@node Graphics, Standards Support, Error System, Top
@chapter Graphics
@cindex graphics

MIT/GNU Scheme has a simple two-dimensional line-graphics interface
that is suitable for many graphics applications.  In particular it is
often used for plotting data points from experiments.  The interface is
generic in that it can support different types of graphics devices in a
uniform manner.  At the present time only one type of graphics device
is implemented on each operating system.

Procedures are available for drawing points, lines, and text; defining
the coordinate system; clipping graphics output; controlling some of the
drawing characteristics; and controlling the output buffer (for devices
that perform buffering).  Additionally, devices may support custom
operations, such as control of colors.

There are some constraints on the arguments to the procedures described
in this chapter.  Any argument named @var{graphics-device} must be a
graphics device object that was returned from a call to
@code{make-graphics-device}.  Any argument that is a coordinate must be
either an exact integer or an inexact real.

@menu
* Opening and Closing of Graphics Devices::
* Coordinates for Graphics::
* Drawing Graphics::
* Characteristics of Graphics Output::
* Buffering of Graphics Output::
* Clipping of Graphics Output::
* Custom Graphics Operations::
* Images::
* X Graphics::                  Graphics on the X Window System
@end menu

@node Opening and Closing of Graphics Devices, Coordinates for Graphics, Graphics, Graphics
@section Opening and Closing of Graphics Devices
@cindex graphics, opening and closing devices

@deffn procedure graphics-type-available? graphics-device-type
This predicate returns @code{#t} if the graphics system named by the
symbol @var{graphics-device-type} is implemented by the Scheme system.
Otherwise it returns @code{#f}, in which case it is an error to attempt
to make a graphics device using @var{graphics-device-type}.
@end deffn

@deffn procedure enumerate-graphics-types
This procedure returns a list of symbols which are the names of all the
graphics device types that are supported by the Scheme system.  The
result is useful in deciding what additional arguments to supply to
@code{make-graphics-device}, as each device type typically has a unique
way of specifying the initial size, shape and other attributes.
@end deffn

@deffn procedure make-graphics-device graphics-device-type object @dots{}
This operation creates and returns a graphics device object.
@var{Graphics-device-type} is a symbol naming a graphics device type,
and both the number and the meaning of the remaining arguments is
determined by that type (see the description of each device type for
details); @var{graphics-device-type} must satisfy
@code{graphics-type-available?}.  @var{Graphics-device-type} may also be
@code{#f}, in which case the graphics device type is chosen by the
system from what is available.  This allows completely portable graphics
programs to be written provided no custom graphics operations are used.
When @var{graphics-device-type} is @code{#f} no further arguments may be
given; each graphics device type will use some ``sensible'' defaults.
If more control is required then the program should use one of the two
procedures above to dispatch on the available types.

This procedure opens and initializes the device, which remains valid
until explicitly closed by the procedure @code{graphics-close}.
Depending on the implementation of the graphics device, if this object
is reclaimed by the garbage collector, the graphics device may remain
open or it may be automatically closed.  While a graphics device remains
open the resources associated with it are not released.
@end deffn

@deffn procedure graphics-close graphics-device
Closes @var{graphics-device}, releasing its resources.  Subsequently it
is an error to use @var{graphics-device}.
@end deffn

@node Coordinates for Graphics, Drawing Graphics, Opening and Closing of Graphics Devices, Graphics
@section Coordinates for Graphics
@cindex graphics, coordinate systems

@cindex coordinates, graphics
@cindex device coordinates, graphics (defn)
@cindex graphics, device coordinates (defn)
@cindex virtual coordinates, graphics (defn)
@cindex graphics, virtual coordinates (defn)
Each graphics device has two different coordinate systems associated
with it: @dfn{device coordinates} and @dfn{virtual coordinates}.  Device
coordinates are generally defined by low-level characteristics of the
device itself, and often cannot be changed.  Most device coordinate
systems are defined in terms of pixels, and usually the upper-left-hand
corner is the origin of the coordinate system, with @var{x} coordinates
increasing to the right and @var{y} coordinates increasing downwards.

