From ZBrush Info
ZBrush lets you mask parts of a 3D object so that when you modify the object with a brush, the operation affects only part of the object. Masking can be applied with variable intensity—up to 256 levels—so that the strength of applied effects varies according to the intensity of the mask. Masking can be applied while editing an object by holding down the Ctrl key as you drag over the object. The Tool:Masking subpalette provides many other functions related to masking, described below.
ViewMask: Causes masking to be visible; the mask appears as a dark gray shading. The darker the mask, the less effect operations applied to it will have. This is automatically turned on when you apply masking when editing an object.
In the illustration below, a mask was applied to the upper part of a cylinder, and then the SBend deformation was applied to the cylinder. The darker shade shows the mask, which prevented SBend from affecting that part of the object.
Note: If you 'paint' a 3D object onto the canvas using any standard method such as Snapshot, all visible masking is included in the image transferred to the canvas.
Inverse: Inverts the masking. Masked areas become unmasked, and unmasked areas become masked.
Clear: Removes any masking.
MaskAll: Masks the entire object. Use MaskAll or any other method to apply a mask before using Row, Col, or Grd, which act only on masked areas.
BlurMask: Blurs the edges of the current mask. Additional presses result in more blurring. This is particularly useful when masking an area that will be sculpted to blend very smoothly into the surrounding area, as you can quickly draw a mask, and then blur it to the desired degree.
SharpenMask: Sharpens the edge of the current mask. This is the inverse of the BlurMask operation (though blurring and then sharpening a mask won't normally give you exactly the original mask; use undo for that.)
Mask By Cavity: Automatically creates a mask based on the cavity geometry of the model. This allows you to, for example, easily paint only the surface of a dragon's scales, not the grooves between them. Cavity masking can be used in many ways in ZBrush 3. Make sure to check out the material in www.ZBrush.info.
The next three settings—Int, Hue, and Sat—let you derive a mask from a texture applied to an object. For more information on texture usage, see the section describing the Texture palette. To use Int, Hue, and Sat, first apply a texture, then click the appropriate button to apply the masking. Then, to best see the result, turn off texturing by selecting the TXTR OFF thumbnail in the Texture inventory.
Mask By Intensity: Derives a mask from the color-intensity values in an applied texture. Darker areas receive higher masking values than do lighter areas. If no texture is active ZBrush 3 will use your model's PolyPainting instead.
Mask By Hue: Derives a mask from the colors in an applied texture. Masking values of different colors are arbitrarily assigned based on the order of colors in the Color Selector, with the masking value at 100% at the left edge, decreasing to 0% at the right edge.
Mask By Saturation: Derives a mask from the color-saturation values in an applied texture. Highly saturated areas receive higher masking values than do less-saturated areas.
Mask By Alpha: Uses the currently selected alpha to apply masking.
Note: The alpha is applied on a per-polygon basis, so be sure the object has a high enough mesh resolution to support the alpha bitmap resolution; otherwise, the result will be blocky. This also applies to Intensity, Hue and Saturation masks.
Intens: The strength by which masking is removed by the Row/Col/Grd commands. Use different Sel/Skp settings with different Int amounts to vary masking strength on different parts of the object. Default is 100%. Range is 0% to 100%.
Blend: The degree to which a new masking operation mixes with an existing mask. This lets you, for instance, combine a Row/Col/Grd mask with an Alpha mask. To use, create a mask, then set the Blend amount, and then apply a second mask, and so on.
Create Alpha: Create an alpha from the current mask. This effectively saves the mask for later use.
HidePt: Hides any completely or partially non-masked parts of an object. This effectively lets you “cut away” part of an object, leaving only the masked areas. HidePt leaves only parts of an object on which the masking is 50% or greater.
ShowPt: Restores visibility of any parts of an object hidden with HidePt, as long as their masking values haven't changed.
