ShaderToys

// common 得到viewRay
v2f vert(appdata v)
{
    // ......
    float4 ndcFarPlanePos = float4(o.uv * 2.0 - 1.0, 1.0, 1.0);
        float4 viewDir = mul(unity_CameraInvProjection, ndcFarPlanePos);
        o.viewRay = viewDir.xyz / viewDir.w;
}

float4 frag(v2f i)
{
    // forward path
    float linear01Depth;
    half3 viewSpaceNormal;
    DecodeDepthNormal(tex2D(_CameraDepthNormalsTexture, uv), linear01Depth, viewSpaceNormal);

    // deferred path
    float decodedDepth = Linear01Depth(tex2D(_CameraDepthTexture, i.uv).r);
    float3 viewSpaceObjPos = decodedDepth * i.viewRay;
    float3 worldSpaceNormal = tex2D(_CameraGBufferTexture2, i.uv).rgb * 2.0 - 1.0;
    float3 viewSpaceNormal = normalize(mul((float3x3)_WorldToView, worldSpaceNormal));
}

  bool CheckDepthCollision(float3 viewPos, out float2 screenPos)
  {
      float4 clipPos = mul(UNITY_MATRIX_P, float4(viewPos, 1.0));
      clipPos = clipPos / clipPos.w;
      screenPos = float2(clipPos.x, clipPos.y) * 0.5 + 0.5;
      float eyeZ = GetEyeZ(screenPos);

      return screenPos.x > 0 && screenPos.y > 0 &&
      screenPos.x < 1.0 && screenPos.y < 1.0 &&
      eyeZ < -viewPos.z &&                 // eyeZ < 当前检测的深度 (说明检测点在当前像素之后)
      eyeZ + _DepthThickness > -viewPos.z; // eyeZ + 物体厚度 > 当前检测的深度 (说明 eyeZ+_DepthThcikness 对应的点在当前检测点之前)
  }

  float3 Layer(float2 UV, float t)
  {
      float2 aspect = float2(2, 1);
      // 把屏幕空间划分为多个Grid
      float2 uv = UV*_Size*aspect;
      uv.y += t * 0.25;

      // frac(uv) - 0.5 使得grid_uv=(0,0)位置为grid的中心点
      // grid_uv 表示grid内每个像素点的grid uv
      float2 grid_uv = frac(uv)-0.5;
      float2 id = floor(uv);

      float n = N21(id); // 0 - 1
      t += n*6.2831;

      // 将屏幕在y方向上划分为10等分，让y方向上的雨滴流动路径错开
      float w = UV.y * 10;

      float x = (n - 0.5)*0.8; // -0.4 - 0.4
      // 模拟雨滴流动过程中x方向的轨迹变化 大部分时间居中 偶尔向左流 偶尔向右流
      x += (0.4-abs(x)) * sin(3*w)*pow(sin(w), 6)*0.45;

      // 模拟雨滴流动时先快后慢的效果
      float y = -sin(t+sin(t+sin(t)*0.5))*0.45;
      // 控制雨滴的形状
      y -= (grid_uv.x - x)*(grid_uv.x - x);

      // (x,y)为当前雨滴的位置 dropToGridPixelDir 为雨滴指向grid内像素的方向向量
      float2 dropToGridPixelDir = (grid_uv - float2(x, y)) / aspect;
      // 限制雨滴的范围
      float drop = S(0.05, 0.03, length(dropToGridPixelDir));

      // grid_uv - float2(x, t*0.25) 抵消掉上面 uv.y+=t*0.25 使得尾部雨滴不随时间移动
      float2 trailToGridPixelDir = (grid_uv - float2(x, t*0.25)) / aspect;
      // 控制尾部雨滴数量
      trailToGridPixelDir.y = (frac(trailToGridPixelDir.y * 8)-0.5)/8;
      // 控制尾部雨滴大小
      float trail = S(0.03, 0.01, length(trailToGridPixelDir));

      // 小于 dropToGridPixelDir.y 的地方是，雨滴还未经过的地方, 下面代码使得小于 dropToGridPixelDir.y 的轨迹为黑色
      float fogTrail = S(-0.05, 0.05, dropToGridPixelDir.y);
      // 模拟轨迹逐渐消失的效果
      fogTrail *= S(0.5, y, grid_uv.y);
      // 模拟尾部雨滴逐渐消失的效果
      trail *= fogTrail;

      // 使用 dropToGridPixelDir.x 限制fogTrail的范围
      fogTrail *= S(0.05, 0.04, abs(dropToGridPixelDir.x));

      float2 offs = drop * dropToGridPixelDir + trail * trailToGridPixelDir;
      return float3(offs, fogTrail);
  }

// drops.xy 存储的是 drop到gridPixel的方向
float3 drops = float3(drop * dropToGridPixelDir + trail * trailToGridPixelDir, fogTrail);

// grabUV沿着dropToGridPixelDir方向偏移进行采样，来模拟折射现象
grabUV += drops.xy * _Distortion * fade;
col = tex2D(_GrabTexture, grabUV);