Creating a Black-to-Transparent Lineart
There are quite a few ways to get a black-to-transparent lineart from a greyscale image, including using channels to grab the selection, but this is one of the easiest and the one I use personally.
1) To begin, we need a lineart. I’m using a quick drawing of my character Geoff. You want to have already cleaned up your lineart so that it has a good level of contrast, but it doesn’t have to be pure black and white: you can use this technique just as well with a shaded greyscale image or a pencil sketch as you can with a inked drawing. If you don’t have a lineart to hand, you can try with a low-res version of mine, here (use right click, save link as).
My starting lineart
2) Our next step is to get that lineart into the quick mask. The first thing we need to do for that is select it all, and copy it to the clipboard. You can either do that with the menus, or much more efficiently just hit ⌘+a to select all, and ⌘+c to copy (ctrl+a and ctrl+c on windows). Photoshop is packed full of handy keyboard shortcuts, and the more you know, the faster things will go.
Next up, hop into quickmask mode with the button or by pressing Q. Paste in your lineart with ⌘+v (ctrl+v on windows)
My lineart pasted into the quickmask
3) Now, if you’ve been paying attention, you’ll know that this mask means the white parts are selected, and the red parts are masked away. This is the opposite of what we want, so let’s change that. Press ⌘+d to get rid of the selection from pasting in the lineart (ctrl+d on windows… you should be getting the idea by now), and then ⌘+i (ctrl+i on windows) to invert everything.
Inverting things to get the mask I actually want
4) Drop back into normal mode, and the marching ants are back, showing the basics of our mask. To create our black-to transparent lineart, we need a transparent layer to work on, so create one above your original lineart layer (via the menu, the new layer button, or by pressing ⌘+shift+n).
Back in normal mode, create a new layer
5) Nearly there. If your lineart level is still set to background, doubleclick it and hit okay in the ‘new layer’ dialogue box that pops up to free it up. Now we can hide your original lineart by clicking the eye icon next to it, to make it easier to see what we’re doing.
Hide the original lineart layer
6) Hide the marching ants with ⌘+h (ctrl+h on windows), and then grab the fill tool (from the tool bar or by hitting g) and the colour of your choice. Then, click ONCE anywhere in your image to get your transparent lineart.
Fill the mask
It is important only to click once, or the fill will be applied again, which will boost the opacity of everything, making it darker and potentially losing you some detail and smoothness.
As the process for doing this is the same every time, you can even set up an action to do it for you at the click of a button, but that is a lesson for another time.
Next, we’re going to look at why you might want to use a black-to-transparent lineart like this.