CACHECHARS(2G) CACHECHARS(2G)
NAME
cachechars, agefont, loadchar, Subfont, Fontchar, Font -
font utilities
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <libg.h>
int cachechars(Font *f, char **s, ushort *c, int n, int
*widp)
int loadchar(Font *f, Rune r, Cacheinfo *c, int h, int
noclr)
void agefont(Font *f)
DESCRIPTION
A Font may contain too many characters to hold in memory
simultaneously. The graphics library and bitblt device (see
bit(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: binit
and its associated font variable, 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 is a set of images for a contiguous range of char-
acters, 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 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 {
short n; /* number of chars in subfont */
char height; /* height of bitmap */
char ascent; /* top of bitmap to baseline */
Fontchar *info; /* n+1 Fontchars */
int id; /* id as known in /dev/bitblt */
} Font;
Page 1 Plan 9 (printed 12/21/24)
CACHECHARS(2G) CACHECHARS(2G)
The bitmap 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 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, p.y). The baseline of the char-
acters is `ascent' rows down from the top of the subfont
bitmap. 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 Bitmap associated with a Subfont s is s->info[s->n].x.
A Font consists of an overall height and ascent and a col-
lection of subfonts together with the ranges of runes (see
utf(6)) they represent. Fonts are described by the follow-
ing structures.
typedef
struct Cachefont {
Rune min; /* rune value of 0th char in subfont */
Rune max; /* rune value+1 of last char in subfont */
int abs; /* name has been made absolute */
char *name;
} Cachefont;
typedef
struct Cacheinfo {
Rune value; /* value of character at this slot in cache */
ushort age;
ulong xright; /* right edge of bits */
Fontchar;
} 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;
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 nsub; /* number of subfonts */
ulong age; /* increasing counter; used for LRU */
Page 2 Plan 9 (printed 12/21/24)
CACHECHARS(2G) CACHECHARS(2G)
int ncache; /* size of cache */
int nsubf; /* size of subfont list */
Cacheinfo *cache;
Cachesubf *subf;
Cachefont **sub; /* as read from file */
} Font;
The `height', `ascent', and `ldepth' 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 corre-
sponds to a line of the file describing the font.
The image for rune r is found in position r-min of the sub-
font.
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 charac-
ters pointed to by *s 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 maxi-
mum 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.
It can return zero if it is unable to make progress because
it is unable to resize the caches.
Loadchar loads a character image into the character cache.
First, if necessary, it loads the subfont containing the
character. 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.
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'
Page 3 Plan 9 (printed 12/21/24)
CACHECHARS(2G) CACHECHARS(2G)
is set to the font age. Thus, characters or subfonts with
small ages are the best candidates for replacement when the
cache is full.
SEE ALSO
graphics(2), balloc(2), bitblt(2), subfalloc(2), bitmap(6),
font(6)
DIAGNOSTICS
All of the functions use the graphics error function (see
graphics(2)).
Page 4 Plan 9 (printed 12/21/24)