ref: f8935b5778397074d41a48205e5c7f87d7b531fe
dir: /man/2/draw-image/
.TH DRAW-IMAGE 2
.SH NAME
Image \-
pictures and drawing
.SH SYNOPSIS
.EX
include "draw.m";
draw := load Draw Draw->PATH;
# compositing operators
SinD: con 1<<3;
DinS: con 1<<2;
SoutD: con 1<<1;
DoutS: con 1<<0;
S: con SinD|SoutD;
SoverD: con SinD|SoutD|DoutS;
SatopD: con SinD|DoutS;
SxorD: con SoutD|DoutS;
D: con DinS|DoutS;
DoverS: con DinS|DoutS|SoutD;
DatopS: con DinS|SoutD;
DxorS: con DoutS|SoutD;
Clear: con 0;
Image: adt
{
r: Rect;
clipr: Rect;
chans: Chans;
depth: int;
repl: int;
display: ref Display;
screen: ref Screen;
draw: fn(dst: self ref Image, r: Rect, src: ref Image,
mask: ref Image, p: Point);
drawop: fn(dst: self ref Image, r: Rect, src: ref Image,
mask: ref Image, p: Point, op: int);
gendraw: fn(dst: self ref Image, r: Rect, src: ref Image,
p0: Point, mask: ref Image, p1: Point);
gendrawop: fn(dst: self ref Image, r: Rect, src: ref Image,
p0: Point, mask: ref Image, p1: Point, op: int);
line: fn(dst: self ref Image, p0,p1: Point,
end0,end1,thick: int,
src: ref Image, sp: Point);
lineop: fn(dst: self ref Image, p0,p1: Point,
end0,end1,thick: int,
src: ref Image, sp: Point, op: int);
poly: fn(dst: self ref Image, p: array of Point,
end0,end1,thick: int,
src: ref Image, sp: Point);
polyop: fn(dst: self ref Image, p: array of Point,
end0,end1,thick: int,
src: ref Image, sp: Point, op: int);
bezspline: fn(dst: self ref Image, p: array of Point,
end0,end1,thick: int,
src: ref Image, sp: Point);
bezsplineop: fn(dst: self ref Image, p: array of Point,
end0,end1,thick: int,
src: ref Image, sp: Point, op: int);
fillpoly: fn(dst: self ref Image, p: array of Point,
wind: int, src: ref Image, sp: Point);
fillpolyop: fn(dst: self ref Image, p: array of Point,
wind: int, src: ref Image, sp: Point, op: int);
fillbezspline: fn(dst: self ref Image, p: array of Point,
wind: int, src: ref Image, sp: Point);
fillbezsplineop: fn(dst: self ref Image, p: array of Point,
wind: int, src: ref Image, sp: Point, op: int);
ellipse: fn(dst: self ref Image, c: Point, a, b,
thick: int, src: ref Image, sp: Point);
ellipseop: fn(dst: self ref Image, c: Point, a, b,
thick: int, src: ref Image, sp: Point, op: int);
fillellipse:fn(dst: self ref Image, c: Point, a, b: int,
src: ref Image, sp: Point);
fillellipseop:fn(dst: self ref Image, c: Point, a, b: int,
src: ref Image, sp: Point, op: int);
arc: fn(dst: self ref Image, c: Point, a, b, thick: int,
src: ref Image, sp: Point, alpha, phi: int);
arcop: fn(dst: self ref Image, c: Point, a, b, thick: int,
src: ref Image, sp: Point,
alpha, phi: int, op: int);
fillarc: fn(dst: self ref Image, c: Point, a, b: int,
src: ref Image, sp: Point, alpha, phi: int);
fillarcop: fn(dst: self ref Image, c: Point, a, b: int,
src: ref Image, sp: Point,
alpha, phi: int, op: int);
bezier: fn(dst: self ref Image, a,b,c,d: Point,
end0,end1,thick: int,
src: ref Image, sp: Point);
bezierop: fn(dst: self ref Image, a,b,c,d: Point,
end0,end1,thick: int,
src: ref Image, sp: Point, op: int);
fillbezier: fn(dst: self ref Image, a,b,c,d: Point, wind:int,
src: ref Image, sp: Point);
fillbezierop: fn(dst: self ref Image, a,b,c,d: Point, wind:int,
src: ref Image, sp: Point, op: int);
arrow: fn(a,b,c: int): int;
text: fn(dst: self ref Image, p: Point, src: ref Image,
sp: Point, font: ref Font, str: string): Point;
textop: fn(dst: self ref Image, p: Point, src: ref Image,
sp: Point, font: ref Font, str: string,
op: int): Point;
textbg: fn(dst: self ref Image, p: Point, src: ref Image,
sp: Point, font: ref Font, str: string,
bg: ref Image, bgp: Point): Point;
