CANVAS(9) CANVAS(9)
NAME
canvas - Create and manipulate canvas widgets
SYNOPSIS
canvas pathName ?options?
STANDARD OPTIONS
-background -selectbackground -takefocus
-borderwidth -selectborderwidth -xscrollcommand
-relief -selectforeground -yscrollcommand
WIDGET-SPECIFIC OPTIONS
-closeenough dist
Specifies a floating-point value indicating how close
the mouse cursor must be to an item before it is con-
sidered to be ``inside'' the item. Defaults to 1.0.
-confine boolean
Specifies a boolean value that indicates whether or not
it should be allowable to set the canvas's view outside
the region defined by the scrollregion option.
Defaults to true, which means that the view will be
constrained within the scroll region.
-height dist
Specifies a desired window height that the canvas wid-
get should request from its geometry manager. The
value may be specified in any of the forms described in
the COORDINATES section below.
-scrollregion list
Specifies a list with four dist coordinates describing
the left, top, right, and bottom coordinates of a rect-
angular region. This region is used for scrolling pur-
poses and is considered to be the boundary of the
information in the canvas. Each of the coordinates may
be specified in any of the forms given in the COORDI-
NATES section below.
-width dist
Specifies a desired window width that the canvas widget
should request from its geometry manager. The value
may be specified in any of the forms described in the
COORDINATES section below.
-xscrollincrement dist
Specifies an increment for horizontal scrolling, in any
of the usual forms permitted for screen distances. If
the value of this option is greater than zero, the hor-
izontal view in the window will be constrained so that
Page 1 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
the canvas x coordinate at the left edge of the window
is always an even multiple of xscrollicrement; fur-
thermore, the units for scrolling (e.g., the change in
view when the left and right arrows of a scrollbar are
selected) will also be xscrollicrement. If the value
of this option is less than or equal to zero, then hor-
izontal scrolling is unconstrained.
-yscrollincrement dist
Specifies an increment for vertical scrolling, in any
of the usual forms permitted for screen distances. If
the value of this option is greater than zero, the ver-
tical view in the window will be constrained so that
the canvas y coordinate at the top edge of the window
is always an even multiple of yscrollicrement; fur-
thermore, the units for scrolling (e.g., the change in
view when the top and bottom arrows of a scrollbar are
selected) will also be yscrollicrement. If the value
of this option is less than or equal to zero, then ver-
tical scrolling is unconstrained.
-buffer what
Specifies how much of the canvas region will be backed
by an offscreen bitmap buffer. What can be one of all
(the entire scroll region will be buffered), visible
(only the visible area), none (no buffering) or auto
(equivalent to either none or visible depending on
whether the canvas is packed inside another canvas or
not).
INTRODUCTION
The canvas command creates a new window (given by the path-
Name argument) and makes it into a canvas widget. Addi-
tional options, described above, may be specified on the
command line to configure aspects of the canvas such as its
colours and 3-D relief. The canvas command returns its
pathName argument. At the time this command is invoked,
there must not exist a window named pathName.
Canvas widgets implement structured graphics. A canvas dis-
plays any number of items, which may be things like rectan-
gles, circles, lines, and text. Items may be manipulated
(e.g. moved or re-coloured) and commands may be associated
with items in much the same way that the bind command allows
commands to be bound to widgets. For example, a particular
command may be associated with the <Button-1> event so that
the command is invoked whenever button 1 is pressed with the
mouse cursor over an item. This means that items in a can-
vas can have behaviours defined by the Tk scripts bound to
them.
Page 2 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
DISPLAY LIST
The items in a canvas are ordered for purposes of display,
with the first item in the display list being displayed
first, followed by the next item in the list, and so on.
Items later in the display list obscure those that are ear-
lier in the display list and are sometimes referred to as
being ``on top'' of earlier items. When a new item is cre-
ated it is placed at the end of the display list, on top of
everything else. Widget commands may be used to re-arrange
the order of the display list.
ITEM IDS AND TAGS
Items in a canvas widget may be named in either of two ways:
by id or by tag. Each item has a unique identifying number
which is assigned to that item when it is created. The id
of an item never changes and id numbers are never re-used
within the lifetime of a canvas widget.
Each item may also have any number of tags associated with
it. A tag is just a string of characters, and it may take
any form except that of an integer. For example, ``x123''
is OK but ``123'' isn't. The same tag may be associated
with many different items. This is commonly done to group
items in various interesting ways; for example, all
selected items might be given the tag ``selected''.
The tag all is implicitly associated with every item in the
canvas; it may be used to invoke operations on all the
items in the canvas.
The tag current is managed automatically by Tk; it applies
to the current item, which is the topmost item whose drawn
area covers the position of the mouse cursor. If the mouse
is not in the canvas widget or is not over an item, then no
item has the current tag.
When specifying items in canvas widget commands, if the
specifier is an integer then it is assumed to refer to the
single item with that id. If the specifier is not an inte-
ger, then it is assumed to refer to all of the items in the
canvas that have a tag matching the specifier. The symbol
tagOrId is used below to indicate that an argument specifies
either an id that selects a single item or a tag that
selects zero or more items. Some widget commands only oper-
ate on a single item at a time; if tagOrId is specified in
a way that names multiple items, then the normal behaviour
is for the command to use the first (lowest) of these items
in the display list that is suitable for the command.
