8.5 Font Metrics

This section discusses how to:

• Load and free fonts

• Obtain and free font names

• Compute character string sizes

• Return logical extents

• Query character string sizes

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:

• The direction member can be either FontLeftToRight or FontRightToLeft. It is just a hint as to whether most XCharStruct elements have a positive (FontLeftToRight) or a negative (FontRightToLeft) character width metric. The core protocol defines no support for vertical text.

• If the min_byte1 and max_byte1 members are both zero, min_char_or_byte2 specifies the linear character index corresponding to the first element of the per_char array, and max_char_or_byte2 specifies the linear character index of the last element.

  If either min_byte1 or max_byte1 are nonzero, both min_char_or_byte2 and max_char_or_byte2 are less than 256, and the 2-byte character index values corresponding to the per_char array element N (counting from 0) are:

  byte1 = N/D + min_byte1
  byte2 = N\D + min_char_or_byte2

  where:

  D = max_char_or_byte2 - min_char_or_byte2 + 1
  / = integer division
  \ = integer modulus

• If the per_char pointer is NULL, all glyphs between the first and last character indexes inclusive have the same information, as given by both min_bounds and max_bounds.

• If all_chars_exist is True, all characters in the per_char array have nonzero bounding boxes.

• The default_char member specifies the character that will be used when an undefined or nonexistent character is printed. The default_char is a 16-bit character (not a 2-byte character). For a font using 2-byte matrix format, the default_char has byte1 in the most-significant byte and byte2 in the least-significant byte. If the default_char itself specifies an undefined or nonexistent character, no printing is performed for an undefined or nonexistent character.

• The min_bounds and max_bounds members contain the most extreme values of each individual XCharStruct component over all elements of this array (and ignore nonexistent characters). The bounding box of the font (the smallest rectangle enclosing the shape obtained by superimposing all of the characters at the same origin [x,y]) has its upper-left coordinate at:

  [x + min_bounds.lbearing, y - max_bounds.ascent]

  Its width is:

  max_bounds.rbearing - min_bounds.lbearing

  Its height is:

  max_bounds.ascent + max_bounds.descent

• The ascent member is the logical extent of the font above the baseline that is used for determining line spacing. Specific characters may extend beyond this.

• The descent member is the logical extent of the font at or below the baseline that is used for determining line spacing. Specific characters may extend beyond this.

• If the baseline is at Y-coordinate y, the logical extent of the font is inclusive between the Y-coordinate values (y - font.ascent) and (y + font.descent - 1). Typically, the minimum interline spacing between rows of text is given by ascent + descent.

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