Unity3d: rotate camera around center of the screen with middle mouse button pressed

Question

I am working on an RTS game and would like to support the camera to rotate around the current center of the screen on mouse button pressed, just like in any RTS, strategy game.

I tried the MouseOrbitImproved script as shown on the unity wiki (copied below), but it needs a target object used by the camera to rotate around. My problem is that my target is always the current center of the screen.

Any idea how to achieve this, but only while the middle mouse button is pressed?

using UnityEngine;
using System.Collections;

[AddComponentMenu("Camera-Control/Mouse Orbit with zoom")]
public class MouseOrbitImproved : MonoBehaviour {

    public Transform target;
    public float distance = 5.0f;
    public float xSpeed = 120.0f;
    public float ySpeed = 120.0f;

    public float yMinLimit = -20f;
    public float yMaxLimit = 80f;

    public float distanceMin = .5f;
    public float distanceMax = 15f;

    private Rigidbody rigidbody;

    float x = 0.0f;
    float y = 0.0f;

    // Use this for initialization
    void Start () 
    {
        Vector3 angles = transform.eulerAngles;
        x = angles.y;
        y = angles.x;

        rigidbody = GetComponent<Rigidbody>();

        // Make the rigid body not change rotation
        if (rigidbody != null)
        {
            rigidbody.freezeRotation = true;
        }
    }

    void LateUpdate () 
    {
        if (target) 
        {
            x += Input.GetAxis("Mouse X") * xSpeed * distance * 0.02f;
            y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;

            y = ClampAngle(y, yMinLimit, yMaxLimit);

            Quaternion rotation = Quaternion.Euler(y, x, 0);

            distance = Mathf.Clamp(distance - Input.GetAxis("Mouse ScrollWheel")*5, distanceMin, distanceMax);

            RaycastHit hit;
            if (Physics.Linecast (target.position, transform.position, out hit)) 
            {
                distance -=  hit.distance;
            }
            Vector3 negDistance = new Vector3(0.0f, 0.0f, -distance);
            Vector3 position = rotation * negDistance + target.position;

            transform.rotation = rotation;
            transform.position = position;
        }
    }

    public static float ClampAngle(float angle, float min, float max)
    {
        if (angle < -360F)
            angle += 360F;
        if (angle > 360F)
            angle -= 360F;
        return Mathf.Clamp(angle, min, max);
    }
}

Answer 1

In your orbit function, where you determine the object to orbit around, raycast a ray from the center of the screen and orbit around the collision:

public LayerMask groundLayerMask; // mask to choose orbitable layers

...

Ray screenRay = Camera.main.ScreenPointToRay(new Vector3(Screen.width/2f, Screen.height/2f, 0f));` 

RaycastHit hitInfo;
Physics.Raycast(screenRay, out hitInfo, Mathf.Infinity, groundLayerMask.value);

Vector3 orbitPosition = hitInfo.point;

You can check if the middle mouse button is pressed with Input.GetMouseButton(2). Before orbiting, check it and only detect mouse input changes if its held down.

In the MouseOrbitImproved code it might look like this in the LateUpdate method:

void LateUpdate () 
{
    // Skip mouse input here if middle mouse button is not pressed
    if (Input.GetMouseButton(2))  
    { 
        x += Input.GetAxis("Mouse X") * xSpeed * distance * 0.02f;
        y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;
        y = ClampAngle(y, yMinLimit, yMaxLimit);
    }

    Quaternion rotation = Quaternion.Euler(y, x, 0);

    distance = Mathf.Clamp(distance - Input.GetAxis("Mouse ScrollWheel")*5, distanceMin, distanceMax);

    Ray screenRay = Camera.main.ScreenPointToRay(new Vector3(Screen.width/2f, Screen.height/2f, 0f));` 

    RaycastHit hitInfo;
    Physics.Raycast(screenRay, out hitInfo, Mathf.Infinity, groundLayerMask.value);

    Vector3 orbitPosition = hitInfo.point;

    Vector3 negDistance = new Vector3(0.0f, 0.0f, -distance);
    Vector3 position = rotation * negDistance + orbitPosition ;

    transform.rotation = rotation;
    transform.position = position;
}

In the class fields, get rid of public Transform target; and add public LayerMask groundLayerMask; and set it in the inspector to only collide with whatever layer your ground is.