Mouse drag rotation script doesn't work when attached to OBJ model

Question

I am new to Unity. I imported an OBJ model and assigned materials to it.

I want to rotate it using the mouse in the game, using this script:

using UnityEngine;

public class MouseDragRotate : MonoBehaviour {
    float rotationSpeed = 0.2f;

    void OnMouseDrag()
    {
        float XaxisRotation = Input.GetAxis("Mouse X")*rotationSpeed;
        float YaxisRotation = Input.GetAxis("Mouse Y")*rotationSpeed;
        // select the axis by which you want to rotate the GameObject
        transform.RotateAround (Vector3.down, XaxisRotation);
        transform.RotateAround (Vector3.right, YaxisRotation);
    }
}

I tried to apply this rotation script to the OBJ asset in my Project folder, but I wasn't able to find an Add Component option in its Inspector.

So I dragged it to the Hierarchy panel to create an instance of the object in my scene. I applied my materials again to this instance.

Then I added the rotation script using the Add Component button in the Inspector.

But when running the game in play mode, I can't rotate it.

I verified that the script above works perfectly for the default Cube or Plane primitives in Unity, but not for this imported object.

How can I make this script work with my OBJ model?

Answer 1

Unity's OnMouseDown / OnMouseDrag / etc MonoBehaviour messages only work when the object has a collider attached.

This is because when you use these methods, under the hood, the engine fires a physics raycast each frame from the main camera, through the position of the mouse, to find out what object is under it, so it knows what object to fire these messages on.

Only colliders will be hit by that raycast — it ignores rendered meshes and such (otherwise the performance cost could get extreme: physics colliders are usually much simpler than detailed display geometry). This also gives you the ability to control which objects should block that raycast — like a solid wall — versus not — like a puff of smoke particles or a translucent glass window.