non-VR portal rendering

Assets/Escape Room Engine/Portal/Portal.cs

@@ -1,10 +1,14 @@
 using System;
 using UnityEngine;
+using UnityEngine.Rendering;
+using UnityEngine.Rendering.Universal;
 
 namespace Escape_Room_Engine.Portal
 {
     public class Portal : MonoBehaviour
     {
+        private static readonly Matrix4x4 HalfRotation = Matrix4x4.Rotate(Quaternion.Euler(0, 180, 0));
+        
         /// <summary>
         /// The portal that is connected with this one.
         /// </summary>
@@ -12,25 +16,65 @@ namespace Escape_Room_Engine.Portal
         /// <summary>
         /// The main camera to take the position for the portal camera from.
         /// </summary>
-        [SerializeField] private Transform playerCamera;
+        [SerializeField] private Camera playerCamera;
         /// <summary>
         /// The camera that will draw the view for this portal.
         /// </summary>
-        [SerializeField] private Transform portalCamera;
+        [SerializeField] private Camera portalCamera;
+        /// <summary>
+        /// The quad where the rendered texture will be drawn on.
+        /// </summary>
+        [SerializeField] private MeshRenderer portalQuad;
 
-        private void Start()
+        private RenderTexture _texture;
+
+        private void Awake()
         {
             // check whether the other portal is set up
             if (!other || other.other != this) throw new Exception("Other Portal not set up correctly.");
             // check whether the player camera is set up
             if (!playerCamera) throw new Exception("No camera initialised.");
+            
+            // create render texture
+            _texture = new RenderTexture(Screen.width, Screen.height, 24, RenderTextureFormat.ARGB32);
         }
 
-        private void Update()
+        private void Start()
         {
-            var portalCameraMatrix = transform.localToWorldMatrix * other.transform.worldToLocalMatrix *
-                                     playerCamera.localToWorldMatrix;
-            portalCamera.SetPositionAndRotation(portalCameraMatrix.GetPosition(), portalCameraMatrix.rotation);
+            // assign render texture
+            portalCamera.targetTexture = _texture;
+            other.portalQuad.material.mainTexture = _texture;
+        }
+
+        private void OnEnable()
+        {
+            RenderPipelineManager.beginCameraRendering += Render;
+        }
+
+        private void OnDisable()
+        {
+            RenderPipelineManager.beginCameraRendering -= Render;
+        }
+
+        private void Render(ScriptableRenderContext scriptableRenderContext, Camera camera)
+        {
+            var t = transform;
+            
+            // position portal camera
+            var portalCameraMatrix = t.localToWorldMatrix * HalfRotation * other.transform.worldToLocalMatrix *
+                                     playerCamera.transform.localToWorldMatrix;
+            portalCamera.transform.SetPositionAndRotation(portalCameraMatrix.GetPosition(), portalCameraMatrix.rotation);
+            
+            // set camera clip plane to portal (otherwise the wall behind the portal would be rendered)
+            // calculating the clip plane: https://computergraphics.stackexchange.com/a/1506
+            var n = -t.forward; // clip plane normal
+            var portalPlane = new Plane(n, t.position); // clip plane in world space
+            var clipPlane = portalCamera.worldToCameraMatrix.inverse.transpose *
+                            new Vector4(n.x, n.y, n.z, portalPlane.distance); // vector format clip plane in camera space
+            portalCamera.projectionMatrix = playerCamera.CalculateObliqueMatrix(clipPlane);
+            
+            // render portal view
+            UniversalRenderPipeline.RenderSingleCamera(scriptableRenderContext, portalCamera);
         }
     }
 }