Exceptions are noted in the widget command descriptions
below.
Page 3 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
COORDINATES
All coordinates related to canvases are stored as fixed-
point numbers. Coordinates and distances are specified as
documented in the dist section of types(9).
TRANSFORMATIONS
Normally the origin of the canvas coordinate system is at
the upper-left corner of the window containing the canvas.
It is possible to adjust the origin of the canvas coordinate
system relative to the origin of the window using the xview
and yview widget commands; this is typically used for
scrolling. Canvases do not support scaling or rotation of
the canvas coordinate system relative to the window coordi-
nate system.
Individual items may be moved or scaled using widget com-
mands described below, but they may not be rotated.
INDICES
Text items support the notion of an index for identifying
particular positions within the item. Indices are used for
commands such as inserting text, deleting a range of charac-
ters, and setting the insertion cursor position. An index
may be specified in any of a number of ways, and different
types of items may support different forms for specifying
indices. Text items support the following forms for an
index. Note that it is possible to refer to the character
just after the last one in the text item; this is necessary
for such tasks as inserting new text at the end of the item.
number A decimal number giving the position of the
desired character within the text item. 0 refers
to the first character, 1 to the next character,
and so on. A number less than 0 is treated as if
it were zero, and a number greater than the length
of the text item is treated as if it were equal to
the length of the text item.
end Refers to the character just after the last one in
the item (same as the number of characters in the
item).
insert Refers to the character just before which the
insertion cursor is drawn in this item.
sel.first Refers to the first selected character in the
item. If the selection isn't in this item then
this form is illegal.
sel.last Refers to the last selected character in the item.
If the selection isn't in this item then this form
Page 4 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
is illegal.
@x,y Refers to the character at the point given by x
and y, where x and y are specified in the coordi-
nate system of the canvas. If x and y lie outside
the coordinates covered by the text item, then
they refer to the first or last character in the
line that is closest to the given point.
WIDGET COMMAND
The canvas command creates a new Tk command whose name is
pathName. This command may be used to invoke various opera-
tions on the widget. It has the following general form:
pathName option ?arg arg ...?
Option and the args determine the exact behaviour of the
command. The following widget commands are possible for
canvas widgets:
pathName addtag tag searchSpec ?arg arg ...?
For each item that meets the constraints specified by
searchSpec and the args, add tag to the list of tags
associated with the item if it isn't already present on
that list. It is possible that no items will satisfy
the constraints given by searchSpec and args, in which
case the command has no effect. This command returns
an empty string as result. SearchSpec and arg's may
take any of the following forms:
above tagOrId
Selects the item just after (above) the one given
by tagOrId in the display list. If tagOrId
denotes more than one item, then the last (top-
most) of these items in the display list is used.
all Selects all the items in the canvas.
below tagOrId
Selects the item just before (below) the one given
by tagOrId in the display list. If tagOrId
denotes more than one item, then the first (low-
est) of these items in the display list is used.
closest x y ?halo? ?start?
Selects the item closest to the point given by x
and y. If more than one item is at the same clos-
est distance (e.g. two items overlap the point),
then the top-most of these items (the last one in
the display list) is used. If halo is specified,
then it must be a non-negative value. Any item
closer than halo to the point is considered to
overlap it. The start argument may be used to
Page 5 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
step circularly through all the closest items. If
start is specified, it names an item using a tag
or id (if by tag, it selects the first item in the
display list with the given tag). Instead of
selecting the topmost closest item, this form will
select the topmost closest item that is below
start in the display list; if no such item
exists, then the selection behaves as if the start
argument had not been specified.
enclosed x1 y1 x2 y2
Selects all the items completely enclosed within
the rectangular region given by x1, y1, x2, and
y2. X1 must be no greater then x2 and y1 must be
no greater than y2.
overlapping x1 y1 x2 y2
Selects all the items that overlap or are enclosed
within the rectangular region given by x1, y1, x2,
and y2. X1 must be no greater then x2 and y1 must
be no greater than y2.
withtag tagOrId
Selects all the items given by tagOrId.
pathName bbox tagOrId ?tagOrId tagOrId ...?
Returns a list with four elements giving an approximate
bounding box for all the items named by the tagOrId
arguments. The list has the form ``x1 y1 x2 y2'' such
that the drawn areas of all the named elements are
within the region bounded by x1 on the left, x2 on the
right, y1 on the top, and y2 on the bottom. The return
value may overestimate the actual bounding box by a few
pixels. If no items match any of the tagOrId arguments
or if the matching items have empty bounding boxes
(i.e. they have nothing to display) then an empty
string is returned.
pathName bind tagOrId ?sequence? ?command?
This command associates command with all the items
given by tagOrId such that whenever the event sequence
given by sequence occurs for one of the items the com-
mand will be invoked. This widget command is similar
to the bind command except that it operates on items in
a canvas rather than entire widgets. See the bind man-
ual entry for complete details on the syntax of
sequence and the substitutions performed on command
before invoking it. If all arguments are specified
then a new binding is created, replacing any existing
binding for the same sequence and tagOrId (if the first
character of command is ``+'' then command augments an
existing binding rather than replacing it). In this
Page 6 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
case the return value is an empty string. If command
is omitted then the command returns the command associ-
ated with tagOrId and sequence (an error occurs if
there is no such binding). If both command and
sequence are omitted then the command returns a list of
all the sequences for which bindings have been defined
for tagOrId.