In contrast, virtual coordinates are more flexible in the units
employed, the position of the origin, and even the direction in which
the coordinates increase.  A virtual coordinate system is defined by
assigning coordinates to the edges of a device.  Because these edge
coordinates are arbitrary real numbers, any Cartesian coordinate system
can be defined.

All graphics procedures that use coordinates are defined on virtual
coordinates.  For example, to draw a line at a particular place on a
device, the virtual coordinates for the endpoints of that line are
given.

When a graphics device is initialized, its virtual coordinate system is
reset so that the left edge corresponds to an x-coordinate of @code{-1},
the right edge to x-coordinate @code{1}, the bottom edge to y-coordinate
@code{-1}, and the top edge to y-coordinate @code{1}.

@deffn procedure graphics-device-coordinate-limits graphics-device
Returns (as multiple values) the device coordinate limits for
@var{graphics-device}.  The values, which are exact non-negative
integers, are: @var{x-left}, @var{y-bottom}, @var{x-right}, and
@var{y-top}.
@end deffn

@deffn procedure graphics-coordinate-limits graphics-device
Returns (as multiple values) the virtual coordinate limits for
@var{graphics-device}.  The values, which are real numbers, are:
@var{x-left}, @var{y-bottom}, @var{x-right}, and @var{y-top}.
@end deffn

@deffn procedure graphics-set-coordinate-limits graphics-device x-left y-bottom x-right y-top
Changes the virtual coordinate limits of @var{graphics-device} to the
given arguments.  @var{X-left}, @var{y-bottom}, @var{x-right}, and
@var{y-top} must be real numbers.  Subsequent calls to
@code{graphics-coordinate-limits} will return the new limits.  This
operation has no effect on the device's displayed contents.

Note: This operation usually resets the clip rectangle, although it is
not guaranteed to do so.  If a clip rectangle is in effect when this
procedure is called, it is necessary to redefine the clip rectangle
afterwards.
@end deffn

@node Drawing Graphics, Characteristics of Graphics Output, Coordinates for Graphics, Graphics
@section Drawing Graphics
@cindex graphics, drawing

The procedures in this section provide the basic drawing capabilities of
Scheme's graphics system.

@deffn procedure graphics-clear graphics-device
Clears the display of @var{graphics-device}.  Unaffected by the current
drawing mode.
@end deffn

@deffn procedure graphics-draw-point graphics-device x y
Draws a single point on @var{graphics-device} at the virtual coordinates
given by @var{x} and @var{y}, using the current drawing mode.
@end deffn

@deffn procedure graphics-erase-point graphics-device x y
Erases a single point on @var{graphics-device} at the virtual
coordinates given by @var{x} and @var{y}.  This procedure is unaffected
by the current drawing mode.
@end deffn

@noindent
This is equivalent to

@example
@group
(lambda (device x y)
  (graphics-bind-drawing-mode device 0
    (lambda ()
      (graphics-draw-point device x y))))
@end group
@end example

@deffn procedure graphics-draw-line graphics-device x-start y-start x-end y-end
@var{X-start}, @var{y-start}, @var{x-end}, and @var{y-end} must be real
numbers.  Draws a line on @var{graphics-device} that connects the points
(@var{x-start}, @var{y-start}) and (@var{x-end}, @var{y-end}).  The line
is drawn using the current drawing mode and line style.
@end deffn

@deffn procedure graphics-draw-text graphics-device x y string
Draws the characters of @var{string} at the point (@var{x}, @var{y}) on
@var{graphics-device}, using the current drawing mode.  The
characteristics of the characters drawn are device-dependent, but all
devices are initialized so that the characters are drawn upright, from
left to right, with the leftmost edge of the leftmost character at
@var{x}, and the baseline of the characters at @var{y}.
@end deffn

@cindex graphics, cursor (defn)
@cindex cursor, graphics (defn)
The following two procedures provide an alternate mechanism for drawing
lines, which is more akin to using a plotter.  They maintain a
@dfn{cursor}, which can be positioned to a particular point and then
dragged to another point, producing a line.  Sequences of connected line
segments can be drawn by dragging the cursor from point to point.

