CACHECHARS(3G)

NAME
cachechars, Subfont, Cachesubf, Fontchar, Font - font utilities

SYNOPSIS
#include <libc.h>
#include <libg.h>

int  cachechars(Font *f, char **s, XChar2b *c, int n, int *widp, ushort *fp)

DESCRIPTION
A Font may contain too many characters to hold in memory simultaneously. The graphics library and X server cooperate to solve this problem by maintaining a cache of recently used character images. The details of this cooperation need not be known by most programs: xtbinit and its associated *p9font X resource, rdfontfile, charwidth, string, and ffree are sufficient for most purposes. The routines described below are used internally by the graphics library to maintain the font cache.

A Subfont may be considered to be a set of images for a contiguous range of characters, stored as a single bitmap with the characters placed side-by-side on a common baseline. It is described by the following data structures.

        typedef
        struct Fontchar {
        	ushort	cwidth;	/* width of glyph */
        	uchar	top;	/* first non-zero scan-line */
        	uchar	bottom;	/* last non-zero scan-line */
        	char	left;	/* offset of baseline */
        	uchar	width;	/* advance to next char's origin */
        } Fontchar;
        typedef
        struct Subfont {
        	short	minrow;	/* first character row in font (for X subfonts) */
        	short	mincol;	/* first character col in font (for X subfonts) */
        	short	minchar;	/* first char code in font (for X subfonts) */
        	short	maxchar;	/* last char code in font (for X subfonts) */
        	short	width;	/* number of chars in row (for X subfonts) */
        	short	n;	/* number of chars in subfont */
        	unsigned char height;	/* height of bitmap */
        	char	ascent;	/* top of bitmap to baseline */
        	Fontchar	*info;	/* n+1 character descriptors */
        	int	id;	/* of font */
        } Subfont;

The actual bitmap for each character is stored on the server but the metrics associated with character c are cached locally in subfont->info[c]. When a character is displayed at Point p in a bitmap, the character rectangle is placed at (p.x+i->left, p.y) and the next character of the string is displayed at (p.x+i->width+(i+1)->left, p.y). The baseline of the characters is rows down from the top of the subfont bitmap. Each Fontchar has two widths associated with it: width is the width of the rectangular bitmap to contain the character; cwidth is the true width of the character, that is, the number of pixels between the leftmost and rightmost pixels of the character glyph.

A Font consists of an overall height and ascent and a collection of subfonts together with the ranges of runes (see utf(4)) they represent. Fonts are described by the following structures.

        typedef
        struct Cachesubf {
        	Rune	min;	/* rune value of 0th char in subfont */
        	Rune	max;	/* rune value+1 of last char in subfont */
        	char	*name;
        	Subfont	*f;	/* attached subfont */
        } Cachefont;
        typedef
        struct Font {
        	char	*name;
        	uchar	height;	/* max height of bitmap, interline spacing */
        	char	ascent;	/* top of bitmap to baseline */
        	char	width;	/* widest so far; used in caching only */
        	char	ldepth;	/* of images */
        	short	id;	/* of font */
        	short	nsubf;	/* number of subfonts */
        	Cachesubf	*subf;	/* as read from file */
        } Font;

The and fields of Font are described in graphics(3). contains pointers to Cachesubfs. A Cachesubf connects runes through inclusive, to the subfont it corresponds to a line of the file describing the font.

The image for rune r is found in position r-min+minchar of the subfont.

For each font, the library, with support from the graphics server, maintains a list of subfonts. The of a font is the maximum of the horizontal extents of the characters in the cache. [ is unused in the X implementation of libg.] String draws a string by calling cachechars and emitting a sequence of X font indices to draw. Cachechars calculates the subfont/index pairs for the characters pointed to by *s. It calls getsubfont to fetch subfonts which are not yet in the subfont list of f. Cachechars translates the character string into a set of subfont/index pairs. It loads the character indices into the array c, up to a maximum of n indices or the length of the string. It also loads the corresponding element of fp with the subfont index for each character. Cachechars returns in c the number of subfont indices emitted, updates *s to point to the next character to be processed, and sets *widp to the total width of the characters processed. Cachechars may return before the end of the string if it cannot proceed. It can return zero if it is unable to make progress because a character code does not covered by any subfont range.

SEE ALSO
graphics(3), balloc(3), bitblt(3), font(4).

DIAGNOSTICS
All of the functions use the graphics error function (see graphics(3)).