Description

DisplayDevice is one of the main classes in VMD. It is a base class for all the objects which are responsible for drawing the current scene, either to an on-screen graphics window, or to a file. It is also responsible for encapsulating the window-manager-specific routines which are different for each type of windowing environment (e.g., GL-based window management and rendering, or X-Windows-based window management and OpenGL rendering, etc.). Finally, it is used to determine which item a pointer is currently selecting when a pointer button is pressed, either by a 2D mouse or by a 3D pointer. There are a large number of virtual functions which each subclass may define (if relevant). The DisplayDevice base class provides default versions for all the virtual functions, however, so that by default a bare DisplayDevice acts as an ``empty'' device, which essentially does nothing but which still functions to the rest of the program as a working DisplayDevice.

The main DisplayDevice instance used by VMD is also responsible for checking for and reporting window events, such as when the window is resized, or when a mouse button is pressed, etc. The main graphics window has one main popup menu (which could be implemented as a pulldown menu if required), which the DisplayDevice must be able to create and acquire events from.

There are two types of subclasses of DisplayDevice used in VMD:

• On-screen rendering devices, which manage a graphics display window on the workstation monitor, and which define new versions for basically all virtual functions in DisplayDevice.
• Image-file rendering devices, which are used to render the current scene to an image file, or to create an input script for some post-rendering processor (such as a ray-tracing program or the Raster3D application). These subclasses are designed to be used to create a new file each time the render routine is called. Since file-rendering DisplayDevice's do not have to manage a window, deal with picking objects with a pointer, etc., they are much simpler and do not define new version of many of the virtual functions. For file rendering devices, many of the internal variables, enumerations, and functions are irrelevant and may be ignored when new file rendering subclasses are written.