Unity edge detection using sobel filter naive implementation

naiveedgedetection_sobel.shader

Shader "matheusfaria/EdgeDetectionSobel"
{
    Properties
    {
        _MainTex ("Base (RGB)", 2D) = "white" {}
        _Threshold ("Threshold", Range(0, 1)) = 1
    }

    Category
    {
        Tags {
            "Queue"="Transparent"
        }

        SubShader
        {
            Pass
            {
                CGPROGRAM

                #pragma vertex vert
                #pragma fragment frag

                #include "UnityCG.cginc"

                sampler2D _MainTex;
                float4 _MainTex_ST;
                float2 _MainTex_TexelSize;

                int _Threshold;

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

                v2f vert (appdata_base v)
                {
                    v2f o;
                    o.pos = UnityObjectToClipPos (v.vertex);
                    o.uv = TRANSFORM_TEX (v.texcoord, _MainTex);
                    return o;
                }

                half4 frag (v2f i) : COLOR
                {
                    float3x3 kernel = {
                        1, 0, -1,
                        2, 0, -2,
                        1, 0, -1
                    };

                    float2 edge = (0, 0);

                    for (int x  = -1; x <= 1; ++x) {
                        for (int y = -1; y <= 1; ++y) {
                            float sample = tex2D(_MainTex,
                                float2(
                                    i.uv.x + x * _MainTex_TexelSize.x,
                                    i.uv.y + y * _MainTex_TexelSize.y
                                )
                            );

                            edge += float2(kernel[y + 1][x + 1],
                                           kernel[x + 1][y + 1]) * sample;
                        }
                    }

                    float color = step(length(edge), _Threshold);
                    return half4(color, color, color, 1.0);
                }
                ENDCG
            }
        }
    }
}