Display, Point, Rectangle, Cursor, initdraw, geninitdraw, drawerror, initdisplay, closedisplay, getwindow, gengetwindow, flushimage, bufimage, lockdisplay, unlockdisplay, cursorswitch, cursorset, openfont, buildfont, freefont, Pfmt, Rfmt, strtochan, chantostr, chantodepth – interactive graphics

#include <u.h>
#include <libc.h>
#include <draw.h>
#include <cursor.h>
int    initdraw(void (*errfun)(Display*, char*), char *font,
char *label)
int    geninitdraw(char *devdir, void(*errfun)(Display*, char*),
char *font, char *label, char *mousedir, char *windir,
int ref)
int    newwindow(char *str)
void    drawerror(Display *d, char *msg)
Display*initdisplay(char *devdir, char *win, void(*errfun)(Display*, char*))
void    closedisplay(Display *d)
int    flushimage(Display *d, int vis)
int    bufimage(Display *d, int n)
int    lockdisplay(Display *d)
int    unlockdisplay(Display *d)
int    getwindow(Display *d, int ref)
int    gengetwindow(Display *d, char *winname,
Image **ip, Screen **sp, int ref)
int    scalesize(Display *d, int n)
void    cursorswitch(Cursor *curs)
void    cursorset(Point p)
Font* openfont(Display *d, char *name)
Font* buildfont(Display *d, char *desc, char *name)
void    freefont(Font *f)
int    Pfmt(Fmt*)
int    Rfmt(Fmt*)
ulong strtochan(char *s)
char* chantostr(char *s, ulong chan)
int    chantodepth(ulong chan)
extern Display *display
extern Image    *screen
extern Screen    *_screen
extern Font     *font

