Unity中的反射效果（基于URP）

一，cubemap

通过采样一张cubemap模拟反射效果。

            v2f vert (appdata v)
            {
                v2f o = (v2f) 0;
                o.vertex = TransformObjectToHClip(v.vertex.xyz);
                o.worldNormal = TransformObjectToWorldNormal(v.normal.xyz);
                return o;
            }

            half4 frag (v2f i) : SV_Target
            {
                Light light=GetMainLight();
                float3 worldLightDir= light.direction;
                float3 reflectDir=normalize(reflect(-worldLightDir,i.worldNormal));

                half4 envCol = texCUBE(_CubeMap, reflectDir);
                half3 envHDRCol = DecodeHDREnvironment(envCol, unity_SpecCube0_HDR);
                return half4(envHDRCol,1);
            }

CubeMapReflection(默认天空球)

二，ReflectionProbe

ReflectionProbe反射探头会采样周围生成一张CubeMap，在Shader中通过变量访问。ReflectionProbe有三种模式，baked，custom，realtime。baked提前烘培，custom可以烘培，也可以在定义一张CubeMap

            v2f vert (appdata v)
            {
                v2f o = (v2f) 0;
                o.vertex = TransformObjectToHClip(v.vertex.xyz);
                o.worldNormal = TransformObjectToWorldNormal(v.normal.xyz);
                return o;
            }

            half4 frag (v2f i) : SV_Target
            {
                Light light=GetMainLight();
                float3 worldLightDir= light.direction;
                float3 reflectDir=normalize(reflect(-worldLightDir,i.worldNormal));

                half4 envCol = SAMPLE_TEXTURECUBE(unity_SpecCube0, samplerunity_SpecCube0, reflectDir);
                half3 envHDRCol = DecodeHDREnvironment(envCol, unity_SpecCube0_HDR);
                return half4(envHDRCol,1);
            }

三，PlanarReflection

适用于表明平整物体的反射，镜子，湖面等。通过反射矩阵将需要反射的物体再渲染一遍。

C#部分

using System;
using UnityEngine.Experimental.Rendering;
using UnityEngine.Serialization;
using Unity.Mathematics;

namespace UnityEngine.Rendering.Universal
{
    [ExecuteAlways]
    public class PlanarReflections : MonoBehaviour
    {
        [Serializable]
        public enum ResolutionMulltiplier
        {
            Full,
            Half,
            Third,
            Quarter
        }

        [Serializable]
        public class PlanarReflectionSettings
        {
            public ResolutionMulltiplier m_ResolutionMultiplier = ResolutionMulltiplier.Third;
            public float m_ClipPlaneOffset = 0.07f;
            public LayerMask m_ReflectLayers = -1;
            public bool m_Shadows;
        }

        [SerializeField]
        public PlanarReflectionSettings m_settings = new PlanarReflectionSettings();

        public GameObject target;
        [FormerlySerializedAs("camOffset")] public float m_planeOffset;

        private static Camera _reflectionCamera;
        private RenderTexture _reflectionTexture;
        private readonly int _planarReflectionTextureId = Shader.PropertyToID("_PlanarReflectionTexture");

        private int2 _oldReflectionTextureSize;

        public static event Action<ScriptableRenderContext, Camera> BeginPlanarReflections;

        private void OnEnable()
        {
            RenderPipelineManager.beginCameraRendering += ExecutePlanarReflections;
        }
        
        private void OnDisable()
        {
            Cleanup();
        }

        private void OnDestroy()
        {
            Cleanup();
        }

        private void Cleanup()
        {
            RenderPipelineManager.beginCameraRendering -= ExecutePlanarReflections;

            if(_reflectionCamera)
            {
                _reflectionCamera.targetTexture = null;
                SafeDestroy(_reflectionCamera.gameObject);
            }
            if (_reflectionTexture)
            {
                RenderTexture.ReleaseTemporary(_reflectionTexture);
            }
        }

        private static void SafeDestroy(Object obj)
        {
            if (Application.isEditor)
            {
                DestroyImmediate(obj);
            }
            else
            {
                Destroy(obj);
            }
        }

        private void UpdateCamera(Camera src, Camera dest)
        {
            if (dest == null) return;

            dest.CopyFrom(src);
            dest.useOcclusionCulling = false;
            if (dest.gameObject.TryGetComponent(out UniversalAdditionalCameraData camData))
            {
                camData.renderShadows = m_settings.m_Shadows;
            }
        }

        private void UpdateReflectionCamera(Camera realCamera)
        {
            if (_reflectionCamera == null)
                _reflectionCamera = CreateMirrorObjects();
            
            Vector3 pos = Vector3.zero;
            Vector3 normal = Vector3.up;
            if (target != null)
            {
                pos = target.transform.position + Vector3.up * m_planeOffset;
                normal = target.transform.up;
            }

            UpdateCamera(realCamera, _reflectionCamera);
            
            var d = -Vector3.Dot(normal, pos) - m_settings.m_ClipPlaneOffset;
            var reflectionPlane = new Vector4(normal.x, normal.y, normal.z, d);

            var reflection = Matrix4x4.identity;
            reflection *= Matrix4x4.Scale(new Vector3(1, -1, 1));

            CalculateReflectionMatrix(ref reflection, reflectionPlane);
            var oldPosition = realCamera.transform.position - new Vector3(0, pos.y * 2, 0);
            var newPosition = ReflectPosition(oldPosition);
            _reflectionCamera.transform.forward = Vector3.Scale(realCamera.transform.forward, new Vector3(1, -1, 1));
            _reflectionCamera.worldToCameraMatrix = realCamera.worldToCameraMatrix * reflection;
            
            var clipPlane = CameraSpacePlane(_reflectionCamera, pos - Vector3.up * 0.1f, normal, 1.0f);
            var projection = realCamera.CalculateObliqueMatrix(clipPlane);
            _reflectionCamera.projectionMatrix = projection;
            _reflectionCamera.cullingMask = m_settings.m_ReflectLayers;
            _reflectionCamera.transform.position = newPosition;
        }

        private static void CalculateReflectionMatrix(ref Matrix4x4 reflectionMat, Vector4 plane)
        {
            reflectionMat.m00 = (1F - 2F * plane[0] * plane[0]);
            reflectionMat.m01 = (-2F * plane[0] * plane[1]);
            reflectionMat.m02 = (-2F * plane[0] * plane[2]);
            reflectionMat.m03 = (-2F * plane[3] * plane[0]);

            reflectionMat.m10 = (-2F * plane[1] * plane[0]);
            reflectionMat.m11 = (1F - 2F * plane[1] * plane[1]);
            reflectionMat.m12 = (-2F * plane[1] * plane[2]);
            reflectionMat.m13 = (-2F * plane[3] * plane[1]);

            reflectionMat.m20 = (-2F * plane[2] * plane[0]);
            reflectionMat.m21 = (-2F * plane[2] * plane[1]);
            reflectionMat.m22 = (1F -