Many graphics operations have an unspecified effect on the cursor.  The
following exceptions are guaranteed to leave the cursor unaffected:

@example
@group
graphics-device-coordinate-limits
graphics-coordinate-limits
graphics-enable-buffering
graphics-disable-buffering
graphics-flush
graphics-bind-drawing-mode
graphics-set-drawing-mode
graphics-bind-line-style
graphics-set-line-style
@end group
@end example

The initial state of the cursor is unspecified.

@deffn procedure graphics-move-cursor graphics-device x y
Moves the cursor for @var{graphics-device} to the point (@var{x},
@var{y}).  The contents of the device's display are unchanged.
@end deffn

@deffn procedure graphics-drag-cursor graphics-device x y
Draws a line from @var{graphics-device}'s cursor to the point (@var{x},
@var{y}), simultaneously moving the cursor to that point.  The line is
drawn using the current drawing mode and line style.
@end deffn

@node Characteristics of Graphics Output, Buffering of Graphics Output, Drawing Graphics, Graphics
@section Characteristics of Graphics Output

@cindex graphics, output characteristics
Two characteristics of graphics output are so useful that they are
supported uniformly by all graphics devices: @dfn{drawing mode} and
@dfn{line style}.  A third characteristic, @dfn{color}, is equally
useful (if not more so), but implementation restrictions prohibit a
uniform interface.

@cindex drawing mode, graphics (defn)
@cindex graphics, drawing mode (defn)
The @dfn{drawing mode}, an exact integer in the range @code{0} to
@code{15} inclusive, determines how the figure being drawn is combined
with the background over which it is drawn to generate the final result.
Initially the drawing mode is set to ``source'', so that the new output
overwrites whatever appears in that place.  Useful alternative drawing
modes can, for example, erase what was already there, or invert it.

Altogether 16 boolean operations are available for combining the source
(what is being drawn) and the destination (what is being drawn over).
The source and destination are combined by the device on a
pixel-by-pixel basis as follows:

@example
@group
Mode    Meaning
----    -------
0       ZERO @r{[erase; use background color]}
1       source AND destination
2       source AND (NOT destination)
3       source
4       (NOT source) AND destination
5       destination
6       source XOR destination
7       source OR destination
8       NOT (source OR destination)
9       NOT (source XOR destination)
10      NOT destination
11      source OR (NOT destination)
12      NOT source
13      (NOT source) OR destination
14      (NOT source) OR (NOT destination)
15      ONE @r{[use foreground color]}
@end group
@end example

@cindex line style, graphics (defn)
@cindex graphics, line style (defn)
The @dfn{line style}, an exact integer in the range @code{0} to @code{7}
inclusive, determines which parts of a line are drawn in the foreground
color, and which in the background color.  The default line style,
``solid'', draws the entire line in the foreground color.
Alternatively, the ``dash'' style alternates between foreground and
background colors to generate a dashed line.  This capability is useful
for plotting several things on the same graph.

Here is a table showing the name and approximate pattern of the
different styles.  A @samp{1} in the pattern represents a foreground
pixel, while a @samp{-} represents a background pixel.  Note that the
precise output for each style will vary from device to device.  The only
style that is guaranteed to be the same for every device is ``solid''.

