Modeling with polygons: Troubleshooting

From TOI-Pedia


Several problems can occur when modeling with polygons. This document describes common issues and the solutions.

Problems when using Extrude (face)

Manipulator Tool isn't showed when using Extrude

Most common cause for this is when your construction history is turned off. Make sure that the Construction History is turned on using the icon in your Status Line. It's located between the Snap options and the icons for Rendering options.

If this doesn't solve your problem, you may want to try to reset your user preferences.

Boolean woes

Possible problems:

  • Boolean seems to be an union, while difference was used (or vice-versa)
  • All geometry disappears
  • Holes seem to have a 'skin' over them


At the root of these problems may lay different causes:

  • open volumes: geometry does not form a closed volume.
  • 'unclean' geometry: duplicate vertices, lamina faces, zero-area faces, etc
  • incorrect or inconsistent face-normal directions: all normals should point outward from a volume
  • no 'overlap' or margin between both volumes: one or more faces of one object are in exactly the same place as the faces of the other object
  • accuracy/tolerance problems: when triangulating the geometry, zero-area triangles are formed due to insufficient precision


Troubleshooting a failing boolean requires some investigation by looking at the symptoms and trying a few diagnostic options. The table below shows which causes should be investigated for certain symptoms.

 difference becomes uniongeometry disappearsholes have a 'skin'
open volume-possible-
unclean geometry-likelypossible
incorrect normalslikely [1]possible [2]-
no 'overlap'/margin-likely-
  1. All face normals are reversed for a volume
  2. Only a few face normals are reversed for a volume

Open Volume

Should be fairly simple to diagnose by looking at your geometry.

Unclean geometry

Duplicate vertices: Switch to Component mode, set your selection mask to vertices. Use the drag window method to select the vertex (vertices) at a single point in your geometry (which should be one single vertex). In the Channelbox, click the line CVs (click to show). A table appears with the (local) X,Y and Z coordinate of the selected vertices. This should be one vertex. If there are more, you geometry is not clean.

There may be other problems than duplicate vertices, which are much harder to diagnose. In that case: just try the fix or fixes and see if it helps.

Incorrect normals

Select your geometry and use Display > Polygons > Face Normals to display the face normals. Each face should get a line at the center, perpendicular to the face. You may have to switch between shaded and wireframe display modes and tumble around your geometry to be able to determine the direction of each normal correctly.

No overlap / margin

Should be fairly simple to diagnose by looking at your geometry.

Accuracy / precision

This can only be diagnosed by looking at the resulting geometry. If a 'skin' apears to be over (some of) the holes made by the boolean, a problem with accuracy is probably the cause.


Open Volume

Make sure you use closed volumes when applying booleans. You may want to use Mesh > Fill Hole.

Unclean geometry

When you geometry has multiple vertices in the same place, take the following steps:

  1. Select all vertices
  2. Edit Mesh > Merge. Set the treshold in the option box to a small number like 0.001 (default). Make sure this number is smaller than the shortest distance between valid vertices in your geometry.
  3. This may leave lamina faces in your geometry, so use Mesh > Cleanup. In the option box, make sure you check Lamina faces and to be sure, check Faces with zero geometry area as well.

In all other cases where you suspect incorrect geometry, just execute the last step in the sequence above. You may want to try some of the other options for Cleanup as well, if a basic cleanup doesn't fix the problem.

When using Cleanup: Make sure you save a backup copy of your scene, as this command can produce unwanted results. It can be undone, but sometimes you discover problems only at a later stage.

Incorrect normals

Use Normals > Reverse to reverse the normals of the faces which have reversed normals.

No overlap / margin

Make sure that there is either an overlap or margin between both volumes. Move the face(s) of the second object so that they don't coincide with the faces of the first object.

Accuracy / precision

Use Edit Mesh > Add Subdivisions to subdivide the large faces in your geometry. You may also use the Split Polygon Tool to subdivide your geometry manually.

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