Shader "Hidden/ChannelMixer"
{
    Properties
    {
        _MainTex ("Red Channel Texture", 2D) = "white"
    }
    SubShader
    {
        Cull Off 
        ZWrite Off 
        ZTest Always

        Pass
        {
            CGPROGRAM

            #pragma vertex vert
            #pragma fragment frag

            #pragma shader_feature_local _HAS_TEX_R
            #pragma shader_feature_local _HAS_TEX_G
            #pragma shader_feature_local _HAS_TEX_B
            #pragma shader_feature_local _HAS_TEX_A

            #include "UnityCG.cginc"

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

            struct v2f {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            v2f vert(appdata v) {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            sampler2D _MainTex;
            int4 _ChannelSource;

            float4 frag (v2f i) : SV_Target
            {
                float4 color = tex2D(_MainTex, i.uv);

                float r = color[_ChannelSource.x];
                float g = color[_ChannelSource.y];
                float b = color[_ChannelSource.z];
                float a = color[_ChannelSource.w];

                return float4(r, g, b, a);
            }
            ENDCG
        }
    }
}