textbgop: fn(dst: self ref Image, p: Point, src: ref Image,
sp: Point, font: ref Font, str: string,
bg: ref Image, bgp: Point, op: int): Point;
border: fn(dst: self ref Image, r: Rect, i: int,
src: ref Image, sp: Point);
borderop: fn(dst: self ref Image, r: Rect, i: int,
src: ref Image, sp: Point, op: int);
readpixels: fn(src: self ref Image, r: Rect,
data: array of byte): int;
writepixels:fn(dst: self ref Image, r: Rect,
data: array of byte): int;
name: fn(im: self ref Image, s: string, in: int): int;
top: fn(win: self ref Image);
bottom: fn(win: self ref Image);
flush: fn(win: self ref Image, func: int);
origin: fn(win: self ref Image, log, scr: Point): int;
};
.EE
.SH DESCRIPTION
The
.B Image
type defines rectangular pictures and the methods to draw upon them;
it is also the building block for higher level objects such as
windows and fonts.
In particular, a window is represented as an
.BR Image ;
no special operators are needed to draw on a window.
Off-screen images can have an alpha channel, which gives each pixel an opacity
factor, which in turn allows non-rectangular images to be defined
(ie, pixels made fully transparent by the alpha channel
do not appear when the image is displayed).
Many drawing operations allow images to be shaped, or partial transparency added, by using the alpha
channel of another image as a mask (also called a `matte').
There are two functions in
.B Image
for each such operation.
One has an
.B op
suffix, and takes an explicit image compositing operator:
.BR S ,
.BR D ,
.BR SinD , ...,
.BR SoverD
and so on.
(See the Porter-Duff paper mentioned below for the meaning of each operation.)
The other function (without the
.B op
suffix) provides as its default operation the most common operation,
.BR SoverD ,
by which the source image, within its matte, is drawn over the destination image.
.PP
An
.B Image
has a pixel channel structure as described in
.IR colour (6),
represented by a value of the
.B Chans
adt,
defined in
.IR draw-display (2).
The channel structure of an image is fixed when the image is allocated.
.PP
.B Image
has the following components:
.TP 10
.B display
Tells on which display the image resides.
.TP
.B screen
If the image is a window on a
.B Screen
(see
.IR draw-screen (2)),
this field refers to that screen; otherwise it is nil.
.TP
.B r
The coordinates of the rectangle in the plane for which the
.B Image
has defined pixel values.
It should not be modified after the image is created.
.TP
.B clipr
The clipping rectangle: operations that read or write
the image will not access pixels outside
.BR clipr .
Frequently,
.B clipr
is the same as
.BR Image.r ,
but it may differ; see in particular the discussion of
.BR Image.repl .
The clipping region may be modified dynamically.
.TP
.B chans
The pixel channel structure of the image; the value
should not be modified after the image is created.
.TP
.B depth
The number of bits per pixel in the picture:
it is simply a convenience since it is necessarily equal to
.BR chans.depth() ,
and it should not be modified after the image is created.
.TP
.B repl
A boolean value specifying whether the image is tiled to cover
the plane when used as a source for a drawing operation.
If
.B Image.repl
is zero, operations are restricted to the intersection of
.B Image.r
and
.BR Image.clipr .
If
.B Image.repl
is set,
.B Image.r
defines the tile to be replicated and
.B Image.clipr
defines the portion of the plane covered by the tiling, in other words,
.B Image.r
is replicated to cover
.BR Image.clipr ;
in such cases
.B Image.r
and
.B Image.clipr
are independent.
