Masking is a way of selecting part of a tool, so that you can isolate it. It can then be hidden, used to create polygroups, or sculpt on particular areas, polypaint, etc.
CREATING A MASK
Hold Ctrl and make a brush stroke to mask an area of your model.
Ctrl + Alt + Click to ‘unpaint’ parts of a mask.
Ctrl, Click and Drag on a blank area of canvas to completely erase an active mask.
As with other tools, Press X to toggle symmetry mode when selecting masks.
Note that since ZBrush is an awkward bollocks, the masked area will appear as a dark area on the model, which is the part which cannot be sculpted on.
Invert a mask by holding Ctrl and clicking on a blank area of the canvas, or by clicking invert in the mask palette.
MASK BRUSHES
The basic Mask Pen brush is used for painting our masks, but there are other selection emthods available.
Ctrl, Click and Drag over an empty canvas area to drag a rectangular selection.
With Ctrl held, you can select different mask brushes.
The Mask Lasso brush is useful for selecting parts of the model such as limbs. Note that it will select both front and back of the model.
NB - Alphas and strokes can be used like stencils to paint masks. Hold Ctrl when picking a new alpha from the toolbar to assign it to the masking function.
ADJUSTING MASKS
In the masking palette, masks can be blurred, sharpened etc.
In the mask palette, use the ShrinkMask, GrowMask and SharpenMask buttons to adjust your mask edges.
Convert a masked area to a polygroup by pressing Ctrl + W, or by clicking the Group Masked button in the polygroups section of the Tool palette.
DELETING GEOMETRY VIA MASKS
Deleting geometry is (like everything else) a little bit awkward in ZBrush.
The simplest way is to mask and hide the part of your mesh that you want to delete.
Mask out the part of the mdoel that you want to delete.
Invert the mask
Choose Visibility > Hide Points in the Palette.
Then in the Geometry Palette, click Modify Topology > Del Hidden. This will delete any geometry that is currently hidden in the current tool.
NB - This will create a hole in your mesh. This can be closed with the Close Holes’ button located in the same tab, but can give triangles or wonky geometry that will need to be fixed later.