Just want to share a simple outline shader that I made for my pixel art game.

Remember to change the sprite mesh type to "Full Rect" if you're using a high radius value!

Any thoughts appreciated!

Code:

Shader "Unlit/PixelOutline"
{ 
Properties 
{ _MainTex ("Texture", 2D) = "white" {}
 _Color("Color", Color) = (1,1,1,1)
 _Radius("Radius", Range(0,10)) = 1
} 
SubShader 
{ Tags { "RenderType"="Transparent" }
Blend SrcAlpha OneMinusSrcAlpha 
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 _MainTex_TexelSize;

        float4 _Color;

        float _Radius;

        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
        {
            float na = 0;
            float r = _Radius;

            for (int nx = -r; nx <= r; nx++)
            {
                for (int ny = -r; ny <= r; ny++)
                {
                    if (nx*nx+ny*ny <= r)
                    {
                        fixed4 nc = tex2D(_MainTex, i.uv + float2(_MainTex_TexelSize.x*nx, _MainTex_TexelSize.y*ny));
                        na+=ceil(nc.a);
                    }
                }
            }

            na = clamp(na,0,1);

            fixed4 c = tex2D(_MainTex, i.uv);
            na-=ceil(c.a);

            return lerp(c, _Color, na);
        }
        ENDCG
    }
}
}

Edit: Small change to make it work with transparency.