Shader to create hole in sprite Unity 2d - unity3d

Question

I'm currently developping a game where a character can move on a background. The idea will be that this character dig this background. I think it should be done by a shader but i'm a beginner with its. .
I can imagine something like if the character is too far from the position of this pixel, so the alpha of this pixel is 0. And to keep the alpha at 0.If the alpha is equals to 0 you don't make the character test.
But for now, i've tried with the sprites/diffuse shader as base and i can't find a way to get the position of the pixels. I tried something in the surf function but with no real results The surf function is supposed to be executed once by pixel no?
Thanks in advance to help me.
Edit
I've tried some things to finally get a vert frag shader. As i said, i'm trying to compute the alpha with the distance of the pixel.
For now I can't figure out where is my mistake but maybe some will be more talkative.
By the way, my sorting layer has just explode when i put this new shader. I've tried to switch off Ztest and Zwrite but it doesn't work. So if you have any idea (But it's not the main problem)
Shader "Unlit/SimpleUnlitTexturedShader"
{
Properties
{
// we have removed support for texture tiling/offset,
// so make them not be displayed in material inspector
[NoScaleOffset] _MainTex("Texture", 2D) = "white" {}
_Position("Position", Vector) = (0,0,0,0)
}
SubShader
{
Pass
{
CGPROGRAM
// use "vert" function as the vertex shader
#pragma vertex vert
// use "frag" function as the pixel (fragment) shader
#pragma fragment frag
// vertex shader inputs
struct appdata
{
float4 vertex : POSITION; // vertex position
float2 uv : TEXCOORD0; // texture coordinate
};
// vertex shader outputs ("vertex to fragment")
struct v2f
{
float2 uv : TEXCOORD0; // texture coordinate
float4 vertex : SV_POSITION; // clip space position
float3 worldpos:WD_POSITION;
};
// vertex shader
v2f vert(appdata v)
{
v2f o;
// transform position to clip space
// (multiply with model*view*projection matrix)
o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
// just pass the texture coordinate
o.uv = v.uv;
o.worldpos= mul(_Object2World,o.vertex).xyz;
return o;
}
// texture we will sample
sampler2D _MainTex;
float4 Position ;
// pixel shader; returns low precision ("fixed4" type)
// color ("SV_Target" semantic)
fixed4 frag(v2f i) : SV_Target
{
// sample texture and return it
fixed4 col = tex2D(_MainTex, i.uv);
col.a = step(2,distance(Position.xyz, i.worldpos));
return col;
}
ENDCG
}
}
}

Answer 1

The effect of "cutting holes" in geometry can be achieved using vertex shaders. This is possible since vertex shaders allow you to determine the 3d world position of a pixel of an object using the _Object2World matrix. If I wasnt on my phone I'd give a code example, but I'll try to describe it. If you take the vertex position, apply an _Object2World matrix, it will return a 3d world position. Here's a quick example:
float3 worldPos = mul (_Object2World, v.vertex).xyz;
In the fragment portion of the shader, you can then set the transparency based on the distance between worldPos and any other position. I apologize if this is still difficult to follow, I am trying to do this on my phone right now. If anyone wants to elaborate on what I said or give code examples that'd be great. Otherwise, you can study up on vertex shaders here:
http://docs.unity3d.com/Manual/SL-VertexFragmentShaderExamples.html
Hope this helps!