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Manual Reference Pages  -  GNOME2::CANVAS (3)

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Gnome2::Canvas - A structured graphics canvas



  use strict;
  use Gtk2 -init;
  use Gnome2::Canvas;
  my $window = Gtk2::Window->new;
  my $scroller = Gtk2::ScrolledWindow->new;
  my $canvas = Gnome2::Canvas->new;
  $scroller->add ($canvas);
  $window->add ($scroller);
  $window->set_default_size (150, 150);
  $canvas->set_scroll_region (0, 0, 200, 200);

  my $root = $canvas->root;
  Gnome2::Canvas::Item->new ($root, Gnome2::Canvas::Text,
                             x => 20,
                             y => 15,
                             fill_color => black,
                             font => Sans 14,
                             anchor => GTK_ANCHOR_NW,
                             text => Hello, World!);
  my $box = Gnome2::Canvas::Item->new ($root, Gnome2::Canvas::Rect,
                                       x1 => 10, y1 => 5,
                                       x2 => 150, y2 => 135,
                                       fill_color => red,
                                       outline_color => black);
  $box->signal_connect (event => sub {
          my ($item, $event) = @_;
          warn "event ".$event->type."\n";



The Gnome Canvas is an engine for structured graphics that offers a rich imaging model, high-performance rendering, and a powerful, high level API. It offers a choice of two rendering back-ends, one based on GDK for extremely fast display, and another based on Libart, a sophisticated, antialiased, alpha-compositing engine. This widget can be used for flexible display of graphics and for creating interactive user interface elements.

To create a new Gnome2::Canvas widget call Gnome2::Canvas->new or Gnome2::Canvas->new_aa for an anti-aliased mode canvas.

A Gnome2::Canvas contains one or more Gnome2::CanvasItem objects. Items consist of graphing elements like lines, ellipses, polygons, images, text, and curves. These items are organized using Gnome2::CanvasGroup objects, which are themselves derived from Gnome2::CanvasItem. Since a group is an item it can be contained within other groups, forming a tree of canvas items. Certain operations, like translating and scaling, can be performed on all items in a group.

There is a special root group created by a Gnome2::Canvas. This is the top level group under which all items in a canvas are contained. The root group is available as $canvas->root.

There are several different coordinate systems used by Gnome2::Canvas widgets. The primary system is a logical, abstract coordinate space called world coordinates. World coordinates are expressed as unbounded double floating point numbers. When it comes to rendering to a screen the canvas pixel coordinate system (also referred to as just canvas coordinates) is used. This system uses integers to specify screen pixel positions. A user defined scaling factor and offset are used to convert between world coordinates and canvas coordinates. Each item in a canvas has its own coordinate system called item coordinates. This system is specified in world coordinates but they are relative to an item (0.0, 0.0 would be the top left corner of the item). The final coordinate system of interest is window coordinates. These are like canvas coordinates but are offsets from within a window a canvas is displayed in. This last system is rarely used, but is useful when manually handling GDK events (such as drag and drop) which are specified in window coordinates (the events processed by the canvas are already converted for you).

Along with different coordinate systems come methods to convert between them. $canvas->w2c converts world to canvas pixel coordinates and canvas->c2w converts from canvas to world. To get the affine transform matrix for converting from world coordinates to canvas coordinates call $canvas->w2c_affine. $canvas->window_to_world converts from window to world coordinates and $canvas->world_to_window converts in the other direction. There are no methods for converting between canvas and window coordinates, since this is just a matter of subtracting the canvas scrolling offset. To convert to/from item coordinates use the methods defined for Gnome2::CanvasItem objects.

To set the canvas zoom factor (canvas pixels per world unit, the scaling factor) call $canvas->set_pixels_per_unit; setting this to 1.0 will cause the two coordinate systems to correspond (e.g., [5, 6] in pixel units would be [5.0, 6.0] in world units).

Defining the scrollable area of a canvas widget is done by calling $canvas->set_scroll_region and to get the current region $canvas->get_scroll_region can be used. If the window is larger than the canvas scrolling region it can optionally be centered in the window. Use $canvas->set_center_scroll_region to enable or disable this behavior. To scroll to a particular canvas pixel coordinate use $canvas->scroll_to (typically not used since scrollbars are usually set up to handle the scrolling), and to get the current canvas pixel scroll offset call $canvas->get_scroll_offsets.






widget = Gnome2::Canvas->new

Create a new empty canvas in non-antialiased mode.

widget = Gnome2::Canvas->new_aa

Create a new empty canvas in antialiased mode.

boolean = CW$canvas->aa

Returns true if $canvas was created in anti-aliased mode.

