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NAME
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cachechars, agefont, loadchar, Subfont, Fontchar, Font – font utilities
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SYNOPSIS
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#include <u.h>
#include <libc.h>
#include <draw.h>
int cachechars(Font *f, char **s, Rune **r, ushort *c, int max,
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int *widp, char **sfname)
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int loadchar(Font *f, Rune r, Cacheinfo *c, int h,
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int noclr, char **sfname)
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void agefont(Font *f)
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DESCRIPTION
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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.
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typedef
struct Fontchar {
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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 */
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} Fontchar;
typedef
struct Subfont {
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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 */
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} Subfont;
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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(7)) they
represent. Fonts are described by the following structures.
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typedef
struct Cachefont {
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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 */
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} Cachefont;
typedef
struct Cacheinfo {
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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;
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} Cacheinfo;
typedef
struct Cachesubf {
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ulong age; /* for replacement */
Cachefont *cf; /* font info that owns us */
Subfont *f; /* attached subfont */
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} Cachesubf;
typedef
struct Font {
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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;
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} Font;
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The height and ascent fields of Font are described in graphics(3).
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.
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SOURCE
SEE ALSO
DIAGNOSTICS
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All of the functions use the graphics error function (see graphics(3)).
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