Before Emacs can draw a character on a graphical display, it must select a font for that character23. See Fonts in The GNU Emacs Manual. Normally, Emacs automatically chooses a font based on the faces assigned to that character—specifically, the face attributes :family, :weight, :slant, and :width (see Face Attributes). The choice of font also depends on the character to be displayed; some fonts can only display a limited set of characters. If no available font exactly fits the requirements, Emacs looks for the closest matching font. The variables in this section control how Emacs makes this selection.
If a given family is specified but does not exist, this variable specifies alternative font families to try. Each element should have this form:
(family alternate-families…)
If family is specified but not available, Emacs will try the other families given in alternate-families, one by one, until it finds a family that does exist.
If there is no font that exactly matches all desired face attributes (:width, :height, :weight, and :slant), this variable specifies the order in which these attributes should be considered when selecting the closest matching font. The value should be a list containing those four attribute symbols, in order of decreasing importance. The default is (:width :height :weight
:slant).
Font selection first finds the best available matches for the first attribute in the list; then, among the fonts which are best in that way, it searches for the best matches in the second attribute, and so on.
The attributes :weight and :width have symbolic values in a range centered around normal. Matches that are more extreme (farther from normal) are somewhat preferred to matches that are less extreme (closer to normal); this is designed to ensure that non-normal faces contrast with normal ones, whenever possible.
One example of a case where this variable makes a difference is when the default font has no italic equivalent. With the default ordering, the italic face will use a non-italic font that is similar to the default one. But if you put :slant before :height, the italic face will use an italic font, even if its height is not quite right.
This variable lets you specify alternative font registries to try, if a given registry is specified and doesn’t exist. Each element should have this form:
(registry alternate-registries…)
If registry is specified but not available, Emacs will try the other registries given in alternate-registries, one by one, until it finds a registry that does exist.
Emacs can make use of scalable fonts, but by default it does not use them.
This variable controls which scalable fonts to use. A value of nil, the default, means do not use scalable fonts. t means to use any scalable font that seems appropriate for the text.
Otherwise, the value must be a list of regular expressions. Then a scalable font is enabled for use if its name matches any regular expression in the list. For example,
(setq scalable-fonts-allowed '("iso10646-1$"))
allows the use of scalable fonts with registry iso10646-1.
This variable specifies scaling for certain faces. Its value should be a list of elements of the form
(fontname-regexp . scale-factor)
If fontname-regexp matches the font name that is about to be used, this says to choose a larger similar font according to the factor scale-factor. You would use this feature to normalize the font size if certain fonts are bigger or smaller than their nominal heights and widths would suggest.
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Licensed under the GNU GPL license.
https://www.gnu.org/software/emacs/manual/html_node/elisp/Font-Selection.html