Let's take a closer look at the coordinate system defined by the viewport, window, and world matrix. (In this context, the term "window" does not refer to a window in the sense of a top-level widget, and the "viewport" has nothing to do with QScrollView's viewport.)
The viewport and the window are tightly bound. The viewport is an arbitrary rectangle specified in physical coordinates. The window specifies the same rectangle, but in logical coordinates. When we do the painting, we specify points in logical coordinates, and those coordinates are converted into physical coordinates in a linear algebraic manner, based on the current window–viewport settings.
By default, the viewport and the window are set to the device's rectangle. For example, if the device is a 320 x 200 widget, both the viewport and the window are the same 320 x 200 rectangle with its top-left corner at position (0, 0). In this case, the logical and physical coordinate systems are the same.
The window–viewport mechanism is useful to make the drawing code independent of the size or resolution of the paint device. We can always do the arithmetic to map logical coordinates to physical coordinates ourselves, but it's usually simpler to let QPainter do the work. For example, if we want the logical coordinates to extend from (-50, -50) to (+50, +50), with (0, 0) in the middle, we can set the window as follows:
painter.setWindow(QRect(-50, -50, 100, 100));
The (-50, -50) pair specifies the origin, and the (100, 100) pair specifies the width and height. This means that the logical coordinates (-50, -50) now correspond to the physical coordinates (0, 0), and the logical coordinates (+50, +50) correspond to the physical coordinates (320, 200). In this example, as is often the case, we don't need to change the viewport.
//////////////////////////////////////
请参考Prentice Hall - C++ GUI Programming with Qt 3.chm
