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+* Velocity after free updating may violate joint constraint,
+* Solve joint velocity to set velocity right,
+* body position may violate joint constraint by error accumulation and discrete time step simulation,
+* so body position must be corrected.
+* Distance joint solve method
+* joint anchor velocity is vp, body velocity is vb, body angular velocity is wb,the vector between body anchor
+* d(vp) = d(vb) + cross(d(wb),l)
+* I is the impulse which joint gives to body. I = m*d(vb), cross(I,l) = Interia*(wb)
+* the constraint is distance(anchor1-anchor2) = constant,it can apply this:
+* project((vp1 - vp2),(anchor1-anchor2)) = 0
+* so,we can solve it by the above equations.
int JointObject_SolveConstraintVelocity (PyJointObject *joint, double stepTime);