用Unity写一个描边shader

下面是一个简单的Unity描边shader的示例代码：

Shader "Custom/Outline" {
    Properties {
        _MainTex ("Texture", 2D) = "white" {}
        _OutlineColor ("Outline Color", Color) = (0,0,0,1)
        _OutlineWidth ("Outline Width", Range (0.002, 0.1)) = 0.01
    }
 
    SubShader {
        Tags {"Queue"="Transparent" "RenderType"="Opaque"}
        LOD 100
 
        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;
            float4 _OutlineColor;
            float _OutlineWidth;
 
            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 {
                // sample the texture
                fixed4 col = tex2D(_MainTex, i.uv);
 
                // calculate the outline color
                fixed4 outline = _OutlineColor;
                float2 texelSize = 1.0 / _ScreenParams.xy;
                float2 offset[4] = float2[](
                    float2(texelSize.x, 0),
                    float2(-texelSize.x, 0),
                    float2(0, texelSize.y),
                    float2(0, -texelSize.y)
                );
                for (int j = 0; j < 4; j++) {
                    outline += tex2D(_MainTex, i.uv + _OutlineWidth * offset[j]);
                }
                outline /= 5.0;
 
                // mix the texture and outline color
                return lerp(outline, col, col.a);
            }
            ENDCG
        }
    }
    FallBack "Diffuse"
}

这个shader的核心思想是在每个像素周围采样一些颜色，并将它们平均值作为描边颜色。可以通过调整 `_OutlineWidth` 属性来控制描边的宽度，通过调整 `_OutlineColor` 属性来控制描边的颜色。