UnityのImage EffectでSobelフィルタによるエッジ検出

UnityのImageEffectでSobelフィルタでエッジを検出するためシェーダーです。
プロパティの_Widthと_Heightにスクリーンサイズを、_Thresholdにエッジと検出するための閾値を設定します。

Shader "ImageEffect/Edge"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Width ("Width", Float) = 640 // Screen Size X
        _Height ("Height", Float) = 480 // Screen Size Y
        _Threshold ("Threshold", Range(0, 1)) = 0.5 // Threshold for edge
    }
    SubShader
    {

        Pass
        {
            CGPROGRAM
            #pragma vertex vert_img
            #pragma fragment frag

            #include "UnityCG.cginc"

            sampler2D _MainTex;
            float _Width;
            float _Height;
            float _Threshold;

            float gray (fixed4 c) {
                return 0.2126 * c.r + 0.7152 * c.g + 0.0722 * c.b;
            }

            fixed4 frag (v2f_img i) : COLOR
            {
                float dx = 1.0 / _Width;
                float dy = 1.0 / _Height;
                float c00 = gray(tex2D(_MainTex, i.uv + float2(-dx, -dy))); 
                float c01 = gray(tex2D(_MainTex, i.uv + float2(-dx, 0.0))); 
                float c02 = gray(tex2D(_MainTex, i.uv + float2(-dx, dy))); 
                float c10 = gray(tex2D(_MainTex, i.uv + float2(0, -dy))); 
                float c12 = gray(tex2D(_MainTex, i.uv + float2(0, dy))); 
                float c20 = gray(tex2D(_MainTex, i.uv + float2(dx, -dy))); 
                float c21 = gray(tex2D(_MainTex, i.uv + float2(dx, 0.0))); 
                float c22 = gray(tex2D(_MainTex, i.uv + float2(dx, dy))); 

                float sx = -1.0 * c00 + -2.0 * c10 + -1.0 * c20 + 1.0 * c02 + 2.0 * c12 + 1.0 * c22;
                float sy = -1.0 * c00 + -2.0 * c01 + -1.0 * c02 + 1.0 * c20 + 2.0 * c21 + 1.0 * c22;

                float g = sqrt(sx * sx + sy * sy);

                return g > _Threshold ? fixed4(1, 1, 1, 1) : fixed4(0, 0, 0, 1);
            }
            ENDCG
        }
    }
}