The only events for which bindings may be specified are
those related to the mouse and keyboard, such as Enter,
Leave, ButtonPress, Motion, and KeyPress. The handling
of events in canvases uses the current item defined in
ITEM IDS AND TAGS above. Enter and Leave events trig-
ger for an item when it becomes the current item or
ceases to be the current item; note that these events
are different than Enter and Leave events for windows.
Mouse-related events are directed to the current item,
if any. Keyboard-related events are directed to the
focus item, if any (see the focus widget command below
for more on this).
It is possible for multiple bindings to match a partic-
ular event. This could occur, for example, if one
binding is associated with the item's id and another is
associated with one of the item's tags. When this
occurs, all of the matching bindings are invoked. A
binding associated with the all tag is invoked first,
followed by one binding for each of the item's tags (in
order), followed by a binding associated with the
item's id. If there are multiple matching bindings for
a single tag, then only the most specific binding is
invoked. A continue command in a binding script termi-
nates that script, and a break command terminates that
script and skips any remaining scripts for the event,
just as for the bind command.
If bindings have been created for a canvas window using
the bind command, then they are invoked in addition to
bindings created for the canvas's items using the bind
widget command. The bindings for items will be invoked
before any of the bindings for the window as a whole.
pathName canvasx screenx ?gridspacing?
Given a window x-coordinate in the canvas screenx, this
command returns the canvas x-coordinate that is dis-
played at that location. If gridspacing is specified,
then the canvas coordinate is rounded to the nearest
multiple of gridspacing units.
pathName canvasy screeny ?gridspacing?
Given a window y-coordinate in the canvas screeny this
command returns the canvas y-coordinate that is
Page 7 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
displayed at that location. If gridspacing is speci-
fied, then the canvas coordinate is rounded to the
nearest multiple of gridspacing units.
pathName cget option
Returns the current value of the configuration option
given by option. Option may have any of the values
accepted by the canvas command.
pathName configure ?option? ?value? ?option value ...?
Query or modify the configuration options of the wid-
get. If no option is specified, returns a list of all
of the available options for pathName. If one or more
option-value pairs are specified, then the command mod-
ifies the given widget option(s) to have the given
value(s); in this case the command returns an empty
string. Option may have any of the values accepted by
the canvas command.
pathName coords tagOrId ?x0 y0 ...?
Query or modify the coordinates that define an item.
If no coordinates are specified, this command returns a
list whose elements are the coordinates of the item
named by tagOrId. If coordinates are specified, then
they replace the current coordinates for the named
item. If tagOrId refers to multiple items, then the
first one in the display list is used.
pathName create type x y ?x y ...? ?option value ...?
Create a new item in pathName of type type. The exact
format of the arguments after type depends on type, but
usually they consist of the coordinates for one or more
points, followed by specifications for zero or more
item options. See the subsections on individual item
types below for more on the syntax of this command.
This command returns the id for the new item.
pathName dchars tagOrId first ?last?
For each item given by tagOrId, delete the characters
in the range given by first and last, inclusive. If
some of the items given by tagOrId don't support text
operations, then they are ignored. First and last are
indices of characters within the item(s) as described
in INDICES above. If last is omitted, it defaults to
first. This command returns an empty string.
pathName delete ?tagOrId tagOrId ...?
Delete each of the items given by each tagOrId, and
return an empty string.
pathName dtag tagOrId ?tagToDelete?
For each of the items given by tagOrId, delete the tag
Page 8 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
given by tagToDelete from the list of those associated
with the item. If an item doesn't have the tag tag-
ToDelete then the item is unaffected by the command.
If tagToDelete is omitted then it defaults to tagOrId.
This command returns an empty string.
pathName find searchCommand ?arg arg ...?
This command returns a list consisting of all the items
that meet the constraints specified by searchCommand
and arg's. SearchCommand and args have any of the
forms accepted by the addtag command. If searchCommand
is enclosed, overlapping, or all, the items are
returned in display-list order, i.e. bottommost first.
pathName focus ?tagOrId?
Set the keyboard focus for the canvas widget to the
item given by tagOrId. If tagOrId refers to several
items, then the focus is set to the first such item in
the display list that supports the insertion cursor.
If tagOrId doesn't refer to any items, or if none of
them support the insertion cursor, then the focus isn't
changed. If tagOrId is an empty string, then the focus
item is reset so that no item has the focus. If
tagOrId is not specified then the command returns the
id for the item that currently has the focus, or an
empty string if no item has the focus.
Once the focus has been set to an item, the item will
display the insertion cursor and all keyboard events
will be directed to that item. The focus item within a
canvas and the focus window on the screen (set with the
focus command) are totally independent: a given item
doesn't actually have the input focus unless (a) its
canvas is the focus window and (b) the item is the
focus item within the canvas. In most cases it is
advisable to follow the focus widget command with the
focus command to set the focus window to the canvas (if
it wasn't there already).
pathName gettags tagOrId
Return a list whose elements are the tags associated
with the item given by tagOrId. If tagOrId refers to
more than one item, then the tags are returned from the
first such item in the display list. If tagOrId
doesn't refer to any items, then an error is returned.
