# Sketcher helper constraint

## Overview

Helper constraint is a regular sketcher constraint that is needed as a part of a more complex constraint, but is exposed in user interface to assist dealing with redundancy. For example, for Snell's Law constraint, the two lines that represent rays of light need to be connected (coincident constraint), and the joint must lie on the interface (Point On Object constraint).

Helper constraints are added automatically when they are needed. The decision for if they are needed is currently made by evaluating the helper constraint error for current state of geometry (this may change in future versions). If the error is small enough, the constraint is considered to be unnecessary, and is not added. In some cases, this logic can lead to errors (the constraint can be satisfied by accident, which can easily happen when Sketcher Grid Snapping is on).

If this happens (a helper constraint is missing, and the required conditions is not satisfied otherwise), the complex constraint will be broken. It will do something, but the actual behavior is undefined. Such a broken constraint can be repaired by adding the missing helper constraint manually.

Helper constraints are currently required for:

- Constraint Tangent (in tangent-via-point mode; two point-on-object constraints are needed)
- Constraint Perpendicular (in perpendicular-via-point mode; two point-on-object constraints are needed)
- Constraint Angle (in angle-via-point mode; two point-on-object constraints are needed)
- Constraint SnellsLaw (coincident constraint and point-on-object constraint are needed)

## Scripting

When constraints requiring helpers are added from Python, no helper constraints are automatically added. One can replicate the automatic decision-making of the UI commands in a script by testing the following functions, specifically added for the purpose and used in the UI routines:

```
Sketch.isPointOnCurve(icurve,x,y)
```

isPointOnCurve tests if a virtual point, given by sketch coordinates `x,y` (float values), happens to satisfy a virtual point-on-object constraint - i.e. lies on curve specified by curve index `icurve`. Returns True if the point is on curve, and False if it doesn't.

```
Sketch.calculateConstraintError(iconstr)
```

calculateConstraintError evaluates an error function of a constraint specified by its index `iconstr` in the sketch. If there is only one error function in the constraint, the return value is the signed return value of the error function. If there is more than one error function associated with the constraint (i.e. the constraint removes more than one degree of freedom), the return value is the RMS of all the error functions (always positive).

## Version

Helper constraints were introduced in v0.15.4387

**The tools:**New sketch, Edit sketch, Leave sketch, View sketch, View section, Map sketch to face, Reorient sketch, Validate sketch, Merge sketches, Mirror sketch

**Sketcher geometries:**Point, Line by 2 point,**Create an arc,**Arc, Arc by 3 Point,**Create a circle**, Circle, Circle by 3 Point,**Create a conic**, Ellipse by center, Ellipse by 3 points, Arc of ellipse, Arc of hyperbola, Arc of parabola,**Create a B-spline**, Create B-spline, Create periodic B-pline, Polyline (multiple-point line), Rectangle,**Create regular polygon**, Triangle, Square, Pentagon, Hexagon, Heptagon, Octagon, Create Regular Polygon, Slot, Fillet, Trimming, Extend, External Geometry, CarbonCopy, Construction Mode

**Sketcher constraints****Geometric constraints**Coincident, Point On Object, Vertical, Horizontal, Parallel, Perpendicular, Tangent, Equal Length, Symmetric, Constrain Block**Dimensional constraints**Lock, Horizontal Distance, Vertical Distance, Distance, Radius, Internal Angle, Snell's Law, Internal Alignment, Toggle reference/driving constraint,

**Sketcher tools**Select solver DOFs, Close Shape, Connect Edges, Select Constraints, Select Origin, Select Vertical Axis, Select Horizontal Axis, Select Redundant Constraints, Select Conflicting Constraints, Select Elements Associated with constraints, Show/Hide internal geometry, Symmetry, Clone, Copy, Move, Rectangular Array, Delete All Geometry, Delete All Constraints

**Sketcher B-spline tools**Show/Hide B-spline degree, Show/Hide B-spline control polygon, Show/Hide B-spline curvature comb, Show/Hide B-spline knot multiplicity, Convert Geometry to B-spline, Increase degree, Increase knot multiplicity, Decrease knot multiplicity

**Sketcher virtual space**Switch Virtual Space

**Getting started****Installation:**Download, Windows, Linux, Mac, Additional components, Docker, AppImage, Ubuntu Snap**Basics:**About FreeCAD, Interface, Mouse navigation, Selection methods, Object name, Preferences, Workbenches, Document structure, Properties, Help FreeCAD, Donate

**Help:**Tutorials, Video tutorials**Workbenches:**Std Base, Arch, Draft, FEM, Image, Inspection, Mesh, OpenSCAD, Part, PartDesign, Path, Points, Raytracing, Reverse Engineering, Sketcher, Spreadsheet, Start, Surface, TechDraw, Test Framework, Web**Deprecated or unmaintained workbenches:**Complete, Drawing, Robot

**Hubs:**User hub, Power users hub, Developer hub