Drawing via QPainter
Before we get started, let me introduce the QPainter class to you. This class performs low-level painting on widgets and other paint devices. In fact, everything drawn on the screen in a Qt application is the result of QPainter. It can draw almost anything, including simple lines and aligned text. Thanks to the high-level APIs that Qt has provided, it's extremely easy to use these rich features.
Qt's paint system consists of QPainter, QPaintDevice, and QPaintEngine. In this chapter, we won't need to deal with the latter two. The relations diagram is sketched as follows:
QPainter is used to perform drawing operations, while QPaintDevice is an abstraction of a two-dimensional space that can be painted on by using QPainter. QPaintEngine provides the interface that the painter uses to draw onto different types of devices. Note that the QPaintEngine class is used internally by QPainter and QPaintDevice. It's also designed to be hidden from programmers...
