Collision between objects that technically don't touch each other

[nizbel]

Hello, I am having a problem with two objects colliding in the following situation:
Object 1 has a circle collider and moves in a vertical line, from top to bottom of the screen.
Object 2 has a box collider, standing still at the center of the screen.
Object 1 is positioned in a way that in the x-axis it’s close to Object 2, but the colliders never overlap. Object 1 is also the child of another object who moves horizontally to the left, on the FixedUpdate loop. In order to negate the horizontal movement, Object 1’s rigidbody has the same horizontal speed, to the right.

So what happens is that Object 1 moves vertically, but once it’s near Object 2, they collide. The transform position and the rigibody position never change in the x-axis, but for some reason the collision happens. If I remove the horizontal movements (from Object 1’s parent and Object 1’s rigidbody) the collision does not happen. I believe it is some sort of calculation in some part of the Physics engine, but so far I have no clue from the docs. Perhaps the collision checks use “both” positions of the object? Can anyone help?

[Kurt-Dekker]

How are you moving these things?

With Physics (or Physics2D), never manipulate the Transform directly. If you manipulate the Transform directly, you are bypassing the physics system and you can reasonably expect glitching and missed collisions and other physics mayhem.

This means you may not change transform.position, transform.rotation, you may not call transform.Translate(), transform.Rotate() or other such methods, and also transform.localScale is off limits. You also cannot set rigidbody.position or rigidbody.rotation directly. These ALL bypass physics.

Always use the .MovePosition() and .MoveRotation() methods on the Rigidbody (or Rigidbody2D) instance in order to move or rotate things. Doing this keeps the physics system informed about what is going on.

Collisions in high speed without unity physic / rigidbody

You say you're not using 2D physics but you're using queries therefore obviously colliders. This IS using 2D physics therefore you need to use it correctly so here are some basic rules: Don't ever modify the Transform if using 2D physics components If you want a 2D Collider to move then add a Rigidbody2D and use its API to move it; many ways to do this. Rigidbody2D don't make it "unresponsive". If you want explicit control then set the body-type to Kinematic; it's what it's for. When you qu…

OnCollisionEnter2D not being called

That’s not related to body-type. You should never drive via the Transform. The main thing to understand here is that the Rigidbody2D role is to write to the Transform after the physics simulation, not the other way around. It acts as a proxy to the Transform because you want physics behaviour. Does “it work” if you write to the Transform and force the Rigidbody2D to read from the Transform? Yes! It works like that because otherwise it’d be reported as a bug and so we’re forced to “support” it.…

[nizbel]

I am moving the Objects using their rigidbodies. Once they are inserted into the scene the transform position is set and the rigidbody receives a velocity.
But while they move, I also have their parent object, which is sort of like a “layer” of elements that should be passing through the player. This parent object always moves on a constant speed to the left, with a transform.position += Vector3.left * speed * Time.fixedDeltatime; on the FixedUpdate loop.

[MelvMay]

That’s not moving with a Rigidbody2D, that’s stomping over the Transform. The whole point of the Rigidbody2D is to simulate physics and write that post to the Transform. Use the Rigidbody2D API to cause movement, never modify the Transform.

[nizbel]

So, in order to achieve the effect of elements in the background colliding with each other and also moving constantly to the left as if in a conveyor belt, could I use rigidbody2d’s MovePositon()? If I get the current position of the transform, the current velocity of the rigidbody and add the constant speed that objects in this layer should be moving (conveyorBeltSpeed), would I still be preserving the calculations made during the physics update?

rb.MovePosition(rb.position + Vector2.left * conveyorBeltSpeed * Time.fixedDeltaTime + rb.velocity * Time.fixedDeltaTime);

[MelvMay]

The scenario doesn’t matter. Use the Rigidbody2D API and all will be well.