Qt的绘图系统

QPainter draws in a "natural" coordinate system, but it is able to perform view and world transformations using the QMatrix class。 One advantage of using QImage as a paint device is that it is possible to guarantee the pixel exactness of any drawing operation in a platform-independent way. pixels can only be accessed through QPainter functions or by converting the QPixmap to a QImage. The most common way to read images is through QImage and QPixmap's constructors, or by calling the QImage::load() and QPixmap::load() functions.

The coordinate system is controlled by the QPainter class. Together with the QPaintDevice and QPaintEngine classes.  The default coordinate system of a paint device has its origin at the top-left corner.The mapping of the logical coordinates to the physical QPaintDevice coordinates are handled by 's transformation matrix, viewport and "window". The logical and physical coordinate systems coincide by default.If you need the same transformations over and over, you can also use objects and the () and () functions.