Object only moving small amount in dragged direction [Unity 2D isometric]

Question

So I'm currently working on an isometric 2D game and I'm trying to drag objects with the mouse. I've added the script below after following some tutorials but the object only moves a little bit in the direction it's being dragged. I have no idea why the object is not just following the mouse's coordinates but if you need any other info I don't mind providing.

private void OnMouseDown()
{
    mouseOffset = MouseWorldPos() - (Vector2)transform.position;
}

private void OnMouseDrag()
{
    transform.position = MouseWorldPos() - mouseOffset;
}

private Vector2 MouseWorldPos()
{
    Vector2 mousePos = Input.mousePosition;
    return Camera.main.ScreenToViewportPoint(mousePos);
}

Answer 1

why do you convert to Viewport coordinates?

Viewport space is normalized and relative to the camera. The bottom-left of the camera is (0,0); the top-right is (1,1). The z position is in world units from the camera.

=> You first do

mouseOffset = MouseWorldPos() - (Vector2)transform.position;

which can maximum be a position 0 to √(2) away from the transform.position.

Then you do

transform.position = MouseWorldPos() - mouseOffset;

which basically keeps it at your original position +- √(2) in any direction.

---

You probably would rather want to do something like e.g.

private readonly struct DragInfo
{
    public Plane Plane { get; }
    public Vector3 StartHitPoint { get; }
    public Vector3 OriginalPosition { get; }

    public DragInfo(Plane plane, Vector3 startHitPoint, Vector3 originalPosition)
    {
        Plane = plane;
        StartHitPoint = startHitPoint;
        OriginalPosition = originalPosition;
    }
}

private DragInfo? dragInfo;

private Camera mainCamera;

private void OnMouseDown()
{
    if(!mainCamera) mainCamera = Camera.main;

    var ray = mainCamera.ScreenPointToRay(Input.mousePosition);

    var plane = new Plane(-mainCamera.transform.forward, transform.position);
    if(!plane.Raycast(ray, out var hitDistance)) return;
    
    var startPoint = ray.GetPoint(hitDistance);

    dragInfo = new DragInfo(Plane, startPoint, transform.position);
}

private void OnMouseDrag()
{
    if(!dragInfo.HasValue) return;

    var ray = mainCamera.ScreenPointToRay(Input.mousePosition);

    if(!dragInfo.Plane.Raycast(ray, out var hitDistance)) return;

    var currentPoint = ray.GetPoint(hitDistance);

    var delta = currentPoint - dragInfo.StartHitPoint;
    transform.position = dragInfo.OriginalPosition + delta;
}

private void OnMouseUp()
{
    dragInfo = null;
}