I dreamed of it for a long time : a non destructive eraser tool ! Take a look at its possibilities in the attached file.
Simply draw a white line on top of any shape or object (better with deep color for the first try). Then group the two objects and apply the filter ; the white line becomes completely transparent. If applied on a group which includes images or light colors, you can adjust the opacity with the second color matrix : for example enter 10 and -9 as the two last numbers in the last row below.
Then if you select the white object with Ctrl+Click, you can modify it, move its nodes, or press Shift and draw new lines, or apply a distortion or any other filter... The possibilities are infinite.
Of course not absolutely perfect but already very usable. It's not made to replace the Object/Mask feature but it can complete it.
Enjoy !
ivan
