/*
 * Copyright (c) Camden Dixie O'Brien
 * SPDX-License-Identifier: AGPL-3.0-only
 */

#version 450

#define MAXITER 1000

layout(location = 0) out vec4 out_colour;

layout(push_constant) uniform Constants {
	vec2 img_size;
	vec2 shift;
	float zoom;
} params;

vec2 ss_offsets[4] = vec2[](
	vec2(-0.25, -0.25),
	vec2( 0.25, -0.25),
	vec2(-0.25,  0.25),
	vec2( 0.25,  0.25)
);

vec2 maptoz(vec2 xy)
{
	return params.zoom * ((xy / params.img_size.xy) * 4.0 - 2.0) + params.shift;
}

float mandelbrot(vec2 c) {
	vec2 z = vec2(0.0, 0.0);
	for (int i = 0; i < MAXITER; ++i) {
		z = vec2(z.x * z.x - z.y * z.y, 2.0 * z.x * z.y) + c;
		if (length(z) > 2.0)
			return float(i) / float(MAXITER);
	}
	return 0.0;
}

vec3 colour(float i) {
	return vec3(i, i * i, i * i * i);
}

void main()
{
	vec2 c;
	vec3 col = vec3(0.0, 0.0, 0.0);
	for (int i = 0; i < 4; ++i) {
		c = maptoz(gl_FragCoord.xy + ss_offsets[i]);
		col += colour(mandelbrot(c));
	}
	out_colour = vec4(col, 1.0);
}