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Re: [h-e-w] Displaying Unicode - font substitution for missing glyphs

From: Jason Rumney
Subject: Re: [h-e-w] Displaying Unicode - font substitution for missing glyphs
Date: Mon, 14 Jan 2013 20:51:37 +0800
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux)

John Brown <address@hidden> writes:

> Can anyone say how emacs identifies a font that contains a glyph
> that is not present in its current default font, and why it failed
> to find DejaVu Sans Mono? It is not that important; I was just
> testing emacs on a UTF-8 demo file.

Step 1: Check in the default font (that is why the character displays
when the chosen default font contains that character).

Step 2: Figure out which script the character belongs to, and
build a shortlist of fonts which claim to have coverage of that script.

Step 3: Discard any fonts that do not cover the characters listed in
`script-representative-characters', as fonts often lie about which
scripts they cover.

Step 4: Look for the actual character in the shortlisted fonts.

Things can go wrong at steps 2 and 3, either because of deficiencies in
the fonts themselves, or in Windows reporting of scripts covered by the

The situation on X is slightly better, as there is an API for querying
whether a character is contained in a font without allocating resources
for opening the font for drawing, so a more exact search can be done
much more quickly.

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