All X clients use fonts for drawing text. The basic object
used in drawing text is XFontStruct,
which identifies the font that contains the images to be drawn.
The desktop already supports fonts by way of the XFontStruct
data structure defined by Xlib; yet, the encoding of the characters
within the font must be known to an internationalized application.
To communicate this information, the program expects that all fonts
at the server are identified by an X Logical Font Description (XLFD)
name. The XLFD name enables users to describe both the base characteristics
and the charset (encoding of font glyphs). The term charset
is used to denote the encoding of glyphs within the font, while
the term code set means the encoding of characters
within the locale. The charset for a given font is determined by
the CharSetRegistry and CharSetEncoding fields of the XLFD name.
Text and symbols are drawn as defined by the codes in the fonts.
A font set (for example, an XFontSet
data structure defined by Xlib) is a collection of one or more fonts
that enables all characters defined for a given locale to be drawn.
Internationalized applications may be required to draw text encoded
in the code sets of the locale where the value of an encoded character
is not identical to the glyph index. Additionally, multiple fonts
may be required to render all characters of the locale using one
or more fonts whose encodings may be different than the code set
of the locale. Since both code sets and charsets may vary from locale
to locale, the concept of a font set is introduced through XFontSet.
While fonts are identified by their XLFD name, font sets are
identified by a list of XLFD names. The list can consist of one
or more XLFD names with the exception that only the base characteristics
are significant; the encoding of the desired fonts is determined
from the locale. Any charsets specified in the XLFD base name list
are ignored and users need only concentrate on specifying the base
characteristics, such as point size, style, and weight. A font set
is said to be locale-sensitive and is used
to draw text that is encoded in the code set of the locale. Internationalized
applications should use font sets instead of font structs to render
text data.
Render tables are collections of renditions that specify how
text is to be rendered. They are summarized elsewhere in this section.
Font Specification |
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The font specification can be either
an X Logical Function Description (XLFD) name or an alias for the
XLFD name. For example, the following are valid font specifications
for a 14-point font:
-dt-application-medium-r-normal-serif-*-*-*-*-p-*-iso8859-1 |
OR
Font Set Specification |
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The font set specification is a list
of names (XLFD names or their aliases) and is sometimes called a
base name list. All names are separated by
commas, with any blank spaces before or after the comma being ignored.
Pattern-matching (wildcard) characters can be specified to help
shorten XLFD names.
Remember that a font set specification is determined by the
locale that is running. For example, the ja_JP Japanese locale defines
three fonts (character sets) necessary to display all of its characters;
the following identifies the set of Gothic fonts needed.
Example of full
XLFD name list:
-dt-mincho-medium-r-normal14-*-*-m-*-jisx0201.1976-0,-dt-mincho-medium-r-normal28-*-*-*-m-*-jisx0208.1983-0: |
Example of single XLFD pattern name:
The preceding two cases can be used with a Japanese locale
as long as fonts exist that match the base name list.
Base
Font Name List Specification |
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The base font name list is a list of
base font names associated with a font set as defined by the locale.
The base font names are in a comma-separated list and are assumed
to be characters from the portable character set; otherwise, the
result is undefined. Blank space immediately on either side of a
separating comma is ignored.
Use of XLFD font names permits international applications
to obtain the fonts needed for a variety of locales from a single
locale-independent base font name. The single base font name specifies
a family of fonts whose members are encoded in the various charsets
needed by the locales of interest.
An XLFD base font name can explicitly name the font's charset
needed for the locale. This enables the user to specify an exact
font for use with a charset required by a locale, fully controlling
the font selection.
If a base font name is not an XLFD name, an attempt is made
to obtain an XLFD name from the font properties for the font.
The following algorithm is used to select the fonts that are
used to display text with font sets.
For each charset required by the locale, the base font name
list is searched for the first of the following cases that names
a set of fonts that exist at the server.
The first XLFD-conforming
base font name that specifies the required charset or a superset
of the required charset in its CharSetRegistry and CharSetEncoding
fields.
The first set of one or more XLFD-conforming base
font names that specify one or more charsets that can be remapped
to support the required charset. The Xlib implementation can recognize
various mappings from a required charset to one or more other charsets
and use the fonts for those charsets. For example, JIS Roman is
ASCII with the ~ (tilde) and \ (backslash) characters replaced
by the yen and overbar characters; Xlib can load an ISO8859-1 font
to support this character set if a JIS Roman font is not available.
The first XLFD-conforming font name, or the first
non-XLFD font name for which an XLFD font name can be obtained,
combined with the required charset (replacing the CharSetRegistry
and CharSetEncoding fields in the XLFD font name). In the first
instance, the implementation can use a charset that is a superset
of the required charset.
The first font name that can be mapped in some locale-dependent
manner to one or more fonts that support imaging text in the charset.
For example, assume a locale requires the following charsets:
You can supply a base font name list that explicitly specifies
the charsets, ensuring that specific fonts are used if they exist,
as shown in the following example:
"-dt-mincho-Medium-R-Normal-*-*-*-*-*-M-*-JISX0208.1983-0,\
-dt-mincho-Medium-R-Normal-*-*-*-*-*-M- \
*-JISX0201.jisx0201\.1976-1,\
-dt-song-Medium-R-Normal-*-*-*-*-*-M-*-GB2312-1980.0,\
-*-default-Bold-R-Normal-*-*-*-*-M-*-ISO8859-1" |
You can supply a base font name list that omits the charsets,
which selects fonts for each required code set, as shown in the
following example:
"-dt-Fixed-Medium-R-Normal-*-*-*-*-*-M-*,\
-dt-Fixed-Medium-R-Normal-*-*-*-*-*-M-*,\
-dt-Fixed-Medium-R-Normal-*-*-*-*-*-M-*,\
-*-Courier-Bold-R-Normal-*-*-*-*-M-*" |
Alternatively, the user can supply a single base font name
that selects from all available fonts that meet certain minimum
XLFD property requirements, as shown in the following example:
"-*-*-*-R-Normal*-*-*-*-*-M-*" |
Render Tables |
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A render table consists of one or more
entries called renditions.
Each rendition is tagged with a name that is used when drawing a
compound string. For internationalized applications, renditions
and render tables should be specified in resource files to ensure
the independence of application binaries from the different needs
of various locales. For detailed information about renditions and
render tables, refer to Motif Programmer's Guide.
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