I’ve just finished flatting Chapter 2 of Oriental Lieutenant. For those who don’t know, flatting is a term some people use for a process that makes it easier to colour line art in Photoshop. You isolate the line art and then start filling in the various shapes in the image on different layers. In the image above, each different shade of blue is a different layer. For a traditional analogue, think of it like the sort of masking you might do for airbrushing or watercolours.

In the past, when I was colouring my flatted pages, I would CTRL-click the layer thumbnail, which would create a selection of the shape of that layer. For example, in the image above, if I CTRL-clicked the layer with the big flat shape of the sail, I’d get a big sail-shaped selection. Then I’d paint within the selection.

This is the wrong way to do it.

Kazu Kibuishi revealed to me the awesome correct way to do it, and it is very simple. It’s the “Lock Transparent Pixels” button, and if you don’t know about how it can improve your life, I’ll explain behind the cut.

That is the Lock Transparent Pixels button, and this is what it does: After you’ve painted your big flat shape, the contents of that layer consist of the thing you painted and a bunch of transparent pixels around it. Clicking this button tells Photoshop not to paint any pixels where there aren’t already pixels.

Here’s what would happen if I painted over that layer without engaging the Lock Transparent Pixels button:

(Note that I’ve hidden the line art layer – you’re just looking at the flatting layers.) And here’s the same thing, except I’ve told Photoshop to not put pixels down where there aren’t some already:

This is functionally similar to what I had been doing previously by creating a selection and then painting within it, but it is much better. For one, it’s faster: once all the layers have their transparency locked, it’s simple to flick through them and start painting, not having to worry about getting paint all over the place and not having to take the extra step of creating the selection and making sure you’re still painting on the correct layer.

But the real reason it’s better is that it is much, much cleaner. Here are the results from using that make-a-selection method, where my layer contents were some black pixels, and I’ve painted red on top of them. Notice the line of filthy half-red, half-black pixels along the edge.

Here’s the same example, except I’ve locked the transparency on that black layer:

Much better. Using the selection method has another disadvantage: creating successive selections, painting within them, and then selecting again, and again, etc. etc. eventually degrades the quality of the shape – it can get aliased (sharp and jagged), which could potentially lead to little specks of colour sticking out of the line art.

If you’re using this sort of method to colour line art in Photoshop but weren’t aware of this little feature, I hope you find it as helpful as I have.