canvas (GCL TK Manual)

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### 2.4 canvas

canvas \- Create and manipulate canvas widgets

#### Synopsis

canvas pathName ?options?

#### Standard Options

    background       insertBorderWidth relief            xScrollCommand
    borderWidth      insertOffTime     selectBackground  yScrollCommand
    cursor           insertOnTime      selectBorderWidth
    insertBackground insertWidth       selectForeground

See options, for more information.

#### Arguments for Canvas

`:closeenough`

  Name=`"closeEnough" Class=`"`CloseEnough"`

  Specifies a floating-point value indicating how close the mouse cursor must be to an item before it is considered to be “inside” the item. Defaults to 1.0.

`:confine`

  Name=`"confine" Class=`"`Confine"`

  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 argument. Defaults to true, which means that the view will be constrained within the scroll region.

`:height`

  Name=`"height" Class=`"`Height"`

  Specifies a desired window height 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.

`:scrollincrement`

  Name=`"scrollIncrement" Class=`"`ScrollIncrement"`

  Specifies a distance used as increment during scrolling: when one of the arrow buttons on an associated scrollbar is pressed, the picture will shift by this distance. The distance may be specified in any of the forms described in the COORDINATES section below.

`:scrollregion`

  Name=`"scrollRegion" Class=`"`ScrollRegion"`

  Specifies a list with four coordinates describing the left, top, right, and bottom coordinates of a rectangular region. This region is used for scrolling purposes 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 COORDINATES section below.

`:width`

  Name=`"width" Class=`"`width"`

  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.

#### Introduction

The canvas command creates a new window (given by the pathName argument) and makes it into a canvas widget. Additional options, described above, may be specified on the command line or in the option database to configure aspects of the canvas such as its colors 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, but pathName’s parent must exist.

Canvas widgets implement structured graphics. A canvas displays any number of items, which may be things like rectangles, circles, lines, and text. Items may be manipulated (e.g. moved or re-colored) 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 canvas can have behaviors defined by the Tcl scripts bound to them.

#### 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 earlier in the display list and are sometimes referred to as being “on top” of earlier items. When a new item is created 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 integer, 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 operate on a single item at a time; if tagOrId is specified in a way that names multiple items, then the normal behavior 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.

#### Coordinates

All coordinates related to canvases are stored as floating-point numbers. Coordinates and distances are specified in screen units, which are floating-point numbers optionally followed by one of several letters. If no letter is supplied then the distance is in pixels. If the letter is m then the distance is in millimeters on the screen; if it is c then the distance is in centimeters; i means inches, and p means printers points (1/72 inch). Larger y-coordinates refer to points lower on the screen; larger x-coordinates refer to points farther to the right.

#### 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 coordinate system.

Indidividual items may be moved or scaled using widget commands 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 characters, 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; if you define new types of text-like items, it would be advisable to support as many of these forms as practical. 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 is illegal.

@x,y

  Refers to the character at the point given by x and y, where x and y are specified in the coordinate 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.

#### A Canvas Widget’s Arguments

The canvas command creates a new Tcl command whose name is pathName. This command may be used to invoke various operations on the widget. It has the following general form:

    pathName option ?arg arg ...?

Option and the args determine the exact behavior 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 (topmost) 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 (lowest) 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 closest 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 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 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 command 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 manual 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 case the return value is an empty string. If command is omitted then the command returns the command associated 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 trigger 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 commands to be bound to a single event sequence for a single object. This occurs, for example, if one command is associated with the item’s id and another is associated with one of the item’s tags. When this occurs, the first matching binding is used. A binding for the item’s id has highest priority, followed by the oldest tag for the item and proceeding through all of the item’s tags up through the most-recently-added one. If a binding is associated with the tag all, the binding will have lower priority than all other bindings associated with the item.

pathName :canvasx screenx ?gridspacing?

  Given a screen x-coordinate screenx this command returns the canvas x-coordinate that is displayed 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 screen y-coordinate screeny this command returns the canvas y-coordinate that is displayed at that location. If gridspacing is specified, then the canvas coordinate is rounded to the nearest multiple of gridspacing units.

pathName :configure ?option? ?value? ?option value ...?

