Demonstrates how Composition Modes work in QPainter .
2 种最常见的合成形式是 源 and SourceOver . 源 用于将不透明对象绘制到描绘设备上。按此模式，源的每个像素会替换目的地的相应像素。在 SourceOver 合成模式，源对象是透明的且会绘制在目的地顶部。
In addition to these standard modes, Qt defines the complete set of composition modes as defined by X. Porter and Y. Duff. See the QPainter 文档编制了解细节。
