Constraints for Revolute Joint

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Hi everyone! I am currenty programming my own 2D physics engine. The constraint system was inspired by jbox2d. I've managed to implement different types of constraints such as distance joints, prismatic joints and revolute joints. All of them work, but I am currently facing a maths problem. I just don't understand how to find the Jacobians and correction impulses for the revolute joint. According to the source code of jbox2d, the velocity constraint for a revolute joint is Cdot(x) = v2 - v1, where indices refer to the bodies. -that's fine Now, the comments in the sourcecode say that the Jacobian is J = [-I -r1_skew I r2_skew] -could someone please tell me what is meant by "r1_skew"? I was thinking that r1_skew is a matrix so that w cross r = r1_skew dot r but I'm not entirely sure about it or how I would implement this in 2 dimensions Any help or links to explanations would be highly apreciated, Thanks in advance :) Jakob

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