CACHECHARS(2)CACHECHARS(2)NAME
cachechars, agefont, loadchar, Subfont, Fontchar, Font - font utili‐
ties
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 simultane‐
ously. 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 pro‐
grams: initdraw and its associated font variable, openfont, string‐
width, string, and freefont are sufficient for most purposes. The rou‐
tines 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
rows down from the top of the subfont image. The array has n+1 ele‐
ments, 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 and fields of Font are described in graphics(2). contains pointers
to Cachefonts. A Cachefont connects runes through inclusive, to the
subfont with file name it corresponds to a line of the file describing
the font.
The characters are taken from the subfont starting at character number
(usually zero) in the subfont, permitting selection of parts of sub‐
fonts. 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 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 load‐
char 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 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, updat‐
ing *sfname as described above. It returns 1 to indicate success.
The fields record when subfonts and characters have been used. The
font is increased every time the font is used (agefont does this). A
character or subfont is set to the font age at each use. Thus, charac‐
ters or subfonts with small ages are the best candidates for replace‐
ment when the cache is full.
SOURCE
/sys/src/libdraw
SEE ALSOgraphics(2), allocimage(2), draw(2), subfont(2), image(6), font(6)DIAGNOSTICS
All of the functions use the graphics error function (see graphics(2)).
CACHECHARS(2)
