bug#21028: Slow font rendering in emacs

[John Mastro]

Eli Zaretskii <eliz@gnu.org> wrote:
>   2) set up a very detailed fontset with explicit ranges of codepoints
>      allotted to each font that supports the respective characters
>      well and whose looks on display you like

Is it possible to get the codepoint ranges supported by a given font
programmatically within Emacs? It sounds like that might help a user
tell Emacs "use this font for everything it supports" without specifying
an overly-broad target (like `unicode') or finding and spelling out the
ranges of codepoints manually.

In other words, I'm imagining something like this:

(dolist (target (targets-supported-by-font "DejaVu Sans Mono"))
  (set-fontset-font "fontset-default" target
                    (font-spec :name "DejaVu Sans Mono" :registry "iso10646-1")
                    nil 'prepend))

Where `targets-supported-by-font' is the hypothetical function I'm
imagining.

Apologies if this doesn't actually make sense :)

        John