Check if a 90 degree rotation will cause a collision

Question

I'm practicing Unity by developing a 2D game where the player has to stack tetrominoes falling from the sky. The problem I've ran into is that upon a 90 degree rotation of the falling piece, the piece sometimes overlaps with the existing tower causing two pieces to be stuck together.

I would like to check if this overlap will happen before rotating my piece.

I'm aware that such a problem would probably be solved by rotating a copy "shadow" object and checking for collision and yet I have found no way to check if an object is colliding directly after spawning it. I'm also aware of the Physics2D.OverlapBox method but I'm really looking for a way to predict collision with a more complicated 2D shape.

Current code for the shadow collider attempt:

GameObject shadow = new GameObject();

shadow.transform.position = currentTetrisPiece.transform.position;
shadow.transform.rotation = currentTetrisPiece.transform.rotation;

PolygonCollider2D shadowCollider = shadow.AddComponent<PolygonCollider2D>();

PolygonCollider2D currentCollider = currentTetrisPiece.GetComponent<PolygonCollider2D>();
shadowCollider.isTrigger = true;
shadowCollider.points = currentCollider.points;

shadow.transform.RotateAround(rotationPoint, new Vector3(0, 0, 1), 90);

Physics2D.SyncTransforms();

bool collided = shadowCollider.IsTouchingLayers();

if (!collided)
{
    currentTetrisPiece.transform.RotateAround(rotationPoint, new Vector3(0, 0, 1), 90);
}

Answer 1

From: Collider2D.IsTouchingLayers

"It is important to understand that checking if colliders are touching or not is performed against the last physics system update i.e. the state of touching colliders at that time. If you have just added a new Collider2D or have moved a Collider2D but a physics update has not yet taken place then the colliders will not be shown as touching. The touching state is identical to that indicated by the physics collision or trigger callbacks."

The physics loop doesn't know about the change when you check for collision. You should wait for a physics update or simulate it.

Physics.Simulate - for simulating physics manually.

You should also check out this thread that started with the same problem as yours: Instant Collision Checking.

Rigidbody.SweepTest - This is probably what you are looking for, but the problem there is no analog for rotation. Someone here: SweepTest for rotation came up with a hack to achieve this but I haven't tested it myself, so I will copy it here.

Collider2D.Cast - a 2D alternative to SweepTest.

[by "forcepusher"]

var rigidbodyCompoennt = GetComponent<Rigidbody>();
var initialRotation = rigidbodyCompoennt.rotation;
rigidbodyCompoennt.rotation = rotationSweep;
var sweepHits = rigidbodyCompoennt.SweepTestAll(new Vector3(0f, 0.01f, 0f));
rigidbodyCompoennt.rotation = initialRotation;

The main idea is to rotate your object, then after a sweep test with unnoticeable movement - reset rotation back to normal.