How I create cel-style drawings
This brief tutorial describes my method of creating cel-style images. I developed this in PaintShopPro, but I updated my description to make it also relevant to Photoshop or Gimp. This is not so much a tutorial on cel-style itself, but rather on how to make it fast.
Perhaps the main characteristic of my method is that I like to work BIG. The work I use as an example is really 2611x2569 pixels. And like true cel-style, I work with a reduced number of colours until the final version. That means no interpolating image manipulations, no anti-aliasing, etc. The big advantage this offers, in my view, is that it remains very easy to select and modify areas of the same colour. When it's finished, I rescale to a publishable size, any anti-alias type smoothing before that point is useless.
First I scan my ink drawing. I always use the 1-bit "lineart" setting of my scanner, at 300 or 600 dpi depending on the size of my drawing.
As you may notice, my scanner is rather crummy, so there is noise and scanning artefacts, also because I often draw on cheap scrap paper, my ink tends to bleed. So my next step is to clean up the lineart. I use the freehand selection tool to select and clear the areas that should be white (background colour set to white). I also use the pencil tool with a thickness of 6-8 (and anti-alias disabled) to clean up lines, and to close any areas that should be closed. Since I obtained a graphic tablet, I've started to completely go over my scan on a separate layer using a pencil tool, this gives cleaner lines and if I don't ink the drawing, the effort is about the same.
I always periodically save my intermediaries as PNG. I distrust proprietary formats, because if ever your workfile gets corrupted so your program can't read it any more, you are so out of luck. With a standard format, you can almost always find some application that lets you recover your work. (Yes this happened to me.)
After cleanup, I increase the colour level to 24bpp, and then usually run the noise despeckle filter a few times.
I duplicate the lineart layer, keeping the top level copy intact. My next step is to fill in the major colour areas using the bucket tool. Since I closed all major areas before, this takes just a couple of mouseclicks.
For the text on the shirt, I removed it from the "lineart layer" once it was filled on the colour layer.
I then duplicate the coloured layer to a new one which I usually call "shading". Leaving the plain colour layer is very handy to select areas with the magic wand tool even after they became multi-coloured on the shading layer.
The next steps add the typical cel-style shading. I usually restrict my art to 2 to 3 colour levels; main, shadow, and highlight. I use a number of different ways to achieve this. In important areas such as the face or hands, I usually draw the boundaries, then fill them with the bucket tool.
Another method I use, particularly for large areas with multiple colours, is to select the colours of the area I'm working on with the "magic wand" tool. Then I use the freehand selection tool to reduce the selection to only the areas I want darkened, and use "Adjust:hue/saturation" to reduce saturation and luminance by 10-20%. I did this for the T-shirt and its text. Depending on the lighting conditions, I may also change the colour balance, for example to make the shadows "cooler" by increasing the blue. The great thing about this method is that you can apply it to different colours at once if you are in a hurry, although I don't recommend this for best results
I used the same "selection" method for shading the eyes.
When I'm satisfied so far, the last step consists of adding a little "soft" touches such as highlights, sparkles, etc. I do this on a separate layer. I usually use the airbrush or "spraycan" tool in this step. I try to use only a little of this, to liven things a little. If you want "pure" cel-style, extra highlights could also be added in the ways described above.
Sometimes "effects" are added over the lineart, such as with gleams on weapons and similar. Since they are on top of the lineart, I have a separate layer for them. Lately I've started to go over the lineart with colored brushes, making it darker shades of the area colours rather than plain black.
If the picture has a background, I often create this as a separate image, often larger than it should be, because the position of the drawing on the background and the best crop is not always easy to plan in advance (and by the time I get to the background, my drawing has become a real memory hog)...
Finally I flatten the layers, run the despeckle filter again, the image is resized for publication, and optionally sharpened.
This concludes the tutorial. Thanks for reading!