Affinity Photo 1.7.0 on MacBook. Goal: Create initial Selection, save to Mask; then come back and add more selections to the existing Mask. The following notes reflect how I used Spare Channel to pack multiple selections into one Mask. It’s probably not the best way to combine multiple selections into one Mask, but it’s the best I could figure out and find on Affinity Forums.
Warning part 1: Saving to Spare Channel, while in Quick Mask red overlay mode, DOES NOT save anything to a Spare Channel. You MUST have marching ants in order to save anything to a Spare Channel. Toggling Quick Mask in the Toolbar toggles between marching ants and red overlay.
Warning part 2: If you load a marching ants selection from the Spare Channel, toggle Quick Mask (in Toolbar), and add to the selection by painting red overlay with the Selection Brush Tool: you will NOT be able to save this as a new Spare Channel! You MUST toggle the Quick Mask again to display the marching ants again. Now you can > Menu: Select > Save Selection > Save Spare Channel. If you stay in red overlay Quick Mode and try to save it as a Spare Channel, the Spare Channel will be blank.
- Selection Brush Tool > Use cursor to create selection > Menu: Select > Save Selection > Save Spare Channel
- Use curser to create another Selection with Selection Brush Tool > Menu: Select > Save Selection > Save Spare Channel
- Channels tab > Choose Spare Channel > Right-click just left of* trash can (to display Options Menu) > Load Selection > Make Mask
- In Layers tab, click on just the Mask thumbnail – then…
- Channels > Choose 2nd Spare Channel > Right-click just left of* trash can > Add to Mask Alpha
*Right-clicking on the Spare Channel thumbnail itself, as recommended in Affinity Forums, does NOT display the Options Menu.