Smoothing groups

Last updated 2019/01/18


It's advisable to have read the "Normals" chapter first.


"Smoothing groups" are a way of defining areas on a mesh that should be rendered with smooth shading. Usually, numbers are used to define the group membership per polygon. One polygon can have one or several numbers and thus one or several groups. The idea behind assigning several groups to one polygon is to have a way of connecting separate groups at specific points. Let's have a look at some examples.

simple smoothing groups

Here we can see a simple smoothing group assignment where two halves of a sphere have two different smoothing groups. As expected, we have a hard non-smoothed border between both smoothing groups.

overlapping smoothing groups

In this example, we add an additional group in between the existing groups and assign both smoothing groups IDs. This way we create a connection between the groups and we get a smooth transition as you can see on the right.

Some 3d applications let the user define hard edges instead of smoothing groups. This way you don't have to think about overlapping IDs and it's generally a bit more user-friendly. However, those hard edge definitions are usually converted back into smoothing groups (or explicit normals) when exporting models into other apps.