DRAW(3) DRAW(3)
NAME
draw - screen graphics
SYNOPSIS
bind -a #i /dev
/dev/draw/new
/dev/draw/n/ctl
/dev/draw/n/data
/dev/draw/n/refresh
DESCRIPTION
The draw device serves a three-level file system providing
an interface to the graphics facilities of the system. The
Limbo Draw module (see draw-intro(2)) implements its func-
tions using this device. Each client of the device connects
by opening /dev/draw/new and reading 12 strings, each 11
characters wide followed by a blank: the connection number
(n), the image id (q.v.) of the display image (always
zero), the channel format of the image, the min.x, min.y,
max.x, and max.y of the display image, and the min.x, min.y,
max.x, and max.y of the clipping rectangle. The channel for-
mat string is described in image(6), and the other fields
are decimal numbers.
The client can then open the directory /dev/draw/n/ to
access the ctl, data, and refresh files associated with the
connection.
Via the ctl and draw files, the draw device provides access
to images and font caches in its private storage, as
described in draw-intro(2). Each image is identified by a
4-byte integer, its id.
Reading the ctl file yields 12 strings formatted as in
/dev/draw/new, but for the current image rather than the
display image. The current image may be set by writing a
binary image id to the ctl file.
A process can write messages to data to allocate and free
images, fonts, and subfonts; read or write portions of the
images; and draw line segments and character strings in the
images. All graphics requests are clipped to their images.
Some messages return a response to be recovered by reading
the data file.
The draw device provides three types of graphical resource:
Images, Screens and Fonts. Resource instances have an
Page 1 Plan 9 (printed 11/23/24)
DRAW(3) DRAW(3)
identification number. Screen identifiers are global to the
device. All other identifiers are local to each client.
Image is the fundamental resource type on which all drawing
primitives are performed. At client connection the physical
display is represented by Image 0.
A Screen manages a set of (overlapping) Images, handling Z-
order and position manipulation and the refreshing of
regions uncovered by such operations. When a Screen is cre-
ated it is associated with an Image on which to render
itself. New images can be associated with a screen when
they are created; they are then treated as windows on that
screen. Screens can be marked as public, meaning that other
clients can import their ID and create new windows on them.
A Font is an image with associated character data. The Font
image provides the bitmap of all the characters in the Font;
the character data is used by the string command to render
characters from the image.
Command messages
The format of messages written to data is a single letter
followed by binary parameters; multibyte integers are trans-
mitted with the low order byte first. Points are two four-
byte numbers: x, y. Rectangles are four four-byte numbers:
min x, min y, max x, and max y. Images, screens, and fonts
have 32-bit identifiers. In the discussion of the protocol
below, the distinction between identifier and actual image,
screen, or font is not made, so that ``the object id''
should be interpreted as ``the object with identifier id''.
The definitions of constants used in the description below
can be found in /module/draw.m or /include/draw.h.
The following requests are accepted by the data file. The
numbers in brackets give the length in bytes of the parame-
ters.
A id[4] imageid[4] fillid[4] public[1]
Allocate a new Screen (see draw-display(2)) with screen
identifier id using backing store image imageid, fill-
ing it initially with data from image fillid. If the
public byte is non-zero, the screen can be accessed
from other processes using the publicscreen interface.
b id[4] screenid[4] refresh[1] chan[4] repl[1] r[4*4]
clipr[4*4] color[4]
Allocate an image with a given id on the screen named
by screenid. The image will have rectangle r and clip-
ping rectangle clipr. If repl is non-zero, the image's
replicate bit will be set (see draw(2)).
Page 2 Plan 9 (printed 11/23/24)
DRAW(3) DRAW(3)
Refresh specifies the method to be used to draw the
window when it is uncovered. Refbackup causes the
server to maintain a backing store, Refnone does not
refresh the image, and Refmesg causes a message to be
sent via the refresh file (q.v.).