If the item contains no tags, then an empty string is
returned.
pathName grab what tagOrId
Does for canvas widgets what grab(9) does for normal tk
widgets: mouse events will only be delivered to
tagOrId. If tagOrId refers to more than one item, then
Page 9 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
the first such item in the display list is grabbed.
What is as described in grab(9).
Note that the canvas grab item, as set by this command, and
the tk grab item, as set by grab(9) are totally independent;
a canvas item doesn't actually grab the mouse unless a) the
canvas itself has grabbed the mouse or b) the mouse events
are being delivered to the canvas as a matter of course.
pathName icursor tagOrId index
Set the position of the insertion cursor for the
item(s) given by tagOrId to just before the character
whose position is given by index. If some or all of
the items given by tagOrId don't support an insertion
cursor then this command has no effect on them. See
INDICES above for a description of the legal forms for
index. Note: the insertion cursor is only displayed
in an item if that item currently has the keyboard
focus (see the widget command focus, below), but the
cursor position may be set even when the item doesn't
have the focus. This command returns an empty string.
pathName index tagOrId index
This command returns a decimal string giving the numer-
ical index within tagOrId corresponding to index.
Index gives a textual description of the desired posi-
tion as described in INDICES above. The return value
is guaranteed to lie between 0 and the number of char-
acters within the item, inclusive. If tagOrId refers
to multiple items, then the index is processed in the
first of these items that supports indexing operations
(in display list order).
pathName insert tagOrId beforeThis string
For each of the items given by tagOrId, if the item
supports text insertion then string is inserted into
the item's text just before the character whose index
is beforeThis. See INDICES above for information about
the forms allowed for beforeThis. This command returns
an empty string.
pathName itemcget tagOrId option
Returns the current value of the configuration option
for the item given by tagOrId whose name is option.
This command is similar to the cget widget command
except that it applies to a particular item rather than
the widget as a whole. Option may have any of the val-
ues accepted by the create widget command when the item
was created. If tagOrId is a tag that refers to more
than one item, the first (lowest) such item is used.
pathName itemconfigure tagOrId ?option? ?value? ?option value
Page 10 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
This command is similar to the configure widget command
except that it modifies item-specific options for the
items given by tagOrId instead of modifying options for
the overall canvas widget. If one or more option-value
pairs are specified, then the command modifies the
given widget option(s) to have the given value(s) in
each of the items given by tagOrId; in this case the
command returns an empty string. The options and val-
ues are the same as those permissible in the create
widget command when the item(s) were created; see the
sections describing individual item types below for
details on the legal options.
pathName lower tagOrId ?belowThis?
Move all of the items given by tagOrId to a new posi-
tion in the display list just before the item given by
belowThis. If tagOrId refers to more than one item
then all are moved but the relative order of the moved
items will not be changed. BelowThis is a tag or id;
if it refers to more than one item then the first (low-
est) of these items in the display list is used as the
destination location for the moved items. This command
returns an empty string.
pathName move tagOrId xAmount yAmount
Move each of the items given by tagOrId in the canvas
coordinate space by adding xAmount to the x-coordinate
of each point associated with the item and yAmount to
the y-coordinate of each point associated with the
item. This command returns an empty string.
pathName raise tagOrId ?aboveThis?
Move all of the items given by tagOrId to a new posi-
tion in the display list just after the item given by
aboveThis. If tagOrId refers to more than one item
then all are moved but the relative order of the moved
items will not be changed. AboveThis is a tag or id;
if it refers to more than one item then the last (top-
most) of these items in the display list is used as the
destination location for the moved items. This command
returns an empty string.
pathName scale tagOrId xOrigin yOrigin xScale yScale
Rescale all of the items given by tagOrId in canvas
coordinate space. XOrigin and yOrigin identify the
origin for the scaling operation and xScale and yScale
identify the scale factors for x- and y-coordinates,
respectively (a scale factor of 1.0 implies no change
to that coordinate). For each of the points defining
each item, the x-coordinate is adjusted to change the
distance from xOrigin by a factor of xScale. Simi-
larly, each y-coordinate is adjusted to change the
Page 11 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
distance from yOrigin by a factor of yScale. This com-
mand returns an empty string.
pathName screenx canvasx
Given an x-coordinate canvasx in the canvas, this com-
mand returns the equivalent screen x-coordinate.
pathName screeny canvasy
Given an x-coordinate canvasy in the canvas, this com-
mand returns the equivalent screen y-coordinate.
pathName see x1 y1 ?x2 y2?
Adjusts the view in the window such that, if possible
the point [x1, y1] (and, if given, the point [x2, y2])
are made visible.
pathName select option ?tagOrId arg?
