Qt 圆形图像 图像缩放 QPixmap QImage

 1 void CommentDialog::getRoundQPixmap(QPixmap & src, QPixmap & mask, QSize size){
 2     QImage resultImage(size, QImage::Format_ARGB32_Premultiplied);
 3     QPainter painter(&resultImage);
 4     painter.setCompositionMode(QPainter::CompositionMode_Source);
 5     painter.fillRect(resultImage.rect(), Qt::transparent);
 6     painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
 7     painter.drawPixmap(0, 0, mask);
 8     painter.setCompositionMode(QPainter::CompositionMode_SourceOut);
 9     if ((src.height() / 4 > size.height()) && (src.width() / 4 > size.width())){
10         src = src.scaled(src.height() / 4, src.height() / 4);
11     }
12     painter.drawPixmap(0, 0, src.scaled(size, Qt::IgnoreAspectRatio, Qt::SmoothTransformation));
13     src = QPixmap::fromImage(resultImage);
14 }

第4行设置mask图片，src显示的形状由mask决定

9~12行可以将较大的图片快速缩小到指定大小，并且具有较高的质量。

 1 //另一种方法使用QImage。这个没加缩放优化
 2 QImage getRoundQImage(QImage & src, QImage & mask, QSize size)
 3 {
 4     QImage resultImage(size, QImage::Format_ARGB32_Premultiplied);
 5     src = src.scaled(size);
 6     QPainter painter(&resultImage);
 7     painter.setCompositionMode(QPainter::CompositionMode_Source);
 8     painter.fillRect(resultImage.rect(), Qt::transparent);
 9     painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
10     painter.drawImage(0, 0, mask);
11     painter.setCompositionMode(QPainter::CompositionMode_SourceOut);
12     painter.drawImage(0, 0, src);
13     painter.setCompositionMode(QPainter::CompositionMode_DestinationOver);
14     painter.end();
15     return resultImage;
16 }