Unity-2D边缘检测(描边效果)

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
}

二、ShaderGraph

抓取图片缓冲，上下左右四个方位平移，乘以描边颜色；

四张图合并，减去原图范围的像素，只剩边缘；

最后将原图和边缘合并（可插值使边缘柔和）；

升级项目到URP，修改projectsetting-graphic-pielinesettings；

导入ShaderGraph包，开始拖拖拽拽，真的香，效果好，速度快，思路清晰；