Demo 91 - Chroma key to transparency

Shader "Custom/GreenToTransparent"
{
    Properties {
        _MainTex ("Base", 2D) = "white" {}
        _Threshold ("Threshold", Range(0, 1)) = 0
    }
    SubShader {
        Pass {
            Tags { "Queue"="Transparent" "RenderType"="Transparent" }
            Blend SrcAlpha OneMinusSrcAlpha
            Cull Off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct v2f {
                float4 pos : SV_POSITION;
                float2 uv1 : TEXCOORD0;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

            v2f vert(appdata_base v) {
                v2f o;
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                o.uv1 = TRANSFORM_TEX(v.texcoord, _MainTex);
                return o;
            }

            float _Threshold;

            fixed4 frag(v2f i) : COLOR {
                fixed4 col1 = tex2D(_MainTex, i.uv1);
                fixed4 val = ceil(saturate(col1.g - col1.r - _Threshold)) * ceil(saturate(col1.g - col1.b - _Threshold));
                return lerp(col1, fixed4(0., 0., 0., 0.), val);
            }

            ENDCG
        }
    }
}