# Performing A Symmetry

This task shows you how to perform a symmetry on a subdivision surface.
There are two types of symmetry:
• no intersection between the subdivision surface and the symmetry plane
• intersection between the subdivision surface and the symmetry plane.

Open the Symmetry1.CATPart document (without intersection) or Symmetry2.CATPart document (with intersection).
1. Click Symmetry .

 A Tools Palette is displayed: The following icons inform you about the input selection: surface plane
2. Select a surface.

 The Plane Selection icon becomes active.
3. Select the plane.

 You can also select the plane first then the surface. In this case, the Surface Selection becomes active.

### No intersection between the subdivision surface and the symmetry plane

 The result is the mathematical symmetry by a plane:

### Intersection between the surface and the plane

 In this case, the surface is made of two parts separated by the plane. A side of the surface is automatically chosen, corresponding to the bigger side: A new icon appears in the Tools Palette to let you choose the side.

1. Select Side Selection or click in the 3D area to invert the side of the symmetry.

• The symmetry keeps the curvature continuity of the subdivision surface, therefore some details of the input mesh will be modified.
• Previous operations performed on the input surface are also kept.
• The result of the symmetry is a subdivision surface. Thus, you can perform a symmetry of a symmetry or copy-paste a symmetry or use this feature to associate other subdivision surfaces.

• If the surface is U-shaped, you cannot select a side as it is automatically chosen:

• If the symmetry plane intersects a S-shaped surface, the symmetry cannot be performed. Below is an example using the green plane: the symmetry cannot be performed. Below is an example using the red plane: the symmetry can be performed.

• If the face of the surface has only one vertex on the chosen side, the result of the symmetry is a mirror surface.

### Copy and Paste a Symmetry

When copying and pasting a subdivision surface that is a symmetry, a dialog box opens:

• Click Yes to create a new subdivision surface.

• Click No to simply create a copy of the symmetry.

### Create a sharp edge

To create a sharp edge at the symmetry plane, the subdivision surface must intersect the plane and a sharp edge must already exist.