.TH DRAW 3 
.SH NAME
draw \- screen graphics
.SH SYNOPSIS
.nf
.B bind -a #i /dev

.B /dev/draw/new

.BI /dev/draw/ n /ctl
.BI /dev/draw/ n /data
.BI /dev/draw/ n /colormap
.BI /dev/draw/ n /refresh

.ft L
#include <u.h>
#include <draw.h>

.ta \w'ushort 'u
ushort	BGSHORT(uchar *p)
ulong	BGLONG(uchar *p)
void	BPSHORT(uchar *p, ushort v)
void	BPLONG(uchar *p, ulong v)
.ft P
.fi
.SH DESCRIPTION
The
.I draw
device serves a three-level file system 
providing an interface to the graphics facilities of the system.
Each client of the device connects by opening
.B /dev/draw/new
and reading 12 strings, each 11 characters wide followed by a blank:
the connection number
.RI ( n ),
the image id
.RI ( q.v. )
of the display image (always zero),
the
channel format
of the image,
the
replicate bit,
the
.BR min.x ,
.BR min.y ,
.BR max.x ,
and
.B max.y
of the display image,
and the
.BR min.x ,
.BR min.y ,
.BR max.x ,
and
.B max.y 
of the clipping rectangle. 
The channel format string is described in 
.IR image (6),
and the other fields are decimal numbers.
.PP
The client can then open the directory
.BI /dev/draw/ n /
to access the
.BR ctl ,
.BR data ,
.BR colormap ,
and
.B refresh
files associated with the connection.
.PP
Via the 
.B ctl
and
.B data 
files, the
.I draw
device provides access to 
images and font caches
in its private storage,
as described in 
.IR graphics (2).
Each image is identified by a 4-byte integer, its
.IR id .
.PP
Reading the
.B ctl 
file yields 12 strings formatted as in
.BR /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
.B ctl
file.
.PP
A process can write messages to 
.B 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
.B data 
file.
.PP
The format of messages written to
.B data
is a single letter
followed by binary parameters;
multibyte integers are transmitted with the low order byte first.
The
.B BPSHORT
and
.B BPLONG
macros place correctly formatted two- and four-byte integers into a character
buffer.
.B BGSHORT
and
.B BGLONG
retrieve values from a character buffer.
Points are two four-byte numbers:
.IR x ,
.IR y .
Rectangles are four four-byte numbers: min 
.IR x ,
min
.IR y ,
max
.IR x ,
and max
.IR 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
.IR id ''
should be interpreted as 
``the object with identifier
.IR id ''.
The definitions of constants used in the description below can be found in
.BR draw.h .
.PP
The following requests are accepted by the
.B data
file.
The numbers in brackets give the length in bytes of the parameters.
.HP .5i
.B A
.IR id [4]
.IR imageid [4]
.IR fillid [4]
.IR public [1]
.br
Allocate a new
.B Screen
(see
.IR window (2))
with
screen identifier
.I id
using
backing store image
.IR imageid ,
filling it initially
with data from image
.IR fillid .
If the
.I public
byte is non-zero, the screen can 
be accessed from other processes
using the
.B publicscreen
interface.
.HP
.B b 
.IR id [4]
.IR screenid [4]
.IR refresh [1]
.IR chan [4]
.IR repl [1]
.IR r [4*4]
.IR clipr [4*4]
.IR color [4]
.br
Allocate an image with a given 
.I id 
on the
screen named by
.IR screenid .
The image will have rectangle
.I r 
and clipping rectangle
.IR clipr .
If 
.I repl
is non-zero, the image's replicate 
bit will be set (see
.IR draw (2)).
.IP
.I Refresh
specifies the method to be used to draw the window
when it is uncovered.
.B Refbackup
causes the server to maintain a backing store,
.B Refnone 
does not refresh the image,
and
.B Refmesg
causes a message to be sent via
the
.B refresh 
file
.RI ( q.v. ).
.IP
The image format is described by
.IR 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
.BR CRed ,
.BR CGreen ,
.BR CBlue ,
.BR CGrey ,
.BR CAlpha ,
.BR CMap ,
and
.BR CIgnore .
See
.IR image (6).
.IP
.I Color
is the catenation of four 8-bit numbers
specifying 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.
.HP
.B c
.IR dstid [4]
.IR repl [1]
.IR clipr [4*4]
.br
Change the replicate bit and clipping rectangle of the 
image
.IR dstid .
This overrides whatever settings were specified in
the allocate message.
.HP
.B d
.IR dstid [4]
.IR srcid [4]
.IR maskid [4]
.IR dstr [4*4]
.IR srcp [2*4]
.IR maskp [2*4]
.br
Use the 
.B draw
operator to combine the rectangle 
.I dstr
of 
image
.I dstid 
with a 
rectangle of image
.IR srcid ,
using a rectangle of image
.IR maskid
as an alpha mask to further control blending.
