How the Solver Works
Read the status pill to find out how much of your sketch is still undecided. A sketch is a set of geometry plus a set of relations between that geometry, and the solver works out from both where everything has to be.
Drawing puts shapes on the plane. Constraining tells the solver which of their freedoms you have decided about.
Degrees of freedom
Every point in a sketch starts with two freedoms: it can move in x, and it can move in y. Four corners carry eight between them. Each constraint removes one or more. Horizontal takes one from a line, coincident takes two from a point, a length dimension takes one. Draw a rectangle and the tool spends four of those eight for you on the spot, so the pill reads 4 rather than 8.
A constraint against a pattern copy costs nothing. The copy is handed to the solver with exactly as many relations as it has freedoms, so dimensioning to the fourth hole of a row leaves the count where it was: a sketch that was fully constrained stays fully constrained.
When the count reaches zero the sketch is fully constrained: there is exactly one arrangement that satisfies everything, and nothing can drift.
Reading the state
The status pill has three states:
| State | What it means | What to do |
|---|---|---|
| Under-constrained · N DOF | N freedoms remain; the shape can still be dragged | Fine while you work. Not fine when you are done |
| Fully constrained | Zero freedoms; one solution | Done — the shape can only change if you change a constraint |
| Over-constrained | The solver cannot satisfy everything you asked for, or it solved but reports constraints in conflict | Remove one of the conflicting chips |
Whichever of the three it is, the sentence is also always on screen. It sits at the bottom right of the viewport for as long as a sketch is open, so the one number that should steer your next click is not hidden behind a window you have to go and open. It only reads out — acting on a constraint happens on its chip, or in the Constraints window.
A fourth word turns up, but not in the pill: redundant. A redundant constraint is satisfiable and says nothing new, so the sketch still solves. It shows as an amber badge on the chip itself and as a count at the foot of the Constraints window. Usually that means you constrained the same thing twice by two routes.
Open a chip and the popover gives you the solver's verdict in full, in three flavours:
- Redundant — already implied by the rest, and the worker took it out, re-solved and checked that removing it frees no degree of freedom. Safe to delete, which is why an amber chip is the one that still deletes on a single click.
- Over-specified — part of it duplicates what the others say, but it is still holding a freedom. Delete it and the sketch goes back to under-constrained.
- Not applied — the solver has no equivalent for those operands, so the constraint holds nothing at all. A tangency between two straight lines is the usual example: what you meant there is parallel plus coincident.
Under-constrained is not an error. It is the normal state of a sketch you are still thinking in. Sketch loosely, then decide. What you should not do is leave a sketch under-constrained and then rely on it not moving, because the first upstream edit will move it.
Dragging is how you interrogate it
Drag a point and the solver moves everything that must move to keep every constraint true. What stays put is what you have decided about.
Use that as a diagnostic. If a shape distorts in a way you did not expect, the relation you thought you had is missing. If it refuses to move at all, it is fully defined.
The constraint chips
Every relation is a chip in the Constraints window, and an icon on the canvas at the place the constraint actually happens. Hover either to highlight the geometry it holds, and you will find the constraint that is fighting you, including the ones you never added by hand: the ones a snap left behind, and the ones a tool emitted when it drew a rectangle for you.
Clicking a chip opens it rather than deleting it, so you can read the verdict, watch the geometry it holds pulse, and then decide. Geometric Constraints has the rest of what a chip does.
Why bother
A fully defined sketch is what makes the rest of the parametric promise work. Change a number at the top of the tree and everything downstream regenerates in a way you can predict, because there is only one arrangement it can regenerate into. A sketch with freedoms left in it can regenerate into several, and the one it picks is not always the one you meant.
Before you build features on a sketch you expect to change later, check that the pill reads Fully constrained.
See also
- Geometric Constraints: the relations themselves
- Dimensions: the ones with a number in them
- Regions and Profiles: what a solved sketch produces