Detecting Vector3 Movement speed.

Questions & Answers
1

Hello, I’m currently making a space type movement system.

My current movement script so far is stated below.

        if (Input.GetKey(KeyCode.W))
            rigidbody.AddRelativeForce (Vector3.forward * speed);
 
        if (Input.GetKey(KeyCode.S))
 			rigidbody.AddRelativeForce (Vector3.back * speed);
 
        if (Input.GetKey(KeyCode.D))
            rigidbody.AddRelativeForce (Vector3.right * speed);
		
		if (Input.GetKey(KeyCode.A))
			rigidbody.AddRelativeForce (Vector3.left * speed);
		
		if (Input.GetKey(KeyCode.Space))
			rigidbody.AddRelativeForce (Vector3.up * speed);
		
		if (Input.GetKey(KeyCode.LeftControl))
			rigidbody.AddRelativeForce (Vector3.down * speed);
 
        rigidbody.AddRelativeTorque ((Input.GetAxis("Vertical"))*turnSpeed,0,0);
		rigidbody.AddRelativeTorque (0,(Input.GetAxis("Horizontal"))*turnSpeed,0);

So far this works perfectly for what I want it for. Now i’m also trying to make it so when a player lets go of the movement the ship auto stops.

For example.

If the ship is moving foward, engage front thrusters to push it back.

I have looked all over and can not find a way to detect if Vector3.foward speed is bigger than 0.

Can anybody help me with this?

Thanks.

3

Dot products (http://en.wikipedia.org/wiki/Dot_product) are useful for this! A useful property of dot products is:

A · B = ||A|| ||B|| cosθ

Where θ is the angle in between A and B.

So, if we take the ship’s velocity vector and take the dot product with the forward vector, we very conveniently get a scalar value that represents how fast the ship is moving in the forward direction (note: this works out because Vector3.forward is normalized, so ||B|| in the equation above is 1, and nicely drops out of the equation).

float forwardSpeed = Vector3.Dot(rigidbody.velocity, Vector3.forward);
if (forwardSpeed > 0) 
{
    // going in the forward direction forwardSpeed units / second!
    // better slow down!
}
else if (forwardSpeed < 0)
{
    // we're going backward at ||forewardSpeed|| units / second!
}
2

function Start(){
rigidbody.drag = 1;
rigidbody.angularDrag = 1;
}

Drag is used to stop a rigidbody when no force is being applied to it.