In my opinion this is unnecessarily difficult (even with the tutorial I was unable to make a simple rectangular blur mask where I need it), I believe it would be much simpler if the mask could be manipulated directly by clicking and dragging on some sort of shape based GUI object in the video preview.
I am running:
Ubuntu 21.10
Shotcut 21.01.31
QT 5.15.2
Thank you for shotcut I use it fairly often and it has been a very useful tool in my arsenal.
In the meantime. I always find it difficult to see where the mask is, especially for blur. So what I do is use the “Invert Colors” filter instead of the “Blur”, which makes the area of the mask standout vividly. Then I modify the mask to what I want. Then I delete the “Invert Colors” filter and replace it with the Blur filter.