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An X font is (yet another) a server-side resource. In
essence, a font is a collection of bitmaps representing
characters. The bitmaps in a single font will have a
similar size and style. GDK allows you to manipulate fonts
with a client-side handle called
GdkFont.
To obtain a GdkFont, call
gdk_font_load() (or use the font
from a preexisting
GtkStyle; more on this in
the section called
GtkStyle and Themes). Fonts are loaded by
name; font names are a fairly involved topic. Names follow
a convention called the "X Logical Font Description," or
XLFD. The best way to get a feel for XLFD is to play with
the xfontsel program that
comes with X. You can also get a list of font names on your
X server with the xlsfonts
program. The standard X distribution includes a 30-page
XLFD manual, but the basics aren't too complicated.
A font name is a string made up of hyphen-separated fields.
Each field describes some aspect of the font. For example:
-misc-fixed-medium-r-normal--0-0-75-75-c-0-iso8859-1
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or
-adobe-new century schoolbook-bold-i-normal--11-80-100-100-p-66-iso8859-1
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The fourteen fields are:
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Foundry: the vendor who created the font, such as Adobe
or Sony. misc is used
for some generic fonts that come with X.
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Family: the typeface or style of the font---Courier,
Times, Helvetica, etc.
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Weight: Bold, demibold, medium, etc.
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Slant: Italic, Roman, Oblique (abbreviated as i,
r, or o
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Set Width: the "proportionate width" of the font;
possible values include normal, condensed,
semicondensed, etc.
-
"Add Style": any additional information about the font
can go in this field; it is used to distinguish two
fonts that otherwise have the same name. There are no
constraints on the string.
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Pixels: the pixel size of the font.
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Points: the point size of the font, in tenths of a
point. A point is 1/72 of an inch; the relationship of
point size to pixel size is determined by the
resolution (dots per inch) the X server thinks the
display has. Typically people do not configure their X
server to match their monitor, so the X server's idea
of the current resolution may be wildly
inaccurate.
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Horizontal Resolution: horizontal display resolution in
dots per inch (dpi) the font is designed for.
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Vertical Resolution: vertical display resolution the
font is designed for.
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Spacing: Monospace (abbreviated m) or proportional (abbreviated
p). Indicates whether
all characters have the same width or different
characters may have different widths.
-
Average Width: the mean width of all characters in the
font, in tenths of a pixel.
-
Character Set Registry: the organization or standard
defining the character set.
-
Character Set Encoding: specifies the specific
character set encoding. These last two fields in
combination specify the character set. For European
languages iso8859-1
will almost always be used. This is the "Latin-1"
character set, which is an 8-bit encoding including
ASCII as a subset.
It is not necessary to specify all 14 fields when
requesting a font. "Wild cards" are allowed: * matches any number of characters, and
? matches a single character.
For example, a 160 point Bold Roman Helvetica font can be
requested like this:
-*-helvetica-bold-r-*-*-*-160-*-*-*-*-*-*
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When you pass a font name to
gdk_font_load(), you should consider it a default
choice only. Users in other countries will certainly want
to use a font appropriate to their language; US and
European users might want to customize fonts too. Also,
there are no fonts guaranteed to exist on all servers. Thus
you should provide a way to customize any fonts you use.
The easiest way is to use fonts from a widget's GtkStyle (again, see the section called GtkStyle and Themes).
gdk_font_load() returns NULL if it fails to find a font
matching the supplied name. When you are done with a font,
you should release it by calling
gdk_font_unref().
Figure 10 summarizes these functions.
When loading fonts, you will need to specify at least the
font name, the weight, the slant, and a size---otherwise
the * wildcards would probably
load a random bold italic font, which is unlikely to be
what you want. The Xlib Programming
Manual suggests that you always specify the font size
in points, so that users with different monitors will get
the right effect; however, X servers do not reliably know
the display resolution, so this is more theory than
reality. Perhaps it's better to specify pixels, because you
do know the pixel size of other elements of your display.
Neither is perfect; be sure you make your application's
fonts configurable.
To use a font, you typically need detailed information
about its metrics. Font metrics
are used to position characters with respect to one
another, and to determine the size of strings rendered in
the font. The most fundamental metrics are the ascent and
descent of the font. Text is placed on a baseline; the baseline is like a rule on
a sheet of notebook paper. The bottom of each character
touches the baseline. Some characters (such as lowercase
"p" and "y") extend below the baseline. A font's descent is the maximum distance
below the baseline reached by its characters. Its ascent is the maximum distance
above the baseline. The height
of a font is the sum of the ascent and the descent. When
rendering multiple lines of text, you should leave at
least the font's height between each baseline.
Ascent and descent are available as members of the GdkFont struct:
typedef struct _GdkFont GdkFont;
struct _GdkFont
{
GdkFontType type;
gint ascent;
gint descent;
};
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The type field
distinguishes fonts from fontsets; fontsets are used to
display non-European languages.
Individual characters in a font have their own ascent and
descent; a character's ascent and descent will always be
less than or equal to the font's ascent and descent. GDK
can calculate the sum of the maximum ascent and descent
for a particular string, rather than an entire font; this
height will be less than or equal to the font's height.
The relevant functions are
gdk_string_height(),
gdk_text_height(), and
gdk_char_height().
gdk_text_height() is different from gdk_string_height() because it accepts
the length of the string as an argument; gdk_string_height() calls strlen() for you. Thus gdk_text_height() is preferred if you
already know a string's length.
In addition to its vertical metrics, each character in a
font has three metrics describing its horizontal
dimensions. The width of a
character is the distance from the left origin of the
character to the start of the next character. Note: the
width is not the distance to the
furthest-right pixel in a character; in some fonts,
especially italic fonts, characters may lean over past
the start of the next character. The left-side-bearing or lbearing is the distance from the left
origin to the leftmost pixel in the character; the right-side-bearing or rbearing is the distance from the origin
to the rightmost pixel in the character. Thus the rbearing can be larger than the
width, in the case of italic fonts that lean over past
the start of the next character.
GDK functions that return a character or string width
return the width of the character, or the sum of the
character widths in the string. If the rbearing of the
rightmost character is greater than its width, a string
may require more space than
gdk_string_width(),
gdk_text_width(), or
gdk_char_width() will return. Like the
height-measurement functions, the _string_ variant computes the length
of the string for you, and the
_text_ variant accepts a precomputed length as an
argument.
The functions ending in
_measure are more often what you want. For a
string with N characters, these
return the sum of the widths of the first N-1 characters, plus the rbearing of the
final character. That is, they take into account the fact
that rbearing may be greater than width. If you're trying
to decide how much space to leave for rendering a string,
you probably want the
gdk_string_measure(),
gdk_text_measure(), or
gdk_char_measure() functions. Sometimes you don't
want to consider the rbearing, however; for example, if
you're centering a string, it may look nicer to use the
width (because a small italic flourish extending past the
width won't "fill" the space and the string will look
slightly left of center).
gdk_text_extents() and gdk_string_extents() return all the metrics for a string,
including both bearings, the width, ascent, and descent.
The returned left-side-bearing is the leftmost pixel in
the string; the right-side-bearing is the rightmost pixel
as returned by
gdk_text_measure(). The width is the sum of
character widths, as returned by
gdk_text_width().
Figure 11
depicts the various font metrics. Figure 12
summarizes the API for querying font metrics. All font
metrics are computed on the client side, so these
functions are not expensive compared to most of the
drawing API.
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