Re: otf ligature rendering support?

[Yuri D'Elia <wavexx@thregr.org>]

On 01/02/2015 05:07 PM, Eli Zaretskii wrote:
>> From: Yuri D'Elia <wavexx@thregr.org>
>> Date: Fri, 02 Jan 2015 17:00:31 +0100
>>
>>> I think you need to define rules for composing characters into
>>> ligatures, in order to have what you want.
>>
>> This is different than composition of unicode ligatures.
>> OTF ligatures are an inherent property of the font along with kerning
>> tables.
> 
> I'm probably missing something, and I'm certainly not an expert on OTF
> fonts, in Emacs or elsewhere.  But if what you want is display several
> characters as a single glyph, the way to do that in Emacs is define
> composition rules.

Composition generates at most a sequence of precomposed utf code points,
such as æ. This is done at the client level and is font-agnostic.

OTF ligatures instead are glypt replacement tables which are provided
for a sequence of characters in the font itself. This is mostly needed
to improve the rendering for languages such as arabic.

The issue is that it needs special support in the client, since
previously drawn glypts might change as a result of typing.

I was under the impression that ligatures were supported by looking at
`fontset.el'.