[Chapter 18] 18.12 The Canvas Widget

[Chapter 18] 18.12 The Canvas Widget



18.12.1 Canvas Creation Methods

To place graphic elements in a canvas, there are several item creation commands:

createArc

Creates an arc contained within the given bounding box. For example, to create an oval bounded

by the box from (0,0) to (40,100):

$canvas->createArc(0,0,40,100, -extent => 360);

The -extent option gives a number between 0 and 360 defining the length of the arc. The

default -extent is 90, or 1/4 of an oval; an extent of 360 gives you a full oval. The complete list

of options to createArc is:

-extent => degrees

Creates an arc of the specified extent. degrees can be any number between 0 and 360, as

described above.

-fill => color

Fills the arc with the specified color.

-outline => color

Draws the arc with the specified color (default = black).

-outlinestipple => bitmap

Draws the outline with the specified bitmap pattern.

-start => degrees

Starts drawing the arc from the specified position, where the position is represented by a

number from 0 to 360. The default is 0, which means to start drawing at the 3 o'clock

position.

-stipple => bitmap

Uses the specified bitmap to fill the arc (if -fill is also specified).

-style => type

Draws the arc as specified. Values are:

'pieslice'

Draws lines from the center to the ends of the arc (the default).

'chord'

Draws a line connecting the two ends of the arc.

'arc'

Draws the arc with no other lines.

-tags => tagnames



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Associates the arc with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-width => amount

The width of the outline. Default is 1.

createBitmap

Inserts a bitmap. For example, to place the "calculator" bitmap at the (0,0) coordinates:

$canvas -> createBitmap(0, 0, -bitmap => 'calculator');

Options are:

-anchor => position

Anchors the bitmap at the specified position. Values are "center" (default), "n", "e",

"s", "w", "ne", "nw", "se", and "sw".

-background => color

Specifies the color to use for the "0" pixels in the bitmap (default is to be transparent).

-bitmap => bitmap

Specifies the bitmap name. For a built-in bitmap, just specify the name; for a local bitmap

file, specify the name with an "@" symbol preceding it.

-foreground => color

Specifies the color to use for the "1" pixels in the bitmap (default is black).

-tags => tagnames

Associates the bitmap with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

createImage

Creates an image. For example, to place an image at (0,0):

$canvas->createImage(0,0, -image => $imgptr);

Options are:

-anchor => position

Anchors the image at the specified position. Values are "center" (default), "n", "e",

"s", "w", "ne", "nw", "se", and "sw".

-image => $imgptr

$imgptr is a pointer to a Photo or Image object made using a GIF or PPM file. For

example:

$imgptr = $mainwindow->Photo(-file => "doggie.gif");

-tags => tagnames

Associate the image with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

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createLine

Creates a line or several adjoining lines. For example, to create a line from (0,0) to (100, 100) and

then back to (100, 0):

$canvas->createLine (0,0,100,100,100,0);

The first four coordinates are required. Any additional coordinates are taken to represent a

continuation of that line. Options are:

-arrow => position

Specifies where to place arrowheads. Values are 'none' (default), 'first', 'last',

and 'both'.

-arrowshape => [ head, length, flare ]

Specifies the dimensions of the arrow as a three-element anonymous list, describing (in

order) the distance from the base to the "head" of the arrow, the distance from the rear

point(s) to the head of the arrow, and the distance from the rear point(s) to the line.

-capstyle => type

Defines the type of arrowhead. Values are "butt" (the default), "projecting", and

"round".

-fill => color

The color to use to draw the line.

-joinstyle => type

Defines how multiple lines are joined. Values are "miter" (default), "bevel", and

"round".

-smooth => boolean

Determines whether the lines are drawn with a Bezier spine. Default is 0.

-splinesteps => n

Determines how smooth the Bezier curve is.

-stipple => bitmap

Draws the line with the specified bitmap pattern.

-tags => tagnames

Associates the line with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-width => amount

The width of the line (default = 1 pixel).

createOval

Creates an oval. For example, to create a circle bounded by the box from (50,50) to (150,150):



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$canvas->createOval(50,50,150,150);

Options are:

-fill => color

Fills the arc with the specified color.

-outline => color

Specifies the color for the outline (default = black).

-stipple => bitmap

Specifies a bitmap to fill the oval with.

-tags => tagnames

Associates the oval with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-width => amount

The width of the outline (default = 1 pixel).

createPolygon

Creates a polygon. At least three sets of coordinates are required; the first point is automatically

connected to the last point to complete the polygon.

$canvas -> createPolygon(0,0,130, 20, 90, -35);

Options are:

-fill => color

The color to use to fill the polygon.

-outline => color

Specifies the color for the outline (default = black).

-smooth => boolean

Determines whether the outline is drawn with a Bezier spine. Default is 0.

-splinesteps => n

Determines how smooth the Bezier curve is.

-stipple => bitmap

Fills the polygon with the specified bitmap pattern.