Manipulates the selection in one of several ways,
depending on option. The command may take any of the
forms described below. In all of the descriptions
below, tagOrId must refer to an item that supports
indexing and selection; if it refers to multiple items
then the first of these that supports indexing and the
selection is used. Index gives a textual description
of a position within tagOrId, as described in INDICES
above.
pathName select adjust tagOrId index
Locate the end of the selection in tagOrId nearest
to the character given by index, and adjust that
end of the selection to be at index (i.e. includ-
ing but not going beyond index). The other end of
the selection is made the anchor point for future
select to commands. If the selection isn't cur-
rently in tagOrId then this command behaves the
same as the select to widget command. Returns an
empty string.
pathName select clear
Clear the selection if it is in this widget. If
the selection isn't in this widget then the com-
mand has no effect. Returns an empty string.
pathName select from tagOrId index
Set the selection anchor point for the widget to
be just before the character given by index in the
item given by tagOrId. This command doesn't
change the selection; it just sets the fixed end
of the selection for future select to commands.
Returns an empty string.
pathName select item
Page 12 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
Returns the id of the selected item, if the selec-
tion is in an item in this canvas. If the selec-
tion is not in this canvas then an empty string is
returned.
pathName select to tagOrId index
Set the selection to consist of those characters
of tagOrId between the selection anchor point and
index. The new selection will include the charac-
ter given by index; it will include the character
given by the anchor point only if index is greater
than or equal to the anchor point. The anchor
point is determined by the most recent select
adjust or select from command for this widget. If
the selection anchor point for the widget isn't
currently in tagOrId, then it is set to the same
character given by index. Returns an empty
string.
pathName type tagOrId
Returns the type of the item given by tagOrId, such as
rectangle or text. If tagOrId refers to more than one
item, then the type of the first item in the display
list is returned. If tagOrId doesn't refer to any
items at all then an empty string is returned.
pathName xview ?args?
This command is used to query and change the horizontal
position of the information displayed in the canvas's
window. It can take any of the following forms:
pathName xview
Returns a list containing two elements. Each ele-
ment is a real fraction between 0 and 1; together
they describe the horizontal span that is visible
in the window. For example, if the first element
is .2 and the second element is .6, 20% of the
canvas's area (as defined by the -scrollregion
option) is off-screen to the left, the middle 40%
is visible in the window, and 40% of the canvas is
off-screen to the right. These are the same val-
ues passed to scrollbars via the -xscrollcommand
option.
pathName xview moveto fraction
Adjusts the view in the window so that fraction of
the total width of the canvas is off-screen to the
left. Fraction must be a fraction between 0 and
1.
pathName xview scroll number what
This command shifts the view in the window left or
Page 13 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
right according to number and what. Number must
be an integer. What must be either units or
pages. If what is units, the view adjusts left or
right in units of the xscrollicrement option, if
it is greater than zero, or in units of one-tenth
the window's width otherwise. If what is pages
then the view adjusts in units of nine-tenths the
window's width. If number is negative then infor-
mation farther to the left becomes visible; if it
is positive then information farther to the right
becomes visible.
pathName yview ?args?
This command is used to query and change the vertical
position of the information displayed in the canvas's
window. It can take any of the following forms:
pathName yview
Returns a list containing two elements. Each ele-
ment is a real fraction between 0 and 1; together
they describe the vertical span that is visible in
the window. For example, if the first element is
.6 and the second element is 1.0, the lowest 40%
of the canvas's area (as defined by the -scrollre-
gion option) is visible in the window. These are
the same values passed to scrollbars via the
-yscrollcommand option.
pathName yview moveto fraction
Adjusts the view in the window so that fraction of
the canvas's area is off-screen to the top. Frac-
tion is a fraction between 0 and 1.
pathName yview scroll number what
This command adjusts the view in the window up or
down according to number and what. Number must be
an integer. What must be either units or pages.
If what is units, the view adjusts up or down in
units of the yscrollicrement option, if it is
greater than zero, or in units of one-tenth the
window's height otherwise. If what is pages then
the view adjusts in units of nine-tenths the
window's height. If number is negative then
higher information becomes visible; if it is pos-
itive then lower information becomes visible.
OVERVIEW OF ITEM TYPES
The sections below describe the various types of items sup-
ported by canvas widgets. Each item type is characterized
by two things: first, the form of the create command used to
create instances of the type; and second, a set of
Page 14 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
configuration options for items of that type, which may be
used in the create and itemconfigure widget commands. Most
items don't support indexing or selection or the commands
related to them, such as index and insert. Where items do
support these facilities, it is noted explicitly in the
descriptions below (at present, only text items provide this
support).
ARC ITEMS
Items of type arc appear on the display as arc-shaped
regions. An arc is a section of an oval delimited by two
angles (specified by the -start and -extent options) and
displayed in one of several ways (specified by the -style
option). Arcs are created with widget commands of the fol-
lowing form:
pathName create arc x1 y1 x2 y2 ?option value option value ...?
The arguments x1, y1, x2, and y2 give the coordinates of two
diagonally opposite corners of a rectangular region enclos-
ing the oval that defines the arc. After the coordinates
there may be any number of option-value pairs, each of which
sets one of the configuration options for the item. These
same option-value pairs may be used in itemconfigure widget
commands to change the item's configuration. The following
options are supported for arcs:
-extent degrees
Specifies the size of the angular range occupied by the
arc. The arc's range extends for degrees degrees
counter-clockwise from the starting angle given by the
-start option. Degrees may be negative. If it is
greater than 360 or less than -360, then degrees modulo
360 is used as the extent.