A Display structure represents a connection to the graphics device, draw(3), holding all graphics resources associated with the connection, including in particular raster image data in use by the client program. The structure is defined (in part) as:
struct Display
void     (*error)(Display*, char*);
Image     *black;
Image     *white;
Image     *opaque;
Image     *transparent;
Image     *image;
Font     *defaultfont;
Subfont *defaultsubfont;
A Point is a location in an Image (see below and draw(3)), such as the display, and is defined as:
struct Point {
int x;
int y;
} Point;
The coordinate system has x increasing to the right and y increasing down.
A Rectangle is a rectangular area in an image.
struct Rectangle {
Point min;       /* upper left */
Point max;       /* lower right */
} Rectangle;
By definition, min.xmax.x and min.ymax.y. By convention, the right (maximum x) and bottom (maximum y) edges are excluded from the represented rectangle, so abutting rectangles have no points in common. Thus, max contains the coordinates of the first point beyond the rectangle.
The Image data structure is defined in draw(3).
A Font is a set of character images, indexed by runes (see utf(7)). The images are organized into Subfonts, each containing the images for a small, contiguous set of runes. The detailed format of these data structures, which are described in detail in cachechars(3), is immaterial for most applications. Font and Subfont structures contain two interrelated fields: ascent, the distance from the top of the highest character (actually the top of the image holding all the characters) to the baseline, and height, the distance from the top of the highest character to the bottom of the lowest character (and hence, the interline spacing). See cachechars(3) for more details.
Buildfont parses the font description in the buffer desc, returning a Font* pointer that can be used by string (see draw(3)) to draw characters from the font. Openfont does the same, but reads the description from the named font. Freefont frees a font. In contrast to Plan 9, font names in Plan 9 from User Space are a small language describing the desired font. See font(7) for details.
A Cursor is defined:
typedef struct
Cursor {
Point offset;
uchar clr[2*16];
uchar set[2*16];
} Cursor;
The arrays are arranged in rows, two bytes per row, left to right in big-endian order to give 16 rows of 16 bits each. A cursor is displayed on the screen by adding offset to the current mouse position, using clr as a mask to draw white at the pixels where clr is one, and then drawing black at the pixels where set is one.
The routine initdraw connects to the display; it returns –1 if it fails and sets the error string. Initdraw sets up the global variables display (the Display structure representing the connection), screen (an Image representing the display memory itself or, if rio(1) is running, the client’s window), and font (the default font for text). The arguments to initdraw include a label, which is written to /dev/label if non-nil so that it can be used to identify the window when hidden (see rio(1)). The font is created by reading the named font file. If font is null, initdraw reads the file named in the environment variable $font; if $font is not set, it imports the default (usually minimal) font from the operating system. (See font(7) for a full discussion of font syntaxes.) The global font will be set to point to the resulting Font structure. The errfun argument is a graphics error function to call in the event of a fatal error in the library; it must never return. Its arguments are the display pointer and an error string. If errfun is nil, the library provides a default, called drawerror. Another effect of initdraw is that it installs print(3) formats Pfmt and Rfmt as %P and %R for printing Points and Rectangles.
The geninitdraw function provides a less automated way to establish a connection, for programs that wish to connect to multiple displays. Devdir is the name of the directory containing the device files for the display (if nil, default /dev); errfun, font, and label are as in initdraw; mousedir and windir are the directories holding the mouse and winname files; and ref specifies the refresh function to be used to create the window, if running under rio(1) (see window(3)).
Initdisplay is part of geninitdraw; it sets up the display structures but does not allocate any fonts or call getwindow. The arguments are similar to those of initdraw; win names the directory, default /dev, in which the files associated with the window reside. Closedisplay disconnects the display and frees the associated data structures. Neither of these routines is needed by most programs, since initdraw calls them as needed.
The data structures associated with the display must be protected in a multi-process program, because they assume only one process will be using them at a time. Multi-process programs should set display−>locking to 1, to notify the library to use a locking protocol for its own accesses, and call lockdisplay and unlockdisplay around any calls to the graphics library that will cause messages to be sent to the display device. Initdraw and geninitdraw initialize the display to the locked state.
Getwindow returns a pointer to the window associated with the application; it is called automatically by initdraw to establish the screen pointer but must be called after each resizing of the window to restore the library’s connection to the window. If rio is not running, it returns display−>image; otherwise it negotiates with rio by looking in /dev/winname to find the name of the window and opening it using namedimage (see allocimage(3)). The resulting window will be created using the refresh method ref (see window(3)); this should almost always be Refnone because rio provides backing store for the window.
Getwindow overwrites the global variables screen, a pointer to the Image defining the window (or the overall display, if no window system is running); and _screen, a pointer to the Screen representing the root of the window’s hierarchy. (See window(3). The overloading of the screen word is an unfortunate historical accident.) Getwindow arranges that screen point to the portion of the window inside the border; sophisticated clients may use _screen to make further subwindows. If getwindow is being called due to a resizing of the window, the resize may be accompanied by a change in screen pixel density (DPI), in which case the value of the Display’s dpi field and any open Font’s height and ascent fields may be updated during the call to getwindow. Programs should discard any cached information about display or font sizes. Gengetwindow’s extra arguments are the full path of the window’s winname file and pointers to be overwritten with the values of the ‘global’ Image and Screen variables for the new window.
Historically, Plan 9 graphics programs have used fixed-size graphics features that assume a narrow range of display densities, around 100 dpi: pixels (or dots) per inch. The new field display−>dpi contains the display’s actual density if known, or else DefaultDPI (100). Scalesize scales the fixed pixel count n by display−>dpi/DefaultDPI, rounding appropriately.
The mouse cursor is always displayed. The initial cursor is an arrow. Cursorswitch causes the argument cursor to be displayed instead. A zero argument causes a switch back to the arrow cursor. Cursorset moves the mouse cursor to position p, provided (if in a window) that the requesting program is executing in the current window and the mouse is within the window boundaries; otherwise cursorset is a no-op.
The graphics functions described in draw(3), allocimage(3), cachechars(3), and subfont(3) are implemented by writing commands to files under /dev/draw (see draw(3)); the writes are buffered, so the functions may not take effect immediately. Flushimage flushes the buffer, doing all pending graphics operations. If vis is non-zero, any changes are also copied from the ‘soft screen’ (if any) in the driver to the visible frame buffer. The various allocation routines in the library flush automatically, as does the event package (see event(3)); most programs do not need to call flushimage. It returns –1 on error.
Bufimage is used to allocate space for n bytes in the display buffer. It is used by all the graphics routines to send messages to the display.
The functions strtochan and chantostr convert between the channel descriptor strings used by image(7) and the internal ulong representation used by the graphics protocol (see draw(3)’s b message). Chantostr writes at most nine bytes into the buffer pointed at by s and returns s on success, 0 on failure. Chantodepth returns the number of bits per pixel used by the format specified by chan. Both chantodepth and strtochan return 0 when presented with bad input.

To reconnect to the window after a resize event,
if(getwindow(display, Refnone) < 0)
sysfatal("resize failed: %r");
To create and set up a new rio(1) window,
Image *screen2;
Screen *_screen2;
srvwsys = getenv("wsys");
if(srvwsys == nil)
sysfatal("can't find $wsys: %r");
rfork(RFNAMEG); /* keep mount of rio private */
fd = open(srvwsys, ORDWR);
if(fd < 0)
sysfatal("can't open $wsys: %r");
/* mount creates window; see
rio(4) */
if(mount(fd, −1, "/tmp", MREPL, "new −dx 300−dy 200") < 0)
sysfatal("can't mount new window: %r");
if(gengetwindow(display, "/tmp/winname",
&screen2, &_screen2, Refnone) < 0)
sysfatal("resize failed: %r");
/* now open /tmp/cons, /tmp/mouse */

/usr/local/plan9/font/bit    directory of fonts


rio(1), addpt(3), allocimage(3), cachechars(3), subfont(3), draw(3), event(3), frame(3), print(3), window(3), draw(3), image(7), font(7)

An error function may call errstr(3) for further diagnostics.

The names clr and set in the Cursor structure are reminders of an archaic color map and might be more appropriately called white and black.
These manual pages contain many references to the now-fictitious /dev/draw.

Space Glenda