The image format is described by chan, a binary version
of the channel format string. Specifically, the image
format is the catenation of up to four 8-bit numbers,
each describing a particular image channel. Each of
these 8-bit numbers contains a channel type in its high
nibble and a bit count in its low nibble. The channel
type is one of CRed, CGreen, CBlue, CGrey, CAlpha,
CMap, and CIgnore. See image(6).
Color is the catenation of four 8-bit numbers specify-
ing the red, green, blue, and alpha channels of the
color that the new image should be initially filled
with. The red channel is in the highest 8 bits, and
the alpha in the lowest. Note that color is always in
this format, independent of the image format.
c dstid[4] repl[1] clipr[4*4]
Change the replicate bit and clipping rectangle of the
image dstid. This overrides whatever settings were
specified in the allocate message.
d dstid[4] srcid[4] maskid[4] dstr[4*4] srcp[2*4] maskp[2*4]
Use the draw operator to combine the rectangle dstr of
image dstid with a rectangle of image srcid, using a
rectangle of image maskid as an alpha mask to further
control blending. The three rectangles are congruent
and aligned such that the upper left corner dstr in
image dstid corresponds to the point srcp in image
srcid and the point maskp in image maskid. See draw-
image(2).
D debugon[1]
If debugon is non-zero, enable debugging output. If
zero, disable it. The meaning of ``debugging output''
is implementation dependent.
e dstid[4] srcid[4] c[2*4] a[4] b[4] thick[4] sp[2*4]
alpha[4] phi[4]
Draw an ellipse in image dst centered on the point c
with horizontal and vertical semiaxes a and b. The
ellipse is drawn using the image src, with the point sp
in src aligned with c in dst. The ellipse is drawn with
thickness 1+2×thick.
If the high bit of alpha is set, only the arc of the
ellipse from degree angles alpha to phi is drawn. For
Page 3 Plan 9 (printed 11/23/24)
DRAW(3) DRAW(3)
the purposes of drawing the arc, alpha is treated as a
signed 31-bit number by ignoring its high bit.
E dstid[4] srcid[4] center[2*4] a[4] b[4] thick[4] sp[2*4]
alpha[4] phi[4]
Draws an ellipse or arc as the e message, but rather
than outlining it, fills the corresponding sector using
the image srcid. The thick field is ignored, but must
be non-negative.
f id[4]
Free the resources associated with the image id.
F id[4]
Free the the screen with the specified id. Windows on
the screen must be freed separately.
i id[4] n[4] ascent[1]
Treat the image id as a font cache of n character
cells, each with ascent ascent.
l cacheid[4] srcid[4] index[2] r[4*4] sp[2*4] left[1]
width[1]
Load a character into the font cache associated with
image cacheid at cache position index. The character
data is drawn in rectangle r of the font cache image
and is fetched from the congruent rectangle in image
srcid with upper left corner sp. Width specifies the
width of the character-the spacing from this character
to the next-while left specifies the horizontal dis-
tance from the left side of the character to the left
side of the cache image. The dimensions of the image
of the character are defined by r.
L dstid[4] p0[2*4] p1[2*4] end0[4] end1[4] thick[4] srcid[4]
sp[2*4]
Draw a line of thickness 1+2×thick in image dstid from
point p0 to p1. The line is drawn using the image
srcid, translated so that point sp in srcid aligns with
p0 in dstid. The end0 and end1 fields specify whether
the corresponding line end should be a square, a disc,
or an arrow head. See line in draw-image(2) for more
details.
N id[4] in[1] j[1] name[j]
If in is non-zero, associate the image id with the
string name. If in is zero and name already corresponds
to the image id, the association is deleted.
n id[4] j[1] name[j]
Introduce the identifier id to correspond to the image
named by the string name.