-fill colour
Fill the region of the arc with colour. If colour is
an empty string (the default), then the arc will not be
filled.
-outline colour
Colour specifies a colour to use for drawing the arc's
outline. This option defaults to black. If colour is
specified as an empty string then no outline is drawn
for the arc.
-start degrees
Specifies the beginning of the angular range occupied
by the arc. Degrees is given in units of degrees mea-
sured counter-clockwise from the 3-o'clock position;
it may be either positive or negative.
-stipple bitmap
Page 15 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
Indicates that the arc should be filled in a stipple
pattern; bitmap specifies the stipple pattern to use.
If the -fill option hasn't been specified then this
option has no effect. If bitmap is an empty string
(the default), then filling is done in a solid fashion.
The results are undefined if bitmap is not a 1-bit
image.
-style type
Specifies how to draw the arc. If type is pieslice
(the default) then the arc's region is defined by a
section of the oval's perimeter plus two line segments,
one between the center of the oval and each end of the
perimeter section. If type is chord then the arc's
region is defined by a section of the oval's perimeter
plus a single line segment connecting the two end
points of the perimeter section. This type is not
implemented at the moment. It behaves as arc. If type
is arc then the arc's region consists of a section of
the perimeter alone. In this last case the -fill
option is ignored.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
-width outlineWidth
Specifies the width of the outline to be drawn around
the arc's region, in any of the forms described in the
COORDINATES section above. If the -outline option has
been specified as an empty string then this option has
no effect. Wide outlines will be drawn centered on the
edges of the arc's region. This option defaults to
1.0.
BITMAP ITEMS
Items of type bitmap appear on the display as images with
two colours, foreground and background. Bitmaps are created
with widget commands of the following form:
pathName create bitmap x y ?option value option value ...?
The arguments x and y specify the coordinates of a point
used to position the bitmap on the display (see the -anchor
option below for more information on how bitmaps are dis-
played). After the coordinates there may be any number of
option-value pairs, each of which sets one of the configura-
tion options for the item. These same option-value pairs
may be used in itemconfigure widget commands to change the
item's configuration. The following options are supported
for bitmaps:
Page 16 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
-anchor anchorPos
AnchorPos tells how to position the bitmap relative to
the positioning point for the item. For example, if
anchorPos is center then the bitmap is centered on the
point; if anchorPos is n then the bitmap will be drawn
so that its top center point is at the positioning
point. This option defaults to center.
-bitmap bitmap
Specifies the bitmap to display in the item.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
IMAGE ITEMS
Items of type image are used to display images on a canvas.
Images are created with widget commands of the following
form:
pathName create image x y ?option value option value ...?
The arguments x and y specify the coordinates of a point
used to position the image on the display (see the -anchor
option below for more information). After the coordinates
there may be any number of option-value pairs, each of which
sets one of the configuration options for the item. These
same option-value pairs may be used in itemconfigure widget
commands to change the item's configuration. The following
options are supported for images:
-anchor anchorPos
AnchorPos tells how to position the image relative to
the positioning point for the item. For example, if
anchorPos is center then the image is centered on the
point; if anchorPos is n then the image will be drawn
so that its top center point is at the positioning
point. This option defaults to center.
-image name
Specifies the name of the image to display in the item.
This image must have been created previously with the
image create command.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item; it may be an empty list.
LINE ITEMS
Page 17 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
Items of type line appear on the display as one or more con-
nected line segments or curves. Lines are created with wid-
get commands of the following form:
pathName create line x1 y1... xn yn ?option value option value ...?
The arguments x1 through yn give the coordinates for a
series of two or more points that describe a series of con-
nected line segments. After the coordinates there may be
any number of option-value pairs, each of which sets one of
the configuration options for the item. These same option-
value pairs may be used in itemconfigure widget commands to
change the item's configuration. The following options are
supported for lines:
-arrow where
Indicates whether or not arrowheads are to be drawn at
one or both ends of the line. Where must have one of
the values none (for no arrowheads), first (for an
arrowhead at the first point of the line), last (for an
arrowhead at the last point of the line), or both (for
arrowheads at both ends). This option defaults to
none.
-arrowshape shape
This option indicates how to draw arrowheads. The
shape argument must be a list with three elements, each
specifying a distance in any of the forms described in
the COORDINATES section above. The first element of
the list gives the distance along the line from the
neck of the arrowhead to its tip. The second element
gives the distance along the line from the trailing
points of the arrowhead to the tip, and the third ele-
ment gives the distance from the outside edge of the
line to the trailing points. If this option isn't
specified then Tk picks a ``reasonable'' shape.
-capstyle style
Specifies the ways in which caps are to be drawn at the
endpoints of the line. Style may one of butt, project-
ing, or round. If this option isn't specified then it
defaults to butt. Where arrowheads are drawn the cap
style is ignored. Note that the first two options cur-
rently have the same effect.
-fill colour
Colour specifies a colour to use for drawing the line.
It may also be an empty string, in which case the line
will be transparent. This option defaults to black.
-smooth boolean
Boolean indicates whether or not the line should be
drawn as a curve. If so, the line is rendered as a set
of Bezier splines: one spline is drawn for the first
Page 18 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
and second line segments, one for the second and third,
and so on. Straight-line segments can be generated
within a curve by duplicating the end-points of the
desired line segment.
