8.4 Updating Damaged Areas

When an area inside the visible area of the canvas has been obscured, e.g. by an overlapping window, and again becomes visible, parts of the canvas must be redrawn. The canvas supports redrawing by updating areas that have been damaged. The damaged area is handled by a damaged-list in the canvas.

Update events originating from the window system - say, a window overlapping the canvas is moved - are handled automatically by Bifrost. When Bifrost receives an update event from the window system, the damaged area is in many cases reported along with the update event. The canvas redraws the damaged area transparently to the application. In cases where the damaged areas is not reported, the whole visible area of the canvas is redrawn.

The process of updating damaged areas originating from application dependent actions - say, removing of a graphical object - is a partly application responsible process; it is not entirely automatic. In this situation the application is responsible of adding damaged areas to the damaged-list. Adding a rectangle to the list is accomplished by the operation damaged. After the application has called the operation Repair, the canvas redraws the visible area using an advanced algorithm to determine which objects must be redrawn.

The traditional way of redrawing is to draw all objects and turn the responsibility of selecting the objects inside the clipping area to the display device (or basic graphics library). Although clipping is a very efficient way of reducing the overhead of the display device in redrawing, it is still necessary to redraw all graphical objects in the canvas.

An alternative is to limit the number of graphical objects and pictures that has to be considered in the redrawing process. When applications use graphics modelling, each picture typically consists of a small set of proximate and related objects, expected to be updated collectively, e.g. by moving the picture. This means, that if a picture is completely outside the region that should be updated, then it is not necessary to further consider the graphical objects inside the picture. The following example illustrates the situation:

Given a clipping rectangle T and some pictures (illustrated by dashed rectangles) and graphical objects (illustrated by small circles) all the graphical objects in pictures that are completely outside T are never considered.

This approach depends on the assumption that the application is using graphics modelling, and that the graphical objects in each picture are close to each other. Consider for example two graphical objects in a picture, that are very distant from each other, the picture becomes very large and it is a more likely that the picture intersects T even though the graphical objects may be outside T. On the other hand, this update approach encourage the application programmer or user to apply graphics modelling to the drawing.

The advanced updating approach in Bifrost does not exclude the possibility to use other updating mechanisms. In cases where graphics modelling can not be used or does not make sense, the application programmer can implement a different approach, e.g. the very advanced method described by Edelsbrunner called 'dynamic rectangle intersection search' [Edelsbrunner 80].