Rotating sprite causes P2 collision fail [SOLUTION]

[onefatman]

Hey all,

 

Just a quick heads up to share an odd solution to a problem that had me tearing my head out for a couple of days, thought it may come in handy for anyone else in the same boat.

 

While working on a game where I was using P2 physics to detect a collision event between 2 physics bodies, I found that whenever the target sprite was rotated further than a certain amount all of my collision events would silently fail. I spent AGES trying different collision methods and none of them worked, when trying to implement a hack-ish workaround that I found on the forums (can't remember where) I realised that my collisions were now working fine, even though the method I was using returned something to the effect of "this doesn't work".

 

Long story short - I found that by resetting a physics body's AABB data after every change in rotation made the collision events work again:

//attackingThing.body.onBeginContact.add(reportSomething, this);//function rotateThing() {    //    thing.rotation += 1;    // reportSomething fails unless you do this    thing.body.data.updateAABB();}

If anyone can enlighten me as to what/why/how this works, that'd be awesome.

 

Phase on, dudes!

 

OFM

[wayfinder]

For a centered, circular shape this makes no difference, but the way the collision handler keeps cpu use low is to (in a first step, the broad phase) generate candidate pairs (of shapes that may collide) from the overlap of bounding boxes of ALL shapes in the world - that's a fairly cheap test to run so it can afford to do it every frame. If you have a shape that's not a centered circle and it rotates, it now has a different bounding box, and updateAABB stores these values. Your collisions were wonky because the handler was testing against the old bounding box.

 

I think setting the rotation should update the AABB automatically, but maybe only when the setter is used instead of setting the value directly? I don't actually know why it didn't happen in your case.

[onefatman]

Wait - I lied! And also omitted some information. Yep the shapes are polygons that I've created in PhysicsEditor, but also I was using this code to rotate them:

thing.angle = x;thing.body.angle = x;

My bad! Not sure if that's the reason that the error was appearing, or if it's just an alternate way of doing it that should have also worked?