[Feature Request] Provide debug info of ligature component and id for each glyph in buffer

[mhosken]

When debugging issues around ligatures and ligature attachment, it would be very helpful to have the buffer serialization be able to output the ligature component and id for each glyph in the buffer.

I would be happy to code this up, but what fields do we want to call them? Do we simply extend --show-flags to include them? Do we want to output them as part of the text output?

[behdad]

I'm very uncomfortable exposing this in the buffer serialization, for various reasons.

However, I'm open to adding buffer messages to ligate_input function..

[mhosken]

It's hard to respond to 'various reasons'. But I will just say that my needs are, without trying to force the method of addressing them.

1. To know when a new value of ligature id or component is set on a glyph
2. To know the ligature ids and components when doing ligature attachment

The reason is that if I am trying to track down why a particular mark attaches to a different ligature component than the one expect, I need to know which component it thinks it is attaching to and how it got that idea in the first place. Since ligature components and ids are an internal concept to a shaping engine, there is no other way to get this information.

[mhosken]

Any news on this. What should we do? I still am trying to debug my ligature problems.

[behdad]

I haven't forgotten this. Let me think more. Is really hard to expose these without leaking a lot of internal details.