@example
@group
Style   Name                    Pattern
-----   -------                 -------
0       solid                   1111111111111111
1       dash                    11111111--------
2       dot                     1-1-1-1-1-1-1-1-
3       dash dot                1111111111111-1-
4       dash dot dot            11111111111-1-1-
5       long dash               11111111111-----
6       center dash             111111111111-11-
7       center dash dash        111111111-11-11-
@end group
@end example

@deffn procedure graphics-bind-drawing-mode graphics-device drawing-mode thunk
@deffnx procedure graphics-bind-line-style graphics-device line-style thunk
These procedures bind the drawing mode or line style, respectively, of
@var{graphics-device}, invoke the procedure @var{thunk} with no
arguments, then undo the binding when @var{thunk} returns.  The value of
each procedure is the value returned by @var{thunk}.  Graphics
operations performed during @var{thunk}'s dynamic extent will see the
newly bound mode or style as current.
@end deffn

@deffn procedure graphics-set-drawing-mode graphics-device drawing-mode
@deffnx procedure graphics-set-line-style graphics-device line-style
These procedures change the drawing mode or line style, respectively, of
@var{graphics-device}.  The mode or style will remain in effect until
subsequent changes or bindings.
@end deffn

@node Buffering of Graphics Output, Clipping of Graphics Output, Characteristics of Graphics Output, Graphics
@section Buffering of Graphics Output
@cindex buffering, of graphics output
@cindex graphics, buffering of output

To improve performance of graphics output, most graphics devices provide
some form of buffering.  By default, Scheme's graphics procedures flush
this buffer after every drawing operation.  The procedures in this
section allow the user to control the flushing of the output
buffer.

@deffn procedure graphics-enable-buffering graphics-device
Enables buffering for @var{graphics-device}.  In other words, after this
procedure is called, graphics operations are permitted to buffer their
drawing requests.  This usually means that the drawing is delayed until
the buffer is flushed explicitly by the user, or until it fills up and
is flushed by the system.
@end deffn

@deffn procedure graphics-disable-buffering graphics-device
Disables buffering for @var{graphics-device}.  By default, all graphics
devices are initialized with buffering disabled.  After this procedure
is called, all drawing operations perform their output immediately,
before returning.

Note: @code{graphics-disable-buffering} flushes the output buffer if
necessary.
@end deffn

@deffn procedure graphics-flush graphics-device
Flushes the graphics output buffer for @var{graphics-device}.  This
operation has no effect for devices that do not support buffering, or if
buffering is disabled for the device.
@end deffn

@node Clipping of Graphics Output, Custom Graphics Operations, Buffering of Graphics Output, Graphics
@section Clipping of Graphics Output
@cindex graphics, clipping
@cindex clipping, of graphics

@cindex clip rectangle, graphics (defn)
Scheme provides a rudimentary mechanism for restricting graphics output
to a given rectangular subsection of a graphics device.  By default,
graphics output that is drawn anywhere within the device's virtual
coordinate limits will appear on the device.  When a @dfn{clip
rectangle} is specified, however, output that would have appeared
outside the clip rectangle is not drawn.

Note that changing the virtual coordinate limits for a device will
usually reset the clip rectangle for that device, as will any operation
that affects the size of the device (such as a window resizing
operation).  However, programs should not depend on this.

@deffn procedure graphics-set-clip-rectangle graphics-device x-left y-bottom x-right y-top
Specifies the clip rectangle for @var{graphics-device} in virtual
coordinates.  @var{X-left}, @var{y-bottom}, @var{x-right}, and
@var{y-top} must be real numbers.  Subsequent graphics output is clipped
to the intersection of this rectangle and the device's virtual
coordinate limits.
@end deffn

@deffn procedure graphics-reset-clip-rectangle graphics-device
Eliminates the clip rectangle for @var{graphics-device}.  Subsequent
graphics output is clipped to the virtual coordinate limits of the
device.
@end deffn

@node Custom Graphics Operations, Images, Clipping of Graphics Output, Graphics
@section Custom Graphics Operations
@cindex custom operations, on graphics device
@cindex graphics, custom operations

In addition to the standard operations, a graphics device may support
@dfn{custom operations}.  For example, most devices have custom
operations to control color.  @code{graphics-operation} is used to
invoke custom operations.

