Box collides differently on left and right sides about matter-js HOT 3 CLOSED

Can you try adding the following after you set the render options:

engine.broadphase.current = 'bruteForce';

This will swap the bruteforce broadphase in rather than the default grid broadphase.

I tested this and it seems to prevent the problem, so it indicates an issue with the grid broadphase.
I'll investigate and try get it fixed, thanks for reporting this!

from matter-js.

I does not look like changing to the bruteForce broadphase works around the problem; I can still reproduce with this method.

By simply adding the line it looks like it fixes the problem, but I think it is changing some of the collision ordering in SAT.collides() which has the result of masking the bug (from what I can tell, of course, a little on this below).

Using the above code, change the rectangle adding order to this:

Matter.World.add(engine.world, Matter.Bodies.rectangle(380.5, 25.5, 102, 102, { isStatic: false, density: 1.0, friction: 0.0, restitution: 1.0 }));
Matter.World.add(engine.world, Matter.Bodies.rectangle(285.5, 323.5, 102, 102, { isStatic: true, density: 1.0, friction: 0.0, restitution: 1.0 }));
Matter.World.add(engine.world, Matter.Bodies.rectangle(193.5, 25.5, 102, 102, { isStatic: false, density: 1.0, friction: 0.0, restitution: 1.0 }));

You'll see this still reproduces the problem when using the bruteForce broadphase.

Side information -- I originally discovered this because on a simpler example it looked like the restitution was wrong with the bruteForce method. Try these two add lines:

Matter.World.add(engine.world, Matter.Bodies.rectangle(321.5, 337.5, 383, 17, { isStatic: true, density: .5, friction: 1, restitution: 0, angle: 0}));
Matter.World.add(engine.world, Matter.Bodies.rectangle(300, 67, 102, 102, { isStatic: false, density: 0.5, friction: 1, restitution: 0.8, angle: 0}));

It doesn't bounce at all. In fact, if you move the box over to the left, it doesn't do a flat bounce:

Matter.World.add(engine.world, Matter.Bodies.rectangle(321.5, 337.5, 383, 17, { isStatic: true, density: .5, friction: 1, restitution: 0, angle: 0}));
Matter.World.add(engine.world, Matter.Bodies.rectangle(200, 67, 102, 102, { isStatic: false, density: 0.5, friction: 1, restitution: 0.8, angle: 0}));

However, changing the ordering makes it bounce as expected:

Matter.World.add(engine.world, Matter.Bodies.rectangle(300, 67, 102, 102, { isStatic: false, density: 0.5, friction: 1, restitution: 0.8, angle: 0}));
Matter.World.add(engine.world, Matter.Bodies.rectangle(321.5, 337.5, 383, 17, { isStatic: true, density: .5, friction: 1, restitution: 0, angle: 0}));

It looks like the difference is that this line is being called in SAT.collides():

        if (Vector.dot(collision.normal, Vector.sub(bodyB.position, bodyA.position)) > 0) {
            collision.normal = Vector.neg(collision.normal);
        }

And this is due to bodyA and bodyB just being swapped when passed into the function. The ordering doesn't seem to matter with the grid broadphase, but let me know if you'd like me to file any individual bugs on these.

from matter-js.

Interesting, thanks for providing these cases, I'll take a look when I can.

from matter-js.