Demonstrates how Composition Modes work in QPainter .
2 種最常見的閤成形式是 源 and SourceOver . 源 用於將不透明對象繪製到描繪設備上。按此模式，源的每個像素會替換目的地的相應像素。在 SourceOver 閤成模式，源對象是透明的且會繪製在目的地頂部。
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 文檔編製瞭解細節。
