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Question by Arshoon · Jul 02, 2014 at 08:55 PM · trajectory

Bullet trajectory based on cannon rotation angle

I'm a new Unity developer, and terrible at complicated math, I never got past algebra, so please excuse me. I have a 2D game with a cannon at the center of the screen that rotates around the Z axis. I want bullets fired to fly off with the trajectory angle the same as which way the gun is pointing. How can I turn the gun angle (which I see as a degree from 0 to 360 on the Z) to x and y values as a velocity to make my bullets fly in the right direction?

I Instantiate the bullet with the same rotation as the cannon, so I tried using the transform.position.forward, but that only changed the Z position.

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avatar image Graham-Dunnett ♦♦ · Jul 02, 2014 at 08:55 PM 0
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I hear cos and sin are good for converting angles into x and y lengths.

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Answer by robertbu · Jul 02, 2014 at 09:05 PM

The easiest solution is to use use a vector based on the transform. Most people implement a cannon/gun so when the rotation is 0.0 it is either facing right, or it is facing up. Let's assume it is facing right. You can do:

 Rigidbody2D.AddForce(amount * transform.right);

'transform.right' will the direction the right side of your object is facing in World space. If you really want to convert an angle into a direction, you can do it with Vector math. First, you need to establish a vector for rotation 0.0. Again this is typically either facing right or up. Let's assume for this one the direction is up. You can do:

 var dir = Quaternion.AngleAxis(angle, Vector3.forward) * Vector3.up;
 Rigidbody2D.AddForce(amount * dir);

Note that the use of 'Vector3.forward' means you are rotating around the 'Z' axis.

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