How do you cut Geometry in Zbrush?

ZBrush provides a robust set of tools specifically designed for precisely cutting, slicing, and shaping your 3D models with both destructive and non-destructive methods. Understanding these brushes and techniques is key to achieving clean edges, sharp details, and complex forms in your digital sculptures.

Key ZBrush Cutting Tools

ZBrush offers several specialized brushes that allow artists to "cut" geometry in different ways, each serving a unique purpose. These tools are often activated using Ctrl + Shift while drawing on your model.

1. The Slice Brushes (SliceCurve, SliceLasso, SliceRect)

The Slice brushes are primarily used to define new edges and create new Polygroups on your model without actually removing any geometry. This is incredibly useful for setting up areas for further sculpting, masking, or splitting the model later.

• How to Use:
  • Hold Ctrl + Shift and select a Slice brush (e.g., SliceCurve).
  • Draw a line or shape across your model. The area encompassed by the line will become a new polygroup.
  • Result: The geometry remains intact, but new edges are inserted along your drawn path, and the affected areas are assigned distinct polygroups.
• Practical Insight: Great for creating panel lines, separating limbs, or defining architectural elements where you want to retain all the original mesh data but need clear divisions.

2. The Knife Brushes (KnifeCurve, KnifeLasso, KnifeRect)

Knife brushes are powerful destructive cutting tools that actively remove geometry and redefine the mesh. They are excellent for creating clean, sharp cuts and extracting new shapes.

• How to Use:
  • Hold Ctrl + Shift and select a Knife brush (e.g., KnifeCurve).
  • Draw a line or shape. The part of the mesh outside the line (or depending on the brush, the side you define) will be cut away.
  • Result: Geometry is cleanly cut away, and the remaining surface is often closed, creating new polygroups along the cut edge.
• Practical Insight: Ideal for creating sharp mechanical parts, refining hard surface models, or making precise surgical cuts in your sculpts. They can create some really cool cuts and shapes.

3. The Trim Brushes (TrimCurve, TrimLasso, TrimRect)

Trim brushes perform planar cuts, effectively shaving off parts of your model to create flat, clean surfaces. They are similar to Knife brushes but are specifically designed for creating perfectly flat, flush cuts.

• How to Use:
  • Hold Ctrl + Shift and select a Trim brush (e.g., TrimCurve).
  • Draw a line or shape. The portion of the mesh on one side of the line will be "trimmed" or flattened down to the plane defined by your stroke.
  • Result: Geometry is removed and flattened, resulting in a crisp, planar surface.
• Practical Insight: Excellent for hard surface modeling, creating flat panels, edges, or for cleaning up areas that need a sharp, defined plane.

4. The Clip Brushes (ClipCurve, ClipLasso, ClipRect)

Clip brushes don't remove geometry in the same way Knife or Trim brushes do. Instead, they push or pull the geometry along a defined plane, effectively flattening or "clipping" it without deleting polygons. The volume of the mesh is maintained.

• How to Use:
  • Hold Ctrl + Shift and select a Clip brush (e.g., ClipCurve).
  • Draw a line or shape. The area on one side of the line will be pushed or pulled to align with the plane you've defined, while the other side remains unaffected.
  • Result: Geometry is re-positioned to create a flat surface, but no polygons are deleted.
• Practical Insight: Perfect for adjusting existing flat surfaces, creating subtle indentations, or for non-destructively flattening areas without losing detail behind the clipped region.

Advanced Techniques for Precise Cuts

Beyond the dedicated cutting brushes, ZBrush offers powerful workflows that combine masking and visibility to achieve complex cuts and separations.

Using Hidden Geometry for Cuts and Splits

A highly effective method to create precise cuts and new shapes is by utilizing ZBrush's masking and visibility features. You can hide parts of the geometry to create some really cool cuts and shapes, especially when combined with splitting functions.

1. Mask the Area: Use masking brushes (hold Ctrl while drawing) to define the part of your model you want to isolate or cut.
2. Hide Unmasked: Invert the mask (Ctrl + I), then hide the unmasked (now masked) portion by Ctrl + Shift + Clicking on the background. Alternatively, directly hide unmasked portions after masking by Ctrl + Shift + Dragging a selection rectangle over the unmasked part of the model.
3. Perform the Cut/Split: With the desired geometry hidden, you have several options:
   • Delete Hidden: Go to Tool > Geometry > Modify Topology and click Del Hidden. This will permanently delete all hidden geometry, resulting in a clean cut along the visible boundary.
   • Split Hidden: Go to Tool > SubTool > Split and click Split Hidden. This will create a new SubTool from the hidden geometry, effectively "cutting" it away from the original model into a separate piece.
   • Work on Visible Area: Simply work on the visible geometry with any sculpting or cutting brush, ensuring your operations only affect the exposed parts.
4. Show All: Once done, click Tool > Visibility > Show All or Ctrl + Shift + Click on the background to reveal your entire model.

Changing Brush Size

For any of these brushes, adjusting the brush size is straightforward:

• Press 'S' key: This brings up the Draw Size slider, allowing you to quickly change the size.
• Draw Size Slider: You can also adjust the Draw Size slider directly in the Draw palette or in the top UI bar.

Tips for Effective Cutting in ZBrush

• Polygroups are Your Friends: Many cutting tools inherently create polygroups. Utilize these for easy selection, masking, and isolating parts of your model.
• Hold Shift for Straight Lines: When using curve-based cutting brushes (e.g., KnifeCurve, SliceCurve), holding Shift after drawing the initial point will snap your line to 90-degree increments, allowing for perfectly straight cuts.
• Tap Alt for Corners: When drawing with curve brushes, you can tap the Alt key to create sharp corners in your line, enabling more complex cut paths.
• Zoom In for Precision: For very fine cuts, zoom in closely on your model to ensure accuracy.
• Save Frequently: Always save your work periodically, especially before performing destructive operations.

Mastering these cutting tools and techniques will significantly enhance your ability to sculpt intricate details, create hard-surface models, and manage your mesh effectively in ZBrush.