NAME
cachechars, agefont, loadchar, Subfont, Fontchar, Font – font utilities

SYNOPSIS
#include <u.h>
#include <libc.h>
#include <draw.h>

int    cachechars(Font *f, char **s, Rune **r, ushort *c, int max,

int *widp, char **sfname)

int    loadchar(Font *f, Rune r, Cacheinfo *c, int h,

int noclr, char **sfname)

void agefont(Font *f)

DESCRIPTION
A Font may contain too many characters to hold in memory simultaneously. The graphics library and draw device (see draw(3)) 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: initdraw and its associated font variable, openfont, stringwidth, string, and freefont are sufficient for most purposes. The routines described below are used internally by the graphics library to maintain the font cache.

A Subfont is a set of images for a contiguous range of characters, stored as a single image with the characters placed side–by–side on a common baseline. It is described by the following data structures.
typedef
struct Fontchar {
int        x;          /* left edge of bits */
uchar      top;        /* first non–zero scan–line */
uchar      bottom;     /* last non–zero scan–line */
char       left;       /* offset of baseline */
uchar      width;      /* width of baseline */
} Fontchar;
typedef
struct Subfont {
char       *name;
short      n;          /* number of chars in subfont */
uchar      height;     /* height of image */
char       ascent;     /* top of image to baseline */
Fontchar *info;      /* n+1 Fontchars */
Image      *bits;      /* of font */
} Subfont;

The image fills the rectangle (0, 0, w, height), where w is the sum of the horizontal extents (of non–zero pixels) for all characters. The pixels to be displayed for character c are in the rectangle (i–>x, i–>top, (i+1)–>x, i–>bottom) where i is &subfont–>info[c]. When a character is displayed at Point p in an image, 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, p.y). The baseline of the characters is ascent rows down from the top of the subfont image. The info array has n+1 elements, one each for characters 0 to n–1 plus an additional entry so the size of the last character can be calculated. Thus the width, w, of the Image associated with a Subfont s is s–>info[s–>n].x.

A Font consists of an overall height and ascent and a collection of subfonts together with the ranges of runes (see utf(6)) they represent. Fonts are described by the following structures.
typedef
struct Cachefont {
Rune        min;        /* value of 0th char in subfont */
Rune        max;        /* value+1 of last char in subfont */
int         offset;     /* posn in subfont of char at min */
char        *name;      /* stored in font */
char        *subfontname;/* to access subfont */
} Cachefont;
typedef
struct Cacheinfo {
ushort      x;          /* left edge of bits */
uchar       width;      /* width of baseline */
schar       left;       /* offset of baseline */
Rune        value;      /* of char at this slot in cache */
ushort      age;
} Cacheinfo;
typedef
struct Cachesubf {
ulong       age;        /* for replacement */
Cachefont *cf;        /* font info that owns us */
Subfont     *f;         /* attached subfont */
} Cachesubf;
typedef
struct Font {
char        *name;
Display     *display;
short       height;     /* max ht of image;interline space*/
short       ascent;     /* top of image to baseline */
short       width;      /* widest so far; used in caching */
short       nsub;       /* number of subfonts */
ulong       age;        /* increasing counter; for LRU */
int         ncache;     /* size of cache */
int         nsubf;      /* size of subfont list */
Cacheinfo *cache;
Cachesubf *subf;
Cachefont **sub;      /* as read from file */
Image       *cacheimage;
} Font;

The height and ascent fields of Font are described in graphics(2). Sub contains nsub pointers to Cachefonts. A Cachefont connects runes min through max, inclusive, to the subfont with file name name; it corresponds to a line of the file describing the font.

The characters are taken from the subfont starting at character number offset (usually zero) in the subfont, permitting selection of parts of subfonts. Thus the image for rune r is found in position r–min+offset of the subfont.

For each font, the library, with support from the graphics server, maintains a cache of subfonts and a cache of recently used character images. The subf and cache fields are used by the library to maintain these caches. The width of a font is the maximum of the horizontal extents of the characters in the cache. String draws a string by loading the cache and emitting a sequence of cache indices to draw. Cachechars guarantees the images for the characters pointed to by *s or *r (one of these must be nil in each call) are in the cache of f. It calls loadchar to put missing characters into the cache. Cachechars translates the character string into a set of cache indices which it loads into the array c, up to a maximum of n indices or the length of the string. Cachechars returns in c the number of cache 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 without destroying active data in the caches. If it needs to load a new subfont, it will fill *sfname with the name of the subfont it needs and return –1. It can return zero if it is unable to make progress because it cannot resize the caches.

Loadchar loads a character image into the character cache. Then it tells the graphics server to copy the character into position h in the character cache. If the current font width is smaller than the horizontal extent of the character being loaded, loadfont clears the cache and resets it to accept characters with the bigger width, unless noclr is set, in which case it just returns –1. If the character does not exist in the font at all, loadfont returns 0; if it is unable to load the character without destroying cached information, it returns –1, updating *sfname as described above. It returns 1 to indicate success.

The age fields record when subfonts and characters have been used. The font age is increased every time the font is used (agefont does this). A character or subfont age is set to the font age at each use. Thus, characters or subfonts with small ages are the best candidates for replacement when the cache is full.

SOURCE
/sys/src/libdraw

SEE ALSO
graphics(2), allocimage(2), draw(2), subfont(2), image(6), font(6)

DIAGNOSTICS
All of the functions use the graphics error function (see graphics(2)).
Copyright © 2017 Alcatel-Lucent. All rights reserved.