.IP
For example, a replicated image with
.B Image.r
set to ((0,\ 0),\ (1,\ 1)) and
.B Image.clipr
set to ((0,\ 0),\ (100,\ 100)),
with the single pixel of
.B Image.r
set to blue,
behaves identically to an image with
.B Image.r
and
.B Image.clipr
both set to ((0,\ 0),\ (100,\ 100)) and all pixels set to blue.
However,
the first image requires far less memory.
The replication flag may be modified dynamically along with the clipping
rectangle.
.TP
.IB dst .draw( r\fP,\fP\ src\fP,\fP\ mask\fP,\fP\ p\fP )
.PD0
.TP
.IB dst .drawop( r\fP,\fP\ src\fP,\fP\ mask\fP,\fP\ p\fP,\fP\ op )
.PD
.B Draw
is the standard drawing function.
Only those pixels within the intersection of
.IB dst .r
and
.IB dst .clipr
will be affected;
.B draw
ignores
.IB dst .repl\fR.
The operation proceeds as follows
(this is a description of the behavior, not the implementation):
.RS
.IP 1.
If
.B repl
is set in
.I src
or
.IR mask ,
replicate their contents to fill
their clip rectangles.
.IP 2.
Translate
.I src
and
.I mask
so
.I p
is aligned with
.IB r .min\fR.
.IP 3.
Set
.I r
to the intersection of
.I r
and
.IB dst .r\fR.
.IP 4.
Intersect
.I r
with
.IB src .clipr\fR.
If
.IB src .repl
is false, also intersect
.I r
with
.IB src .r\fR.
.IP 5.
Intersect
.I r
with
.IB mask .clipr\fR.
If
.IB mask .repl
is false, also intersect
.I r
with
.IB mask .r\fR.
.IP 6.
For each location in
.IR r ,
combine the
.I dst
pixel using the alpha value corresponding to the
.I mask
pixel.
If the
.I mask
has an explicit alpha channel, the alpha value corresponding to the
.I mask
pixel is simply that pixel's alpha channel.
Otherwise, the alpha value is the NTSC greyscale equivalent of the colour value,
with white meaning opaque and black transparent.
.RE
.IP
In terms of the Porter-Duff compositing algebra,
.I draw
replaces the
.I dst
pixels with
.RI ( src
in
.IR mask )
over
.IR dst .
.I Drawop
is almost identical, but applies the compositing operation
.I op
instead:
.RI ( src
in
.IR mask )
.I op
.IR dst .
.IP
The various
pixel channel formats
involved need not be identical.
If the channels involved are smaller than 8-bits, they will
be promoted before the calculation by replicating the extant bits;
after the calculation, they will be truncated to their proper sizes.
For
.B draw
and
.B gendraw
only,
if
.I mask
is nil, no mask is used.
.TP
\f2dst\fP.\f5gendraw(\f2r\fP, \f2src\fP, \f2p0\fP, \f2mask\fP, \f2p1\fP)\fP
.PD0
.TP
\f2dst\fP.\f5gendrawop(\f2r\fP, \f2src\fP, \f2p0\fP, \f2mask\fP, \f2p1\fP\f5, \f2op\fP)\fP
.PD
Similar to \f5draw()\fP except that it aligns the source and mask differently:
.I src
is aligned so
.I p0
corresponds to
.IB r . min
and
.I mask
is aligned so
.I p1
corresponds to
.IB r . min .
For most purposes with simple masks and source images,
.B draw
is sufficient, but
.B gendraw
is the general operator and the one the other drawing primitives are built upon.
.TP
\f2dst\fP.\f5line(\f2p0\fP, \f2p1\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5lineop(\f2p0\fP, \f2p1\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Line
draws in
.I dst
a line of width
.RI 1+2* thick
pixels joining points
.I p0
and
.IR p1 .
The line is drawn using pixels from the
.I src
image aligned so
.I sp
in the source corresponds to
.I p0
in the destination.
The line touches both
.I p0
and
.IR p1 ,
and
.I end0
and
.I end1
specify how the ends of the line are drawn.
.B Draw->Endsquare
terminates the line perpendicularly to the direction of the line; a thick line with
.B Endsquare
on both ends will be a rectangle.
