8.5 Font Metrics

A font is a graphical description of a set of characters that are used to increase efficiency whenever a set of small, similar sized patterns are repeatedly used.

This section discusses how to:

The X server loads fonts whenever a program requests a new font. The server can cache fonts for quick lookup. Fonts are global across all screens in a server. Several levels are possible when dealing with fonts. Most applications simply use XLoadQueryFont() to load a font and query the font metrics.

Characters in fonts are regarded as masks. Except for image text requests, the only pixels modified are those in which bits are set to 1 in the character. This means that it makes sense to draw text using stipples or tiles (for example, many menus gray-out unusable entries).

The XFontStruct structure contains all of the information for the font and consists of the font-specific information as well as a pointer to an array of XCharStruct structures for the characters contained in the font. The XFontStruct, XFontProp, and XCharStruct structures contain:

typedef struct {
	short lbearing;			/* origin to left edge of raster */
	short rbearing;			/* origin to right edge of raster */
	short width;			/* advance to next char's origin */
	short ascent;			/* baseline to top edge of raster */
	short descent;			/* baseline to bottom edge of raster */
	unsigned short attributes;	/* per char flags (not predefined) */
} XCharStruct;

typedef struct {
	Atom	name;
	unsigned long card32;
} XFontProp;

typedef struct {	/* normal 16 bit characters are two bytes */
    unsigned char byte1;
    unsigned char byte2;
} XChar2b;

typedef struct {
	XExtData *ext_data;		/* hook for extension to hang data */
	Font fid;			/* Font id for this font */
	unsigned direction;		/* hint about the direction font is painted */
	unsigned min_char_or_byte2;	/* first character */
	unsigned max_char_or_byte2;	/* last character */
	unsigned min_byte1;		/* first row that exists */
	unsigned max_byte1;		/* last row that exists */
	Bool all_chars_exist;		/* flag if all characters have nonzero size */
	unsigned default_char;		/* char to print for undefined character */
	int n_properties;		/* how many properties there are */
	XFontProp *properties;		/* pointer to array of additional properties */
	XCharStruct min_bounds;		/* minimum bounds over all existing char */
	XCharStruct max_bounds;		/* maximum bounds over all existing char */
	XCharStruct *per_char;		/* first_char to last_char information */
	int ascent;			/* logical extent above baseline for spacing */
	int descent;			/* logical decent below baseline for spacing */
} XFontStruct;

X supports single byte/character, two bytes/character matrix, and 16-bit character text operations. Note that any of these forms can be used with a font, but a single byte/character text request can only specify a single byte (that is, the first row of a 2-byte font). You should view 2-byte fonts as a two-dimensional matrix of defined characters: byte1 specifies the range of defined rows and byte2 defines the range of defined columns of the font. Single byte/character fonts have one row defined, and the byte2 range specified in the structure defines a range of characters.

The bounding box of a character is defined by the XCharStruct of that character. When characters are absent from a font, the default_char is used. When fonts have all characters of the same size, only the information in the XFontStruct min and max bounds are used.

The members of the XFontStruct have the following semantics:

For a character origin at [x,y], the bounding box of a character (that is, the smallest rectangle that encloses the character's shape) described in terms of XCharStruct components is a rectangle with its upper-left corner at:

[x + lbearing, y - ascent]

Its width is:

rbearing - lbearing

Its height is:

ascent + descent

The origin for the next character is defined to be:

[x + width, y]

The lbearing member defines the extent of the left edge of the character ink from the origin. The rbearing member defines the extent of the right edge of the character ink from the origin. The ascent member defines the extent of the top edge of the character ink from the origin. The descent member defines the extent of the bottom edge of the character ink from the origin. The width member defines the logical width of the character.

Note that the baseline (the y position of the character origin) is logically viewed as being the scanline just below nondescending characters. When descent is zero, only pixels with Y-coordinates less than y are drawn, and the origin is logically viewed as being coincident with the left edge of a nonkerned character. When lbearing is zero, no pixels with X-coordinate less than x are drawn. Any of the XCharStruct metric members could be negative. If the width is negative, the next character will be placed to the left of the current origin.

The X protocol does not define the interpretation of the attributes member in the XCharStruct structure. A nonexistent character is represented with all members of its XCharStruct set to zero.

A font is not guaranteed to have any properties. The interpretation of the property value (for example, long or unsigned long) must be derived from a priori knowledge of the property. A basic set of font properties is specified in the X Consortium standard X Logical Font Description Conventions.

Next: Loading and Freeing Fonts

Christophe Tronche, ch@tronche.com