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(elisp)Font Selection

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Font Selection
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   "Selecting a font" means mapping the specified face attributes for a
character to a font that is available on a particular display.  The
face attributes, as determined by face merging, specify most of the
font choice, but not all.  Part of the choice depends on what character
it is.

   For multibyte characters, typically each font covers only one
character set.  So each character set (Note: Character Sets) specifies
a registry and encoding to use, with the character set's
`x-charset-registry' property.  Its value is a string containing the
registry and the encoding, with a dash between them:

     (plist-get (charset-plist 'latin-iso8859-1)
                'x-charset-registry)
          => "ISO8859-1"

   Unibyte text does not have character sets, so displaying a unibyte
character takes the registry and encoding from the variable
`face-default-registry'.

 - Variable: face-default-registry
     This variable specifies which registry and encoding to use in
     choosing fonts for unibyte characters.  The value is initialized
     at Emacs startup time from the font the user specified for Emacs.

   If the face specifies a fontset name, that fontset determines a
pattern for fonts of the given charset.  If the face specifies a font
family, a font pattern is constructed.

   Emacs tries to find an available font for the given face attributes
and character's registry and encoding.  If there is a font that matches
exactly, it is used, of course.  The hard case is when no available font
exactly fits the specification.  Then Emacs looks for one that is
"close"--one attribute at a time.  You can specify the order to
consider the attributes.  In the case where a specified font family is
not available, you can specify a set of mappings for alternatives to
try.

 - Variable: face-font-selection-order
     This variable specifies the order of importance of the face
     attributes `:width', `:height', `:weight', and `:slant'.  The
     value should be a list containing those four symbols, in order of
     decreasing importance.

     Font selection first finds the best available matches for the first
     attribute listed; 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.

     The default is `(:width :height :weight :slant)', which means first
     find the fonts closest to the specified `:width', then--among the
     fonts with that width--find a best match for the specified font
     height, and so on.

     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.

 - Variable: face-font-family-alternatives
     This variable lets you specify alternative font families to try,
     if a given family is specified and doesn't exist.  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.

 - Variable: face-font-registry-alternatives
     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, since the use of too many or too big scalable fonts can crash
XFree86 servers.

 - Variable: scalable-fonts-allowed
     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 '("muleindian-2$"))

     allows the use of scalable fonts with registry `muleindian-2'.

 - Function: clear-face-cache &optional unload-p
     This function clears the face cache for all frames.  If UNLOAD-P
     is non-`nil', that means to unload all unused fonts as well.