Page 4 Plan 9 (printed 11/23/24)
DRAW(3) DRAW(3)
o id[4] r.min[2*4] scr[2*4]
Position the window id so that its upper left corner is
at the point scr on its screen. Simultaneously change
its internal (logical) coordinate system so that the
point log corresponds to the upper left corner of the
window.
p dstid[4] n[2] end0[4] end1[4] thick[4] srcid[4] sp[2*4]
dp[2*2*(n+1)]
Draw a polygon of thickness 1+2×thick. It is conceptu-
ally equivalent to a series of n line-drawing messages
(see L above) joining adjacent points in the list of
points dp. The source image srcid is translated so that
the point sp in srcid aligns with the first point in
the list dp. The polygon need not be closed: end0 and
end1 specify the line endings for the first and last
point on the polygon. All interior lines have rounded
ends to make smooth joins.
P dstid[4] n[2] wind[4] ignore[2*4] srcid[4] sp[2*4]
dp[2*2*(n+1)]
Draw a polygon as the p message, but fill it rather
than outlining it. The winding rule parameter wind
resolves ambiguities about what to fill if the polygon
is self-intersecting. If wind is ~0, a pixel is inside
the polygon if the polygon's winding number about the
point is non-zero. If wind is 1, a pixel is inside if
the winding number is odd. Complementary values (0 or
~1) cause outside pixels to be filled. The meaning of
other values is undefined. The polygon is closed with
a line if necessary.
r id[4] r[4*4]
Cause the next read of the data file to return the
image pixel data corresponding to the rectangle r in
image id.
s dstid[4] srcid[4] fontid[4] p[2*4] clipr[4*4] sp[2*4] n[2]
n*(index[2])
Draw in the image dstid the text string specified by
the n cache indices into font fontid, starting with the
upper left corner at point p in image dstid. The image
drawn is taken from image srcid, translated to align sp
in srcid with dp in dstid. All drawing is confined to
the clipping rectangle clipr in dstid.
x dstid[4] srcid[4] fontid[4] dp[2*4] clipr[4*4] sp[2*4]
n[2] bgid[4] bp[2*4] n*(index[2])
Like the string drawing s command, but fill the back-
ground of each character with pixels from image bgid.
The image bgid is translated so that the point bp
aligns with the point dp in dstid.
Page 5 Plan 9 (printed 11/23/24)
DRAW(3) DRAW(3)
S id[4] chan[4] Attach to the public screen with the speci-
fied id. It is an error if the screen does not exist,
is not public, or does not have the channel descriptor
chan for its associated image.
t top[1] n[2] n*id[4]
Send n windows to the top (if t is non-zero) or bottom
(if t is zero) of the window stack. The window is
specified by the list of n image ids are moved as a
group, maintaining their own order within the stack.
v
Flush changes from a soft screen, if any, to the dis-
play buffer.
y id[4] r[4*4] buf[x*1]
Y id[4] r[4*4] buf[x*1]
Replace the rectangle r of pixels in image id with the
pixel data in buf. The pixel data must be in the format
dictated by id's image channel descriptor (see
image(6)). The y message uses uncompressed data, while
the Y message uses compressed data. In either case, it
is an error to include more data than necessary.
The refresh file is read-only. As windows owned by the
client are uncovered, if they cannot be refreshed by the
server (such as when they have refresh functions associated
with them), a message is made available on the refresh file
reporting what needs to be repainted by the client. The
message has five decimal integers formatted as in the ctl
message: the image id of the window and the coordinates of
the rectangle that should be refreshed.
SOURCE
/emu/port/devdraw.c
/emu/*/win.c
/os/port/devdraw.c
/os/*/screen.c
/libmemdraw
SEE ALSO
draw-intro(2), colour(6), image(6), font(6)
DIAGNOSTICS
Most messages to draw can return errors; these can be
detected by a system call error on the write(see sys-
read(2)) of the data containing the erroneous message. The
most common error is a failure to allocate because of insuf-
ficient free resources. Most other errors occur only when
the protocol is mishandled by the application. The error
string (see sys-intro(2)) will report details.
Page 6 Plan 9 (printed 11/23/24)
DRAW(3) DRAW(3)
BUGS
The Refmesg refresh method is not fully implemented.
Page 7 Plan 9 (printed 11/23/24)