Unity 2D: Area Hue Shift Effect

Question

I am trying to create a transparent GameObejct in Unity that if active, changes the color hue of anything behind it by 180 degrees. The effect can be easily seen in Photoshop if my explanation is not good enough.

I can guess I will need a shader for this, but my shader making knowledge is quite limited as of now, so any help would be appreciated.

Thank you!

Example made in photoshop of the desired effect:

Answer 1

You can take the shader from this post and make the following adjustments:

1) Add a GrabPass here

 SubShader 
  {
    GrabPass {"_GrabTexture"} // <--

2) Then add a variable for the grab texture here:

  float _HueShift;
  float _SaturationShift;
  sampler2D _GrabTexture; // <--

3) Change the HueShift property to a range (for ease of use):

 Properties 
  {
      _HueShift("HueShift", Range(0.0, 1.0)) = 0 // <--
      _SaturationShift("SaturationShift", Float) = 1.0
  }

4) Change the Input struct to contain screenPos:

 struct Input {
        float4 screenPos; // <--
    };

5) Then finally update the fragment shader to read from the GrabTexture:

void surf (Input IN, inout SurfaceOutput o) {
        float2 uv = IN.screenPos.xy / IN.screenPos.w; // <--
        half4 c = tex2D (_GrabTexture, uv);// <--

        o.Albedo = c.rgb;// <--

        float3 hsv = rgb_to_hsv_no_clip(o.Albedo.xyz);
        hsv.x+=_HueShift;
        hsv.y = _SaturationShift;

        if ( hsv.x > 1.0 ) { hsv.x -= 1.0; }
        o.Albedo = half3(hsv_to_rgb(hsv));
        o.Alpha = c.a;// <--
  }

The final result looks like this:

Here is the final shader for reference and in case I forgot to mention one of the changes:

Shader "Custom/HueRotation" 
{
  Properties 
  {
      _HueShift("HueShift", Range(0.0, 1.0)) = 0
      _SaturationShift("SaturationShift", Float) = 1.0
  }

  SubShader 
  {
    GrabPass {"_GrabTexture"}
    Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType" = "Transparent" }
    ZWrite Off
    Blend SrcAlpha OneMinusSrcAlpha
    Cull Off
    CGPROGRAM
    #pragma surface surf Lambert alpha
    #pragma target 3.0
    #include "UnityCG.cginc"

    struct Input {
        float4 screenPos;
    };

    float3 rgb_to_hsv_no_clip(float3 RGB)
    {
            float3 HSV;

        float minChannel, maxChannel;
        if (RGB.x > RGB.y) {
        maxChannel = RGB.x;
        minChannel = RGB.y;
        }
        else {
        maxChannel = RGB.y;
        minChannel = RGB.x;
        }

        if (RGB.z > maxChannel) maxChannel = RGB.z;
        if (RGB.z < minChannel) minChannel = RGB.z;

            HSV.xy = 0;
            HSV.z = maxChannel;
            float delta = maxChannel - minChannel;             //Delta RGB value
            if (delta != 0) {                    // If gray, leave H  S at zero
                HSV.y = delta / HSV.z;
                float3 delRGB;
                delRGB = (HSV.zzz - RGB + 3*delta) / (6.0*delta);
                if      ( RGB.x == HSV.z ) HSV.x = delRGB.z - delRGB.y;
                else if ( RGB.y == HSV.z ) HSV.x = ( 1.0/3.0) + delRGB.x - delRGB.z;
                else if ( RGB.z == HSV.z ) HSV.x = ( 2.0/3.0) + delRGB.y - delRGB.x;
            }
            return (HSV);
    }

    float3 hsv_to_rgb(float3 HSV)
    {
            float3 RGB = HSV.z;

                float var_h = HSV.x * 6;
                float var_i = floor(var_h);   // Or ... var_i = floor( var_h )
                float var_1 = HSV.z * (1.0 - HSV.y);
                float var_2 = HSV.z * (1.0 - HSV.y * (var_h-var_i));
                float var_3 = HSV.z * (1.0 - HSV.y * (1-(var_h-var_i)));
                if      (var_i == 0) { RGB = float3(HSV.z, var_3, var_1); }
                else if (var_i == 1) { RGB = float3(var_2, HSV.z, var_1); }
                else if (var_i == 2) { RGB = float3(var_1, HSV.z, var_3); }
                else if (var_i == 3) { RGB = float3(var_1, var_2, HSV.z); }
                else if (var_i == 4) { RGB = float3(var_3, var_1, HSV.z); }
                else                 { RGB = float3(HSV.z, var_1, var_2); }

        return (RGB);
    }

      float _HueShift;
      float _SaturationShift;
      sampler2D _GrabTexture;

      void surf (Input IN, inout SurfaceOutput o) {
        float2 uv = IN.screenPos.xy / IN.screenPos.w;
        half4 c = tex2D (_GrabTexture, uv);

        o.Albedo = c.rgb;

        float3 hsv = rgb_to_hsv_no_clip(o.Albedo.xyz);
        hsv.x+=_HueShift;
        hsv.y = _SaturationShift;

        if ( hsv.x > 1.0 ) { hsv.x -= 1.0; }
        o.Albedo = half3(hsv_to_rgb(hsv));
        o.Alpha = c.a;
      }

      ENDCG
    }
    Fallback "Diffuse"
  }

Credits to Beeky for the original code.