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
2D TextureMask
Define a 2D Mask applicable on any 2D Renderer or 2D Modifier
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and pressing the "Create Group" shortcut:
[CTRL] + [SHIFT] + [G]
.