#version 330

out vec4 fragColor;

uniform vec2 res;
uniform float time;

const float sharpness = 300;

const float PI = 3.14159265358979323846;


mat2 rot(float a) {
	return mat2(
		cos(a), -sin(a),
		sin(a), cos(a)
	);
}

float square(vec2 m, float a, float r, vec2 p) {
	vec2 diff = abs((m - p) * rot(r));
	float d = max(diff.x, diff.y);

	return clamp((a - d)*sharpness, 0.0, 1.0);
}

void main(void) {
	vec2 m = res/2;
	float s = min(m.x, m.y);
	vec2 p = (gl_FragCoord.xy - m) / s;

	float scene = 0;

	int n = 24;
	for (int i = 0; i < n; i++) {
		float c = square(vec2(0.5, 0) * rot(PI/2*(4.0*i/n)) * vec2(1, 1), 0.3, PI/2*(24.0*i/n - time/800), p);
		scene = abs(scene - c);
	}

	fragColor = vec4(vec3(1, 1, 1) * scene, 1);
}