Erosion fade in shader used in "La Última Flor de Lazlar"

ImageFadeIn.cs

using UnityEngine;
using UnityEngine.UI;

// Set _ScaleAndOffset in the shader so that our math works as expected with textures in atlases :)
[ExecuteInEditMode]
public class ImageFadeIn : MonoBehaviour 
{
	[SerializeField]
	private Image target;

	void Awake()
	{
		if (target == null) target = FindDefaultTarget ();

		if (target == null) Destroy (this);
	}

	void Start ()
	{
		Texture2D texture = target.sprite.texture;
		var rectInTexture = target.sprite.textureRect;

		Vector4 scaleAndOffset;
		scaleAndOffset.x = texture.width / rectInTexture.width;
		scaleAndOffset.y = texture.height / rectInTexture.height;
		scaleAndOffset.z = (rectInTexture.xMin / texture.width) * scaleAndOffset.x;
		scaleAndOffset.w = (rectInTexture.yMin / texture.height) * scaleAndOffset.y;

		var material = target.material;
		material.SetVector("_ScaleAndOffset", scaleAndOffset);
	}

	private Image FindDefaultTarget() {
		var defaultTarget = GetComponent<Image> ();
		if (defaultTarget.material.name == "ImageFadeIn")
			return defaultTarget;

		return null;
	}

}

ImageFadeIn.shader

Shader "Custom/ImageFadeIn"
{
	Properties
	{
		[PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
		_FadeInTex ("Fade In Texture", 2D) = "white" {}
		_Color ("Tint", Color) = (1,1,1,1)
		_Softness ("Softness", Range (0.001, 1)) = 1
		[MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
	}

	SubShader
	{
		Tags
		{ 
			"Queue"="Transparent" 
			"IgnoreProjector"="True" 
			"RenderType"="Transparent" 
			"PreviewType"="Plane"
			"CanUseSpriteAtlas"="True"
		}

		Cull Off
		Lighting Off
		ZWrite Off
		Blend One OneMinusSrcAlpha

		Pass
		{
		CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma multi_compile _ PIXELSNAP_ON
			#include "UnityCG.cginc"
			
			struct appdata_t
			{
				float4 vertex   : POSITION;
				float4 color    : COLOR;
				float2 texcoord : TEXCOORD0;
			};

			struct v2f
			{
				float4 vertex   : SV_POSITION;
				fixed4 color    : COLOR;
				half2 texcoord  : TEXCOORD0;
				half2 fadeincoord : TEXCOORD1;
			};
			
			fixed4 _Color;
			uniform float4 _ScaleAndOffset;

			v2f vert(appdata_t IN)
			{
				v2f OUT;
				OUT.vertex = UnityObjectToClipPos(IN.vertex);
				OUT.texcoord = IN.texcoord;
				OUT.fadeincoord = IN.texcoord * _ScaleAndOffset.xy - _ScaleAndOffset.zw;
				OUT.color = IN.color * _Color;
				#ifdef PIXELSNAP_ON
				OUT.vertex = UnityPixelSnap (OUT.vertex);
				#endif

				return OUT;
			}

			sampler2D _MainTex;
			sampler2D _AlphaTex;
			sampler2D _FadeInTex;
			float _AlphaSplitEnabled;
			float _Softness;

			fixed4 SampleSpriteTexture (float2 uv)
			{
				fixed4 color = tex2D (_MainTex, uv);
				if (_AlphaSplitEnabled)
					color.a = tex2D (_AlphaTex, uv).r;

				return color;
			}

			fixed4 frag(v2f IN) : SV_Target
			{
				fixed4 colorMultiply = IN.color;
				colorMultiply.a = 1;
				fixed4 c = SampleSpriteTexture (IN.texcoord) * colorMultiply;
				
				fixed4 fadeInColor = tex2D(_FadeInTex, IN.fadeincoord);
				float fadeInBrightness = fadeInColor.r;
				float lerpLevel = clamp(0,1,(IN.color.a-(fadeInBrightness))/_Softness);
				c.a = lerp(0, c.a, lerpLevel);

				c.rgb *= c.a;
				
				return c;
			}
		ENDCG
		}
	}
}