bitmap-pane

A class of window that provides automatic backing-store. When things are drawn on a bitmap-pane, the window stores everything that has been drawn in an unseen memory bitmap (which is similar to a bitmap-stream). When the window is later uncovered, the built-in redisplay-window method for bitmap-panes copies the stored image back to the visible window.

This built-in behavior refreshes the window more quickly than would a non-trivial custom redisplay-window method that redraws everything from scratch. It also allows the application to add to the drawing at any time rather than encapsulating the drawing code into a custom redisplay-window method as usual. (An application should never add a redisplay-window method to a bitmap-pane class.)

The drawback is that significant memory is required for the memory bitmap. Usually it is best to instead use a more general type of window such as a frame-window or non-refreshing-window, unless the quicker redisplay provided by the backing-store is needed.

A special technique for switching a bitmap-pane to a completely new drawing very quickly is described under with-delayed-redraw, with an example.

Typically this class is instantiated indirectly by creating a bitmap-window, whose frame-with-single-child features are recommended when a toolbar or status-bar is to be added to the window. Otherwise, a title-bar and/or border could be added directly to a standalone bitmap-pane if these options are explicitly specified when calling make-window.

Copyright (c) 1998-2001, Franz Inc. Berkeley, CA., USA. All rights reserved.
Documentation for Allegro CL version 6.1 update # 1. This page was not revised.
Created 2001.12.15.