.B Draw->Enddisc
terminates the line by drawing a disc of diameter
.RI 1+2* thick
centered on the end point.
.B Draw->Endarrow
terminates the line with an arrowhead whose tip touches the endpoint.
See the description of
.B arrow
for more information.
.IP
.B Line
and the other geometrical operators are equivalent to calls to
.B gendraw
using a mask produced by the geometric procedure.
.TP
\f2dst\fP.\f5poly(\f2p\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5polyop(\f2p\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Poly
draws a general polygon; it
is equivalent to a series of calls to
.B line
joining adjacent points in the array of
.B Points
.IR p .
The ends of the polygon are specified as in
.BR line ;
interior lines are terminated with
.B Enddisc
to make smooth joins.
The source is aligned so
.I sp
corresponds to
.IB p [0]\f1.
.TP
\f2dst\fP.\f5bezspline(\f2p\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5bezsplineop(\f2p\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Bezspline
takes the same arguments as
.B poly
but draws a quadratic B-spline (despite its name) rather than a polygon.
If the first and last points in
.I p
are equal, the spline has periodic end conditions.
.TP
\f2dst\fP.\f5fillpoly(\f2p\fP, \f2wind\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5fillpolyop(\f2p\fP, \f2wind\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Fillpoly
is like
.B poly
but fills in the resulting polygon rather than outlining it.
The source is aligned so
.I sp
corresponds to
.IB p [0]\f1.
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 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.
.TP
\f2dst\fP.\f5fillbezspline(\f2p\fP, \f2wind\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5fillbezsplineop(\f2p\fP, \f2wind\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Fillbezspline
is like
.B fillpoly
but fills the quadratic B-spline rather than the polygon outlined by
.IR p .
The spline is closed with a line if necessary.
.TP
\f2dst\fP.\f5ellipse(\f2c\fP, \f2a\fP, \f2b\fP, \f2thick\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5ellipseop(\f2c\fP, \f2a\fP, \f2b\fP, \f2thick\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Ellipse
draws in
.I dst
an ellipse centered on
.I c
with horizontal and vertical semiaxes
.I a
and
.IR b .
The source is aligned so
.I sp
in
.I src
corresponds to
.I c
in
.IR dst .
The ellipse is drawn with thickness
.RI 1+2* thick .
.TP
\f2dst\fP.\f5fillellipse(\f2c\fP, \f2a\fP, \f2b\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5fillellipseop(\f2c\fP, \f2a\fP, \f2b\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Fillellipse
is like
.B ellipse
but fills the ellipse rather than outlining it.
.TP
.IB dst .arc(\fIc\fP,\ \fIa\fP,\ \fIb\fP,\ \fIthick\fP,\ \fIsrc\fP,\ \fIsp\fP,\ \fIalpha\fP,\ \fIphi\fP)
.PD0
.TP
.IB dst .arcop(\fIc\fP,\ \fIa\fP,\ \fIb\fP,\ \fIthick\fP,\ \fIsrc\fP,\ \fIsp\fP,\ \fIalpha\fP,\ \fIphi\fP,\ \fIop\fP)
.PD
.I Arc
is like
.IR ellipse ,
but draws only that portion of the ellipse starting at angle
.I alpha
and extending through an angle of
.IR phi .
The angles are measured in degrees counterclockwise from the positive
.I x
axis.
.TP
.IB dst .fillarc(\fIc\fP,\ \fIa\fP,\ \fIb\fP,\ \fIsrc\fP,\ \fIsp\fP,\ \fIalpha\fP,\ \fIphi\fP)
.PD0
.TP
.IB dst .fillarcop(\fIc\fP,\ \fIa\fP,\ \fIb\fP,\ \fIsrc\fP,\ \fIsp\fP,\ \fIalpha\fP,\ \fIphi\fP,\ \fIop\fP)
.PD
.I Fillarc
is like
.IR arc ,
but fills the sector with the source color.
.TP
\f2dst\fP.\f5bezier(\f2a\fP, \f2b\fP, \f2c\fP, \f2d\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5bezierop(\f2a\fP, \f2b\fP, \f2c\fP, \f2d\fP, \f2end0\fP, \f2end1\fP, \f2thick\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Bezier
draws the
cubic Bezier curve defined by
.B Points
.IR a ,
.IR b ,
.IR c ,
and
.IR d .