  Query or modify the configuration options of the widget. If no option is specified, returns a list describing all of the available options for pathName (see Tk_ConfigureInfo for information on the format of this list). If option is specified with no value, then the command returns a list describing the one named option (this list will be identical to the corresponding sublist of the value returned if no option is specified). 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 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 given by tagToDelete from the list of those associated with the item. If an item doesn’t have the tag tagToDelete 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.

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, or if the item contains no tags, then an empty string is returned.

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 numerical index within tagOrId corresponding to index. Index gives a textual description of the desired position as described in INDICES above. The return value is guaranteed to lie between 0 and the number of characters 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 :itemconfigure tagOrId ?option? ?value? ?option value ...?

  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 no option is specified, returns a list describing all of the available options for the first item given by tagOrId (see Tk_ConfigureInfo for information on the format of this list). If option is specified with no value, then the command returns a list describing the one named option (this list will be identical to the corresponding sublist of the value returned if no option is specified). 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 values 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 position 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 (lowest) 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 :postscript ?option value option value ...?

  Generate a Postscript representation for part or all of the canvas. If the :file option is specified then the Postscript is written to a file and an empty string is returned; otherwise the Postscript is returned as the result of the command. The Postscript is created in Encapsulated Postscript form using version 3.0 of the Document Structuring Conventions. The option\-value argument pairs provide additional information to control the generation of Postscript. The following options are supported:

  :colormap varName

    VarName must be the name of a global array variable that specifies a color mapping to use in the Postscript. Each element of varName must consist of Postscript code to set a particular color value (e.g. “1.0 1.0 0.0 setrgbcolor”). When outputting color information in the Postscript, Tk checks to see if there is an element of varName with the same name as the color. If so, Tk uses the value of the element as the Postscript command to set the color. If this option hasn’t been specified, or if there isn’t an entry in varName for a given color, then Tk uses the red, green, and blue intensities from the X color.

  :colormode mode

    Specifies how to output color information. Mode must be either color (for full color output), gray (convert all colors to their gray-scale equivalents) or mono (convert all colors to black or white).

  :file fileName

    Specifies the name of the file in which to write the Postscript. If this option isn’t specified then the Postscript is returned as the result of the command instead of being written to a file.

  :fontmap varName

    VarName must be the name of a global array variable that specifies a font mapping to use in the Postscript. Each element of varName must consist of a Tcl list with two elements, which are the name and point size of a Postscript font. When outputting Postscript commands for a particular font, Tk checks to see if varName contains an element with the same name as the font. If there is such an element, then the font information contained in that element is used in the Postscript. Otherwise Tk attempts to guess what Postscript font to use. Tk’s guesses generally only work for well-known fonts such as Times and Helvetica and Courier, and only if the X font name does not omit any dashes up through the point size. For example, \fB\-*\-Courier\-Bold\-R\-Normal\-\-*\-120\-* will work but \fB*Courier\-Bold\-R\-Normal*120* will not; Tk needs the dashes to parse the font name).

  :height size

    Specifies the height of the area of the canvas to print. Defaults to the height of the canvas window.

  :pageanchor anchor

    Specifies which point of the printed area should be appear over the positioning point on the page (which is given by the :pagex and :pagey options). For example, :pageanchor n means that the top center of the printed area should be over the positioning point. Defaults to center.

  :pageheight size

    Specifies that the Postscript should be scaled in both x and y so that the printed area is size high on the Postscript page. Size consists of a floating-point number followed by c for centimeters, i for inches, m for millimeters, or p or nothing for printer’s points (1/72 inch). Defaults to the height of the printed area on the screen. If both :pageheight and :pagewidth are specified then the scale factor from the later option is used (non-uniform scaling is not implemented).

  :pagewidth size

    Specifies that the Postscript should be scaled in both x and y so that the printed area is size wide on the Postscript page. Size has the same form as for :pageheight. Defaults to the width of the printed area on the screen. If both :pageheight and :pagewidth are specified then the scale factor from the later option is used (non-uniform scaling is not implemented).

  :pagex position

    Position gives the x-coordinate of the positioning point on the Postscript page, using any of the forms allowed for :pageheight. Used in conjunction with the :pagey and :pageanchor options to determine where the printed area appears on the Postscript page. Defaults to the center of the page.

  :pagey position

    Position gives the y-coordinate of the positioning point on the Postscript page, using any of the forms allowed for :pageheight. Used in conjunction with the :pagex and :pageanchor options to determine where the printed area appears on the Postscript page. Defaults to the center of the page.