@deffn procedure graphics-operation graphics-device name object @dots{}
Invokes the graphics operation on @var{graphics-device} whose name is
the symbol @var{name}, passing it the remaining arguments.  This
procedure can be used to invoke the standard operations, as well as
custom operations that are specific to a particular graphics device
type.  The names of the standard graphics operations are formed by
removing the @code{graphics-} prefix from the corresponding procedure.
For example, the following are equivalent:

@example
@group
(graphics-draw-point device x y)
(graphics-operation device 'draw-point x y)
@end group
@end example

For information on the custom operations for a particular device, see
the documentation for its type.
@end deffn

@node Images, X Graphics, Custom Graphics Operations, Graphics
@section Images
@cindex graphics, images
@cindex images, graphics
@cindex graphics, bitmaps
@cindex bitmaps, graphics

Some graphics device types support images, which are rectangular pieces
of picture that may be drawn into a graphics device.  Images are often
called something else in the host graphics system, such as bitmaps or
pixmaps.  The operations supported vary between devices, so look under
the different device types to see what operations are available.  All
devices that support images support the following operations.

@defop operation graphics-device create-image width height
Images are created using the @code{create-image} graphics operation,
specifying the @var{width} and @var{height} of the image in device
coordinates (pixels).

@example
(graphics-operation device 'create-image 200 100)
@end example

@noindent
The initial contents of an image are unspecified.

@code{create-image} is a graphics operation rather than a procedure
because the kind of image returned depends on the kind of graphics
device used and the options specified in its creation.  The image may be
used freely with other graphics devices created with the same
attributes, but the effects of using an image with a graphics device
with different attributes (for example, different colors) is undefined.
Under X, the image is display dependent.
@end defop

@defop operation graphics-device draw-image x y image
The image is copied into the graphics device at the specified position.
@end defop

@defop operation graphics-device draw-subimage x y image im-x im-y w h
Part of the image is copied into the graphics device at the specified
(@var{x}, @var{y}) position.  The part of the image that is copied is the
rectangular region at @var{im-x} and @var{im-y} and of width @var{w} and
height @var{h}.  These four numbers are given in device coordinates
(pixels).
@end defop

@deffn procedure image? object
Returns @code{#t} if @var{object} is an image, otherwise returns
@code{#f}.
@end deffn

@deffn procedure image/destroy image
This procedure destroys @var{image}, returning storage to the system.
Programs should destroy images after they have been used because even
modest images may use large amounts of memory.  Images are reclaimed by
the garbage collector, but they may be implemented using memory outside
of Scheme's heap.  If an image is reclaimed before being destroyed, the
implementation might not deallocate that non-heap memory, which can
cause a subsequent call to @code{create-image} to fail because it is
unable to allocate enough memory.
@end deffn

@c @deffn procedure image/descriptor image
@c The procedure returns the implementation dependent image.  Its use is
@c discouraged as it is non-portable.
@c @end deffn

@deffn procedure image/height image
Returns the height of the image in device coordinates.
@end deffn

@deffn procedure image/width image
Returns the width of the image in device coordinates.
@end deffn

@deffn procedure image/fill-from-byte-vector image bytes
The contents of @var{image} are set in a device-dependent way, using one
byte per pixel from @var{bytes} (a string).  Pixels are filled row by
row from the top of the image to the bottom, with each row being filled
from left to right.  There must be at least @code{(* (image/height
@var{image}) (image/width @var{image}))} bytes in @var{bytes}.
@end deffn

@node X Graphics,  , Images, Graphics
@section X Graphics
@cindex X graphics

@cindex X window system
MIT/GNU Scheme supports graphics in the X window system (version 11).
Arbitrary numbers of displays may be opened, and arbitrary numbers of
graphics windows may be created for each display.  A variety of
operations is available to manipulate various aspects of the windows, to
control their size, position, colors, and mapping.  The X graphics
device type supports images, which are implemented as Xlib @code{XImage}
objects.  X display, window, and image objects are automatically closed
if they are reclaimed by the garbage collector.

