The Logical Tv and the Tv Tool

The programmer works with a logical display device called a Tv. Tools that require graphical display devices can either reuse existing display devices (via calls to the appropriate pointer access functions) or must provide their own Tv device. During program execution a Tv is created using the New Tv Tool button of the main tool. Within reason any number of Tv's can be created. Tv's support the following button proceses. Tv's support both transient and retained graphical facilities. Transient graphics are lost following zooming or other repaint facilities, while retained graphics are not. Generally, transient graphics will not be displayed on cloned Tv's.

• install. The user associates the most recently selected Tv tool from a subtool selection menu to a logical display device.
• clone. Multiple copies of the same Tv can be installed on different Tv Tool's. These copies are not accessable from the main application program and can be therefore used to store graphical results for visual comparison.
• init. An aplication can supply an initialisation function which selects a default geometry for display (eg; an image can be scaled to fit the display screen). Using this button will return the displayed data to the standard projection after zooming or rotating.
• repaint. When the screen is exposed or resized repaint routines are called to refresh the display appropriately. Users can invoke a repaint directly by pressing this button.

Interaction with the tool takes place through the mouse (assumed to have three buttons). The mouse generates 9 different event types from the combinations of these buttons with up, down and drag.