The end styles are determined by
.I end0
and
.IR end1 ;
the thickness of the curve is
.RI 1+2* thick .
The source is aligned so
.I sp
in
.I src
corresponds to
.I a
in
.IR dst .
.TP
\f2dst\fP.\f5fillbezier(\f2a\fP, \f2b\fP, \f2c\fP, \f2d\fP, \f2wind\fP, \f2src\fP, \f2sp\fP)
.PD0
.TP
\f2dst\fP.\f5fillbezierop(\f2a\fP, \f2b\fP, \f2c\fP, \f2d\fP, \f2wind\fP, \f2src\fP, \f2sp\fP, \f2op\fP)
.PD
.B Fillbezier
is to
.B bezier
as
.B fillpoly
is to
.BR poly .
.TP
.BI arrow( "a,\ b,\ c" )
.B Arrow
is a function to describe general arrowheads; its result is passed as
.I end
parameters to
.BR line ,
.BR poly ,
etc.
If all three parameters are zero, it produces the default arrowhead,
otherwise,
.I a
sets the distance along line from end of the regular line to tip,
.I b
sets the distance along line from the barb to the tip,
and
.I c
sets the distance perpendicular to the line from edge of line to the tip of the barb,
all in pixels.
.TP
.IB dst .border( r\fP,\fP\ i\fP,\fP\ src\fP,\fP\ sp\fP)
.PD0
.TP
.IB dst .borderop( r\fP,\fP\ i\fP,\fP\ src\fP,\fP\ sp\fP,\ \f2op\fP)
.PD
.I Border
draws in
.I dst
an outline of rectangle
.I r
in the given
.I src
colour.
The outline has width
.IR i ;
if positive, the border goes inside the rectangle; negative, outside.
The source is aligned so
.I sp
corresponds to
.IB r .min .
.TP
.IB dst .text( p\fP,\fP\ src\fP,\fP\ sp\fP,\fP\ font\fP,\fP\ str\fP)
.PD0
.TP
.IB dst .textop( p\fP,\fP\ src\fP,\fP\ sp\fP,\fP\ font\fP,\fP\ str\fP,\ \f2op\fP)
.TP
.IB dst .textbg( p\fP,\fP\ src\fP,\fP\ sp\fP,\fP\ font\fP,\fP\ str\fP,\ \f2bg\fP,\ \f2bgp\fP)
.PD0
.TP
.IB dst .textbgop( p\fP,\fP\ src\fP,\fP\ sp\fP,\fP\ font\fP,\fP\ str\fP,\ \f2bg\fP,\ \f2bgp\fP,\ \f2op\fP)
.PD
.B Text
draws in
.I dst
characters specified by the string
.I str
and font
.IR font ;
it is equivalent to a series of calls to
.B gendraw
using source
.I src
and masks determined by the character shapes.
The text is positioned with the left of the first character at
.IB p .x
and the top of the line of text at
.IB p .y\f1.
The source is positioned so
.I sp
in
.I src
corresponds to
.I p
in
.IR dst .
.B Text
returns a
.B Point
that is the position of the next character that would be drawn if the string were longer.
.IP
For characters with undefined
or zero-width images in the font, the character at font position 0 (NUL) is drawn.
.IP
.B Text
draws the text leaving the background intact.
.B Textbg
draws the background colour
.I bg
behind the characters, with the alignment specified by point
.IR bgp ;
it is otherwise the same as
.BR text .
.TP
.IB src .readpixels( r\fP,\fP\ data )
.B Readpixels
fills the
.I data
array with pixels from the specified rectangle of the
.I src
image.
The pixels are presented one horizontal line at a time,
starting with the top-left pixel of
.IR r .
Each scan line starts with a new byte in the array,
leaving the last byte of the previous line partially empty, if necessary.
Pixels are packed as tightly as possible within
.IR data ,
regardless of the rectangle being extracted.
Bytes are filled from most to least significant bit order,
as the
.I x
coordinate increases, aligned so
.IR x =0
would appear as the leftmost pixel of its byte.
