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

[ExecuteInEditMode]
public class RayMarchingCamera : MonoBehaviour {
 private Matrix4x4 frustumCorners = Matrix4x4.identity;
 public Material material;
 public Camera myCamera;
 public Transform cameraTransform;
 // Use this for initialization
 private void Start()
 {
  myCamera.depthTextureMode = DepthTextureMode.Depth;
 }

 private void OnRenderImage(RenderTexture source, RenderTexture destination)
 {
  //field of view
  float fov = myCamera.fieldOfView;
  //近裁面距離
  float near = myCamera.nearClipPlane;
  //橫縱比
  float aspect = myCamera.aspect;
  //近裁面一半的高度
  float halfHeight = near * Mathf.Tan(fov * 0.5f * Mathf.Deg2Rad);
  //向上和向右的向量
  Vector3 toRight = myCamera.transform.right * halfHeight * aspect;
  Vector3 toTop = myCamera.transform.up * halfHeight;

  //分別得到相機到近裁面四個角的向量
  //depth/dist=near/|topLeft|
  //dist=depth*(|TL|/near)
  //scale=|TL|/near
  Vector3 topLeft = cameraTransform.forward * near + toTop - toRight;
  float scale = topLeft.magnitude / near;

  topLeft.Normalize();
  topLeft *= scale;

  Vector3 topRight = cameraTransform.forward * near + toTop + toRight;
  topRight.Normalize();
  topRight *= scale;

  Vector3 bottomLeft = cameraTransform.forward * near - toTop - toRight;
  bottomLeft.Normalize();
  bottomLeft *= scale;

  Vector3 bottomRight = cameraTransform.forward * near - toTop + toRight;
  bottomRight.Normalize();
  bottomRight *= scale;

  //給矩陣賦值
  frustumCorners.SetRow(0, bottomLeft);
  frustumCorners.SetRow(1, bottomRight);
  frustumCorners.SetRow(2, topRight);
  frustumCorners.SetRow(3, topLeft);
  //將向量傳給定點著色器，將屏幕畫面?zhèn)鱾€shader
  material.SetMatrix("_FrustumCornorsRay", frustumCorners);
  material.SetTexture("_MainTex", source);
  Graphics.Blit(source, destination,material,0);
 }
}

Shader "Unlit/Fog"
{
 Properties
 {
 _MainTex ("Texture", 2D) = "white" {}
 _NoiseTex("NoiseTex",2D)="white"{}
 //霧起始高度
 _FogStartHeight("FogStartHeight",float)=0.0
 //霧終點高度
 _FogEndHeight("FogEndHeight",float)=1.0
 //霧x位移速度
 _FogXSpeed("FogXSpeed",float)=0.1
 //霧y位移速度
 _FogYSpeed("FogYSpeed",float)=0.1
 }
 SubShader
 {
 Tags { "RenderType"="Opaque" }
 LOD 100

 Pass
 {
 CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 #include "UnityCG.cginc"
 sampler2D _MainTex;
 sampler2D _NoiseTex;
 sampler2D _CameraDepthTexture;
 float _FogStartHeight;
 float _FogEndHeight;
 float _FogXSpeed;
 float _FogYSpeed;
 //獲得相機近裁面四個角向量
 float4x4 _FrustumCornorsRay;

 struct a2v{
 float4 vertex:POSITION;
 float2 uv:TEXCOORD0;
 };

 struct v2f{
 float4 pos:SV_POSITION;
 float2 uv:TEXCOORD0;
 float4 interpolatedRay:TEXCOORD1;
 };

 v2f vert(a2v v){
 v2f o;
 o.pos=UnityObjectToClipPos(v.vertex);
 o.uv=v.uv;
 int index=0;
 if (v.uv.x<0.5&&v.uv.y<0.5)
 {
  index = 0;
 }
 else if (v.uv.x>0.5&&v.uv.y<0.5) {
  index = 1;
 }
 else if (v.uv.x>0.5&&v.uv.y>0.5) {
  index = 2;
 }
 else {
  index = 3;
 }
 //安排uv4個角對應(yīng)四個角向量
 o.interpolatedRay = _FrustumCornorsRay[index];
 return o;
 }

 float4 frag(v2f i):SV_Target{
 //觀察空間下的線性深度值
 float linearDepth=LinearEyeDepth(SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture,i.uv));
 //像素世界坐標(biāo)=世界空間相機位置+像素相對相機距離
 float3 worldPos=_WorldSpaceCameraPos+linearDepth*i.interpolatedRay.xyz;
 float speedX=_Time.y*_FogXSpeed;
 float speedY=_Time.y*_FogYSpeed;
 float noise=tex2D(_NoiseTex,i.uv+float2(speedX,speedY));
 //讓噪聲圖向黑色靠攏，黑白差距不要太大
 noise=pow(noise,0.5);
 //霧濃度=世界高度/指定范圍
 float fogDensity=(_FogEndHeight-worldPos.y)/(_FogEndHeight-_FogStartHeight);
 fogDensity=smoothstep(0.0,1.0,fogDensity*noise);
 float4 color=tex2D(_MainTex,i.uv);
 //根據(jù)霧濃度混合場景和霧顏色
 color.rgb=lerp(color.rgb,float3(0.5,0.5,0.5),fogDensity);
 return color;
 }

 ENDCG
 }
 }
}