  :rotate boolean

    Boolean specifies whether the printed area is to be rotated 90 degrees. In non-rotated output the x-axis of the printed area runs along the short dimension of the page (“portrait” orientation); in rotated output the x-axis runs along the long dimension of the page (“landscape” orientation). Defaults to non-rotated.

  :width size

    Specifies the width of the area of the canvas to print. Defaults to the width of the canvas window.

  :x position

    Specifies the x-coordinate of the left edge of the area of the canvas that is to be printed, in canvas coordinates, not window coordinates. Defaults to the coordinate of the left edge of the window.

  :y position

    Specifies the y-coordinate of the top edge of the area of the canvas that is to be printed, in canvas coordinates, not window coordinates. Defaults to the coordinate of the top edge of the window.

pathName :raise tagOrId ?aboveThis?

  Move all of the items given by tagOrId to a new position 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 (topmost) 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. Similarly, each y-coordinate is adjusted to change the distance from yOrigin by a factor of yScale. This command returns an empty string.

pathName :scan option args

  This command is used to implement scanning on canvases. It has two forms, depending on option:

  pathName :scan :mark x y

    Records x and y and the canvas’s current view; used in conjunction with later scan dragto commands. Typically this command is associated with a mouse button press in the widget and x and y are the coordinates of the mouse. It returns an empty string.

  pathName :scan :dragto x y.

    This command computes the difference between its x and y arguments (which are typically mouse coordinates) and the x and y arguments to the last scan mark command for the widget. It then adjusts the view by 10 times the difference in coordinates. This command is typically associated with mouse motion events in the widget, to produce the effect of dragging the canvas at high speed through its window. The return value is an empty string.

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. including 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 currently 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 command 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

    Returns the id of the selected item, if the selection is in an item in this canvas. If the selection 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 character 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 index

  Change the view in the canvas so that the canvas position given by index appears at the left edge of the window. This command is typically used by scrollbars to scroll the canvas. Index counts in units of scroll increments (the value of the scrollIncrement option): a value of 0 corresponds to the left edge of the scroll region (as defined by the scrollRegion option), a value of 1 means one scroll unit to the right of this, and so on. The return value is an empty string.

pathName :yview index

  Change the view in the canvas so that the canvas position given by index appears at the top edge of the window. This command is typically used by scrollbars to scroll the canvas. Index counts in units of scroll increments (the value of the scrollIncrement option): a value of 0 corresponds to the top edge of the scroll region (as defined by the scrollRegion option), a value of 1 means one scroll unit below this, and so on. The return value is an empty string.

#### Overview Of Item Types

The sections below describe the various types of items supported 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 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 following 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 enclosing 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.

  :fill color

    Fill the region of the arc with color. Color may have any of the forms accepted by Tk_GetColor. If color is an empty string (the default), then then the arc will not be filled.

  :outline color

    Color specifies a color to use for drawing the arc’s outline; it may have any of the forms accepted by Tk_GetColor. This option defaults to black. If the arc’s style is arc then this option is ignored (the section of perimeter is filled using the :fill option). If color 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 measured counter-clockwise from the 3-o’clock position; it may be either positive or negative.

  :stipple bitmap

    Indicates that the arc should be filled in a stipple pattern; bitmap specifies the stipple pattern to use, in any of the forms accepted by Tk_GetBitmap. 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.

  :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. If type is arc then the arc’s region consists of a section of the perimeter alone. In this last case there is no outline for the arc and the :outline 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 colors, 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 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 configuration. The following options are supported for bitmaps:

  :anchor anchorPos

    AnchorPos tells how to position the bitmap relative to the positioning point for the item; it may have any of the forms accepted by Tk_GetAnchor. 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.

  :background color

    Specifies a color to use for each of the bitmap pixels whose value is 0. Color may have any of the forms accepted by Tk_GetColor. If this option isn’t specified, or if it is specified as an empty string, then the background color for the canvas is used.

  :bitmap bitmap

    Specifies the bitmap to display in the item. Bitmap may have any of the forms accepted by Tk_GetBitmap.

  :foreground color

    Specifies a color to use for each of the bitmap pixels whose value is 1. Color may have any of the forms accepted by Tk_GetColor and defaults to black.

  :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.

