基于Unity实现2D边缘检测

这篇文章主要介绍了如何利用Unity实现2D边缘检测，从而达到人物描边效果。文中的示例代码讲解详细，感兴趣的小伙伴可以了解一下

一、ShaderLab

1.Alpha值边缘检测

根据图片的Alpha值边缘判定，向内扩一段距离做边缘，颜色设置未描边颜色；

片元着色阶段，向上下左右四个方向做检测，有一个点的透明度为0，判定为边缘；

Shader "2DOutline" { Properties { _MainTex("Texture", 2D) = "white" {} _LineWidth("Width",Range(0,0.4)) = 1.0 _LineColor("LineColor",color) = (1,1,1,1) _Intensity("Intensity",Range(1,10)) = 1.0 } SubShader { Tags { "RenderType" = "Opaque" "Queue" = "Transparent"} Blend SrcAlpha OneMinusSrcAlpha Pass { CGPROGRAM #pragma vertex vert #pragma fragment frag #include "UnityCG.cginc" struct appdata { float4 vertex : POSITION; float2 uv : TEXCOORD0; }; struct v2f { float2 uv : TEXCOORD0; float4 vertex : SV_POSITION; }; sampler2D _MainTex; float4 _MainTex_ST; fixed _LineWidth; float4 _LineColor; fixed _Intensity; v2f vert(appdata v) { v2f o; o.vertex = UnityObjectToClipPos(v.vertex); o.uv = TRANSFORM_TEX(v.uv, _MainTex); return o; } fixed4 frag(v2f i) : SV_Target { fixed4 col = tex2D(_MainTex, i.uv); // 采样周围4个点 float2 up_uv = i.uv + float2(0, 1) * _LineWidth * 1 / 10 * _MainTex_ST.xy; float2 down_uv = i.uv + float2(0,-1) * _LineWidth * 1 / 10 * _MainTex_ST.xy; float2 left_uv = i.uv + float2(-1,0) * _LineWidth * 1 / 10 * _MainTex_ST.xy; float2 right_uv = i.uv + float2(1,0) * _LineWidth * 1 / 10 * _MainTex_ST.xy; // 如果有一个点透明度为0 说明是边缘 float w = tex2D(_MainTex,up_uv).a * tex2D(_MainTex,down_uv).a * tex2D(_MainTex,left_uv).a * tex2D(_MainTex,right_uv).a; if (w == 0) { col.rgb = lerp(_LineColor * _Intensity, col.rgb, w); } return col; } ENDCG } } }

如果图片内容恰好铺满整张图，没有alpha值，方法不适用；下图底部边缘消失了；

2.卷积边缘检测

在屏幕后处理阶段，使用卷积做边缘检测；

卷积：根据像素周围八个方向的像素的计算出新的像素值；

边缘检测卷积算子，都包含水平和竖直两个方向的卷积核；

梯度公式：G = sqrt(Gx*Gx + Gy*Gy)；

考虑性能问题，使用：G = |Gx|+|Gy|;

顶点着色器计算卷积纹理采样坐标，减少计算量（片元数量更多）；

片元着色阶段Sobel卷积计算，插值获得片元像素颜色；

Sobel计算结果和梯度Gradient比较，大于梯度和EdgeColor做插值；

屏幕后效调用OnRenderImage接口；

Shader "EdgeDetection" { Properties{ _MainTex("Base (RGB)", 2D) = "white" {} _EdgeColor("Edge Color", Color) = (0, 0, 0, 1) //卷积梯度 _Gradient("Gradient",float) =0.0 } SubShader{ Pass { ZTest Always Cull Off ZWrite Off CGPROGRAM #include "UnityCG.cginc" #pragma vertex vert #pragma fragment frag sampler2D _MainTex; uniform half4 _MainTex_TexelSize; //fixed _EdgeOnly; fixed4 _EdgeColor; //fixed4 _BackgroundColor; fixed _Gradient; struct v2f { float4 pos : SV_POSITION; half2 uv[9] : TEXCOORD0; }; v2f vert(appdata_img v) { v2f o; o.pos = UnityObjectToClipPos(v.vertex); half2 uv = v.texcoord; o.uv[0] = uv + _MainTex_TexelSize.xy * half2(-1, -1); o.uv[1] = uv + _MainTex_TexelSize.xy * half2(0, -1); o.uv[2] = uv + _MainTex_TexelSize.xy * half2(1, -1); o.uv[3] = uv + _MainTex_TexelSize.xy * half2(-1, 0); o.uv[4] = uv + _MainTex_TexelSize.xy * half2(0, 0); o.uv[5] = uv + _MainTex_TexelSize.xy * half2(1, 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; } fixed luminance(fixed4 color) { return  0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b; } half Sobel(v2f i) { const half Gx[9] = {    -1,  0,  1, -2,  0,  2, -1,  0,  1}; const half Gy[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 frag(v2f i) : SV_Target { half edge = Sobel(i); fixed4 col = tex2D(_MainTex, i.uv[4]); if(edge> _Gradient) col = lerp(_EdgeColor, tex2D(_MainTex, i.uv[4]), edge); return col; } ENDCG } } FallBack Off }