Layers, masks, and selections

[Gary Cramblitt]

I've been playing a bit with Krita and can't seem to find the functionality 
I'm looking for.

First, how to create a mask of a selection, i.e., a B&W mask of the selection?  
Answer: Create a drop shadow with offset of 0 and blur of 0.

How to turn such a mask back into a selection.? Answer: Make the layer active 
and use the "Select a contiguous area tool".

What I can't seem to find is how to apply the selection to another layer.  
Note, I don't want to copy the contents of the image selection, I want to 
copy the defined selection to another layer so I can apply a filter (or 
whatever) to that selection, but on another layer.  Stated another way, how 
can I use one layer to define the selection on another layer?    Hope this 
makes sense. 

I don't have a specific project in mind.  I'm recalling some of the 
capabilities that were in Fractal Painter.  As I recall, it was very useful 
when working with an image containing multiple parts (subjects) to be able to 
create a bunch of selection masks and keep them around.  Then, when you 
wanted to apply a filter or paint operation on only one part or subject, you 
could use the saved masks to help you.  This was especially helpful after 
spending an hour or more creating a very fine selection mask around a 
subject.  You could save your hard work and then play with the image without 
fear of having to redo all your hard selection work.

Painter had another mask feature that was very useful.  You could toggle a 
layer so that only the selection mask is visible (B&W), then "paint" on the 
mask to alter it.  That can be simulated using the drop shadow trick, except 
I don't see a way to apply the modified selection mask back onto the layer it 
came from.

Separate question: What does the "Sample merged" checkbox on the Contiguous 
Area Selection panel do?  Its mentioned in the selection tutorial, but not 
explained.

BTW, might be useful if we started a Krita Tips & Tricks page on wiki.kde.org?

-- 
Gary Cramblitt (aka PhantomsDad)