Mask layers and Inherit Alpha in Paint by Zaweba

A layer mask belongs to one layer. That's fine until you want one mask to affect a whole stack, or you want to paint only inside the shapes you've already drawn. Paint by Zaweba has two more tools for those cases: mask layers and Inherit Alpha. Both work inside groups, and both are worth knowing because they save you copying the same mask again and again.

Mask layers

A mask layer is a layer in its own right whose job is to mask. Where it's light it reveals, where it's dark it hides, and it does this to everything below it in its group.

That makes it a shared mask. Put a mask layer at the top of a group with five layers under it, and all five are shaped by it at once. Change the mask layer and they all change together. Compare that with five separate layer masks that you'd have to edit one by one.

Because it's a layer, you can do layer things with it. Paint on it with any brush, fill it with a gradient, move it, transform it or change its opacity. A gradient on a mask layer fades a whole group into the background in one stroke.

Inherit Alpha

Inherit Alpha is a setting on a layer. When it's on, the layer shows only where the layers below it in its group have pixels. Anywhere they're transparent, the layer is hidden.

It's the painter's friend. Draw a character's shape on one layer, then add a new layer above with Inherit Alpha on, and every brush stroke you make stays inside the character. Shading, texture and highlights can be painted loosely without spilling over the edge. Each can sit on its own layer, so you can change one without touching the others.

It's also good for text and shapes. Fill a word with a photo or a texture by putting it above the text with Inherit Alpha on.

Which one to use

Use a mask layer when you have a shape or gradient that should hide parts of a group, and you want to control it as one thing. Vignettes, fades and cut-out shapes over a set of layers all suit it.

Use Inherit Alpha when the shape already exists in the layers below and you want new work to stay within it. Shading and texturing are the classic cases.

Both are confined to the group they're in. That's a feature: wrap the layers you want affected in a group, and nothing outside it will change.

Why a mask seems ignored

Putting the mask layer in the wrong place is the commonest. It only masks what's below it in its group. If something is above it, or outside the group, it won't be affected.

With Inherit Alpha, remember it follows all the layers below in the group, not just the one directly beneath. If you add a new shape lower down, the inheriting layer will show over that too.

Forgetting that light reveals on a mask layer can lead to the opposite of what you meant. Paint with black to hide and white to show.

These two features make Paint by Zaweba feel closer to a painting application for illustrators while still working as a photo editor. Once you start painting with Inherit Alpha, going back to selecting the shape before every brush stroke feels slow.