Architecture of the Canvas

The GNOME canvas was originally based on the canvas widget in the Tk toolkit[9], which is in turn based on Joel Bartlett's ezd program, which provides structured graphics in a Scheme environment. The main enhancements that the GNOME canvas provides to the original Tk design are integration with the GTK+ object and signal/slot system[5], nested groups of items, generalized affine transformations, and a high-quality antialiased rendering mode. From the standpoint of the user, the canvas presents the following characteristics:

• The user is able to create graphical items like lines, boxes, ellipses, and text, place them on the canvas, and refer to them later for manipulation. The attributes of an object can be changed at any time; these include the color if the item, its line style, and its position.

• Canvas items receive events just as if they were normal X windows or other GTK+ widgets. Applications receive these events with the normal signal/slot system in GTK+. An application can then connect to the event signals of its canvas items and define the particular behavior it requires. Common actions include moving an item when the user clicks and drags it with the mouse, or highlighting an item by changing its color when the mouse pointer enters the area occupied by the item.

• Items can be grouped together in canvas item groups, and these groups can be nested within each other to form a tree structure. A canvas has a single root group which does not have a parent. Operations such as deleting or moving a group apply to all its items, thus making it easy to create hierarchical drawings. It also has performance advantages since the canvas can use recursive bounding boxes to cull out items for drawing and hit testing operations.

• Items support arbitrary affine transformations, so they can be translated, scaled, rotated, and sheared in any way. Affine transformations apply to item groups as a whole, so the children of a group will obtain the same base transformation as its parent.

• The zooming factor of the canvas can be changed at any time, and the canvas will handle all scrolling issues by itself.

• The canvas takes care of all drawing operations so that it never flickers, and so that the user does not have to worry about repainting the items he wants to display.