@menu
* X Graphics Type::
* Utilities for X Graphics::
* Custom Operations on X Graphics Devices::
@end menu

@node X Graphics Type, Utilities for X Graphics, X Graphics, X Graphics
@subsection X Graphics Type


A graphics device for X windows is created by passing the symbol
@code{x} as the graphics device type name to
@code{make-graphics-device}:

@example
(make-graphics-device 'x #!optional @var{display} @var{geometry} @var{suppress-map?})
@end example

@noindent
where @var{display} is either a display object, @code{#f}, or a string;
@var{geometry} is either @code{#f} or a string; and @var{suppress-map?}
is a boolean or a vector (see below).  A new window is created on the
appropriate display, and a graphics device representing that window is
returned.

@findex x-open-display
@var{Display} specifies which X display the window is to be opened on;
if it is @code{#f} or a string, it is passed as an argument to
@code{x-open-display}, and the value returned by that procedure is used
in place of the original argument.  @var{Geometry} is an X geometry
string, or @code{#f} which means to use the default geometry (which is
specified as a resource).

@var{Suppress-map?}, if given, may take two forms.  First, it may be a
boolean: if @code{#f} (the default), the window is automatically mapped
after it is created; otherwise, @code{#t} means to suppress this
automatic mapping.  The second form is a vector of three elements.  The
first element is a boolean with the same meaning as the boolean form of
@var{suppress-map?}.  The second element is a string, which specifies an
alternative resource name to be used for looking up the window's
resources.  The third element is also a string, which specifies a class
name for looking up the window's resources.  The default value for
@var{suppress-map?} is @code{#f}.

The default resource and class names are @code{"schemeGraphics"} and
@code{"SchemeGraphics"} respectively.

@cindex resources, X graphics
@cindex X resources, graphics
The window is initialized using the resource and class names specified
by @var{suppress-map?}, and is sensitive to the following resource
properties:

@example
@group
Property        Class           Default
--------        -----           -------
geometry        Geometry        512x384+0+0
font            Font            fixed
borderWidth     BorderWidth     2
internalBorder  BorderWidth     @r{[border width]}
background      Background      white
foreground      Foreground      black
borderColor     BorderColor     @r{[foreground color]}
cursorColor     Foreground      @r{[foreground color]}
pointerColor    Foreground      @r{[foreground color]}
@end group
@end example

The window is created with a @code{backing_store} attribute of
@code{Always}.  The window's name and icon name are initialized to
@code{"scheme-graphics"}.


@node Utilities for X Graphics, Custom Operations on X Graphics Devices, X Graphics Type, X Graphics
@subsection Utilities for X Graphics

@deffn procedure x-graphics/open-display display-name
@cindex display, X graphics
@cindex X display, graphics
Opens a connection to the display whose name is @var{display-name},
returning a display object.  If unable to open a connection, @code{#f}
is returned.  @var{Display-name} is normally a string, which is an X
display name in the usual form; however, @code{#f} is also allowed,
meaning to use the value of the unix environment variable
@code{DISPLAY}.
@end deffn

@deffn procedure x-graphics/close-display display
Closes @var{display}; after calling this procedure, it is an error to
use @var{display} for any purpose.  Any windows that were previously
opened on @var{display} are destroyed and their resources returned to
the operating system.
@end deffn

@deffn procedure x-close-all-displays
Closes all open connections to X displays.  Equivalent to calling
@code{x-close-display} on all open displays.
@end deffn

@deffn procedure x-geometry-string x y width height
@cindex geometry string, X graphics
@cindex X geometry string, graphics
This procedure creates and returns a standard X geometry string from the
given arguments.  @var{X} and @var{y} must be either exact integers or
@code{#f}, while @var{width} and @var{height} must be either exact
non-negative integers or @code{#f}.  Usually either @var{x} and @var{y}
are both specified or both @code{#f}; similarly for @var{width} and
@var{height}.  If only one of the elements of such a pair is specified,
it is ignored.

