This topic provides an introduction to two classes that embody the important GUI features of device independence and the ability to zoom.
Two desirable facilities of a GUI are:
These two facilities are embodied in the two classes MGraphicsDeviceMap and TZoomFactor .
Device independence is facilitated by storing drawings and text in device-independent form and then converting the device-independent units (twips) into device-dependent units (pixels) for different devices such as screens and printers. The conversion interface is MGraphicsDeviceMap , and it is implemented by graphics devices.
TZoomFactor also implements this interface. It takes a mapping between twips and pixels, and then also applies a zooming factor: the larger the zooming factor, the more pixels per twip. The relationship between the classes is shown below:
Note that TZoomFactor not only implements MGraphicsDeviceMap but also uses it. The point is that the MGraphicsDeviceMap -supporting object used can either be a graphics device, or another TZoomFactor . This allows zooming to be applied to an already zoomed mapping.
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