-tags => tagnames

Associates the polygon with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-width => amount

The width of the outline (default = 1 pixel).



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createRectangle

Creates a rectangle. For example, to create a square with one corner at (0,0) and another at

(100,100):

$canvas->createRectangle(0,0,100,100);

Options are:

-fill => color

The color to use to fill the rectangle.

-outline => color

Specifies the color for the outline (default = black).

-stipple => bitmap

Fills the rectangle with the specified bitmap pattern.

-tags => tagnames

Associates the rectangle with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-width => amount

The width of the outline (default = 1 pixel).

createText

Places text in a canvas widget. For example, to write "Broadway" centered at the position

(130,-40):

$canvas->createText(130,-40, -text => "Broadway");

Options are:

-anchor => position

Anchors the text at the specified position. Values are "center" (default), "n", "e", "s",

"w", "ne", "nw", "se", and "sw".

-fill => color

The color to use for the text.

-font => fontname

The font for the text.

-justify => position

The justification of the text (any of 'left', 'right', and 'center'). The default is

'left'.

-stipple => bitmap

Fills the text with the specified bitmap pattern.

-tags => tagnames

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Associates the text with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-text => string

Specifies the text to display.

-width => amount

The maximum length of each line of text. Default is 0, which means that lines are only

broken at explicit newline characters.

There is a set of methods for manipulating text items within a Canvas widget. For each of these

methods, the first argument is the tag name or tag ID, and subsequent arguments use text indexes

as described for the Text widget.

dchars

Deletes characters from a text item, given the tag name or ID, and indexes of the first and

last characters to delete.

icursor

Places the insert cursor at the specified index.

index

Gets a numerical index from a named one.

insert

Adds a string to the text item.

createWindow

Embeds another widget inside of a canvas. The widget must have been already created as a child

of the canvas or of the canvas's parent. Options are:

-anchor => position

Anchors the widget at the specified position. Values are "center" (default), "n", "e",

"s", "w", "ne", "nw", "se", and "sw".

-height => amount

Specifies the height of the widget.

-tags => tagnames

Associates the widget with the specified tag(s). Multiple tag names can be supplied as an

anonymous list.

-width => amount

The width of the widget.

-window => $widget

Specifies the widget to embed.

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18.12.2 Item Tags and IDs

Each item in a Canvas Widget is given a unique ID when it is created. This ID is returned from the

canvas creation command. In addition, each item can have a tag associated with it, either when created or

with the addtag method. You can use either the ID or the tag to refer to an item in the canvas. Unlike

IDs, tags do not have to be unique, which makes it possible to configure several items as a group.

Two special tags are created automatically. The "all" tag refers to all items in the canvas. The "current"

tag refers to the item that the cursor is currently over, if any.



18.12.3 Canvas Methods

In addition to configure and cget, the following methods are supported by the Canvas widget.

addtag

Defines a tag for an already-created canvas item. For example, to assign a tag called

"everything" to all items in a canvas:

$canvas->addtag("everything", "all");

To change the tag for an item from "tmp" to "circle":

$canvas->addtag("circle", "withtag", "tmp");

To assign the tag "origin" to the item closest to the coordinates (0,0):

$canvas->addtag("origin", "closest", 0, 0);

The full list of identifiers is:

above

Assigns the tag to the item above the specified item in the display list.

all

Assigns the tag to all items in the canvas.

below

Assigns the tag to the item below the specified item in the display list.

closest

Assigns the tag to the item closest to the specified x,y coordinate.

enclosed

Assigns the tag to all items that are completely enclosed within the specified bounding box.

overlapping

Assigns the tag to all items that are even partially inside the specified bounding box.

withtag

Assigns the tag to all items with the specified tag.

bind

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Binds a callback to an item. (To bind a callback to the canvas widget itself, you must specify

Tk::bind.)

bbox

Returns the bounding box of an item. For example, to get the bounding box for all items in the

canvas:

$canvas->bbox("all");

itemconfigure

Configures one of the items within the canvas. Works just like the configure method for

widgets, but the first argument is the tag name or ID for the canvas item.

itemcget

Gets configuration information for one of the items within the canvas. Works just like the cget

method for widgets, but the first argument is the tag name or ID for the canvas item.

move

Moves an item on the canvas by adding the specified x and y distances to it.

$canvas->move("circle1", 100, 100);

coords

Gets the current x,y coordinates for an item, or moves an item to an explicit x,y coordinate.

lower

Sets the priority for the item in the display list to be lower than the item identified by the specified

tag or ID.

raise

Sets the priority for the item in the display list to be higher than the item identified by the specified

tag or ID.

delete

Removes an item from the canvas. You can specify as many tags or IDs in the argument list as you

want.

find

Finds the specified items. The first argument can be any of:

above

Finds the item above the specified item in the display list.

all

Finds all items in the canvas.

below

Finds the item below the specified item in the display list.

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