Thus, for a 1-bit deep greyscale image,
the pixel at
.I x
offset 165 within the rectangle will be in a
.I data
byte with mask value
.B 16r04
regardless of the overall
rectangle: 165 mod 8 equals 5, and
.B "16r80\ >>\ 5" equals
.BR 16r04 .
It is an error to call
.B readpixels
with an array that is too small to hold the rectangle's pixels.
The return value is the number of bytes copied.
The arrangement of pixels in arrays of bytes is described in
.IR image (6).
.TP
.IB dst .writepixels( r\fP,\fP\ data )
.B Writepixels
copies pixel values from the
.I data
array to the specified rectangle in the
.I dst
image.
The format of the data is that produced by
.BR readpixels .
The return value is the number of bytes copied.
It is an error to call
.B writepixels
with an array that is too small to fill the rectangle.
.TP
.IB im .name( s , in )
Publish the image
.I im
on its display under name
.IR s ,
if
.I in is non-zero;
otherwise,
.I s
must be an already published name and it is withdrawn from publication.
A published image can be retrieved using
.B Display.namedimage
(see
.IR draw-display (2)).
This function returns -1 on error, typically because the name is already in use
(for
.I in
non-zero), or does not exist
(for
.I in
zero).
.TP
.IB win .top()
If the image
.I win
is a window,
.B top
pulls it to the ``top'' of the stack of windows on its
.BR Screen ,
perhaps obscuring other images.
If
.I win
is not a window,
.B top
has no effect.
.TP
.IB win .bottom()
If the image
.I win
is a window,
.B bottom
pulls it to the ``bottom'' of the stack of windows on its
.BR Screen ,
perhaps obscuring it.
If
.I win
is not a window,
.B bottom
has no effect.
.TP
.IB image .flush( flag )
The connection to a display has a buffer used to gather graphics requests
generated by calls to the draw library.
By default, the library flushes the buffer at the conclusion of any
call that affects the visible display
image itself.
The
.B flush
routine allows finer control of buffer management.
The
.I flag
has three possible values:
.B Flushoff
turns off all automatic flushing caused by writes to
.IR image ,
typically a window or the display image itself
(buffers may still be written when they fill or when other objects on the display
are modified);
.B Flushnow
causes the buffer to be flushed immediately;
and
.B Flushon
restores the default behaviour.
.TP
\f2win\fP.\f5origin(\f2log\fP, \f2scr\fP)
When a window is created (see
.IR draw-screen (2)),
the coordinate system within the window is identical to that of the screen:
the upper left corner of the window rectangle is its physical location on the display,
not for example (0, 0).
This symmetry may be broken, however:
.B origin
allows control of the location of the window on the display and the coordinate
system used by programs drawing on the window.
The first argument,
.IR log ,
sets the upper left corner of the logical (in-window) coordinate system without
changing the position of the window on the screen.
The second argument,
.IR scr ,
sets the upper left corner of physical (on-screen) coordinate system, that is, the
window's location on the display, without changing the internal coordinate system.
Therefore, changing
.I scr
without changing
.I log
moves the window without requiring the client using it to be notified of the change;
changing
.I log
without changing
.I scr
allows the client to set up a private coordinate system regardless of the window's
location.
It is permissible for values of
.I scr
to move some or all of the window off screen.
.B Origin
returns \-1 if the image is not a window or, in the case of changes to
.IR scr ,
if there are insufficient resources available to move the window;
otherwise it returns 1.
.SH SOURCE
.B /libdraw
.SH SEE ALSO
.IR draw-intro (2),
.IR draw-display (2),
.IR draw-point (2),
.IR draw-rect (2),
.IR draw-screen (2),
.IR colour (6),
.IR image (6),
.IR font (6)
.IR utf (6)
.PP
T. Porter, T. Duff.
``Compositing Digital Images'',
.I "Computer Graphics
(Proc. SIGGRAPH), 18:3, pp. 253-259, 1984.
.SH DIAGNOSTICS
These functions raise exceptions if argument images are nil,
except for
.B draw
and
.B gendraw
where the mask image is optional and may be nil.
.SH BUGS
Anti-aliased characters can be drawn by defining a font
with multiple bits per pixel, but there are
no anti-aliasing geometric primitives.