Non-destrcutive workflow using Masks additive (revealing from both masks instead of hiding)

[NotMyFault]

Hi,

i searched for an workflow to non-destructively combine 2 masks additively:

• Normally, if you add 2 or more masks to an layer, the resulting effect is subtractive: Only pixels visible in both masks become visible.
• If you want to combine 2 masks additive, your were out of luck.
• The mathematical principle of double inversion comes to your help: By inversion of all masks individually, and inversion of the result again, you get additive masks.
• A simple and - despite all current issues with handling of alpha channel in Affinity Photo - working solution is to use the channel mixer adjustment
• Combining 2 or more masks is quite helpful if you need one mask with matting, e.g. for hair, and another mask with really hard edges, e.g. the rest of the body. It is impossible (at least for me) to achieve this in only one mask layer, as always introduces unwanted heavy spill to otherwise hard edges.

For details, please have a look at my video attached.

Feedback welcome.

1913236495_2021-05-1820-49-13.mkv

[NotMyFault]

PS:

for those who need a shortcut: 

1. add a channels mixer adjustmenet
2. select alpha channel
3. set alpha to minus 100%
4. set offset to plus 100
5. duplicate 2 times
6. nest to each mask layer
7. Put 3rd on top of masks layers

This will allow to use multiple masks in an additive way.