Concatenating Curves

This task explains how to concatenate a 3D multi-cell mono-domain FreeStyle curve or separate contiguous curves into a mono-cell curve.
The resulting curve is a curvature continuous curve (C2).
Open the FreeStyle_Part_22.CATPart document.
This document contains two join-type elements, and a set of connex curves, each identified by different colors.
  1. Click the Concatenate icon .
    The Concatenate dialog box is displayed.

  2. Set the tolerance value.


This value defines the maximum parametric deviation allowed between the selected elements and the resulting curve when concatenating. 

The Maximum deviation value displayed in the dialog box (see step 4) is computed as the standard distance analysis and therefore is not necessarily the minimum tolerance value to be set in order to concatenate the curves.

If the tolerance value you entered is not adequate, a message is issued prompting you to increase it. However, to help you, the minimum tolerance value necessary to obtain a result is then automatically displayed.
If you are using the Auto Update Tolerance mode, the minimum tolerance value is automatically modified; therefore no message is issued.

  1. Select the curves you wish to concatenate and click Apply.

Here we selected Join.2 from the specification tree, and set the tolerance value to 1.6.


A single curve is previewed, with its segment limits, provided the tolerance is compatible. If not, no concatenated curve is previewed, and the OK button is grayed.

  1. Click More>> in the dialog box.


  1. Check the Information option.
    The corresponding information is displayed on the resulting curve, that is:

  • N: number of control points

  • o: the curve order

  • s: the number of segments in the curve

  • the maximum deviation

In case you did not check the Information button, the maximum deviation is also indicated in the dialog box.


  1. Check the Auto Update Tolerance option to automatically update the tolerance value in case the value you set is too low.
    The minimum value to enable the concatenation is displayed and the concatenation is performed.

  1. Click OK in the Concatenate dialog box.
    A curve is created, the result of the concatenation of the elements making up the initial Join.


  • Once you have clicked Apply, if no resulting curve is displayed, increase the tolerance value.
  • Multi-selection is available using the Control key. This means you can select several Join elements for example, and each one will generate a concatenated curve.
    (Join.1 in green is still visible behind Curve.19, but you can notice the deviation due to the discontinuity of the initial element.)

  • Similarly, you can select several contiguous curves using the Control key, or a trap, to generate a single curve. 
    If the selected curves are not connex (if there is a gap between two curves), the Apply button is grayed, and you cannot create the concatenated curve.

Set of independent connex curves

Resulting concatenated curve

  • If the selected elements present a curvature discontinuity (as illustrated below), you will need to increase the tolerance value accordingly.
    In this case too, the maximum deviation will increase considerably.

Initial element presenting a discontinuity

Deviation onto a discontinuous element

  • Use the Converter Wizard prior to selecting non 3D NUPBS multi-cells curves for concatenation.
Available capabilities from the Dashboard are: keep original and furtive display.