godot circle shader transition for canvasgroups

shader_type canvas_item;
render_mode unshaded;

uniform float circle_size : hint_range(0.0, 1.05);
uniform float screen_width;
uniform float screen_height;
uniform sampler2D screen_texture : hint_screen_texture, repeat_disable, filter_nearest;
void fragment() {
	float ratio = screen_width / screen_height;
	float dist = distance(vec2(0.5, 0.5), vec2(mix(0.5, UV.x, ratio), UV.y));
	COLOR.a = step(dist,circle_size);
	
	//canvasgroupshader
	vec4 c = textureLod(screen_texture, SCREEN_UV, 0.0);
	if (c.a > 0.0001) {
		c.rgb /= c.a;
	}
	COLOR *= c;
}