Sketcher ConstrainInternalAlignment

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Sketcher ConstrainInternalAlignment.svg Sketcher ConstrainInternalAlignment

Menu location
Sketch → Sketcher constraints → Constrain internal alignment
Default shortcut
Introduced in version
See also
Sketcher Show/Hide Internal Geometry, Sketcher Ellipse by Center


The InternalAlignment constraint aligns lines and points to particular places of a complex sketcher element (there is just one "complex" element so far, the Ellipse).

For Sketcher CreateEllipseByCenter.svg Ellipse and Sketcher CreateArcOfEllipse.svg Arc of ellipse, it supports constraining lines to become major and minor diameters, and constraining Sketcher CreatePoint.svg points to positions of ellipse's foci.

The constraint is for experienced users because its usage is not as straight forward as for the other constraints. There is a helper tool called Sketcher RestoreInternalAlignmentGeometry.svg Show/Hide internal geometry to avoid the need to invoke the InternalAlignment constraint manually.

Operation on Ellipse

  1. Select elements to be aligned and an ellipse. The ellipse must be selected last. Accepted are up to two lines and up to two points.
  2. Invoking the constraint can be done several ways:
    • Pressing the Sketcher ConstrainInternalAlignment.svg Constrain internal alignment button in the toolbar.
    • Using the Sketch → Sketcher constraints → Sketcher ConstrainInternalAlignment.svg Constrain internal alignment entry from the top menu.

The first line that was selected gets aligned to become ellipse's major diameter (but if it is not occupied already by another line, otherwise it will become minor diameter). The second line is aligned to become minor radius. The lines are automatically switched to construction.

Likewise, the first point is constrained to become the first unoccupied focus, and the second point goes to the other focus.

Note: By default new ellipses have an internal construction geometry. When this defines the ellipse already completely, you cannot directly use the InternalAlignment constraint. You first need to delete the construction geometry or parts of it. In case you don't see the construction geometry, select the ellipse and use the tool Sketcher RestoreInternalAlignmentGeometry.svg Show/Hide internal geometry to make it visible.


Sketch.addConstraint(Sketcher.Constraint('InternalAlignment:EllipseMajorDiameter', index_of_line, index_of_ellipse))
Sketch.addConstraint(Sketcher.Constraint('InternalAlignment:EllipseMinorDiameter', index_of_line, index_of_ellipse))
Sketch.addConstraint(Sketcher.Constraint('InternalAlignment:EllipseFocus1', index_of_point, 1, index_of_ellipse))
Sketch.addConstraint(Sketcher.Constraint('InternalAlignment:EllipseFocus2', index_of_point, 1, index_of_ellipse))


Sketch is a sketch object.
Number 1 in the focus calls stands for starting point of a point element (it is ignored).
The index_of_point argument must be a point, it cannot be used to e.g. denote an edge's extremity.

The Sketcher scripting page explains the values which can be used for index_of_line, index_of_point and index_of_ellipse and contains further examples on how to create constraints from Python scripts.