($bx1, CW$by1, CW$bx2, CW$by2) = Gnome2::Canvas->get_butt_points ($x1, CW$y1, CW$x2, CW$y2, CW$width, CW$project)

o $x1 (double)
o $y1 (double)
o $x2 (double)
o $y2 (double)
o $width (double)
o $project (integer)

(wx, wy) = CW$canvas->c2w ($cx, CW$cy)

o $cx (integer)
o $cy (integer)

boolean = CW$canvas->get_center_scroll_region

CW$canvas->set_center_scroll_region ($center_scroll_region)

o $center_scroll_region (boolean)

list = CW$canvas->get_color ($spec)

o $spec (string)
Returns an integer indicating the success of the color allocation and a GdkColor.

unsigned = CW$canvas->get_color_pixel ($rgba)

o $rgba (integer)

rgbdither = CW$canvas->get_dither

CW$canvas->set_dither ($dither)

o $dither (Gtk2::Gdk::RgbDither)

item = CW$canvas->get_item_at ($x, CW$y)

o $x (double)
o $y (double)

($mx1, CW$my1, CW$mx2, CW$my2) = Gnome2::Canvas->get_miter_points ($x1, CW$y1, CW$x2, CW$y2, CW$x3, CW$y3, CW$width)

o $x1 (double)
o $y1 (double)
o $x2 (double)
o $y2 (double)
o $x3 (double)
o $y3 (double)
o $width (double)

double = CW$canvas->get_pixels_per_unit

Fetch $canvas’ scale factor.

CW$canvas->set_pixels_per_unit ($n)

o $n (double)
Set the zooming factor of $canvas by specifying the number of screen pixels that correspond to one canvas unit.

double = Gnome2::Canvas->polygon_to_point ($poly_ref, CW$x, CW$y)

o $poly_ref (arrayref) coordinate pairs that make up the polygon
o $x (double)
o $y (double)
Return the distance from the point $x,$y to the polygon described by the vertices in $poly_ref, or zero if the point is inside the polygon.

CW$canvas->request_redraw ($x1, CW$y1, CW$x2, CW$y2)

o $x1 (integer)
o $y1 (integer)
o $x2 (integer)
o $y2 (integer)

group = CW$canvas->root

(cx, cy) = CW$canvas->get_scroll_offsets

(x1, y1, x2, y2) = CW$canvas->get_scroll_region

CW$canvas->set_scroll_region ($x1, CW$y1, CW$x2, CW$y2)

o $x1 (double)
o $y1 (double)
o $x2 (double)
o $y2 (double)

CW$canvas->scroll_to ($cx, CW$cy)

o $cx (integer)
o $cy (integer)

CW$canvas->set_stipple_origin ($gc)

o $gc (Gtk2::Gdk::GC)


(cx, cy) = CW$canvas->w2c ($wx, CW$wy)

o $wx (double)
o $wy (double)

CW$affine = CW$canvas->w2c_affine

Fetch the affine transform that converts from world coordinates to canvas pixel coordinates.

Note: This method was completely broken for all $Gnome2::Canvas::VERSION < 1.002.

(cx, cy) = CW$canvas->w2c_d ($wx, CW$wy)

o $wx (double)
o $wy (double)

(worldx, worldy) = CW$canvas->window_to_world ($winx, CW$winy)

o $winx (double)
o $winy (double)

(winx, winy) = CW$canvas->world_to_window ($worldx, CW$worldy)

o $worldx (double)
o $worldy (double)


’aa’ (boolean : default false : readable / writable / construct-only) The antialiasing mode of the canvas.
’focused-item’ (Gnome2::Canvas::Item : default undef : readable / writable)


<B>draw-backgroundB> (Gnome2::Canvas, Gtk2::Gdk::Drawable, integer, integer, integer, integer)
<B>render-backgroundB> (Gnome2::Canvas, gpointer)


    enum Gtk2::Gdk::RgbDither



Gnome2::Canvas::index(3pm) lists the generated Perl API reference PODs.

Frederico Mena Quintero’s whitepaper on the GNOME Canvas:

The real GnomeCanvas is implemented in a C library; the Gnome2::Canvas module allows a Perl developer to use the canvas like a normal gtk2-perl object. Like the Gtk2 module on which it depends, Gnome2::Canvas follows the C API of libgnomecanvas-2.0 as closely as possible while still being perlish. Thus, the C API reference remains the canonical documentation; the Perl reference documentation lists call signatures and argument types, and is meant to be used in conjunction with the C API reference.

GNOME Canvas Library Reference Manual

perl(1), Glib(3pm), Gtk2(3pm).

To discuss gtk2-perl, ask questions and flame/praise the authors, join at


muppet <scott at asofyet dot org>, with patches from Torsten Schoenfeld <kaffetisch at web dot de>.

The DESCRIPTION section of this page is adapted from the documentation of libgnomecanvas.


Copyright 2003-2004 by the gtk2-perl team.

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Library General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details.

You should have received a copy of the GNU Library General Public License along with this library; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307 USA.

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perl v5.20.3 GNOME2::CANVAS (3) 2016-04-05

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