基于Unity实现2D边缘检测

来源：jb51　　时间：2022/4/11 18:16:20　　对本文有异议

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
}