Extrapolating Surfaces

In this task you will learn how to extrapolate a surface, that is add an extra surface to the initial one while taking continuity constraints into account.

Open the FreeStyle_Part_21.CATPart document.

  1. Select a surface boundary.
    A manipulator is displayed.

  2. Click the Styling Extrapolate icon .


The Extrapolation dialog box is displayed. Two options are available:
  • Tangential: the surface is extended along the tangent to the surface on the selected boundary and of a given distance (G1 continuity).
  • Curvature: the surface is extrapolated by the specified value, complying with the surface curvature (G2 continuity). No manipulator is displayed.


  1. Click Tangential and pull on the manipulator till you reach 60 in the Length field of the Extrapolation dialog box.
    The extrapolation is pre-visualized and dynamically evolves as you modify the extension length.

You could just as well enter the value directly in the Length field.


  1. Click OK.
    The extrapolated surface is created.
    Here are two views of the extrapolated surface, illustrating the tangency constraint on the selected edge.

Similarly, if an edge is selected, and the Curvature option active, the surface is extrapolated according its initial curvature:

With Curvature option

With Tangential option

  • To extrapolate a set of surface boundaries, you need to select these, using the Ctrl-key, before clicking the Extrapolate icon .
  • The curvilinear length between the middle of the selected original surface edge and the middle of the extended surface is displayed.
  • Whether in Tangential or Curvature mode, check the Exact option if you want to switch to the exact mode. The figures below show a surface extrapolated when Exact mode is checked and unchecked.
    • when the Exact option is checked, computation is analytical and the resulting surface presents the same degree as the initial surface.
      Since you chose this constraint, there may be no possible result in some cases.
    • when the Exact option is not checked, the extrapolation is computed according to the geometric mode.

Exact mode not checked

Exact mode checked