Combine traditional mask with shape based mask - ELI5

[christerdk]

Normally it's fairly easy combine a stack of masks and they'll all add up nicely. In this case however, I'm using a white shape as a mask to align with the plate. But as you can see, there is some of the pretzel bagel sticking out. I then created Mask 1 with the intent of showing these parts by selecting and filling the selection with white. But it didn't work as expected. They were behaving almost as if the mask and the shape were negating each other instead.

But then I added another mask below (name Magic Mask?), and now the two layers above combine to the create the result I initially expected.

What's going on here - is my expectation unrealistic or am I going about it the wrong way?

Thanks!

Affinity Photo 2.5.7

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[lepr]

1 hour ago, christerdk said:

What's going on here

As far as I have seen, a vector object is always considered to be an additive layer of a Compound Mask (CM), no matter where it is in the CM stack. If it is at the bottom of the stack in the CM, it has no effect because it is being added to the completely revealing base state of the CM. In contrast, a raster mask at the bottom of the CM stack is considered to be multiplicative. 

(For that reason, if you layer the vector Ellipse on top of the painted raster mask, instead of below it, there is no need for an empty raster mask at the bottom of the stack inside the Compound Mask.)

Edited February 8 by lepr
rethink

[christerdk]

Dang, I thought I had tried everything. But I kept trying to change things at the compound mask level. Thanks for the quick response, I appreciate it. 

[NotMyFault]

A dirty trick: use a (white) mask layer in the compound, then add the rectangle or any shape into masking position of the mask.

set the add/subtract mode on the mask, not the rectangle.

you can add multiple additional mask layers of this kind, or simple mask layers.

this allows to mix both raster masks  and vector shapes in compound masks.

i will write a tutorial explaining this method in more detail.