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40.12.12 Low-Level Font Representation

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 in The GNU Emacs Manual.

:family
:foundry
:weight
:slant
:width

These have the same meanings as the face attributes of the same name. See Face Attributes. :family and :foundry are strings, while the other three are symbols. As example values, :slant may be italic, :weight may be bold and :width may be normal.

: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.

:dpi

The resolution in dots per inch for which the font is designed. The value must be a non-negative number.

:spacing

The spacing of the font: proportional, dual, mono, or charcell. The value should be either an integer (0 for proportional, 90 for dual, 100 for mono, 110 for charcell) or a one-letter symbol (one of P, D, M, or C).

:avgwidth

The average width of the font in 1/10 pixel units. The value should be a non-negative number.

:script

The script that the font must support (a symbol).

:lang

The language that the font should support. The value should be a symbol whose name is a two-letter ISO-639 language name. On X, the value is matched against the “Additional Style” field of the XLFD name of a font, if it is non-empty. On MS-Windows, fonts matching the spec are required to support codepages needed for the language. Currently, only a small set of CJK languages is supported with this property: ‘ja’, ‘ko’, and ‘zh’.

:otf

The font must be an OpenType font that supports these OpenType features, provided Emacs is compiled with a library, such as ‘libotf’ on GNU/Linux, that supports complex text layout for scripts which need that. 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. For the list of OpenType script, language, and feature tags, see the list of registered OTF tags.

:type

The symbol that specifies the font backend used to draw the characters. The possible values depend on the platform and on how Emacs was configured at build time. Typical values include ftcrhb and xfthb on X, harfbuzz on MS-Windows, ns on GNUstep, etc. It can also be nil if left unspecified, typically in a font-spec.

Function: font-put font-spec property value

Set the font property property in the font-spec font-spec to value. The property can any of the ones described above.

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. The property can any of the ones that font-spec supports.

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, and the value of the :otf property is a cons of the form (gsub . gpos), where gsub and gpos are lists representing OpenType features supported by the font, of the form

((script-tag (langsys-tag feature…) …) …)

where script-tag, langsys-tag, and feature are symbols representing OpenType layout tags.

If font is a font object, the special property :combining-capability is non-nil if the font backend of font supports rendering of combining characters for non-OpenType fonts.

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.

Function: font-xlfd-name font &optional fold-wildcards

This function returns the XLFD (X Logical Font Descriptor), a string, matching font. See Fonts in The GNU Emacs Manual, for information about XLFDs. If the name is too long for an XLFD (which can contain at most 255 characters), the function returns nil.

If the optional argument fold-wildcards is non-nil, consecutive wildcards in the XLFD are folded into one.

The following two functions return important information about a font.

Function: font-info name &optional frame

This function returns information about a font specified by its name, a string, as it is used on frame. If frame is omitted or nil, it defaults to the selected frame.

The value returned by the function is a vector of the form [opened-name full-name size height baseline-offset relative-compose default-ascent max-width ascent descent space-width average-width filename capability]. Here’s the description of each components of this vector:

opened-name

The name used to open the font, a string.

full-name

The full name of the font, a string.

size

The pixel size of the font.

height

The height of the font in pixels.

baseline-offset

The offset in pixels from the ASCII baseline, positive upward.

relative-compose
default-ascent

Numbers controlling how to compose characters.

max-width

The maximum advance width of the font.

ascent
descent

The ascent and descent of this font. The sum of these two numbers should be equal to the value of height above.

space-width

The width, in pixels, of the font’s space character.

average-width

The average width of the font characters. If this is zero, Emacs uses the value of space-width instead, when it calculates text layout on display.

filename

The file name of the font as a string. This can be nil if the font back-end does not provide a way to find out the font’s file name.

capability

A list whose first element is a symbol representing the font type, one of x, opentype, truetype, type1, pcf, or bdf. For OpenType fonts, the list includes 2 additional elements describing the GSUB and GPOS features supported by the font. Each of these elements is a list of the form ((script (langsys feature …) …) …), where script is a symbol representing an OpenType script tag, langsys is a symbol representing an OpenType langsys tag (or nil, which stands for the default langsys), and each feature is a symbol representing an OpenType feature tag.

Function: query-font font-object

This function returns information about a font-object. (This is in contrast to font-info, which takes the font name, a string, as its argument.)

The value returned by the function is a vector of the form [name filename pixel-size max-width ascent descent space-width average-width capability]. Here’s the description of each components of this vector:

name

The font name, a string.

filename

The file name of the font as a string. This can be nil if the font back-end does not provide a way to find out the font’s file name.

pixel-size

The pixel size of the font used to open the font.

max-width

The maximum advance width of the font.

ascent
descent

The ascent and descent of this font. The sum of these two numbers gives the font height.

space-width

The width, in pixels, of the font’s space character.

average-width

The average width of the font characters. If this is zero, Emacs uses the value of space-width instead, when it calculates text layout on display.

capability

A list whose first element is a symbol representing the font type, one of x, opentype, truetype, type1, pcf, or bdf. For OpenType fonts, the list includes 2 additional elements describing the GSUB and GPOS features supported by the font. Each of these elements is a list of the form ((script (langsys feature …) …) …), where script is a symbol representing an OpenType script tag, langsys is a symbol representing an OpenType langsys tag (or nil, which stands for the default langsys), and each feature is a symbol representing an OpenType feature tag.

The following four functions return size information about fonts used by various faces, allowing various layout considerations in Lisp programs. These functions take face remapping into consideration, returning information about the remapped face, if the face in question was remapped. See Face Remapping.

Function: default-font-width

This function returns the average width in pixels of the font used by the current buffer’s default face, as that face is defined for the selected frame.

Function: default-font-height

This function returns the height in pixels of the font used by the current buffer’s default face, as that face is defined for the selected frame.

Function: window-font-width &optional window face

This function returns the average width in pixels for the font used by face in window. The specified window must be a live window. If nil or omitted, window defaults to the selected window, and face defaults to the default face in window.

Function: window-font-height &optional window face

This function returns the height in pixels for the font used by face in window. The specified window must be a live window. If nil or omitted, window defaults to the selected window, and face defaults to the default face in window.

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