Unity Shader 像素图描边

思路：在片元着色器中，处理像素p，针对p的上下左右四个像素采样（使用一个变量width来控制描边宽度，也就是处理上下左右多远的像素），若p本身是透明像素，则

• 若上下左右存在非透明像素，则当前像素p返回描边颜色
• 若上下左右都是透明像素，则返回透明即可

若 p 本身非透明像素，则返回本身颜色即可

Shader "Custom_Shader/ImageOuterOutline"
{
    Properties
    {
        _MainTex ("Sprite Texture", 2D) = "white" {}
        _OutlineWidth ("Outline Width", float) = 1
        _OutlineColor ("Outline Color", Color) = (1.0, 1.0, 1.0, 1.0)
        _AlphaValue ("Alpha Value", Range(0, 1)) = 0.1
        [ToggleUI] _IsWhite ("IsWhite", Float) = 0
    }
    SubShader
    {
        Blend SrcAlpha OneMinusSrcAlpha

        Tags
        {
            "Queue" = "Transparent"
        }
        Pass
        {
            ZTest Off
            Blend SrcAlpha OneMinusSrcAlpha
            Cull Off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            sampler2D _MainTex;
            float4 _MainTex_ST;
            half4 _MainTex_TexelSize;
            float _OutlineWidth;
            float4 _OutlineColor;
            float _AlphaValue;
            float _IsWhite;

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                float4 normal : NORMAL;
            };

            struct v2f
            {
                float4 vertex : SV_POSITION;
                half2 uv : TEXCOORD0;
                half2 left : TEXCOORD1;
                half2 right : TEXCOORD2;
                half2 up : TEXCOORD3;
                half2 down : TEXCOORD5;
            };

            v2f vert(appdata i)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(i.vertex);
                o.uv = TRANSFORM_TEX(i.uv, _MainTex);
                o.left = o.uv + half2(-1, 0) * _MainTex_TexelSize.xy * _OutlineWidth;
                o.right = o.uv + half2(1, 0) * _MainTex_TexelSize.xy * _OutlineWidth;
                o.up = o.uv + half2(0, 1) * _MainTex_TexelSize.xy * _OutlineWidth;
                o.down = o.uv + half2(0, -1) * _MainTex_TexelSize.xy * _OutlineWidth;
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                const float transparent = tex2D(_MainTex, i.left).a + tex2D(_MainTex, i.right).a + tex2D(_MainTex, i.up)
                    .a + tex2D(_MainTex, i.down).a;
                fixed4 color = tex2D(_MainTex, i.uv);

                fixed4 result;
                if (color.a < 0.1)
                {
                    if (_IsWhite > 0.1)
                    {
                        //闪白
                        result = step(_AlphaValue, transparent) * (1,1,1,1);
                    }
                    else
                    {
                        result = step(_AlphaValue, transparent) * _OutlineColor;    
                    }
                }
                else
                {
                    if (_IsWhite > 0.1)
                    {
                        //闪白
                        result = (1.0, 1.0, 1.0, 1.0);
                    }
                    else
                    {
                        result = color;
                    }
                }
                return result;
            }
            ENDCG
        }
    }
}

参考：https://www.jianshu.com/p/c68a730e9a8b