How does MS word renders different fonts?

Question

My main goal is described here.

How can Microsoft Word or Wordpad or other word editing software render fonts when these fonts seems to not follow the same rules?

How do they render characters glyph based on adjacent(or sometimes even further neighboring) characters?

For example look at this font(it is free). opening it using font forge i learned that code-points from U+FE00 to U+FE87 are assigned to glyphs that do not represent those code points characters. still when you type in Word using that font and entering the keystrokes for those glyphs, the glyphs are rendered correctly.

Note: These glyph represent characters that do not have specific keys on keyboard, but are different forms of other characters. for example "ﻟ" is initial form of "ل" so Word must have some instruction(built-in instructions or instructions stored in font file) to render the desired form instead of isolated form.

Another example is the font IranNastaliq(also free) which is a very popular font in farsi and it's output is like calligraphy done in farsi. so it has lots of glyphs for combination of characters, etc.

1. How does Word render these?
2. How can I mimic it?
3. How can I read the instructions(if there is any) inside of the font file?

Answer 1

Those code points are variation sequences, which are combining marks that you use to select the visual variant of the prior code point, so that a single code point can have multiple visual representations. This is similar to diacritical marks, but instead of rendering on top of the glyph of the previous code point, they specify a different glyph or visual representation.

The rendering of unicode is specified in the unicode standard. All operating systems or rendering systems that implement the unicode standard can render variation sequences.

In fonts variation sequences are supported by mapping tables, which specify how to map particular code point sequences to indexes inside the font's table of glyphs.

Answer 2

Thanks to Joeri's answer I found out that along many other things, there is a table in ttf fonts called GSUB. In the GSUB table there are multiple sub-tables. These tables provide information to software like word so that they can choose the appropriate glyphs depending on where the character is in the word and what other character are adjacent to it and so on.

For using these table I could not find an easy way so i used fonttools to convert ttf data to xml format. inside the xml you can see that GSUB table has some lookup tables. To me the most interesting ones where the ones that where corresponding to initial,medial,final and isolated ("init","medi","fina" and "isol" respectively) characters. Using them I was able to determine the correct code-point of desired glyph. Although the problem with fonts like W_japan has been solved, for fonts like IranNastaliq there are many many details(about 300 subtables) to go through which is very hard.