#version 450
#extension GL_ARB_separate_shader_objects : enable

#define WIDTH 3200
#define HEIGHT 2400
#define WORKGROUP_SIZE 32
layout (local_size_x = WORKGROUP_SIZE, local_size_y = WORKGROUP_SIZE, local_size_z = 1 ) in;

struct Pixel{
  vec4 value;
};

layout(std140, binding = 0) buffer buf {
   Pixel imageData[];
};

void main() {
	/*
	In order to fit the work into workgroups, some unnecessary threads are launched.
	We terminate those threads here. 
	*/
	if(gl_GlobalInvocationID.x >= WIDTH || gl_GlobalInvocationID.y >= HEIGHT)
		return;

	float x = float(gl_GlobalInvocationID.x) / float(WIDTH);
	float y = float(gl_GlobalInvocationID.y) / float(HEIGHT);

	/*
	What follows is code for rendering the mandelbrot set.
	*/
	vec2 uv = vec2(x,y);
	float n = 0.0;
	vec2 c = vec2(-.445, 0.0) +  (uv - 0.5)*(2.0+ 1.7*0.2  );
	vec2 z = vec2(0.0);
	const int M =128;
	for(int i = 0; i < M; i++) {
		z = vec2(z.x*z.x - z.y*z.y, 2.*z.x*z.y) + c;
		if (dot(z, z) > 2) break;
		n++;
	}
          
	// we use a simple cosine palette to determine color:
	// http://iquilezles.org/www/articles/palettes/palettes.htm         
	float t = float(n) / float(M);
	vec3 d = vec3(0.3, 0.3 ,0.5);
	vec3 e = vec3(-0.2, -0.3 ,-0.5);
	vec3 f = vec3(2.1, 2.0, 3.0);
	vec3 g = vec3(0.0, 0.1, 0.0);
	vec4 color = vec4( d + e*cos( 6.28318*(f*t+g) ) ,1.0);
	      
	// store the rendered mandelbrot set into a storage buffer:
	imageData[WIDTH * gl_GlobalInvocationID.y + gl_GlobalInvocationID.x].value = color;
}