#### Line Items

Items of type line appear on the display as one or more connected line segments or curves. Lines are created with widget 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 connected 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 element 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 have any of the forms accepted by Tk_GetCapStyle (butt, projecting, or round). If this option isn’t specified then it defaults to butt. Where arrowheads are drawn the cap style is ignored.

  :fill color

    Color specifies a color to use for drawing the line; it may have any of the forms acceptable to Tk_GetColor. It may also be an empty string, in which case the line will be transparent. This option defaults to black.

  :joinstyle style

    Specifies the ways in which joints are to be drawn at the vertices of the line. Style may have any of the forms accepted by Tk_GetCapStyle (bevel, miter, or round). If this option isn’t specified then it defaults to miter. If the line only contains two points then this option is irrelevant.

  :smooth boolean

    Boolean must have one of the forms accepted by Tk_GetBoolean. It 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 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.

  :splinesteps number

    Specifies the degree of smoothness desired for curves: each spline will be approximated with number line segments. This option is ignored unless the :smooth option is true.

  :stipple bitmap

    Indicates that the line should be filled in a stipple pattern; bitmap specifies the stipple pattern to use, in any of the forms accepted by Tk_GetBitmap. If bitmap is an empty string (the default), then filling is done in a solid fashion.

  :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 enclosing 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; otherwise 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 commands to change the item’s configuration. The following options are supported for ovals:

  :fill color

    Fill the area of the oval with color. Color may have any of the forms accepted by Tk_GetColor. If color is an empty string (the default), then then the oval will not be filled.

  :outline color

    Color specifies a color to use for drawing the oval’s outline; it may have any of the forms accepted by Tk_GetColor. This option defaults to black. If color is 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, in any of the forms accepted by Tk_GetBitmap. 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.

  :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 configuration. The following options are supported for polygons:

  :fill color

    Color specifies a color to use for filling the area of the polygon; it may have any of the forms acceptable to Tk_GetColor. If color is an empty string then the polygon will be transparent. This option defaults to black.

  :smooth boolean

    Boolean must have one of the forms accepted by Tk_GetBoolean It 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.

  :splinesteps number

    Specifies the degree of smoothness desired for curves: each spline will be approximated with number line segments. This option is ignored unless the :smooth option is true.

  :stipple bitmap

    Indicates that the polygon should be filled in a stipple pattern; bitmap specifies the stipple pattern to use, in any of the forms accepted by Tk_GetBitmap. If bitmap is an empty string (the default), then filling is done in a solid fashion.

  :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.

#### 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 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 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 rectangles:

  :fill color

    Fill the area of the rectangle with color, which may be specified in any of the forms accepted by Tk_GetColor. If color is an empty string (the default), then then the rectangle will not be filled.

  :outline color

    Draw an outline around the edge of the rectangle in color. Color may have any of the forms accepted by Tk_GetColor. This option defaults to black. If color 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, in any of the forms accepted by Tk_GetBitmap. 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.

  :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 selection, along with the following text-related canvas widget 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 configuration. 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; it may have any of the forms accepted by Tk_GetAnchor. 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 color

    Color specifies a color to use for filling the text characters; it may have any of the forms accepted by Tk_GetColor. If this option isn’t specified then it defaults to black.

  :font fontName

    Specifies the font to use for the text item. FontName may be any string acceptable to Tk_GetFontStruct. 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, in any of the forms accepted by Tk_GetBitmap. If bitmap is an empty string (the default) then the text is drawn in a solid fashion.

  :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.

  :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 abov. 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 displayed 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 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 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; it may have any of the forms accepted by Tk_GetAnchor. 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 pixels

  Specifies the height to assign to the item’s window. Pixels may have any of the forms described in the COORDINATES section above. If this option isn’t specified, or if it is specified as an empty string, then the window 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 existing tags for the item. TagList may be an empty list.

:width pixels

  Specifies the width to assign to the item’s window. Pixels may have any of the forms described in the COORDINATES section above. If this option isn’t specified, or if it is specified as an empty string, then the window 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.

#### Application-Defined Item Types

It is possible for individual applications to define new item types for canvas widgets using C code. The interfaces for this mechanism are not presently documented, and it’s possible they may change, but you should be able to see how they work by examining the code for some of the existing item types.

#### Bindings

In the current implementation, new canvases are not given any default behavior: you’ll have to execute explicit Tcl commands to give the canvas its behavior.

#### 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.

#### Keywords

canvas, widget

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