Low-Level Font - GNU Emacs Lisp Reference Manual

Normally, it is not necessary to manipulate fonts directly. In case you need to do so, this section explains how.

In Emacs Lisp, fonts are represented using three different Lisp object types: font objects, font specs, and font entities.

— Function: fontp object &optional type

Return t if object is a font object, font spec, or font entity. Otherwise, return nil.
The optional argument type, if non-nil, determines the exact type of Lisp object to check for. In that case, type should be one of font-object, font-spec, or font-entity.

A font object is a Lisp object that represents a font that Emacs has opened. Font objects cannot be modified in Lisp, but they can be inspected.

— Function: font-at position &optional window string

Return the font object that is being used to display the character at position position in the window window. If window is nil, it defaults to the selected window. If string is nil, position specifies a position in the current buffer; otherwise, string should be a string, and position specifies a position in that string.

A font spec is a Lisp object that contains a set of specifications that can be used to find a font. More than one font may match the specifications in a font spec.

— Function: font-spec &rest arguments

Return a new font spec using the specifications in arguments, which should come in property-value pairs. The possible specifications are as follows:
:name
The font name (a string), in either XLFD, Fontconfig, or GTK format. See Fonts.
:family
:foundry
:weight
:slant
:width
These have the same meanings as the face attributes of the same name. See Face Attributes.
:size
The font size—either a non-negative integer that specifies the pixel size, or a floating-point number that specifies the point size.
:adstyle
Additional typographic style information for the font, such as ‘sans’. The value should be a string or a symbol.

:registry
The charset registry and encoding of the font, such as ‘iso8859-1’. The value should be a string or a symbol.
:script
The script that the font must support (a symbol).
:otf
The font must be an OpenType font that supports these OpenType features, provided Emacs is compiled with support for ‘libotf’ (a library for performing complex text layout in certain scripts). The value must be a list of the form
               (script-tag langsys-tag gsub gpos)
where script-tag is the OpenType script tag symbol; langsys-tag is the OpenType language system tag symbol, or nil to use the default language system; gsub is a list of OpenType GSUB feature tag symbols, or nil if none is required; and gpos is a list of OpenType GPOS feature tag symbols, or nil if none is required. If gsub or gpos is a list, a nil element in that list means that the font must not match any of the remaining tag symbols. The gpos element may be omitted.

— Function: font-put font-spec property value

Set the font property property in the font-spec font-spec to value.

A font entity is a reference to a font that need not be open. Its properties are intermediate between a font object and a font spec: like a font object, and unlike a font spec, it refers to a single, specific font. Unlike a font object, creating a font entity does not load the contents of that font into computer memory. Emacs may open multiple font objects of different sizes from a single font entity referring to a scalable font.

— Function: find-font font-spec &optional frame

This function returns a font entity that best matches the font spec font-spec on frame frame. If frame is nil, it defaults to the selected frame.

— Function: list-fonts font-spec &optional frame num prefer

This function returns a list of all font entities that match the font spec font-spec.
The optional argument frame, if non-nil, specifies the frame on which the fonts are to be displayed. The optional argument num, if non-nil, should be an integer that specifies the maximum length of the returned list. The optional argument prefer, if non-nil, should be another font spec, which is used to control the order of the returned list; the returned font entities are sorted in order of decreasing “closeness” to that font spec.

If you call set-face-attribute and pass a font spec, font entity, or font name string as the value of the :font attribute, Emacs opens the best “matching” font that is available for display. It then stores the corresponding font object as the actual value of the :font attribute for that face.

The following functions can be used to obtain information about a font. For these functions, the font argument can be a font object, a font entity, or a font spec.

— Function: font-get font property

This function returns the value of the font property property for font.
If font is a font spec and the font spec does not specify property, the return value is nil. If font is a font object or font entity, the value for the :script property may be a list of scripts supported by the font.

— Function: font-face-attributes font &optional frame

This function returns a list of face attributes corresponding to font. The optional argument frame specifies the frame on which the font is to be displayed. If it is nil, the selected frame is used. The return value has the form
          (:family family :height height :weight weight
             :slant slant :width width)
where the values of family, height, weight, slant, and width are face attribute values. Some of these key-attribute pairs may be omitted from the list if they are not specified by font.

37.12.12 Low-Level Font Representation