Doubles And Overlapping Geometry: Why They Break Boolean Modifiers And How To Fix Them
Understanding Boolean Modifiers
Boolean modifiers in 3D modeling refer to operations that combine two 3D objects using Boolean logic to create a new resulting 3D object. The two most common Boolean operations are union, which combines two objects into one, and difference, which subtracts one object from another.
Doubles refer to duplicate vertices in the same mesh that occupy the exact same spatial coordinates. Overlapping geometry refers to two 3D objects that occupy the same spatial area but do not share the same vertices. Both situations can cause unexpected and undesired results when using Boolean modifiers.
What are doubles and overlapping geometry?
Doubles are duplicate vertices occupying identical spatial locations within a single mesh. They most often occur when joining new geometry to an existing mesh. For example, if you model a cube and then extrude one face to create another cube attached to the first, the connecting faces would contain duplicated vertices at the same coordinates where the cubes intersect.
Overlapping geometry refers to separate objects whose meshes occupy the same space but do not actually share vertices. This can happen when manually positioning objects in close proximity without joining them. If two cube objects are placed in the same location but kept as separate meshes, they would be overlapping without shared vertices.
How boolean modifiers work
Boolean modifiers work by using mathematical Boolean operations to combine or subtract meshes. The union modifier merges the meshes of two objects into one combined object. It does this by creating new faces along the intersections of the two meshes. The difference modifier cuts out one object from the other by removing the faces of the subtracted mesh from the main mesh.
In order for these operations to work properly, the boolean modifier requires clean geometry with no doubles or overlaps. Doubles can confuse the modifier when it tries to create new faces along intersections. Overlapping geometry also causes problems when trying to subtract one object from the other since it is unsure which faces to remove.
Common issues with boolean modifiers
Doubles
Doubles are problematic for boolean operations because the duplicate vertex locations confuse the process of generating new geometry along surface intersections. As the boolean modifier tries to create new connecting faces between the two objects, the duplicate vertices can result in missing faces or holes in the resulting mesh.
This also creates non-manifold geometry which appears completely joined but actually has edges that are disconnected under the surface. Non-manifold meshes can render incorrectly in certain situations and cause problems if the model needs to be 3D printed or converted to other formats.
Overlapping Geometry
Overlapping geometry poses issues for Boolean modifiers because when performing a difference operation, the modifier does not know which object to subtract from the other since their meshes are inhabiting the same coordinates. This can result in unexpected subtractions or failing to subtract anything at all.
In a union operation, overlapping geometry can also generate connections where none should exist or fail to create connections between meshes. The end result is often void areas or extraneous faces that should not be present.
Fixing Problems
Removing doubles
The easiest way to fix doubles is by using the “Remove Doubles” tool available in most 3D modeling software. This automatically searches for vertices sharing the same coordinates within a threshold distance and merges them together. The threshold allows controlling how close vertices must be to qualify as doubles.
For blender specifically, you would enter Edit Mode, select all vertices with “A”, then under Mesh -> Clean Up -> Merge by Distance to remove doubles. Sometimes adjusting the “Merge Distance” value is needed to successfully remove all duplicate vertices.
Fixing overlapping geometry
Fixing overlapping geometry involves either separating or joining the offending objects. To separate them, simply move one of the objects to an empty area of the scene. This eliminates the spatial overlap.
To join them, you can use Boolean modifiers to combine them into a single mesh. The union Boolean modifier works well for this. Apply the union to the overlapping objects to merge them into one object. This ensures they share vertices and occupy the same space without conflict.
Another option is to manually join the geometry by deleting faces where they intersect and creating new faces to bridge the remaining mesh segments. This takes more effort but allows finer control over the final model.
Example Code
Here is some example code using Blender Python API that demonstrates finding and removing doubles:
import bpy
import bmesh
# Get the active mesh
obj = bpy.context.edit_object
me = obj.data
# Get a BMesh representation
bm = bmesh.from_edit_mesh(me)
# Find doubles and store indices
doubles = []
for v in bm.verts:
if v.co.x == v.co.y == v.co.z == 0:
doubles.append(v)
# Remove the double vertices
bmesh.ops.remove_doubles(bm, verts=doubles)
# Show the updates in the viewport
bmesh.update_edit_mesh(me)
And here is example code to join two overlapping cubes using a Boolean modifier:
import bpy
# Get the objects
cube1 = bpy.data.objects["Cube1"]
cube2 = bpy.data.objects["Cube2"]
# Add a union boolean modifier
boolean_mod = cube1.modifiers.new("Boolean", 'BOOLEAN')
boolean_mod.object = cube2
boolean_mod.operation = 'UNION'
# Apply the modifier
bpy.context.view_layer.objects.active = cube1
bpy.ops.object.modifier_apply(modifier="Boolean")
Alternative Approaches
Using booleans sparingly
Due to the complexity of troubleshooting boolean modifier issues, it is best to use them sparingly and only when no other modeling techniques will suffice. Oftentimes constructively modeling geometry using primitives and simpler operators can achieve the same design goals without runtime conflicts.
Limiting use of booleans will help accelerate overall workflow and provide more predictable, reliable results when exported to other downstream applications like animation, rigging, texturing, and rendering.
Modeling without booleans
Many cases where boolean operations would be used can also be constructed manually without modifiers. For example, instead of subtracting one object from another, you could delete the intersecting faces from the main object and manually retopologize the hole left behind. This takes more time but grants full control over the topology.
Constructive solid geometry techniques involve building models using primitives like cubes, cylinders, sphere segments and other simple shapes. Connecting them together with bridging geometry and filling holes gives results free of procedural mesh complications like bad normals, texture stretching or clipping issues.
The disadvantage is that adjusting details on primitives requires rebuilding them from scratch. So shape flexibility can be more restrictive.
Summary
In summary, doubles and overlapping geometry are common mesh issues that derail boolean modifiers by causing mathematical uncertainty in the procedural mesh calculations. Fixing these problems involves removing doubles to consolidate vertices and separating or joining overlaps via union booleans.
When possible, it is best to model 3D geometry without relying solely on heavy use of boolean modifiers. This avoids unpredictable problems down the line and maintains maximum artistic flexibility.