Examples:

@example
@group
(x-geometry-string #f #f 100 200) @result{} "100x200"
(x-geometry-string 2 -3 100 200) @result{} "100x200+2-3"
(x-geometry-string 2 -3 #f #f) @result{} "+2-3"
@end group
@end example

Note that the @var{x} and @var{y} arguments cannot distinguish between
@code{+0} and @code{-0}, even though these have different meanings in X.
If either of those arguments is @code{0}, it means @code{+0} in X
terminology.  If you need to distinguish these two cases you must create
your own geometry string using Scheme's string and number primitives.
@end deffn

@node Custom Operations on X Graphics Devices,  , Utilities for X Graphics, X Graphics
@subsection Custom Operations on X Graphics Devices

Custom operations are invoked using the procedure
@code{graphics-operation}.  For example,

@example
(graphics-operation device 'set-foreground-color "blue")
@end example

@defop operation x-graphics-device set-background-color color-name
@defopx operation x-graphics-device set-foreground-color color-name
@defopx operation x-graphics-device set-border-color color-name
@defopx operation x-graphics-device set-mouse-color color-name
@findex graphics-clear
These operations change the colors associated with a window.
@var{Color-name} must be a string, which is the X server's name for the
desired color.  @code{set-border-color} and @code{set-mouse-color}
immediately change the border and mouse-cursor colors.
@code{set-background-color} and @code{set-foreground-color} change the
colors to be used when drawing, but have no effect on anything drawn
prior to their invocation.  Because changing the background color
affects the entire window, we recommend calling @code{graphics-clear} on
the window's device afterwards.  Color names include both mnemonic
names, like @code{"red"}, and intensity names specified in the
@code{"#@var{rrggbb}"} notation.
@end defop

@defop operation x-graphics-device draw-arc x y radius-x radius-y angle-start angle-sweep fill?
@cindex drawing arcs and circles, graphics
@cindex graphics, drawing arcs and circles
@cindex circles, drawing
@findex draw-arc

Operation @code{draw-arc} draws or fills an arc.  An arc is a segment of
a circle, which may have been stretched along the x- or y- axis to form
an ellipse.

The parameters @var{x}, @var{y}, @var{radius-x} and @var{radius-y}
describe the circle and @var{angle-start} and @var{angle-sweep} choose
which part of the circle is drawn.  The arc is drawn on the graphics
device with the center of the circle at the virtual coordinates given by
@var{x} and @var{y}.  @var{radius-x} and @var{radius-y} determine the
size of the circle in virtual coordinate units.

The parameter @var{angle-start} determines where the arc starts.  It is
measured in degrees in an anti-clockwise direction, starting at 3
o'clock.  @var{angle-sweep} determines how much of the circle is drawn.
It too is measured anti-clockwise in degrees.  A negative value means
the measurement is in a clockwise direction.

Note that the angles are determined on a unit circle before it is
stretched into an ellipse, so the actual angles that you will see on the
computer screen depends on all of: @var{radius-x} and @var{radius-y},
the window size, and the virtual coordinates.

If @var{fill?} is @code{#f} then just the segment of the circle is
drawn, otherwise the arc is filled in a pie-slice fashion.

This draws a quarter circle pie slice, standing on its point, with point
at virtual coordinates (3,5):

@example
(graphics-operation g 'draw-arc 3 5 .5 .5 45 90 #t)
@end example

@end defop

@defop operation x-graphics-device draw-circle x y radius
@defopx operation x-graphics-device fill-circle x y radius
@cindex drawing arcs and circles, graphics
@cindex graphics, drawing arcs and circles
@cindex circles, drawing
@findex draw-circle
@findex fill-circle
These operations draw a circle (outline) or a filled circle (solid) at
on the graphics device at the virtual coordinates given by @var{x} and
@var{y}.  These operations could be implemented trivially interms of the
@code{draw-arc} operation.
@end defop

@defop operation x-graphics-device set-border-width width
@defopx operation x-graphics-device set-internal-border-width width
@findex graphics-clear
These operations change the external and internal border widths of a
window.  @var{Width} must be an exact non-negative integer, specified in
pixels.  The change takes place immediately.  Note that changing the
internal border width can cause displayed graphics to be garbled; we
recommend calling @code{graphics-clear} on the window's device after
doing so.
@end defop

