Let us consider for a certainty angular hinge constraints. There are three of them. Two of them are equality constraints assuring that bodies have a common axis. And one is an inequality constraint limiting their relative angle of rotation aroud that axis. Let us consider a situation where there's only one iteration, so there's no need to accumulate an impulse, for simplicity.

So what do we do? We compule lagrange multipliers corresponding to a reaction force: f = -effMass * Cdot, where f and Cdot are three-dimensional vectors ('cause there are three constraints). Then we clamp f3 (let the inequality constraint be the third) to make sure that it does not

*suck*. And here the subtlety comes: that clamping leads to necessity of correction of another impulse components: f1 and f2. Indeed, f is a solution to

Code: Select all

```
[ K11 K12 K13 ] (f1) (Cdot1)
[ K21 K22 K23 ] (f2) = - (Cdot2)
[ K31 K32 K33 ] (f3) (Cdot3)
```

Code: Select all

```
[ K11 K12 K13 ] (f1') (Cdot1)
[ K21 K22 K23 ] (f2') = - (Cdot2)
(f3')
```

**Why?**I don't see any reasonable reason. We don't accumulate impulse when correct positions? So what? That correction is needed in that case too. Optimization of the price of a bit accuracy? It's tiny compared to the effective mass matrix computation.

Finaly, a bit more clearly, and less Box2D-related: whether position corrections should be managed the same way as velocities corrections (recompute "equalitiy impulses" f1 and f2 after clamping "inequality impulse" f3)? I'm quite sure they should be treated the same way, but Box2D code made me question it.