Applying mirror constraints while curve editing

While performing curve editing using its control points, it is possible to apply constraints that will control the profile shape with respect to its virtual mirror. This would be useful in those designing processes where the symmetry is obtained by developing only half of the part/profile and then mirroring it to get the complete part/profile. The Mirror mode functionality enables you to achieve perfect symmetry while maintaining the right continuity.

Take the example of the curve shown below. To apply a mirror constrain about the YZ-planeY-axis, you can proceed as follows:

  1. For the purpose of visualization, activate the virtual mirror of the curve about the plane on which you intend to apply the mirror constraint. To do so, use the Virtual Mirror command.

    If need be, change the virtual mirror plane by using the Position: option in the Document Properties-View-Advanced category. For this tutorial, it has been set to ZY. The virtual mirror will appear as shown below.



  2. Start the Modify Curve through Control Points command on the curve to be edited.

  3. You can select the Mirror mode check box to apply the mirror constraint on the curve (as per the default settings). However, if there is a need to change the default settings, it is advisable to select this check box after making the new settings.

  4. In the Set On drop-down list, select the position of the mirroring planeaxis.
    XY planeX Axis The XY-plane of the coordinate system is the mirroring planeThe X-axis of the coordinate system is the mirroring axis
    XZ planeY Axis The XZ-plane of the coordinate system is the mirroring planeThe Y-axis of the coordinate system is the mirroring axis
    YZ planeAxis The YZ-plane of the coordinate system is the mirroring planeYou need to define a custom axis to act as the mirroring axis
    Mirror plane The virtual mirror plane is the mirroring plane
    Plane You need to define a custom plane to act as the mirroring plane by using the Reference Plane options. See the "How to define a plane" document for details.

    For the purpose of this illustration, it has been set to YZ planeY Axis.

  5. In the Continuity drop-down list, select the kind of continuity constraint you want to apply on the curve:
    Position Positional continuity is maintained between the original curve and its virtual mirror
    Tangent Tangential continuity is maintained between the original curve and its virtual mirror
    Smooth curvature Curvature continuity is maintained between the original curve and its virtual mirror

  6. Select the control points that you want to move to apply the mirror constraint. Note that it is necessary to select the first control point if it is not already lying on the mirror planeaxis. It will be automatically moved when the constraint is applied.



    Note: To achieve the mirror constraint, the first control point HAS to be present over the mirror planeaxis. Hence its selection is necessary.

  7. Additional options are available under the Advanced Options node.

    The Minimize drop-down list enables you to define the criteria as per which the control points are moved to achieve the specified constraint:
    Max. distance of point displacement The control points are moved in such a way that their maximum displacement is kept to a minimum. As a result the displacement of each control point is the same. This is the default option.
    Average distance of point displacement The control points are moved in such a way that average of individual displacements is kept to a minimum. As a result the displacement is minimal for all control points except the one at the extremity of the selection.
    Average distance of curves The control points are moved in such a way that average distance between the modified and original curve is kept to the minimum. This needs more computation time.

  8. The Projected continuity position option primarily controls the movement of the first control point.
  9. After making the settings, select the Mirror mode check box. The selected control points will be moved to achieve the applied constraint. In this illustration, the curve maintains a tangential continuity with its virtual mirror.


    The virtual mirror curve has been highlighted in green for visualization only.

  10. Due to the tangency mirror constraint, when the control points are further moved, the tangency is till maintained with its virtual mirror.

Note
If the mirror constraint cannot be applied under the prevailing conditions, then the Show Warnings () button is available in the selection list. On selecting it, a warning will be displayed to inform you about the blocking issue.

In the following situation the Smooth curvature continuity has been applied as a result of which the (grey) curvature plot is G1 continuous with its virtual mirror. A comparison is shown with the (yellow) curvature plot for Tangent continuity.


Grey plot - Curvature continuity, Yellow plot - Tangent continuity