Godot Shader for FishEye 2D effect

fisheyeeffect.shader

shader_type canvas_item;
// Based on http://www.geeks3d.com/20140213/glsl-shader-library-fish-eye-and-dome-and-barrel-distortion-post-processing-filters/2/


const float PI = 3.1415926535;
uniform float BarrelPower;

vec2 distort(vec2 p) {
	
	if(p.x > 0.0){
		float angle = p.y / p.x;
		float theta = atan(angle);
		float radius = length(p);
		radius = pow(radius, BarrelPower);
		
		p.x = radius * cos(-theta);
		p.y = radius * sin(-theta);
	} else {
		float angle = p.y / p.x;
		float theta = atan(angle);
		float radius = length(p);
		radius = pow(radius, BarrelPower);
		
		p.y = radius * sin(theta);
		p.x = radius * cos(theta);
		p.x = - p.x;
	}
	
	
	return 0.5 * (p + vec2(1.0,1.0));
}

void fragment(){
	vec2 xy = vec2(2, 2) * UV;
	xy.x = xy.x-1.0;
	xy.y = xy.y-1.0;
	
	float d = length(xy);
	if(d < 1.5){
		xy = distort(xy);
	}
	else{
		xy = UV;
	}
	COLOR = texture(SCREEN_TEXTURE, xy);
}