Booleans

A boolean combines two meshes as if they were solid volumes: one object can "cut into, combine with, or isolate part of another object" (GarageFarm blog: Hard Surface Modeling in Blender). It is the quickest way to make holes, recesses, slots and merged parts in hard-surface modeling, and the main reason hard-surface meshes end up with messy topology.

#The three operations

Blender's Boolean modifier names the three operations clearly (Blender Manual: Boolean Modifier):

• Union adds the target to the modified mesh, "removing any interior faces".
• Difference subtracts the target, keeping everything outside it. This is the one you use most: a cylinder becomes a bolt hole, a box becomes a vent recess.
• Intersect keeps only what is "inside both the target mesh and the modified mesh".

The object doing the cutting is usually called the cutter (Blender calls it the target or operand). It can be a single object or a whole collection (Blender Manual: Boolean Modifier).

#Topology produced by booleans

A boolean finds where two surfaces cross and rebuilds faces along that line, without regard for edge flow. The result usually has n-gons, long thin triangles and vertices where the cut met an existing edge.

On a flat face with hard edges, that is mostly fine: the face is flat, so its normals all point the same way. The problems start when you:

• smooth-shade across the cut, so vertex normals get averaged over thin faces and you see smears and gradients;
• subdivide, because n-gons and triangles subdivide unpredictably;
• bevel the result, because thin faces and stray vertices give the bevel tool bad geometry to work with.

FrankPolygon notes: "Most boolean operations will leave behind some unnecessary geometry" (Polycount forum: Sketchbook: Frank Polygon).

#Solvers: exact, fast and manifold

A solver is the algorithm that computes the cut. Blender has offered several:

• Exact "uses a mathematically complex solver which offers the best results" and handles overlapping geometry, but is much slower (Blender Manual: Boolean Modifier).
• Fast "uses a mathematically simple solver which offers the best performance" but "lacks support for overlapping geometry" (Blender Manual: Boolean Modifier). In Blender 4.5 it was renamed Float (All3DP: New Blender 4.5 LTS Adds Point Cloud Object…).
• Manifold, added in Blender 4.5 and based on the Manifold library, is described as "as fast as, and sometimes faster" than Float (CG Channel: Blender 4.5 LTS is out: check out its 5 key…; All3DP: New Blender 4.5 LTS Adds Point Cloud Object…). It "only works on manifold geometry, where each edge is adjacent to exactly two faces" (CG Channel: Blender 4.5 LTS is out: check out its 5 key…).

Whatever the solver, inputs matter. Blender's manual warns that only manifold meshes are guaranteed to give proper results; open meshes usually work but can produce glitches (Blender Manual: Boolean Modifier). Keep cutters closed and watertight, with normals facing out.

#Cleanup strategies

You have three broad options, from least to most effort.

1. Leave it. If the result is flat, shaded with hard edges or weighted (custom) normals, and will not be subdivided, the n-gons may be harmless. Check the shading under a moving light before you touch anything.
2. Triangulate and dissolve. Apply the boolean, then remove unnecessary vertices and edges. FrankPolygon applies booleans only after approval, then cleans up with triangulation and limited dissolve (Polycount forum: Sketchbook: Frank Polygon).
3. Rebuild locally. Add a few manual cuts to route edges from the hole to nearby corners so faces are closer to quads. Do this only where a bevel or subdivision needs it.

Add-ons can help. MESHmachine includes a Boolean Cleanup tool that merges vertices along the transitional edge between two boolean operands (machin3.io: MESHmachine documentation).

#Bevel after boolean

Booleans give you sharp edges along the cut; real objects have rounded ones (see bevels and chamfers). The usual modifier stack in Blender is: base mesh → Boolean (one or more) → Bevel → Weighted Normal. The bevel then runs on the live boolean result.

This only works if the boolean result is clean enough for the bevel. Stray vertices on the cut edge give uneven bevel width; very thin faces can make the bevel overlap itself. For this reason, tools such as Hard Ops combine booleans and bevels in one system: it advertises a "Non-Destructive / Destructive boolean cutting system" and multi-level bevel control (Read the Docs: Hard Ops official manual).

#Live booleans

A live (non-destructive) boolean keeps the cutter as a separate, editable object. You can move a hole at any time until you apply it.

• Blender modifier stack. The Boolean modifier is live by default. BoxCutter draws cutters directly on a surface, and they stay editable "until you apply the modifiers" (BlenderNation: Blender add-ons review — BoxCutter and Hard…).
• ZBrush Live Boolean. Each subtool gets an additive, subtractive or intersecting Boolean Process setting, and the result is shown as a render preview "without being applied as geometry changes" (ZBrush for iPad documentation: Live Boolean Essentials). To make it real, use Make Boolean Mesh, which creates a new tool prefixed "UMesh_" (ZBrushCentral: ZBrush Live Boolean).
• Plasticity. Works on exact NURBS solids rather than polygons. Its Boolean command can "combine, subtract, or intersect multiple objects", and a Keep Tools option leaves the cutters intact (Plasticity documentation: Boolean). See CAD and NURBS modeling for games.

#In different tools

#Further reading and watching