The three rectangles are congruent and aligned such that
the upper left corner
.I dstr
in image
.I dstid
corresponds to
the point 
.I srcp
in image
.I srcid
and
the point
.I maskp
in image
.IR maskid .
See
.IR draw (2).
.HP
.B D
.IR debugon [1]
.br
If
.I debugon
is non-zero, enable debugging output.
If zero, disable it.
The meaning of ``debugging output'' is implementation dependent.
.HP
.B e
.IR dstid [4]
.IR srcid [4]
.IR c [2*4]
.IR a [4]
.IR b [4]
.IR thick [4]
.IR sp [2*4]
.IR alpha [4]
.IR phi [4]
.br
Draw an ellipse in image
.I dst
centered on the point
.I c
with horizontal and vertical semiaxes
.I a
and
.IR b .
The ellipse is drawn using the image
.IR src ,
with
the point
.I sp
in 
.I src
aligned with
.I c
in
.IR dst .
The ellipse is drawn with thickness
.RI 1+2× thick .
.IP
If the high bit of
.I alpha
is set, 
only the arc of the ellipse from degree angles
.I alpha
to
.I phi 
is drawn.
For the purposes of drawing the arc,
.I alpha
is treated as a signed 31-bit number
by ignoring its high bit.
.HP
.B E
.IR dstid [4]
.IR srcid [4]
.IR center [2*4]
.IR a [4]
.IR b [4]
.IR thick [4]
.IR sp [2*4]
.IR alpha [4]
.IR phi [4]
.br
Draws an ellipse or arc as the
.B e
message, but rather than outlining it, fills
the corresponding sector using the image
.IR srcid .
The 
.I thick 
field is ignored, but must be non-negative.
.HP
.B f
.IR id [4]
.br
Free the resources associated with the image
.IR id .
.HP
.B F
.IR id [4]
.br
Free the screen with the specified
.IR id .
Windows on the screen must be freed separately.
.HP
.B i
.IR id [4]
.IR n [4]
.IR ascent [1]
.br
Treat the image
.I id
as a font cache of 
.I n
character cells, each with
ascent
.IR ascent .
.HP
.B l
.IR cacheid [4]
.IR srcid [4]
.IR index [2]
.IR r [4*4]
.IR sp [2*4]
.IR left [1]
.IR width [1]
.br
Load a character into the font cache associated with image
.I cacheid
at cache position
.IR index .
The character data is drawn in rectangle
.I r
of the font cache image
and is fetched from
the congruent rectangle in image
.I srcid
with upper left corner
.IR sp .
.I Width
specifies the width of the character\(emthe spacing from this character to the next\(emwhile
.I left
specifies
the horizontal distance 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
.IR r .
.HP
.B L
.IR dstid [4]
.IR p0 [2*4]
.IR p1 [2*4]
.IR end0 [4]
.IR end1 [4]
.IR thick [4]
.IR srcid [4]
.IR sp [2*4]
.br
Draw a line of thickness
.RI 1+2× thick
in image
.I dstid
from point
.I p0
to
.IR p1 .
The line is drawn using the image
.IR srcid ,
translated so that point
.I sp
in
.I srcid
aligns with
.I p0
in 
.IR dstid .
The 
.I end0
and
.I end1
fields specify whether the corresponding
line end should be a square, a disc,
or an arrow head.
See 
.I line
in
.IR draw (2)
for more details.
.HP
.B N
.IR id [4]
.IR in [1]
.IR j [1]
.IR name [ j ]
.br
If 
.I in
is non-zero, associate the image
.I id
with the string
.IR name .
If 
.I in
is zero and 
.I name
already corresponds to the
image
.IR id ,
the association is deleted.
.HP
.B n
.IR id [4]
.IR j [1]
.IR name [ j ]
.br
Introduce the identifier 
.I id
to correspond to the image named
by the string
.IR name .
.HP
.B o
.IR id [4]
.IR r.min [2*4]
.IR scr [2*4]
.br
Position the window (layer)
.I id
so that its upper left corner is at the
point
.I scr
on its screen.
Simultaneously change its internal (logical) coordinate system
so that the point
.I r.min
corresponds to the upper left corner of the window, see
.IR memlorigin (2).
.HP
.B O
.IR op [1]
.br
Set the compositing operator to
.I op
for the next draw operation.
(The default is
.BR SoverD ).
.HP
.B p
.IR dstid [4]
.IR n [2]
.IR end0 [4]
.IR end1 [4]
.IR thick [4]
.IR srcid [4]
.IR sp [2*4]
.IR dp [2*2*(n+1)]
.br
Draw a polygon of thickness
.RI 1+2× thick .
