Hi, all.
Currently, I am constructing a UI to control a mobile robot in ROS.
The following is the window of my ui:
1.png
Where both of the background of ui and buttons are set as transparent, and a png figure is set to every button.
Qt Code: 
  1. MainWindow::MainWindow(int argc, char** argv, QWidget *parent)
  2. 	: QMainWindow(parent)
  3. 	, qnode(argc,argv)
  4. {
  5. 	//Transparent
  6. 	setWindowFlags(Qt::Widget | Qt::FramelessWindowHint);
  7. 	setParent(0); // Create TopLevel-Widget
  8. 	setAttribute(Qt::WA_NoSystemBackground, true);
  9. 	setAttribute(Qt::WA_TranslucentBackground, true);  
  10.  
  11. 	ui.setupUi(this); // Calling this incidentally connects all ui's triggers to on_...() callbacks in this class.
  12.  
  13. 	setWindowIcon(QIcon(":/images/icon.png"));
  14.         QObject::connect(&qnode, SIGNAL(rosShutdown()), this, SLOT(close()));
  15.  
  16.     //buttons
  17. 	ButtonBackground(ui.up_button, QPixmap("/home/yrj/manny/src/jmr_qt/resources/images/up_up.png"));
  18. 	ButtonBackground(ui.down_button, QPixmap("/home/yrj/manny/src/jmr_qt/resources/images/down_up.png"));
  19. 	ButtonBackground(ui.left_button, QPixmap("/home/yrj/manny/src/jmr_qt/resources/images/left_up.png"));
  20. 	ButtonBackground(ui.right_button, QPixmap("/home/yrj/manny/src/jmr_qt/resources/images/right_up.png"));
  21. 	ButtonBackground(ui.stop_button, QPixmap("/home/yrj/manny/src/jmr_qt/resources/images/stop.png"));
  22. 	ButtonBackground(ui.exit_button, QPixmap("/home/yrj/manny/src/jmr_qt/resources/images/exit.png"));
  23.  
  24. void MainWindow::ButtonBackground(QPushButton *button, QPixmap pixmap)
  25. {
  26. 	//Bitmap
  27. 	int width = button->size().width();
  28.         int height = button->size().height();
  29.         QIcon ButtonIcon(pixmap);
  30.         button->setFlat(true);
  31.         button->setStyleSheet("background-color: transparent;");
  32.         button->setIcon(ButtonIcon);
  33.         button->setIconSize(QSize(width, height));
  34. }
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However, a shadow always exist, see the middle button of above figure.
And also the bottom button in the following figure:
2.png

Is there any method to hide or delete this shadow?

Thank you very much for your help.