In this “Unity Sprite Outline” tutorial we’ll discover how to create both a “Sprite Innerline” and a “Outer Sprite Outline”.
Simply, the Inner Sprite Outline shader is drawn “inside” the sprite, while “Outer” is the opposite

Read more about “Texture’s Texel Size” here.

HLSL

Inner Sprite Outline HLSL

Shader "Unlit/InnerSpriteOutline HLSL"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
		_OutlineColor("Outline Color", Color) = (1,1,1,1)
    }
    SubShader
    {
		Tags
		{
			"RenderType" = "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;
            float4 _MainTex_TexelSize;
 
			fixed4 _OutlineColor;
 
            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);
 
				fixed leftPixel = tex2D(_MainTex, i.uv + float2(-_MainTex_TexelSize.x, 0)).a;
				fixed upPixel = tex2D(_MainTex, i.uv + float2(0, _MainTex_TexelSize.y)).a;
				fixed rightPixel = tex2D(_MainTex, i.uv + float2(_MainTex_TexelSize.x, 0)).a;
				fixed bottomPixel = tex2D(_MainTex, i.uv + float2(0, -_MainTex_TexelSize.y)).a;
 
				fixed outline = (1 - leftPixel * upPixel * rightPixel * bottomPixel) * col.a;
 
                return lerp(col, _OutlineColor, outline);
            }
            ENDCG
        }
    }
}

Outer Sprite Outline HLSL

To have achieve an “Outline” instead of an “Innerline”, just change the “outline” line (in the previous code snippet) into this:

fixed outline = max(max(leftPixel, upPixel), max(rightPixel, bottomPixel)) - col.a;

Amplify Shader

Inner Sprite Outline ASE

Outer Sprite Outline ASE