ScaleDrag.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ScaleDrag : MonoBehaviour
{
    Camera _cam;
    Transform _dragObject;
    Vector3 _localPos;
    Quaternion _localRot;
    Vector3 _localScale;
    float? _lenToWall;
    Vector3 _prevMouse;
    Vector3 _downMouse;

    private void Awake()
    {
        _cam = GetComponent<Camera>();
    }

    // Start is called before the first frame update
    void Start()
    {
        
    }

    // Update is called once per frame
    void Update()
    {
        if (Input.GetKey(KeyCode.W))
        {
            _cam.transform.position += _cam.transform.forward * Time.deltaTime;
        }
        if (Input.GetKey(KeyCode.S))
        {
            _cam.transform.position -= _cam.transform.forward * Time.deltaTime;
        }
        if (Input.GetKey(KeyCode.A))
        {
            _cam.transform.position -= _cam.transform.right * Time.deltaTime;
        }
        if (Input.GetKey(KeyCode.D))
        {
            _cam.transform.position += _cam.transform.right * Time.deltaTime;
        }

        Vector3 mousePos = Input.mousePosition;
        mousePos.z = 10f;
        var ray = _cam.ScreenPointToRay(mousePos);
        var prevRay = _cam.ScreenPointToRay(_prevMouse);
        var downRay = _cam.ScreenPointToRay(_downMouse);

        if (Input.GetMouseButtonDown(0))
        {
            _downMouse = mousePos;
            _dragObject = null;
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit))
            {
                if (hit.collider is SphereCollider)
                {
                    _dragObject = hit.transform;
                    _localPos = _cam.transform.worldToLocalMatrix.MultiplyPoint(_dragObject.position);
                    _localRot = Quaternion.Inverse(_cam.transform.rotation) * _dragObject.rotation;
                    _localScale = _dragObject.localScale;

                    _dragObject.GetComponent<SphereCollider>().enabled = false;
                    if (Physics.Raycast(ray, out hit))
                    {
                        _lenToWall = hit.distance;
                    }else{
                        _lenToWall = null;
                    }
                }
            }
        }else if (Input.GetMouseButton(0))
        {
            float dot = Vector3.Dot(prevRay.direction, ray.direction);
            if (dot < 1f)
            {
                Vector3 axis = Vector3.Cross(prevRay.direction, ray.direction).normalized;
                var rot = Quaternion.AngleAxis(Mathf.Acos(dot) * Mathf.Rad2Deg * 2f, -axis);
                _cam.transform.rotation = rot * _cam.transform.rotation;
            }

            if (_dragObject!=null)
            {
                _dragObject.position = _cam.transform.localToWorldMatrix.MultiplyPoint(_localPos);
                _dragObject.rotation = _cam.transform.rotation * _localRot;

                if (_lenToWall != null)
                {
                    RaycastHit hit;
                    if (Physics.Raycast(downRay, out hit))
                    {
                        float distScale = hit.distance / _lenToWall.Value;
                        Vector3 toObj = _dragObject.position - _cam.transform.position;
                        _dragObject.localScale = _localScale * distScale;
                        toObj *= distScale;
                        _dragObject.position = _cam.transform.position + toObj;
                    }
                }
            }


        }
        else if (Input.GetMouseButtonUp(0))
        {
            if (_dragObject != null)
            {
                _dragObject.GetComponent<SphereCollider>().enabled = true;
                if (_dragObject.GetComponent<Rigidbody>() == null)
                {
                    _dragObject.gameObject.AddComponent<Rigidbody>().useGravity = true;
                }
                _dragObject = null;
            }
        }

        _prevMouse = mousePos;
    }
}

錯覚を利用したオブジェクトスケールのテスト。