It is conceptually equivalent to a series of
.I n
line-drawing messages (see
.B L
above)
joining adjacent points in the list of points
.IR dp .
The source image
.I srcid
is translated so that the point
.I sp
in
.I srcid
aligns with the first point
in the list
.IR dp .
The polygon need not be closed: 
.I end0
and
.I end1
specify the line endings for the first and
last point on the polygon.
All interior lines have rounded ends
to make smooth joins.
.HP
.B P
.IR dstid [4]
.IR n [2]
.IR wind [4]
.IR ignore [2*4]
.IR srcid [4]
.IR sp [2*4]
.IR dp [2*2*(n+1)]
.br
Draw a polygon as the
.B p
message, but
fill it rather than outlining it.
The winding rule parameter
.I wind
resolves ambiguities about what to fill if the polygon is self-intersecting.
If
.I wind
is
.BR ~0 ,
a pixel is inside the polygon if the polygon's winding number about the point
is non-zero.
If
.I wind
is
.BR 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.
.HP
.B r
.IR id [4]
.IR r [4*4]
.br
Cause the next read of the
.B data
file to return the image pixel data corresponding to the
rectangle
.I r
in image
.IR id .
.HP
.B s
.IR dstid [4]
.IR srcid [4]
.IR fontid [4]
.IR dp [2*4]
.IR clipr [4*4]
.IR sp [2*4]
.IR n [2]
.IR n*(index [2])
.br
Draw in the image
.I dstid
the text string specified by the 
.I n 
cache
.I indices
into font
.IR fontid ,
starting with the upper left corner at point
.I p
in image
.IR dstid .
The image drawn is taken from image
.IR srcid ,
translated to align
.I sp
in
.I srcid
with
.I dp
in
.IR dstid.
All drawing is confined to the clipping rectangle
.I clipr 
in
.IR dstid .
.HP
.B x
.IR dstid [4]
.IR srcid [4]
.IR fontid [4]
.IR dp [2*4]
.IR clipr [4*4]
.IR sp [2*4]
.IR n [2]
.IR bgid [4]
.IR bp [2*4]
.IR n*(index [2])
.br
Like the string drawing 
.B s
command, but fill the background of each character
with pixels from image
.IR bgid .
The image
.I bgid
is translated so that the point
.I bp
aligns with the
point 
.I dp
in 
.IR dstid .
.HP
.B S
.IR id [4]
.IR chan [4]
Attach to the public screen with the specified
.IR id .
It is an error if the screen does not exist, is not public, or does not
have the channel descriptor
.I chan
for its associated image.
.HP
.B t
.IR top [1]
.IR n [2]
.IR n*id [4]
.br
Send 
.I n
windows to the top (if
.I top
is non-zero) or bottom (if
.I top
is zero) of the window stack.
The window is specified by the list of
.I n
image
.IR id s
are moved as a group, maintaining their own order within the stack.
.HP
.B v
.br
Flush changes from a soft screen, if any, to the display buffer.
.HP
.B y
.IR id [4]
.IR r [4*4]
.IR buf [x*1]
.br
.ti -0.5i
.B Y
.IR id [4]
.IR r [4*4]
.IR buf [x*1]
.br
Replace the rectangle 
.I r
of pixels in image
.I id
with the pixel data in 
.IR buf .
The pixel data must be in the format dictated by
.IR id 's
image channel descriptor (see
.IR image (6)).
The 
.B y
message uses uncompressed data,
while the
.B Y
message uses compressed data. 
In either case, 
it is an error to include more data than necessary.
.PP
Reading the
.B colormap
returns the system color map used on 8-bit displays.
Each color map entry consists of a single line containing
four space-separated decimal strings.
The first is an index into the map, and the remaining three are
the red, green, and blue values associated with that index.
The color map can be changed by writing entries in the
above format to
the 
.B colormap
file.
Note that changing the system color map 
does not change the color map used for
calculations involving
.B m8
images, which is immutable.
.PP
The
.B 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
.B refresh
file reporting what needs to be repainted by the client.
The message has five decimal integers formatted as in the
.B ctl
message: the image id of the window and the coordinates of the rectangle
that should be refreshed.
.SH SOURCE
.B /sys/src/9/port/devdraw.c
.br
.B /sys/src/libmemdraw
.SH DIAGNOSTICS
Most messages to
.B draw
can return errors;
these can be detected by a system call error
on the
.IR write (see
.IR read (2))
of the data containing the erroneous message.
The most common error is a failure to allocate
because of insufficient free resources.  Most other errors occur
only when the protocol is mishandled by the application.
.IR Errstr (2)
will report details.
.SH BUGS
The 
.B Refmesg
refresh method is not fully implemented.
.br
The
.B colormap
files only reference the system color map, and as 
such should be called
.B /dev/colormap
rather than
.BI /dev/draw/ n /colormap\f1.