The Background layer, and when you want one
Layer > Background in Paint by Zaweba makes a locked, opaque bottom layer, or turns it back into a normal layer. What changes and why it helps.
Many image editors open a photo as a special locked layer called the Background. People coming from those editors expect it; people new to layered editing find it puzzling. Paint by Zaweba takes a middle road. New documents are unchanged, so you don't start with a locked layer you didn't ask for, but you can make a Background layer whenever it helps and turn it back again.
Making and removing one
Layer > Background > Background from Layer turns the current layer into a Background layer. It becomes a locked, opaque bottom layer. Layer from Background reverses that, making it an ordinary layer again.
What changes on a Background layer
Being opaque is the heart of it. A Background layer can't have transparent pixels. So the eraser doesn't erase to transparency on it; it leaves the background colour. Delete on a selection doesn't clear it to transparency either; it fills the area with the background colour.
Being at the bottom and locked means it stays where it is and acts as a fixed base for everything above.
Why you might want one
A Background layer protects you from a few common accidents. If you're retouching a photo and erase a bit too much, you get the background colour rather than a hole you might not notice until you export to a format that keeps transparency. If you're preparing artwork for print, where there is no such thing as transparent paper, having an opaque base matches the final output.
It's also a reassuring base for beginners. The photo sits at the bottom, solid, and everything you add goes on top. You can't drag it out of place.
It suits work on scans and photos destined for JPEG, which has no transparency anyway, so an opaque base is a better preview of what you'll get.
Why you might not
Design work with transparency is the obvious case against. Logos, icons, cut-outs and stickers all need transparent backgrounds. There, an ordinary layer is what you want, and since new documents in Paint by Zaweba are unchanged, that's what you'll already have.
Moving or transforming the bottom layer is the other case. If you need to rotate the base photo or slide it across the canvas, make it an ordinary layer first with Layer from Background.
A simple way to think about it
Ask whether your final picture will have see-through parts. If it won't, a Background layer is a sensible safety net. If it will, or might, leave the bottom layer as a normal layer.
You can switch either way at any time, so this isn't a choice that locks you in.
Surprises on an opaque base
Expecting the eraser to make transparency on a Background layer leads to confusion. You'll see the background colour appear instead. That's the layer doing its job. Convert it with Layer from Background if you really want transparency.
Being surprised that Delete fills with colour is the same thing in another form. Check which layer is active and whether it's a Background layer.
And don't make a Background layer from a cut-out you've worked hard on. Its transparent areas will be filled, and while undo will rescue you, it's easier not to do it.
This is a small feature, but it gives people who learned on other editors the base layer they're used to, without forcing it on anyone else.
