Adam D. Moss wrote:
The idea isn't that the layer mask usurps image alpha, but
that traditional paint/fill tools are generally used to increase
opacity and define colour simultaneously (they do), while layer
masks are extremely handy ways to safely experiment with eroding
opacity away again as a logically separate composition step rather
than a destructive processing of the RGBA data.
I like this distinction. This should be written down somewhere.
Probably in the manual (if it isn't already). This clearly describes
precisely what approach to use when.
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