Problem: Painting (e.g. with wacom-board) complex athmospheric
perspective with overlapping objects.
Solution suggestion: A parent base layer determines alpha values for a
dependent stack of child layers above the base layer.
Then the last layer on top of the child stack e.g. could be the
"athmosphere color" for the silhouette of the base layer.
People suggest as workaround the "alpha to selection" feature.
But if the silhouette of the base layer has transparency gradients, e.g.
when painting clouds, every addition of color to child layers will
nevertheless end up opaque in the child layer.
Especially when painting on alpha masks of the child layers to evolve
the forms gradually a "clipping mask layer" is in fact the only
possibilty of producing complex scenes with athmospheric depth.
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