There are a number of reasons why you might like to draw offscreen: you might want to compose a collection of images and show them one after another (this is called double-buffering, which you can do to avoid screen painting flicker when you draw onscreen), or write a program that generates image files directly.

As I mentioned in the previous section, Qt provides several classes for offscreen drawing, each with different advantages and disadvantages. These classes are QImage, QPixmap, QBitmap, and QPicture. Under normal circumstances, you need to choose between QImage and QPixmap.

On the other hand, if you're working primarily with offscreen rendering for the purposes of display performance or double-buffering...