Get rigidbody.velocity in OnCollisionEnter

Questions & Answers
1

Hi, please if anybody can help with this.

I am trying to make a customized bounce effect that can makes a 2d rigidbody always bounce off along 45 degree by the collision normal when colliding. (not like the 2d physics material that simply bonce objects reflectively which will invert a perpendicular incoming collider, and this is what I try to avoid.)

The problem is the rigidbody2d.velocity in OnCollisionEnter2D is always return Vector2.zero.

So,how do I get the income direction/velocity of a collision? Are there any official ways to deal with this?

2

I have figured out this myself, the answer is the “collision.relativeVelocity”.
And I wish unity can supply more customizable physics material that we can much easily setup limitation for bouncing angle or apply more complex bouncing trajectory/behaviour.

Anyway, Here is my solution of customized bounce, which divides situations in two ways, 1-) The inDir more parallel to impact plane, i.e., the angle between inDir and normal is larger than 45 degree; Use simple Refection . 2-) The inDir more perpendicular to impact plane,i.e. the angle between inDir and normal is smaller than 45 degree. Determine on which side (Left or Right) of normal and then apply a perfect 45 degree refection;

function OnCollisionEnter2D(collision:Collision2D){
	var dirIn:Vector2; // Direction In
	var a0:float; //angle to normal
	var dirOut:Vector2; //Direction Out
	var dir3:Vector3;
	
	var check:int; //Check incoming direction on which side of normal, left or right or parallel 
	
	var rotateDir:Vector3;

	dirIn = collision.relativeVelocity.normalized;//***THIS IS WHAT I NEED !***/
	a0 =Vector3.Angle(-collision.contacts[0].normal , dirIn);
	
	if(a0> 45.0){ //direction more parallel to the impact plane (more perpendicular to the normal), use Reflect will be good
		dir3 = Vector3.Reflect(dirIn, collision.contacts[0].normal );
		dirOut.x = dir3.x;
		dirOut.y = dir3.y;
	}else{ //direction more parallel to the normal, we need to avoid inverse or concave outcome, the reflect out angle restrict to 45 degree 
                
		check = CheckLeftRight(-dirIn, collision.contacts[0].normal);
		if(check == 0){///Right
			rotateDir = Quaternion.Euler(0,0,-45.0) * collision.contacts[0].normal;

		}else if(check == 1){//Left
			rotateDir = Quaternion.Euler(0,0,45.0) * collision.contacts[0].normal;
		}else{//Parallel, randomly choose left or right
			if(Random.value > 0.5){
				rotateDir = Quaternion.Euler(0,0,-45.0) * collision.contacts[0].normal;
			}else{
				rotateDir = Quaternion.Euler(0,0,45.0) * collision.contacts[0].normal;
			}	
		}
		
		dirOut.x = rotateDir.x;
		dirOut.y = rotateDir.y;
	}

	rigidbody2D.velocity = Vector2.zero;
	rigidbody2D.AddForce(300.0* dirOut); //Deploy the bouncing force 
	
}


function CheckLeftRight(v0:Vector2, v1:Vector2):int{
	var dot:float;
	var dot2:float;
	
	var ccwV1:Vector2 =  Quaternion.Euler(0,0,-90.0) * v1;
	dot = Vector2.Dot(v0, ccwV1);
	
	if(dot < 0){ //On the Right
		return 0;
	}else if(dot >0){ // On the Left
		return 1;
	}else{// forward or back
		dot2 = Vector2.Dot(v0, v1);
		if(dot2 >= 0){// same direction
			return 2;
		}else{ //inverse direction
			return 3;
		}	
	}
}