-stipple bitmap
Indicates that the line should be filled in a stipple
pattern; bitmap specifies the stipple pattern to use.
If bitmap is an empty string (the default), then fill-
ing is done in a solid fashion. The results are unde-
fined if bitmap is not a 1-bit image.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
-width lineWidth
LineWidth specifies the width of the line, in any of
the forms described in the COORDINATES section above.
Wide lines will be drawn centered on the path specified
by the points. If this option isn't specified then it
defaults to 1.0.
OVAL ITEMS
Items of type oval appear as circular or oval regions on the
display. Each oval may have an outline, a fill, or both.
Ovals are created with widget commands of the following
form:
pathName create oval x1 y1 x2 y2 ?option value option value ...?
The arguments x1, y1, x2, and y2 give the coordinates of two
diagonally opposite corners of a rectangular region enclos-
ing the oval. The oval will include the top and left edges
of the rectangle not the lower or right edges. If the
region is square then the resulting oval is circular; other-
wise it is elongated in shape. After the coordinates there
may be any number of option-value pairs, each of which sets
one of the configuration options for the item. These same
option-value pairs may be used in itemconfigure widget com-
mands to change the item's configuration. The following
options are supported for ovals:
-fill colour
Fill the area of the oval with colour. If colour is an
empty string (the default), then then the oval will not
be filled.
-outline colour
Colour specifies a colour to use for drawing the oval's
outline. This option defaults to black. If colour is
Page 19 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
an empty string then no outline will be drawn for the
oval.
-stipple bitmap
Indicates that the oval should be filled in a stipple
pattern; bitmap specifies the stipple pattern to use.
If the -fill option hasn't been specified then this
option has no effect. If bitmap is an empty string
(the default), then filling is done in a solid fashion.
The results are undefined if bitmap is not a 1-bit
image.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
-width outlineWidth
outlineWidth specifies the width of the outline to be
drawn around the oval, in any of the forms described in
the COORDINATES section above. If the -outline option
hasn't been specified then this option has no effect.
Wide outlines are drawn centered on the oval path
defined by x1, y1, x2, and y2. This option defaults to
1.0.
POLYGON ITEMS
Items of type polygon appear as polygonal or curved filled
regions on the display. Polygons are created with widget
commands of the following form:
pathName create polygon x1 y1 ... xn yn ?option value option value ...?
The arguments x1 through yn specify the coordinates for
three or more points that define a closed polygon. The
first and last points may be the same; whether they are or
not, Tk will draw the polygon as a closed polygon. After
the coordinates there may be any number of option-value
pairs, each of which sets one of the configuration options
for the item. These same option-value pairs may be used in
itemconfigure widget commands to change the item's configu-
ration. The following options are supported for polygons:
-fill colour
Colour specifies a colour to use for filling the area
of the polygon. If colour is an empty string then the
polygon will be transparent. This option defaults to
the empty string (transparent).
-outline colour
Colour specifies a colour to use for drawing the
polygon's outline. If colour is an empty string then
Page 20 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
no outline will be drawn for the polygon. This option
defaults to black.
-smooth boolean
Boolean indicates whether or not the polygon should be
drawn with a curved perimeter. If so, the outline of
the polygon becomes a set of Bezier splines, one spline
for the first and second line segments, one for the
second and third, and so on. Straight-line segments
can be generated in a smoothed polygon by duplicating
the end-points of the desired line segment.
-stipple bitmap
Indicates that the polygon should be filled in a stip-
ple pattern; bitmap specifies the stipple pattern to
use. If bitmap is an empty string (the default), then
filling is done in a solid fashion. The results are
undefined if bitmap is not a 1-bit image.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
-winding type
Specifies the winding rule to use when filling the
polygon. Type can be either nonzero (the default) or
odd See fillpoly in draw-image(2) for an explanation.
-width outlineWidth
OutlineWidth specifies the width of the outline to be
drawn around the polygon, in any of the forms described
in the COORDINATES section above. If the -outline
option hasn't been specified then this option has no
effect. This option defaults to 1.0.
Polygon items are different from other items such as rectan-
gles, ovals and arcs in that interior points are considered
to be ``inside'' a polygon (e.g. for purposes of the find
closest and find overlapping widget commands) even if it is
not filled. For most other item types, an interior point is
considered to be inside the item only if the item is filled
or if it has neither a fill nor an outline. If you would
like an unfilled polygon whose interior points are not con-
sidered to be inside the polygon, use a line item instead.
RECTANGLE ITEMS
Items of type rectangle appear as rectangular regions on the
display. Each rectangle may have an outline, a fill, or
both. Rectangles are created with widget commands of the
Page 21 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
following form:
pathName create rectangle x1 y1 x2 y2 ?option value option value ...?
The arguments x1, y1, x2, and y2 give the coordinates of two
diagonally opposite corners of the rectangle (the rectangle
will include its upper and left edges but not its lower or
right edges). After the coordinates there may be any number
of option-value pairs, each of which sets one of the config-
uration options for the item. These same option-value pairs
may be used in itemconfigure widget commands to change the
item's configuration. The following options are supported
for rectangles:
-fill colour
Fill the area of the rectangle with colour. If colour
is an empty string (the default), then the rectangle
will not be filled.
