Explosive tangent impulses causes extreme rotation

Torsten · 2014-06-20T07:26:17

Hi there, i got my html5/javascript 2D Sequential-Impulse based physics engine working - yay - but got issues with some weird behaviours :-( It looks stable, but contacts creates explosive tangent impulses which increases the angular velocity. Here are a video of some stacking circles which shows the weird effect (after forcing the collapse, the circles rotation goes faster and faster. Another video showing the issue on a smaller scale: I nearly use the same approach as box2d - except i added the position correction directly to the velocity bias. The friction code is identical and should be a direct port with one minor difference: i calculate the tangent normal once in the contact initialization - because the normal does not change for each contact/frame. Source for contact solver + initialization: http://pastebin.com/70pz2YXG No idea what causing these problems. I also tried to use a static/dynamic friction method, but had some more worse effects. Would be really great, if you can help me to identify the issue. Thanks in regards, Final

Math and Physics Programming

Started by Finalspace June 17, 2014 08:51 PM

11 comments, last by Finalspace 10 years, 8 months ago

ferrous

6,164

June 19, 2014 08:00 PM

Don't get too discouraged, if you are doing it for the purposes of learning a physics engine, I think the current path you are heading down is completely fine, and a little frustration and debugging are normal and part of the process.

On the other hand, if your desire is to just get the game part started, then I'd probably just expand box2d lite.

Finalspace

Author

1,222

June 20, 2014 07:08 AM

Oh it seems i fixed it. The relative velocity for angular rotation was incorrect - now it works :-)

Original box2d lite:

// Relative velocity at contact

dv = b2->velocity + Cross(b2->angularVelocity, c->r2) - b1->velocity - Cross(b1->angularVelocity, c->r1);

my old impl:

var tempVec = Vec2Pool.get();

var rva = Vec2Pool.get();

var rvb = Vec2Pool.get();

math.vec2CrossSV(tempVec, bodyA.angularVelocity, contact.rA);

math.vec2Add(rva, bodyA.velocity, tempVec);

math.vec2CrossSV(tempVec, bodyB.angularVelocity, contact.rB);

math.vec2Sub(rvb, bodyB.velocity, tempVec);

math.vec2Sub(relVelocity, rvb, rva);

new working impl:

var tmp = Vec2Pool.get();

math.vec2CrossSV(tmp, bodyB.angularVelocity, contact.rB);

math.vec2Add(relVelocity, bodyB.velocity, tmp);

math.vec2Sub(relVelocity, relVelocity, bodyA.velocity);

math.vec2CrossSV(tmp, bodyA.angularVelocity, contact.rA);

math.vec2Sub(relVelocity, relVelocity, tmp);

Javascript does not support operator overload - therefore i needed to make this ugly code -.-

My youtube channel: https://www.youtube.com/user/F1nalspace

Finalspace

Author

1,222

June 20, 2014 07:26 AM

Ah i am really happy right now, now i can go to the next step, implement contact caching + warm starting, better broadphase and finally polygon bodies.

If you want to see my progress, you can check my youtube channel frequently - i upload there things i have made - but mostly in javascript

Thread can be closed.

My youtube channel: https://www.youtube.com/user/F1nalspace

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