Sketcher ConstrainCoincident

Description
Create a coincident constraint on the selected item

This constraint tool takes two points as its argument and serves to make the two points coincident. (Meaning to make them as-one-point).

In practical terms this constraint tool is useful when there is a break in a profile for example - where two lines end near each other and need to be joined - a coincident constraint on their end-points will close the gap.

Usage
As stated above, this tool takes two arguments - both are points. the command will cause the two points to become coincident and be replaced by a single point.
 * 1) First, it is necessary to highlight two distinct points. ( this will not work if, for example, you attempt to select the start and end point of the same line).
 * 2) Highlighting of a drawing item is achieved by moving the mouse over the item and clicking the left-mouse-button.
 * 3) A highlighted item will change its color to green. (This color can be customized in )
 * 4) Subsequent items can be highlighted by repeating the above procedure(s).  it's unnecessary to hold-down any special key like  to achieve multiple item selection in a drawing.
 * 5) Once you have two points highlighted, you can invoke the command using several methods:
 * 6) * Pressing on the constraint button in the toolbar.
 * 7) * Using the keyboard shortcut.
 * 8) * Using the entry in the top menu.

In order to make two points coincident, FreeCAD must out of necessity move one (or both) of the original points.

Scripting
The constraint can be created from macros and from the python console by using the following command:

where :
 * is a sketch object
 * is the number of the line, that will not move by applying the constraint
 * indicates which vertex of has to fulfill the constraint
 * is the number of the line, that will move by applying the constraint
 * indicates which vertex of has to fulfill the constraint

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

Alternatives to Coincident constraint
The two constrained items must be starting point (1) or end point (2) vertices, or center points (3). The coincident constraint cannot be used directly with edges (0), and straight lines do not have a center point. The other combinations can be emulated using other constraints:
 * The Sketcher_ConstrainSymmetric.svgtric constraint can be used to place an extremity on the midpoint of a straight line. This emulates 3 (center) for straight lines.
 * A midpoint-to-midpoint placement can be achieved by creating a new Sketcher_CreatePoint.svg and using two Sketcher_ConstrainSymmetric.svgtric constraints so that it lays on the midpoint of both edges. This emulates 3 (center) for straight lines.
 * Instead of using the values 1 or 2 (vertex) and 0 (edge), a vertex can be constrained to lie along on an edge using a Sketcher_ConstrainPointOnObject.svgOnObject constraint. Note that with this constraint, the point can lie anywhere on the full extension of a segment or curve (i.e. before the start point or outside of the end point).
 * Instead of using the values 0 (edge) and 0 (edge), a co-linear placement can be obtained by combining a Sketcher_ConstrainPointOnObject.svgOnObject constraint and a Sketcher_ConstrainParallel.svglel constraint
 * Instead of using the values 0 (edge) and 0 (edge), two edges can be made identical by using two Sketcher_ConstrainCoincident.svgident constraint, one for each pair of extremities.
 * Instead of using the values 0 (edge) and 0 (edge), two Sketcher_CreateCircle.svge can be made identical by creating a new Sketcher_CreateLine.svg and using two Sketcher_ConstrainCoincident.svgident constraint to place both centers on one extremity of the line, and two Sketcher_ConstrainPointOnObject.svgOnObject constraints to place the other extremity on the circumference of both circles. To get rid of the extra degree of freedom, a horizontal or vertical constraint can be used to lock the line in place.