Alpha mask on shader for Godot

mask.gd

shader_type canvas_item;

uniform sampler2D base_mask: texture;
uniform vec2 size;
uniform vec2 scale;
uniform vec2 position;
uniform vec2 region;

vec2 uv(vec2 uv) {
    vec2 s = vec2(size.x / scale.x, size.x / scale.y);
    vec2 p = vec2(position.x / (size.x / s.x), position.y / (size.y / s.y));
    return vec2(uv.x * s.x - p.x, uv.y * s.y - p.y);
}

void fragment() {
    vec4 base = texture(TEXTURE, UV).rgba;
    base.a = texture(base_mask, uv(UV)).a;
    COLOR = base;
}

I needed a shrinked masked image however when I did this UV seems weird so I had to cut. Btw you don't use region uniform

this is not my code. I only collect.