-outline colour
Draw an outline around the edge of the rectangle in
colour. This option defaults to black. If colour is
an empty string then no outline will be drawn for the
rectangle.
-stipple bitmap
Indicates that the rectangle should be filled in a
stipple pattern; bitmap specifies the stipple pattern
to use. If the -fill option hasn't been specified then
this option has no effect. If bitmap is an empty
string (the default), then filling is done in a solid
fashion. The results are undefined if bitmap is not a
1-bit image.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
-width outlineWidth
OutlineWidth specifies the width of the outline to be
drawn around the rectangle, in any of the forms
described in the COORDINATES section above. If the
-outline option hasn't been specified then this option
has no effect. Wide outlines are drawn centered on the
rectangular path defined by x1, y1, x2, and y2. This
option defaults to 1.0.
TEXT ITEMS
A text item displays a string of characters on the screen in
one or more lines. Text items support indexing and selec-
tion, along with the following text-related canvas widget
Page 22 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
commands: dchars, focus, icursor, index, insert, select.
Text items are created with widget commands of the following
form:
pathName create text x y ?option value option value ...?
The arguments x and y specify the coordinates of a point
used to position the text on the display (see the options
below for more information on how text is displayed). After
the coordinates there may be any number of option-value
pairs, each of which sets one of the configuration options
for the item. These same option-value pairs may be used in
itemconfigure widget commands to change the item's configu-
ration. The following options are supported for text items:
-anchor anchorPos
AnchorPos tells how to position the text relative to
the positioning point for the text. For example, if
anchorPos is center then the text is centered on the
point; if anchorPos is n then the text will be drawn
such that the top center point of the rectangular
region occupied by the text will be at the positioning
point. This option defaults to center.
-fill colour
Colour specifies a colour to use for filling the text
characters. If this option isn't specified then it
defaults to black.
-font font
Specifies the font to use for the text item. If this
option isn't specified, it defaults to a system-
dependent font.
-justify how
Specifies how to justify the text within its bounding
region. How must be one of the values left, right, or
center. This option will only matter if the text is
displayed as multiple lines. If the option is omitted,
it defaults to left.
-stipple bitmap
Indicates that the text should be drawn in a stippled
pattern rather than solid; bitmap specifies the stipple
pattern to use. If bitmap is an empty string (the
default) then the text is drawn in a solid fashion.
The results are undefined if bitmap is not a 1-bit
image.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
existing tags for the item. TagList may be an empty
list.
Page 23 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
-text string
String specifies the characters to be displayed in the
text item. Newline characters cause line breaks. The
characters in the item may also be changed with the
insert and delete widget commands. This option
defaults to an empty string.
-width lineLength
Specifies a maximum line length for the text, in any of
the forms described in the COORDINATES section above.
If this option is zero (the default) the text is broken
into lines only at newline characters. However, if
this option is non-zero then any line that would be
longer than lineLength is broken just before a space
character to make the line shorter than lineLength;
the space character is treated as if it were a newline
character.
WINDOW ITEMS
Items of type window cause a particular window to be dis-
played at a given position on the canvas. Window items are
created with widget commands of the following form:
pathName create window x y ?option value option value ...?
The arguments x and y specify the coordinates of a point
used to position the window on the display (see the -anchor
option below for more information on how bitmaps are dis-
played). After the coordinates there may be any number of
option-value pairs, each of which sets one of the configura-
tion options for the item. These same option-value pairs
may be used in itemconfigure widget commands to change the
item's configuration. The following options are supported
for window items:
-anchor anchorPos
AnchorPos tells how to position the window relative to
the positioning point for the item. For example, if
anchorPos is center then the window is centered on the
point; if anchorPos is n then the window will be drawn
so that its top center point is at the positioning
point. This option defaults to center.
-height dist
Specifies the height to assign to the item's window.
Dist may have any of the forms described in the COORDI-
NATES section above. If this option isn't specified,
or if it is specified as an empty string, then the win-
dow is given whatever height it requests internally.
-tags tagList
Specifies a set of tags to apply to the item. TagList
consists of a list of tag names, which replace any
Page 24 Plan 9 (printed 12/21/24)
CANVAS(9) CANVAS(9)
existing tags for the item. TagList may be an empty
list.
-width dist
Specifies the width to assign to the item's window.
Dist may have any of the forms described in the COORDI-
NATES section above. If this option isn't specified,
or if it is specified as an empty string, then the win-
dow is given whatever width it requests internally.
-window pathName
Specifies the window to associate with this item. The
window specified by pathName must either be a child of
the canvas widget or a child of some ancestor of the
canvas widget. PathName may not refer to a top-level
window.
BINDINGS
New canvases are not given any default behaviour. Use
bind(2) commands to give the canvas its behaviour.
CREDITS
Tk's canvas widget is a blatant ripoff of ideas from Joel
Bartlett's ezd program. Ezd provides structured graphics in
a Scheme environment and preceded canvases by a year or two.
Its simple mechanisms for placing and animating graphical
objects inspired the functions of canvases.
SEE ALSO
options(9), types(9)
Page 25 Plan 9 (printed 12/21/24)