A 2d group mask enables to compute and blend some masks together during an intermediary rendering pass.
Once masks are grouped, the overall Opacity
of the group can be modulated or inverted.
A 2d group mask can be creating by selecting
Define a 2D Mask applicable on any 2D Renderer or 2D Modifier
s and pressing the "Create Group" shortcut:
[CTRL] + [SHIFT] + [G]