CameraBehaviour

using UnityEngine;
using System.Collections;

public class CameraBehaviour : MonoBehaviour {

    public Transform target;
    public float distance = 3.0f;
    public float minDistance = 1.0f;
    public float height = 2.0f;
    public float smooth = 3.0f;
    public float zoomSmooth = 25.0f;
    public LayerMask collideLayers;

    void LateUpdate () {
        //Early out if there's no target
        if (!target) {
            Debug.LogWarning ("The camera behaviour script doesn't have an target to follow");
            return;
        }

        //Calculate the position to go if there's nothing in front of camera
        Vector3 wantedPos = target.position;
        wantedPos = target.forward * (-distance) + wantedPos;
        wantedPos.y = target.position.y + height;

        //Check if there is something in front of camera and return hit info
        RaycastHit hit = new RaycastHit ();
        bool isColliding = Physics.Linecast (target.position,
                                             wantedPos,
                                             out hit,
                                             collideLayers);

        //Add a little less distance from collision point to avoid camera entering meshes
        hit.point += transform.TransformDirection (Vector3.forward) * 0.05f;

        //If there isn't anything in front of camera, then go to the calculate position
        if (!isColliding) {
            transform.position = Vector3.Slerp (transform.position,
                                                wantedPos,
                                                smooth * Time.deltaTime);
        }

        //Else if there is and the hit distance is grater than min distance, go to hit point
        else if (hit.distance > minDistance){
            transform.position = Vector3.Slerp (transform.position, hit.point ,zoomSmooth * Time.deltaTime);
        }

        //Else if there's something in front of camera and the distance is smaller than hit distance,
        //check if there's something above target, if there is, go to the hit point above target
        //else go (min distance * 2) up
        else {
            //Create a vector3 to check the collision up
            Vector3 upVector = target.position;
            upVector.y += minDistance * 2;

            isColliding = Physics.Linecast (target.position,
                                            upVector,
                                            out hit,
                                            collideLayers);

            hit.point -= transform.TransformDirection (Vector3.forward) * 0.12f;

            if (isColliding)
                transform.position = Vector3.Slerp (transform.position, hit.point ,zoomSmooth * Time.deltaTime);

            else
                transform.position = Vector3.Slerp (transform.position, upVector ,zoomSmooth * Time.deltaTime);

            //Draw linecast on editor
            Debug.DrawLine (target.position, upVector, Color.red);
        }

        //Always look at target
        transform.LookAt (target.position);

        //Draw linecast on editor
        Debug.DrawLine (target.position, wantedPos, Color.black);
    }
}