Camera frustum plane-projecting gizmo

CameraFrustumPlaneGizmo.cs

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

// @kurtdekker
//
// Projects a Camera frustum onto an arbitrary Transform plane
//
// You can (probably) drop this script on any GameObject.
//
// It will try and find the Main Camera if present, or you can specify a Camera.
//
// You must specify a ProjectionPlane transform as well.
//
[ExecuteAlways]
public class CameraFrustumPlaneGizmo : MonoBehaviour
{
	public enum ProjAxis
	{
		X,
		Y,
		Z,
	}

	[Header("Projects on the Z == 0 of this transform.", order = 1)]
	public Transform ProjectionPlane;

	[Header("Will try to find Main Camera if empty")]
	public Camera CameraToUse;

	[Header("Axis orientation.")]
	public ProjAxis ProjectionAxis = ProjAxis.Z;

	private void Start()
	{
		if (!CameraToUse)
		{
			CameraToUse = Camera.main;
		}

		// TODO: you might want to destroy this when not in editor...
		//if (!Application.isEditor)
		//{
		//	Destroy(this);
		//}
	}

	bool valid;
	Vector3[] ProjectedPoints;

	Vector2[] ViewportCorners = new Vector2[]
	{
		new Vector2( 0, 0),
		new Vector2( 0, 1),
		new Vector2( 1, 1),
		new Vector2( 1, 0),
	};

	private void Update()
	{
		// presume invalidity
		valid = false;

		// print out some warnings if you like...
		if (!CameraToUse)
		{
			return;
		}
		if (!ProjectionPlane)
		{
			return;
		}

		// malloc
		if (ProjectedPoints == null || ProjectedPoints.Length != ViewportCorners.Length)
		{
			ProjectedPoints = new Vector3[ ViewportCorners.Length];
		}

		// all four corners must hit
		for (int cornerIndex = 0; cornerIndex < ProjectedPoints.Length; cornerIndex++)
		{
			Vector2 viewportPosition = ViewportCorners[cornerIndex];

			Ray ray = CameraToUse.ViewportPointToRay(viewportPosition);

			// by default: this projects on back of the Z == 0
			Plane plane = new Plane(inNormal: ProjectionPlane.forward, inPoint: ProjectionPlane.position);

			// choose orientation
			switch( ProjectionAxis)
			{
				case ProjAxis.X:
					plane = new Plane(inNormal: ProjectionPlane.right, inPoint: ProjectionPlane.position);
					break;

				case ProjAxis.Y:
					plane = new Plane(inNormal: ProjectionPlane.up, inPoint: ProjectionPlane.position);
					break;

				case ProjAxis.Z:		// defaulted above
					break;
			}

			// cast
			float enter;
			if (plane.Raycast( ray, out enter))
			{
				ProjectedPoints[cornerIndex] = ray.GetPoint(enter);
			}
			else
			{
				// can't hit the plane from this camera viewport; give up fully
				return;
			}
		}

		// only executed if we hit all four corners
		valid = true;
	}

	private void OnDrawGizmos()
	{
		if (valid)
		{
			for (int cornerIndex = 0; cornerIndex < ProjectedPoints.Length; cornerIndex++)
			{
				// main bounding rect / trapezoid
				int nextCorner = (cornerIndex + 1) % ProjectedPoints.Length;

				var point1 = ProjectedPoints[cornerIndex];
				var point2 = ProjectedPoints[nextCorner];

				Gizmos.DrawLine(point1, point2);

				// crossing bars (X down the middle)
				if (cornerIndex < 2)
				{
					nextCorner = (cornerIndex + 2) % ProjectedPoints.Length;

					point1 = ProjectedPoints[cornerIndex];
					point2 = ProjectedPoints[nextCorner];

					Gizmos.DrawLine(point1, point2);
				}
			}
		}
	}
}