How to make a QGraphicsItem of translucent color?

Question

I am working on a QT GUI that annotates pictures from a graphics View via various shapes, currently developing the easiest one which is a rectangle. I managed to add a rectangle to the image once a button is pressed and move it around with setFlag function. What i need to do now is make sure the rectangle is translucent so that the user can see what is exactly annotated. My code for the rectangle button:

void MainWindow::on_pushButton_11_clicked()  // rectangle shape creator
{
    QBrush redBrush(Qt::red);
    QBrush blueBrush(Qt::blue);
    QPen blackpen(Qt::black);
    blackpen.setWidth(3);
    rectangle = scene->addRect(-100,-100,50,50,blackpen,blueBrush);
    rectangle->setFlag(QGraphicsItem::ItemIsMovable);
}

I have researched the Qt documentation and found that the opacity function from QGraphicsItem library would probably be the solution to this but I couldn't find a way to implement this. Any help or suggestions are appriciated thanks.

Answer 1

There are several solutions:

• Use the setOpacity() method that will make the items transparent in the fill and the border color.

  rectangle->setOpacity(.2);
  

• If you want to set the transparency in the fill color then you must set a transparent QColor to the QBrush.

  QColor brush_color(Qt::blue);
  brush_color.setAlpha(50);
  QPen blackpen(Qt::black);
  blackpen.setWidth(3);
  rectangle = scene->addRect(-100, -100, 50, 50, blackpen, brush_color);
  rectangle->setFlag(QGraphicsItem::ItemIsMovable);
  

• If you want to set transparency at the border then you must set a transparent QColor to the QPen.

  QBrush blueBrush(Qt::blue);
  QColor black_color(Qt::black);
  black_color.setAlpha(50);
  QPen blackpen(black_color);
  blackpen.setWidth(3);
  rectangle = scene->addRect(-100, -100, 50, 50, blackpen, blueBrush);
  rectangle->setFlag(QGraphicsItem::ItemIsMovable);