【Shader】屏幕后处理-利用卷积算子计算边缘检测

Shader "Edge Detection" {
    Properties {
        _MainTex ("Base (RGB)", 2D) = "white" {}
        _EdgeOnly ("Edge Only", Float) = 1.0
        _EdgeColor ("Edge Color", Color) = (0, 0, 0, 1)
        _BackgroundColor ("Background Color", Color) = (1, 1, 1, 1)
    }
    SubShader {
        Pass {  
            ZTest Always 
            Cull Off 
            ZWrite Off

            CGPROGRAM

            #include "UnityCG.cginc"

            #pragma vertex vert  
            #pragma fragment fragSobel

            sampler2D _MainTex;  
            uniform half4 _MainTex_TexelSize;
            fixed _EdgeOnly;
            fixed4 _EdgeColor;
            fixed4 _BackgroundColor;

            struct v2f {
                float4 pos : SV_POSITION;
                half2 uv[9] : TEXCOORD0;
            };

            v2f vert(appdata_img v) {
                v2f o;
                // 顶点投影坐标
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                // uv坐标
                half2 uv = v.texcoord;

                // 纹素值*Sobel算子
                o.uv[0] = uv + _MainTex_TexelSize.xy * half2(-1, -1);
                o.uv[1] = uv + _MainTex_TexelSize.xy * half2(0, -2);
                o.uv[2] = uv + _MainTex_TexelSize.xy * half2(1, -1);
                o.uv[3] = uv + _MainTex_TexelSize.xy * half2(-2, 0);
                o.uv[4] = uv + _MainTex_TexelSize.xy * half2(0, 0);
                o.uv[5] = uv + _MainTex_TexelSize.xy * half2(2, 0);
                o.uv[6] = uv + _MainTex_TexelSize.xy * half2(-1, 1);
                o.uv[7] = uv + _MainTex_TexelSize.xy * half2(0, 1);
                o.uv[8] = uv + _MainTex_TexelSize.xy * half2(1, 1);

                return o;
            }

            // 固定参数，求亮度
            fixed luminance(fixed4 color) {
                return  0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b; 
            }

            half Sobel(v2f i) {
                const half Gy[9] = {-1,  0,  1,
                                    -2,  0,  2,
                                    -1,  0,  1};

                const half Gx[9] = {-1, -2, -1,
                                    0,  0,  0,
                                    1,  2,  1};     

                half texColor;
                half edgeX = 0;
                half edgeY = 0;
                for (int it = 0; it < 9; it++) {
                    texColor = luminance(tex2D(_MainTex, i.uv[it]));
                    edgeX += texColor * Gx[it];
                    edgeY += texColor * Gy[it];
                }

                half edge = 1 - abs(edgeX) - abs(edgeY);

                return edge;
            }

            fixed4 fragSobel(v2f i) : SV_Target {
                // edge值越小，表示越可能是边缘点
                half edge = Sobel(i);

                // i.uv[4]是0，0偏移，等同于UV坐标
                // 边缘+彩色
                fixed4 withEdgeColor = lerp(_EdgeColor, tex2D(_MainTex, i.uv[4]), edge);

                // 边缘
                fixed4 onlyEdgeColor = lerp(_EdgeColor, _BackgroundColor, edge);

                // 边缘彩色混合
                return lerp(withEdgeColor, onlyEdgeColor, _EdgeOnly);
            }

            ENDCG
        } 
    }
    FallBack Off
}