kaxaml | lightweight XAML editor that gives you a `` split view | Form library

Full disclosure: I'm working on my libui GUI framework's text API. This wraps DirectWrite on Windows, Core Text on OS X, and Pango (which uses HarfBuzz for OpenType shaping) on other Unixes. One of the text formatting attributes I want to specify is a collection of OpenType features to use, which all three provide; DirectWrite's is IDWriteTypography.

Now, when you draw some text with these libraries, by default you'll get a few useful OpenType features enabled, such as the standard ligatures (liga) like the f+i ligature. I thought this was font-specific, but it turns out this is specific to the script of the text being shaped. Microsoft provides guidelines for all the scripts supported by OpenType (under "Script-specific Development"), and I can see rather complex logic for doing it all in HarfBuzz itself to confirm it.

On Core Text and Pango, if I enable other attributes, they'll be added on top of these defaults. But with DirectWrite, in particular IDWriteTextLayout::SetTypography(), doing so removes the defaults:

The program that produces this output is can be found here.

Obviously my first option would be to ask how to get the default features on DirectWrite. Someone did so already on this site, though, and the answer seems to be "no".

I am guessing that DirectWrite is allowing me to be in complete control of the list of features to apply to some text. This is nice, except that I can't do this with the other APIs unless I explicitly disable the default features somehow! Of course, I don't know if this list will ever change, so hardcoding it might not be the best idea.

Even if hardcoding is an option, I could just grab HarfBuzz's list for each script, but a) it's rather complicated b) there are multiple possible shapers for a script, depending on (I think) version compatibility (for instance, Myanmar).

So why not use HarfBuzz's lists to recreate the default list of features for DirectWrite anyway? It seems to want to be accurate to other shapers anyway, so this should work, right? Well I would need to do two things: figure out what script to use, and figure out which attributes to use on which characters for script where the position of a character in the word matters.

DirectWrite provides an interface IDWriteTextAnalyzer that provides facilities to perform shaping. I could use this, but it seems the script data is returned in a DWRITE_SCRIPT_ANALYSIS structure, and the description for the script ID says "The zero-based index representation of writing system script.".

This doesn't help, so I wrote a program to just dump the script numbers for text I type in. Running it on the input string