alt Hacker NewsIn Naskh, each letter has only four forms (for the most part — there are a few ligatures etc. but I think ‘only four forms’ remains basically true). The choice between forms is determined almost entirely by position within a word (initial/medial/final/isolated). All the letters are aligned along the baseline and connect to each other in basically the same way.
By contrast Nastaliq is a much more complicated style. Many letters and letter combinations take on several different forms depending on which other letters surround them. Letter joins are usually diagonal, so letters earlier in a word need to be shifted above the baseline by a variable amount. Having to shift letters vertically as well as horizontally greatly complicates other aspects of the style too.
(I recall seeing a nice table some time ago showing all the various different possibilities for letter joins in Nastaliq. Unfortunately I can’t seem to find it again. Still, you might get some idea by consulting the documentation of one of the existing Nastaliq fonts, e.g. Awami Nastaliq: https://software.sil.org/awami/what-is-special/)
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afaik this is a non-issue with modern text rendering engines. Modern font files include rulesets to determine the forms and shaping engines apply these rules to eventually reach the desired "shape" (i.e. order, position and which glyphs to render). For example, if you use HarfBuzz it should be able to calculate the Glyphs and offsets you need for a properly set script.
I personally spent way to much time trying to understand it, but at least according to this video (https://www.youtube.com/watch?v=VaA0v0V4RsU) it really is not that difficult if you leave out all the font-selection and emoji shenanigans.
I think at least FreeType (glyph rendering) and HarfBuzz (text shaping) make it needlessly complex through their documentation. It is extensive in describing what the parts do, but the only way to figure out what you need is by fiddling around. As soon as you want to do more complex stuff you're on your own. Especially figuring out which parts you don't need is annoying.
SIL's Nastaliq font uses their own Graphite engine, which is included in Firefox but not other browsers (Demo page: https://graphite.sil.org/graphite_fontdemo ), but e.g. Noto Nastaliq Urdu also exists https://fonts.google.com/noto/specimen/Noto+Nastaliq+Urdu?pr... and does a decent job in non-Graphite engines, certainly better than Awami Nastaliq without Graphite.
So the real question is why Android doesn't make it easy to put Noto Nastaliq Urdu in the font stack.
Yeah, but the difficulty isn't in rendering the fonts, it's for the font creator. So once the font is ready with all the combinations it rendering and using a Nastaliq font doesn't differ with rendering a Naskh. Nastaliq fonts are available in Persian, not sure if true for other languages, but it's just more complexity on making the font. For using a ready font the only thing needed is permission to change the font.