Mask Shader over Multiple Sprites in World Space

hey, i'm trying to write a shader over multiple sprites, because the characters in my game are made of multiple sprites. i 'm new to shaders so pls don't expect a code that makes sense ^^:

Shader "Custom/ShaderMaskLit"{
Properties {
_MainTex ("_MainTex", 2D) = "white" { }
_AlphaMask ("_AlphaMask", 2D) = "white" { }
}
SubShader {
Tags {
"Queue"="Transparent"
"RenderType"="Transparent"
"IgnoreProjector"="True"
"PreviewType"="Plane"
"CanUseSpriteAtlas"="True"
}
Cull Off
Lighting Off
ZWrite Off
Blend One OneMinusSrcAlpha

CGPROGRAM
#pragma surface surf Lambert vertex:vert nofog nolightmap nodynlightmap keepalpha
#include "UnityCG.cginc"
float4 _MainTex_ST;
sampler2D _MainTex;
sampler2D _AlphaMask;
struct Input {
fixed4 color;
float2 _uv_MainTex;
float2 _uv_STD;
float4 rect_Sprite;
};

void vert (inout appdata_full i, out Input o)
{
UNITY_INITIALIZE_OUTPUT(Input,o);
o.rect_Sprite = float4(0,0,1,1);
o._uv_MainTex = TRANSFORM_TEX(i.texcoord,_MainTex);
o._uv_STD = float2((i.texcoord.x - o.rect_Sprite.x)/o.rect_Sprite.z,(i.texcoord.y - o.rect_Sprite.y)/o.rect_Sprite.w);
//o_uv_STD = TRANSFORM_TEX(o._uv_STD,_MainTex);
}

void surf (Input i, inout SurfaceOutput o)
{
float4 result = float4(0,0,0,0);

float2 uv_alpha1 = i._uv_STD;
float2 center_alpha1 = float2(0.5,0.5);
//float3 worldPos = mul(unity_ObjectToWorld, i._uv_STD); //WorldPos
//uv_alpha1 = worldPos;
float4 color_alpha1 = tex2D(_AlphaMask, uv_alpha1);
float2 uv_ROOT = i._uv_STD;
float4 color_ROOT = tex2D(_MainTex,uv_ROOT);
color_ROOT = float4(color_ROOT.rgb, color_ROOT.a * color_alpha1.a);
result = float4(color_ROOT.rgb,color_ROOT.a*1);
o.Albedo = result.rgb*result.a;
o.Alpha = result.a;
}
ENDCG
}
fallback "Standard"
}

when i now add i a texture to the alpha mask i get this result:


so i know i have to multiply unity_ObjectToWorld with the uv at some point, so if i uncomment these lines
float3 worldPos = mul(unity_ObjectToWorld, i._uv_STD); //WorldPos
uv_alpha1 = worldPos
i get this result:


but i cant manage to get a feathered mask that starts at the feet of the character and ends at the middle. is this even possible? thanks for any help in advance