Shader "Unlit/Crystal"
{
 Properties
 {
 _Cube("Skybox",Cube)=""{}
 //折射角度
 _EtaRatio("EtaRatio", Range(0, 1)) = 0
 //菲涅爾系數(shù)
 _FresnelBias("FresnelBias",float)=0.5
 _FresnelScale("FresnelScale",float)=0.5
 _FresnelPower("FresnelPower",float)=0.5
 }
 SubShader
 {
 Tags { "RenderType"="Opaque" }
 LOD 100

 Pass
 {
  CGPROGRAM
  #pragma vertex vert
  #pragma fragment frag

  
  #include "UnityCG.cginc"

  struct appdata
  {
  float4 vertex : POSITION;
  float3 normal:NORMAL;
  };

  struct v2f
  {
  float3 normalDir:TEXCOORD0;
  float4 vertex : SV_POSITION;
  float3 viewDir:TEXCOORD1;
  };

  samplerCUBE _Cube;
  float _EtaRatio;
  float _FresnelBias;
  float _FresnelScale;
  float _FresnelPower;
  
  //計算視線反射方向（入射角，法線）
  float3 caculateReflectDir(float3 I, float3 N) {
  float3 R = I - 2.0f*N*dot(I,N);
  return R;
  }

  //計算視線折射方向
  float3 caculateRefractDir(float3 I, float3 N, float etaRatio) {
  float cosTheta = dot(-I, N);
  float cosTheta2 = sqrt(1.f - pow(etaRatio, 2) * (1 - pow(cosTheta, 2)));
  float3 T = etaRatio * (I + N * cosTheta) - N * cosTheta2;
  return T;
  }

  //計算菲涅爾效應(yīng)
  float caculateFresnel(float3 I, float3 N) {
  float fresnel = max(0, min(1, _FresnelBias + _FresnelScale * pow(min(0.0, 1.0 - dot(I, N)), _FresnelPower)));
  return fresnel;
  }

  v2f vert (appdata v)
  {
  v2f o;
  o.vertex = UnityObjectToClipPos(v.vertex);
  //視線方向
  o.viewDir = normalize(mul(unity_ObjectToWorld, v.vertex).xyz - _WorldSpaceCameraPos);
  //法線方向
  o.normalDir = normalize(mul((float3x3)unity_ObjectToWorld, v.normal));
  return o;
  }
  
  fixed4 frag (v2f i) : SV_Target
  {
  //采樣反射折射后的天空盒顏色
  float3 reflectDir = caculateReflectDir(i.viewDir,i.normalDir);
  float4 reflectCol = texCUBE(_Cube, reflectDir);
  float3 refractDir = caculateRefractDir(i.viewDir, i.normalDir, _EtaRatio);
  float4 refractCol = texCUBE(_Cube, refractDir);
  //視線越垂直折射越小
  float fresnel = caculateFresnel(i.viewDir, i.normalDir);
  float4 col = lerp(refractCol, reflectCol, fresnel);
  return col;
  }
  ENDCG
 }
 }
}