shader_type canvas_item;

uniform float intensity: hint_range(0.0, 5.0, 0.1) = 1.0;
uniform float threshold: hint_range(0.0, 2.0, 0.1) = 1.0;

uniform sampler2D screen_texture : hint_screen_texture, filter_nearest;

float vector_angle(vec2 v) {
    if (abs(v.x) < 0.0001 && abs(v.y) < 0.0001) {
        return 0.0; // Default angle for zero vector
    }
    return atan(v.y, v.x);
}

vec2 rotate_vector(vec2 v, float cos_theta, float sin_theta) {
    return vec2(
        v.x * cos_theta - v.y * sin_theta,
        v.x * sin_theta + v.y * cos_theta
    );
}

void fragment() {
	vec2 r_displacement = vec2(-1.0, 0.0);
	vec2 g_displacement = vec2(0.0, 0.0);
	vec2 b_displacement = vec2(1.0, 0.0);

	vec2 center = vec2(0.5);
	vec2 dir = SCREEN_UV - center;
	float angle = vector_angle(dir);
	float dist = 2.0 * length(dir);
	float effect = exp(intensity * (dist - threshold));

	r_displacement = rotate_vector(effect * intensity * r_displacement, cos(angle), sin(angle));
	g_displacement = rotate_vector(effect * intensity * g_displacement, cos(angle), sin(angle));
	b_displacement = rotate_vector(effect * intensity * b_displacement, cos(angle), sin(angle));

	float r = texture(screen_texture, fma(r_displacement, SCREEN_PIXEL_SIZE, SCREEN_UV), 0.0).r;
	float g = texture(screen_texture, fma(g_displacement, SCREEN_PIXEL_SIZE, SCREEN_UV), 0.0).g;
	float b = texture(screen_texture, fma(b_displacement, SCREEN_PIXEL_SIZE, SCREEN_UV), 0.0).b;
	float a = texture(screen_texture, SCREEN_UV).a;

	COLOR = vec4(r, g, b, a);
}