Techniques For Cutting Custom Shapes And Holes In Blender Meshes

Cutting Holes in Meshes

Cutting holes and custom shapes in blender meshes allows for more complex and interesting models. There are two main techniques for cutting holes and shapes:

Using the Boolean modifier
Using the Knife Project tool

Each technique has advantages and disadvantages that make them suitable for different applications.

When to Use Booleans vs. Knife Project

The Boolean modifier cuts using another mesh object, resulting in a clean cut but at the cost of increased geometry. The Knife Project tool projects vertices onto the surface, resulting in reduced geometry but a less precise cut.

Table of Contents

Use Boolean modifiers when:

You need a clean, precise cut
The base mesh and cutter mesh are closed solids
You are willing to have more geometry

Use Knife Project when:

You want to reduce geometry
You are cutting an open surface
You need an approximate cut

Setting Up the Base Mesh

Properly setting up the base mesh is critical for good Boolean and Knife Project results. Consider the following when setting up:

Topology: Quads with mostly perpendicular faces work best
Density: Enough geometry to capture details from cutter mesh
Normals: Make sure normals are consistent and point outwards
Scale and dimensions: Boolean works best with meshes of similar scale and dimension

Also ensure there is some thickness to the mesh surfaces and avoid n-gons and triangles.

Using the Boolean Modifier

The Boolean modifier in Blender cuts or merges meshes using Boolean operations. With proper setup, it can cut very precise and complex shapes.

Boolean Operation Types

There are three types of Boolean operations:

Intersect – Keeps the volume common to all meshes
Union – Keeps the complete volume of all meshes together
Difference – Subtracts one mesh from another

“Difference” is most commonly used for cutting holes and shapes.

Positioning and Aligning Meshes

To properly cut shapes with Boolean difference, the base and cutter meshes must be positioned and aligned precisely:

Overlap the cutter and base mesh in the area you want to cut
Line up features from each mesh
Position close but not quite touching to avoid issues
Parent meshes together or snap transforms to align

It often helps to temporarily hide other objects and focus on just the cutter and base mesh alignment.

Applying Modifiers and Troubleshooting Issues

Proper order of modifier application avoids issues:

Apply all modifiers on cutter mesh
Add Boolean modifier to base mesh targeting cutter
Apply Boolean modifier
Add any remaining modifiers to base mesh

Common Boolean issues and fixes:

Geometry errors – Improve base mesh topology or scale
Cutter disappears – Set Boolean to “Fast” calculation
Incorrect results – Double check normals, overlapping geometry, and order of operation

Knife Project Workflow

Knife project cuts geometry by projecting vertices from one object onto the surface of another object.

Projecting from View

Projecting vertices from the view onto a surface with Knife Project:

Add a mesh object and edit it to prepare vertices to project
With object selected, enter Edit Mode and select vertices to project
Press Knife Project button in toolbar with surface object as target
Tweak the Cut Through threshold value in Operator Box
Adjust viewport and vertex positions to get desired projection results

Use this method for simple cuts across flat surfaces.

Projecting from Object

For better control, project vertices from another mesh object:

Create or add a mesh object to use as the projector
Position projector object how you want vertices projected
With projector selected, enter Edit Mode and select vertices
Select surface object and press Knife Project button
Adjust projector object position to get desired results

This allows polygons, curves, or custom extruded vertices as projectors.

Finishing Cuts and Extruding Faces

Once mesh is cut, additional work is often needed:

Remove doubles to fix overlapping vertices
Re-adjust seams for proper UV unwrapping
Extrude faces to add thickness if needed
Fill holes or open edges depending on model needs

For open cuts, select edges then press Alt+E to extrude/inset faces, allowing the cut to have some thickness.

Optimizing Topology Flow

When doing extensive cutting of meshes, topology flow is important for best results with subsurf modifiers and good edge flow:

Plan seams and cuts tofollow existing topology flow
Minimize number of poles from cuts
Redirect topology flow after cuts if needed
Utilize edge loops and creases for smoother deformation

Take time to analyze mesh flow before and adjusting as needed after extensive cuts.

Extra Tips and Tricks

Use non-manifold meshes for more cutting flexibility
Add edge split modifier for extra shape definition
Use wireframe modifier for quick cuts
Duplicate and separate pieces to control results
Save iterations in case you need to go back
Apply transforms on objects to avoid issues