Re: Ligatures

[조성빈]

Stefan Monnier <address@hidden> 작성:

> > The crucial problem is that we currently perform layout decisions one
> > grapheme cluster at a time, whereas what HarfBuzz people say is that
> > we should basically do that one screen line at a time.
>
> I wonder how it is supposed to work and it works in other applications:

I don’t know how much you know about text rendering, (I’m fairly confident  
that a previous Emacs maintainer to know more about this than me) but for  
people who are curious about this, I found the ’Text Rendering Hates  
You’[0] article which was very helpful for understanding the problem.

[0]: https://gankra.github.io/blah/text-hates-you/

> Disregarding the theoretical question of whether a font can use
> ligatures that involve the LF character (and hence affect the definition
> of what is a line), I still see a chicken-and-egg problems:
> How do you know where the current "screen line" ends if you don't know
> how narrow/wide the font and its ligatures will render the text?
>
> Do current applications use a heuristic like "ligatures won't reduce the
> size by more than a factor 2, so estimate the lower bound on the final
> size to be at most half of what the font metrics say", so they will send
> up to twice as much text to be shaped as needed, and then they throw
> away the left overs?

According to the article I mentioned, it’s just passing the total text  
repeatedly until it runs out of space.

> You have to assume that your text fits on a single line and shape it  
> until you run out of space. At that point you can perform layout  
> operations and figure out where to break the text and start the next  
> line. Repeat until everything is shaped and laid out.

>         Stefan