Re: Increase default `line-spacing' to 0.05, 0.10 or 0.15 [proposal]

[Eli Zaretskii <eliz@gnu.org>]

> From: Augusto Stoffel <arstoffel@gmail.com>
> Date: Thu, 06 May 2021 11:26:45 +0200
> Cc: Jim Porter <jporterbugs@gmail.com>, emacs-devel@gnu.org
> 
> Comparing the picture in [1] and what I see if I put the block cursor
> over an "H", I conclude that monospaced fonts come with some built-in
> leading.  In other words, they are "bastard fonts" [2], and a nil for
> `line-spacing' doesn't mean "solid" typesetting.

I don't think this is true.  First, what you cite refers to so-called
"manual typesetting", not to digital typography.  And second, the
monospaced fonts we use define ascent and descent for each glyph, and
we use that and nothing else (unless the font is broken, in which case
we have fallbacks in place, but that's not relevant to the issue at
hand).  AFAIU, the ascent and descent of each font glyph in a
monospaced font is set up such that they accommodate both the tallest
glyph and the lowest one.  Perhaps that is why you think they have
some "leading", because you probably didn't examine all of the font's
glyphs (we generally use as default fonts that cover many popular
scripts).

> Kerning, tracking and other forms of microtypography are case-by-case
> adjustments, intended to be basically subliminal, so I'd say yes, the
> overall condensedness is pretty much a fixed characteristic of the font.
> And these things are out of reach for Emacs anyway.

They are not out of reach for us, because the shaping engine(s) we use
know how to apply these features.  We just don't use them by default
with most characters, that's all: we don't ask the text-shaping engine
how to render each sequence of characters we need to display, we only
ask it about some relatively rare sequences.  The reason for that is
that under the current design of the display engine, calling the
shaping engine is very expensive, as it requires calling out to Lisp,
which then calls back into C.  So we only do that when necessary.  But
a Lisp program can change that.