Unity3D How can I select multiple objects in 3D with a drag and select / lasso select? - unity3d

Question

I am struggling to find a good tutorial or informations that would allow me to select multiple objects in 3D in a user friendly manner.
So far, the best tutorial I found is this one : https://sharpcoderblog.com/blog/unity-3d-rts-style-unit-selection. The tutorial works by using the transform.position of the selectable objects and checking if it within the user's selection.
What I wish is to have the user be able to select a unit even if it is only partially within the user's selection such as most RTS games do ( both in 2D and 3D ).
One possibility would be to create a temporary mesh using the camera's clipping distances and the user's selection and then check for collisions but I was not able to find any tutorials using this method nor do I know if it is the best approach to the subject.

Answer 1

If I understand correctly you want to
somehow start a selection
collect every object that was "hit" during the collection
somehow end the collection
Couldn't you simply use raycasting? I will assume simple mouse input for now but you could basically port this to whatever input you have.
// Just a little helper class for an event in the Inspector you can add listeners to
[SerializeField]
public class SelectionEvent : UnityEvent<HashSet<GameObject>> { }
public class SelectionController : MonoBehaviour
{
// Adjust via the Inspector and select layers that shall be selectable
[SerializeField] private LayerMask includeLayers;
// Add your custom callbacks here either via code or the Inspector
public SelectionEvent OnSelectionChanged;
// Collects the current selection
private HashSet<GameObject> selection = new HashSet<GameObject>();
// Stores the current Coroutine controlling he selection process
private Coroutine selectionRoutine;
// If possible already reference via the Inspector
[SerializeField] private Camera _mainCamera;
// Otherwise get it once on runtime
private void Awake ()
{
if(!_mainCamera) _mainCamera = Camera.main;
}
// Depending on how exactly you want to start and stop the selection
private void Update()
{
if(Input.GetMouseButtonDown(0))
{
StartSelection();
}
if(Input.GetMouseButtonUp(0))
{
EndSelection();
}
}
public void StartSelection()
{
// if there is already a selection running you don't wanr to start another one
if(selectionRoutine != null) return;
selectionRoutine = StartCoroutine (SelectionRoutine ());
}
public void EndSelection()
{
// If there is no selection running then you can't end one
if(selectionRoutine == null) return;
StopCoroutine (selectionRoutine);
selectionRoutine = null;
// Inform all listeners about the new selection
OnSelectionChanged.Invoke(new HashSet<GameObject>(selection);
}
private IEnumerator SelectionRoutine()
{
// Start with an empty selection
selection.Clear();
// This is ok in a Coroutine as long as you yield somewhere within it
while(true)
{
// Get the ray shooting forward from the camera at the mouse position
// for other inputs simply replace this according to your needs
var ray = _mainCamera.ScreenPointToRay(Input.mousePosition);
// Check if you hit any object
if(Physics.Raycast(ray, out var hit, layerMask = includeLayers ))
{
// If so Add it once to your selection
if(!selection.Contains(hit.gameObject)) selection.Add(hit.gameObject);
}
// IMPORTANT: Tells Unity to "pause" here, render this frame
// and continue from here in the next frame
// (without this your app would freeze in an endless loop!)
yield return null;
}
}
}
Ofcourse you could do it directly in Update in this example but I wanted to provide it in a way where you can easily exchange the input method according to your needs ;)
From UX side you additionally might want to call a second event like OnSelectionPreviewUpdate or something like this every time you add a new object to the selection in order to be able to e.g. visualize the selection outcome.
I might have understood this wrong and it sounds like you rather wanted to get everything inside of a drawn shape.
This is slightly more complex but here would be my idea for that:
Have a dummy selection Rigidbody object that by default is disabled and does nothing
don't even have a renderer on it but a mesh filter and mesh collider
while you "draw" create a mesh based on the input
then use Rigidbody.SweepTestAll in order to check if you hit anything with it
Typed on smartphone but I hope the idea gets clear

Answer 2

I think I would try to create a PolygonCollider2D because it is quite simple comparing to creating a mesh. You can set its path (outline) by giving it 2D points like location of your pointer/mouse. Use the SetPath method for it. You can then use one of its methods to check if another point in space overlaps with that collider shape.
While the PolygonCollider2D interacts with 2D components you can still use its Collider2D.OverlapPoint method to check positions/bounds of your 3D objects after translating it to 2D space.
You can also use its CreateMesh method to create a mesh for drawing your selection area on the screen.
You can read more about the PolygonCollider2D here.
Hope it makes sens and hope it helps.