Working With Your Canvas: Alphas
From ZBrush Info
Alphas are another big part of the painting equation. An alpha is simply a gray map, and it can be used to affect lots of things. Alphas can represent depths, masks, transparencies, and virtually any effect that depends on intensity. 3D maps such as cavity and bump maps are really just alphas.
It's worth noting that many (maybe even most) programs that make use of alphas use 8-bit alphas, allowing only 256 different levels of gray. This can result in a 'stair-stepping' effect in many cases. ZBrush uses 16-bit alphas, allowing for more than 65,000 levels of gray. This ensures a smooth effect, no matter how the alphas are used.
ZBrush comes with a bunch of predefined alphas, that can be found in the alphas popup; either to the left of the canvas, or in the Alpha Palette. It's also easy to create or import your own alphas, but we won't go into that, because it's not really part of how alphas affect painting.
The major effect of alphas is to modify the shape of the brush. Basically, gray parts of an alpha mean the brush has less effect, and white parts of the alpha mean it has more effect. (Dark area mean it has an 'opposite' effect.)
Let's show this with a bit more depth painting. In the figure below, I just started up ZBrush, and 'clicked' three paint strokes. (I didn't drag them out or anything like that.) All of the ZBrush settings were left at their default values except for the alpha.
The stroke at the left shows the effect of the default alpha (Brush 01). Since that alpha is an image with white at the center, fading to gray at the outside, we get an effect where the center of the clicked stroke is raised, and the area around it fades smoothly into the depth of the rest of the canvas.
The center of the image shows the effect when using alpha Brush 17, the 'bullseye'.
Finally, the right of the image shows the effect of a more interesting alpha, Brush 30 I think of this as the "soccer ball" alpha.
