The Volume Brush Tool is one of the two 3D Sculpting Brush Tools available in Meshmixer. Please be sure to have read the Sculpt Tool topic before this one, as here we only describe the functionality specific to Volume Brush.
The main thing to understand about the Volume Brush is that it is quite different than the sculpting brushes in tools like MudBox or Maya. Rather than a stamp "on the surface", the Volume Brush actually applies a stamp in 3D space. The Brush Region is defined by a sphere centered at a point in 3D space. Often this point is on the model surface, but it doesn't have to be, which is one of the superpowers of Volume Brush. So, many of the Volume Brush Operations are actually local spatial deformations which you are simply applying in real-time.
However, because it is actually a volumetric deformation instead of a surface deformation, Volume Brush cannot support some things that you would find in other sculpting tools, like Alpha Masks or Stencils. To use these, see the Surface Brush topic.
Brush Operation
Drag: This Operation is quite powerful. As you click-and-drag the cursor, the area inside the sphere is "pulled" through 3D space. Instead of moving on the surface, when you apply this brush the cursor moves on a plane parallel to the viewpoint, centered at the surface click point.
Draw: Similar to other 3D sculpting tools, this locally displaces the vertices inside the brush along a fixed direction, defined by the average normal over the brush region.
Draw2: A variant of Draw that has a stronger directional effect.
Flatten: We find the average position and normal of the vertices inside the brush region, and move the vertices towards this plane.
Inflate: Vertices are pushed along their individual normal directions
Pinch: Vertices are pulled towards the brush center.
Move: The same as Drag, but with mesh refinement disabled by default (for convenience, mainly).
Spikes: Similar to Draw2 with a very high Strength. Useful for quickly adding detail.
PaintVertex: Paints vertex colors using the selected color. Note that this brush by default still does refinement, so if you use a very small brush, additional vertices will be added to be able to capture the stroke.
Attract: Vertices are pulled towards the current Target Object. This is similar to the Edit > Attract to Target tool, but applied locally and interactively. An active target object must be set.
BubbleSmooth: A smoothing brush that smooths "outwards", instead of shrinking. This is a much safer type of smoothing for 3D sculpting novices, as thin shapes will not collapse like they do in other sculpting tools.
ShrinkSmooth: The standard smoothing brush included in nearly all 3D sculpting programs.
RobustSmooth: This is a variant of ShrinkSmooth that properly handles variable triangle density. However, it is much more computationally intensive than standard smoothing, and often smooths quite slowly if the mesh does have highly variable resolution.
Refine: does not deform the shape, but simply adds more triangles in the brush region
Reduce: Removes triangles in the brush region, basically a local application of the Edit > Reduce tool in that strength controls the amount of reduction.
AdaptiveReduce: same as Reduce but uses adaptivity, similar to Adaptive mode in the Edit > Reduce tool.
ZipperEdge: Pulls open boundaries together. This brush is mainly for use in a multi-step workflow to close complex open boundaries; see the Edit > Weld Boundaries tool for details.
Falloff
The Falloff flyout allows you to select different falloff shapes that modulate the Strength of the Operation you are using. This is similar to a brush alpha mask, but it is a volumetric shape. The icons show 2D representations, but the actual falloff is a 3D function that is quite hard to visualize.
The available Falloffs are shown below. These do not always have a very strong effect on the active brush, but changing from the far left 'tabletop' falloff to the far-right 'spike' falloff will produce a very noticeable difference. You will have to experiment to get a sense of which Falloff is useful for the effect you are trying to achieve.
Main Panel
The main panel for the Volume Brush is shown below. We discussed the Strength, Size, Symmetry, and Flow parameters on the main Sculpt Toolpage. The Depth setting is explained below.
The Lazyness parameter influences the smoothness of your brush stroke. When Lazyness is 0, the brush stamps are applied directly below the cursor positions. As you increase lazyness, the brush stamps will "lag" behind your cursor, as if the brush location were connected to your cursor with a stretchy rubber band. The higher the setting, the slower the brush will move, and as a result your strokes will be significantly smoother.
As we described above, the Volume brush area-of-affect is a sphere around the stamp center. By un-checking the Volumetric toggle, you can modulate this area-of-effect in certain cases. Specifically, imagine sculpting on a thin surface with a large brush. Both sides of the surface will be inside the brush sphere, and hence affected. In non-Volumetric mode, we filter out any regions not geometrically connected to the brush/surface hit-point, within the brush sphere. So, in that case the brush would only be applied to one side of the thin surface. This can be quite useful, however keep in mind that if you are near the edge of a thin surface, the filtering may not work as the brush can "escape" around the edge.
By default for the volume brush, the symmetric stamp is applied at the mirrored 3D position, regardless of whether it is actually on the model surface. When SymSnap is enabled, we project the stamp center onto the closest point on the surface before applying the stamp. This is useful mainly when sculpting on surfaces that are not quite symmetric, such as 3D scans.
Depth
The Depth setting is key to understanding the behaviour of the Volume brush. As we described above, the brush is actually a sphere in 3D space. So, when you click on the surface, the simplest thing would be for the spherical brush stamp to be applied at the surface point. However, we could also offset the stamp center into or away from the surface, along the surface normal.
This is what the Depth parameter controls. At Depth = 0, the stamp is applied on the surface. At Depth = 50, the stamp center is pushed into the surface by half the brush radius, and at Depth = -50 it is pushed outwards the same distance, as in the image show below. The second row shows that if the surface is thin, we only offset into the surface at most to the midpoint between the first and second ray intersections.
The default Depth varies depending on the brush. For example, with the Drag brush the default depth is 50, so that when you drag on thin regions, you are pulling from the middle and not the nearer surface. For Pinch the default is 0, but you can also create two different styles of creases by setting the Depth to positive or negative values.
Filters
The Filters panel has additional controls for the Volume Brush, as described below.
By default, when you sculpt near an open boundary, it is free to move. The image below-middle shows what happens when using the Drag brush near the edge of a square. If you wish to keep the boundary from moving, check the Fixed Boundary box, or toggle it on/off using the b hotkey. The result below-right is for a similar stroke as the middle, but the boundary keeps its position.
Note that with this option the boundary doesn't just stay in place, it is not refined at all. So, when this option is checked, the boundary will remain completely unmodified. This is helpful if you wish to stitch it together with a compatible boundary later. See the Close Cracks tool for an explanation of how to use this capability to do "masking: working on sub-regions of a mesh while hiding the rest.
The Preserve Groups option is actually a control on the remeshing algorithm, basically turning on the same thing that Preserve Group Borders does in the Edit > Remesh tool. With this capability enabled, the remesher attempts to maintain the shape of group boundaries (although this is not always strictly possible). You can also toggle Preserve Groups using the g hotkey.
Similar to Fixed Boundary, the Restrict to Group option controls the brush around group boundaries. When this option is enabled, then when you begin a stroke, the face group of the triangle under the cursor is saved and used as a filter to the sculpting operations. Only the vertices of triangles with the same facegroup can be modified by the brush. So, in the example below, we can first drag part of the hexagonal region downwards, then part of the grey region up, without either stroke affecting the other face groups. Again, this capability can be used for "masking" effects.
Note that in the above examples we also have the Preserve Groups option enabled.