plan9fox/sys/man/2/cachechars

.TH CACHECHARS 2
.SH NAME
cachechars, agefont, loadchar, Subfont, Fontchar, Font  \- font utilities
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.br
.B #include <draw.h>
.PP
.ta \w'\fLCacheinfo 'u
.PP
.B
int  cachechars(Font *f, char **s, Rune **r, ushort *c, int max,
.PP
.B
	   int *widp, char **sfname)
.PP
.B
int  loadchar(Font *f, Rune r, Cacheinfo *c, int h,
.PP
.B
	   int noclr, char **sfname)
.PP
.B
void agefont(Font *f)
.SH DESCRIPTION
A
.I Font
may contain too many characters to hold in memory
simultaneously.
The graphics library and draw device (see
.IR 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:
.I initdraw
and its associated
.I font
variable,
.IR openfont ,
.IR stringwidth ,
.IR string ,
and
.I freefont
are sufficient for most purposes.
The routines described below are used internally by the graphics library
to maintain the font cache.
.PP
A
.B 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.
.IP
.EX
.ta 6n +\w'Fontchar 'u +\w'bottom;   'u
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;
.EE
.PP
The image fills the rectangle
\fL(0, 0, \fIw\fP, height)\fR,
where
.I w
is the sum of the horizontal extents (of non-zero pixels)
for all characters.
The pixels to be displayed for character
.I c
are in the rectangle
\fL(\fIi\fP->x, \fIi\fP->top, (\fIi\fP+1)->x, \%\fIi\fP->bottom)\fR
where
.I i
is
.B
&subfont->info[\fIc\fP]\fR.
When a character is displayed at
.B Point
.B p
in an image,
the character rectangle is placed at
.BI (p.x+ i ->left,
.B p.y)
and the next character of the string is displayed at
.BI (p.x+ i ->width,
.BR p.y) .
The baseline of the characters is
.L ascent
rows down from the top of the subfont image.
The
.L info
array has
.B n+1
elements, one each for characters 0 to
.BR n-1
plus an additional entry so the size of the last character
can be calculated.
Thus the width,
.IR w ,
of the
.B Image
associated with a
.B Subfont
.B s
is
.BR s->info[s->n].x .
.PP
A
.B Font
consists of an overall height and ascent
and a collection of subfonts together with the ranges of runes (see
.IR utf (6))
they represent.
Fonts are described by the following structures.
.IP
.EX
.ta 6n +\w'Cacheinfo 'u +\w'height;   'u
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;
.EE
.PP
The
.LR height
and
.LR ascent
fields of Font are described in
.IR graphics (2).
.L Sub
contains
.L nsub
pointers to
.BR Cachefonts .
A
.B Cachefont
connects runes
.L min
through
.LR max ,
inclusive, to the subfont
with file name
.LR name ;
it corresponds to a line of the file describing the font.
.PP
The characters
are taken from the subfont starting at character number
.L offset
(usually zero) in the subfont, permitting selection of parts of subfonts.
Thus
the image for rune
.I r
is found in position
.IB r -min+offset
of the subfont.
.PP
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
.B subf
and
.B cache
fields are used by the library to maintain these caches.
The
.L width
of a font is the maximum of the horizontal extents of the characters
in the cache.
.I String
draws a string by loading the cache and emitting a sequence of
cache indices to draw.
.I Cachechars
guarantees the images for the characters pointed to by
.I *s
or
.I *r
(one of these must be nil in each call)
are in the cache of
.IR f .
It calls
.I loadchar
to put missing characters into the cache.
.I Cachechars
translates the character string into a set of cache indices
which it loads into the array
.IR c ,
up to a maximum of
.I n
indices or the length of the string.
.I Cachechars
returns in
.I c
the number of cache indices emitted,
updates
.I *s
to point to the next character to be processed, and sets
.I *widp
to the total width of the characters processed.
.I 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
.B *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.
.PP
.I Loadchar
loads a character image into the character cache.
Then it tells the graphics server to copy the character 
into position
.I h
in the character cache.
If the current font
.L width
is smaller than the horizontal extent of the character being loaded,
.I loadfont
clears the cache and resets it to
accept characters with the bigger width, unless
.I noclr
is set, in which case it just returns \-1.
If the character does not exist in the font at all,
.I loadfont
returns 0; if it is unable to load the character
without destroying cached information, it returns \-1,
updating
.B *sfname
as described above.
It returns 1 to indicate success.
.PP
The
.L age
fields record when
subfonts and characters have been used.
The font
.L age
is increased every time the font is used
.RI ( agefont
does this).
A character or subfont
.L 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.
.SH SOURCE
.B /sys/src/libdraw
.SH SEE ALSO
.IR graphics (2),
.IR allocimage (2),
.IR draw (2),
.IR subfont (2),
.IR image (6),
.IR font (6)
.SH DIAGNOSTICS
All of the functions use the graphics error function (see
.IR graphics (2)).