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 straight line; selecting the start and end points of an arc will produce a closed circle or ellipse but will constrain the location of the seam to be on that point).
 * 2) Highlighting of a drawing item is achieved by moving the mouse over the item and clicking the left-mouse-button.
 * 3) It is also possible to highlight all items inside a rectangle by clicking and dragging. When dragging from left to right (with any vertical motion), only the shapes which are entirely contained within the rectangle will be highlighted; in the other direction all shapes which intersect with the selection rectangle will be highlighted. This can be used to select only the vertices without selecting the edges, by dragging a small recangle around some vertices from left to right, as long as there are no edges which are fully contained within the rectangle.
 * 4) A highlighted item will change its color to green. (This color can be customized in )
 * 5) 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.
 * 6) Once you have two points highlighted, you can invoke the command using several methods:
 * 7) * Pressing on the constraint button in the toolbar.
 * 8) * Using the keyboard shortcut.
 * 9) * 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

As the names and  indicate, if both constrained vertices are free to move in any direction, the first one (first to be selected in the Gui) will remain fixed and the other one will move. In the presence of existing constraints, however, both edges may move.

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 an entire edge (0). Circles and Conics (ellipses) have a center point, but cannot be used to refer to the center point of a straight Lines. The combinations which are not possible with Coincident 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 using a Sketcher_ConstrainTangent.svgnt constraint between the two straight lines, or 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 using a Sketcher_ConstrainCoincident.svgident constraint to merge the centers, and an Sketcher_ConstrainEqual.svg constraint applied to the edges of the two circles. For arcs, this will ensure both arcs are part of the same circle, while allowing them to have different starting and ending points.