Obviously in digital there's no way to get a blob of pixels to raise off the surface of your screen, but you can at least capture some of the effect with Painter's impasto. But a note of warning, this effect can be very easily overdone and look like a gimmicky trick that distracts from your piece; if you use it, use it with extreme subtlety. Much like texture, it should be used as an accent and should not draw attention to itself, especially because real impasto has such organic shapes that the calculated strokes of Painter will look unnatural if it catches your attention.
With that said, let's get down to business. Painter has a number of default impasto brushes which you can use, or you can make your own simply by opening a suitable brush in the brush creator, going under the impasto tab and changing the Draw To option to either Depth or Color and Depth. Depth will simply add impasto and not actually change any of the pixels that are on the canvas, Color and Depth will lay down new paint as well as add impasto to the surface. For my impasto brush, I started with the Wet Oily Brush in the Arists' Oils category, changed the settings so it would not fade out so quickly, and set it to Color and Depth with a depth of 100%. Here's an example of what it looks like, using just one color:
The impasto feature will take the opacity of a stroke and apply depth to it; the more opaque an area is the more depth it has. It then lights the canvas and this depth catches the light, creating an impasto effect (As a side note, beware of the canvas lighting changing the values in your piece, go up to Canvas -> Surface Lighting to adjust your surface lighting and be sure it's similar to the values when there is no impasto).
Now, this effect is useful not only for creating thick looking strokes, but also for creating surface texture that layers over time. I can take that same image above and apply a brush that does not apply any depth and look what happens:
You can see that the striations from the impasto brush are left over even when I paint normal brush strokes over top of it, this effect adds a really nice variety to your markmaking and can make your strokes feel very traditional. Now, there's a danger to impasto that we must address, and that is the overexposed highlights and pixelated edges it often creates. When you use impasto brushes, their varying levels of depth can create these artifacts that look pretty hideous:
This is the reason why a lot of digital impasto just looks cheap, but it can easily be fixed with a little extra effort, and the end result can be very satisfying. Essentially what you need to do is create a brush that will blend out the strokes; I made mine out of the same captured dab I showed in yesterday's post, and set the impasto Color and Depth, depth method Uniform, Depth 50%, and expression set to Pressure. The captured dab Impasto isn't using the hard strokes that the Artists' Oils does so it can be used to blend out those artifacts, and though you could just use a captured dab for all your impasto effects I find that it can build up in weird ways and the Artists' Oils looks more subtle and appealing. A mix of the two works best in my opinion. Here's the same image with the artifacts gently blended out:
And here is the same image with no impasto effects:
As you can see, after pixelation issues have been solved, impasto can add a very paint-like quality to your strokes. This becomes very useful because you can use the impasto to define stroke direction and texture, and use regular brushes to handle most of the painting without the impasto getting overdone. Just be sure you clean up the artifacts after laying down the impasto, and you'll have some powerful effects on your hands!
So, to conclude this series, you now have a bunch of ways to make interesting, painterly brush strokes, but how do you apply it all? The secret is in combining all of them in to one to create a seemingly natural and effortless brush stroke; you can use paper textures to create surface variety, custom brushes to create natural strokes, and impasto to create direction and depth.
And that's it for this series! Hopefully it was useful, I'll try and get up a step-by-step up sometime to show this being used in an actual painting of some sort. Until then, happy painting!