls are intended to control discretionary typographic features (as in the case of the Gabriola ligatures in your other test), not fundamental features of writing systems. The init, medi and fina features are applied by default to the Arabic script, and cannot be turned off. If one wants to disable such shaping, one has to do so via Unicode control characters such as zero-width non-joiner (U+200C).

[Note that what you describe in the MSDN forum as an Arabic 'ligature' is not, in fact a ligature, but an init glyph followed by a fina glyph.]

Now that you said it, it's obvious. Thanks.

Regarding my mistaken use of the term 'ligature', which feature tags are used to make those substitutions, init and fina ?

Yes. I recommend reading the MS Arabic layout engine and font specification.

Thanks John, I've just found the feature tags in that spec ("init" and "fina"). Shame on me for asking this silly question.

Hello,

Thanks Bob H, Theunis de Jong and John Hudson for replying.

As I figured it, and correctly pointed out by John Hudson, the CMAP table was corrupt for my font.

The solution: I created a new font file from scratch and copy pasted the glyphs from old to new file.
Little bit of hassle, but the font is in initial stages so it was not a big deal.

As an aside, I would request the community here to create a template for Basic Arabic font, and provide it as an starter for beginners. Or, if such resource exists provide a pointer to it.

Thanks again for all the helpful answers.