Qt displays the QImage image on the label and maintains an adaptive size.

The way Qt uses labels to display images:

        //将QImage的大小收缩或拉伸，与label的大小保持一致。这样label中能显示完整的图片
        QImage imageScale = image.scaled(QSize(ui->label_Pixmap->width(), 
        ui->label_Pixmap->height()));
        QPixmap pixmap = QPixmap::fromImage(imageScale);
        ui->label_Pixmap->setPixmap(pixmap);