@defop operation x-graphics-device set-font font-name
Changes the font used when drawing text in a window.  @var{Font-name}
must be a string that is a font name known to the X server.  This
operation does not affect text drawn prior to its invocation.
@end defop

@defop operation x-graphics-device set-mouse-shape shape-number
Changes the shape of the mouse cursor.  @var{Shape-number} is an exact
non-negative integer that is used as an index into the mouse-shape font;
when multiplied by 2 this number corresponds to an index in the file@*
@file{/usr/include/X11/cursorfont.h}.
@end defop

@defop operation x-graphics-device map-window
@defopx operation x-graphics-device withdraw-window
These operations control the mapping of windows.  They correspond
directly to Xlib's @code{XMapWindow} and @code{XWithdrawWindow}.
@end defop

@defop operation x-graphics-device resize-window width height
Changes the size of a window.  @var{Width} and @var{height} must be
exact non-negative integers.  The operation corresponds directly to
Xlib's @code{XResizeWindow}.

This operation resets the virtual coordinate system and the clip
rectangle.
@end defop

@defop operation x-graphics-device move-window x y
Changes the position of a window on the display.  @var{X} and @var{y}
must be exact integers.  The operation corresponds directly to Xlib's
@code{XMoveWindow}.  Note that the coordinates @var{x} and @var{y} do
not take the external border into account, and therefore will not
position the window as you might like.  The only reliable way to
position a window is to ask a window manager to do it for you.
@end defop

@defop operation x-graphics-device get-default resource property
This operation corresponds directly to Xlib's @code{XGetDefault}.
@var{Resource} and @var{property} must be strings.  The operation
returns the character string corresponding to the association of
@var{resource} and @var{property}; if no such association exists,
@code{#f} is returned.
@end defop

@defop operation x-graphics-device copy-area source-x-left source-y-top width height destination-x-left destination-y-top
This operation copies the contents of the rectangle specified by
@var{source-x-left}, @var{source-y-top}, @var{width}, and @var{height} to
the rectangle of the same dimensions at @var{destination-x-left} and
@var{destination-y-top}.
@end defop

@defop operation x-graphics-device font-structure font-name
Returns a Scheme equivalent of the X font structure for the font named
@var{font-name}.  If the string @var{font-name} does not name a font
known to the X server, or names a 16-bit font, @code{#f} is returned.
@end defop

@deffn procedure x-font-structure/name font-structure
@deffnx procedure x-font-structure/direction font-structure
@deffnx procedure x-font-structure/all-chars-exist font-structure
@deffnx procedure x-font-structure/default-char font-structure
@deffnx procedure x-font-structure/min-bounds font-structure
@deffnx procedure x-font-structure/max-bounds font-structure
@deffnx procedure x-font-structure/start-index font-structure
@deffnx procedure x-font-structure/character-bounds font-structure
@deffnx procedure x-font-structure/max-ascent font-structure
@deffnx procedure x-font-structure/max-descent font-structure
These procedures extract the components of the font description
structure returned by the X graphics operation @code{font-structure}.  A
more complete description of these components appears in documentation
of the @code{XLoadQueryFont} Xlib call.  @code{start-index} is the index
of the first character available in the font.  The @code{min-bounds} and
@code{max-bounds} components are structures of type
@code{x-character-bounds}, and the @code{character-bounds} component is
a vector of the same type.
@end deffn

@deffn procedure x-character-bounds/lbearing character-bounds
@deffnx procedure x-character-bounds/rbearing character-bounds
@deffnx procedure x-character-bounds/width character-bounds
@deffnx procedure x-character-bounds/ascent character-bounds
@deffnx procedure x-character-bounds/descent character-bounds
These procedures extract components of objects of type
@code{x-character-bounds}.  A more complete description of them appears
in documentation of the@* @code{XLoadQueryFont} Xlib call.
@end deffn
