Demo 84 - Sprite dissolve

Shader "Custom/2D/Dissolve"
{
    Properties {
        [PerRendererData] _MainTex ("Main texture", 2D) = "white" {}
        _DissolveTex ("Dissolution texture", 2D) = "gray" {}
        _Threshold ("Threshold", Range(0., 1.01)) = 0.
    }

    SubShader {

        Tags { "Queue"="Transparent" }
        Blend SrcAlpha OneMinusSrcAlpha

        Pass {

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct v2f {
                float4 pos : SV_POSITION;
                float2 uv : TEXCOORD0;
            };

            sampler2D _MainTex;

            v2f vert(appdata_base v) {
                v2f o;
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                o.uv = v.texcoord;
                return o;
            }

            sampler2D _DissolveTex;
            float _Threshold;

            fixed4 frag(v2f i) : SV_Target {
                float4 c = tex2D(_MainTex, i.uv);
                float val = tex2D(_DissolveTex, i.uv).r;

                c.a *= step(_Threshold, val);
                return c;
            }
            ENDCG
        }
    }
}