Star trails in C+SDL2 (inspired by @rlaneth's code in https://startrail.rlaneth.com/)

startrail.c

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <inttypes.h>

#include <SDL.h>

/*
 * Tuto nice:
 * https://lazyfoo.net/tutorials/SDL/index.php
 */

/*
 * Build instructions:
 *
 * gcc sdl.c -o star -O3 `pkg-config --cflags --libs sdl2`
 */

typedef uint8_t u8;

#define WIDTH   1920
#define HEIGHT  1080
#define PI 3.141592653589793

static int centerX, centerY;
static u8 *buff;

#define MAX_STARS 100

static SDL_Window* window;
static SDL_Renderer* renderer;

static int screen_width  = WIDTH;
static int screen_height = HEIGHT;

static int IsWindowResized = 1;

int min_w_h;
struct star {
	double x;
	double y;
	double size;
	double speed;
	double angle;
} stars[MAX_STARS];

static void resize(int width, int height) {
	if (width > 599) {
		centerY = height - 250;
		centerX = width  - 250;
	} else {
		centerY = height;
		centerX = width / 2;
	}
	if (width <= height)
		min_w_h = width;
	else
		min_w_h = height;
}

static void create_stars(void) {
	for (int i = 0; i < MAX_STARS; i++) {
		double radius = drand48() * (double)min_w_h;
		double angle  = drand48() * PI * 2.0;
		stars[i].x = (double)centerX + SDL_cos(angle) * radius;
		stars[i].y = (double)centerY + SDL_sin(angle) * radius;
		stars[i].size  = 2;
		stars[i].speed = (drand48() * 0.0002) + 0.0002;
		stars[i].angle = angle;
	}
}

void drawcircle(int centerX, int centerY, int radius)
{
	int x   = radius - 1;
	int y   = 0;
	int dx  = 1;
	int dy  = 1;
	int err = dx - (radius * 2);
	while (x >= y) {
		SDL_RenderDrawPoint(renderer, centerX + x, centerY + y);
		SDL_RenderDrawPoint(renderer, centerX + y, centerY + x);
		SDL_RenderDrawPoint(renderer, centerX - y, centerY + x);
		SDL_RenderDrawPoint(renderer, centerX - x, centerY + y);
		SDL_RenderDrawPoint(renderer, centerX - x, centerY - y);
		SDL_RenderDrawPoint(renderer, centerX - y, centerY - x);
		SDL_RenderDrawPoint(renderer, centerX + y, centerY - x);
		SDL_RenderDrawPoint(renderer, centerX + x, centerY - y);
		if (err <= 0) {
			y++;
			err += dy;
			dy  += 2;
		}
		if (err > 0) {
			x--;
			dx  += 2;
			err += dx - (radius * 2);
		}
	}
}

static void draw_stars(void) {
	SDL_SetRenderDrawColor(renderer, 0,4,20, 12);
	SDL_RenderFillRect(renderer,
		&(SDL_Rect){
			.x = 0, .y = 0,
			.w = screen_width, .h = screen_height
		}
	);
	SDL_SetRenderDrawColor(renderer, 255,255,255, 255);
	for (int i = 0; i < MAX_STARS; i++)
		drawcircle(stars[i].x, stars[i].y, 2);
}

static void update_stars(void) {
	double radius;
	for (int i = 0; i < MAX_STARS; i++) {
		radius =
			SDL_sqrt(
				SDL_pow(stars[i].x - centerX, 2)
					+
				SDL_pow(stars[i].y - centerY, 2)
			);
		stars[i].angle += stars[i].speed;
		stars[i].x = centerX + SDL_cos(stars[i].angle) * radius;
		stars[i].y = centerY + SDL_sin(stars[i].angle) * radius;
	}
}

static void animate(void) {
	/* Check if resize. */
	if (IsWindowResized) {
		IsWindowResized = 0;
		resize(screen_width, screen_height);
		create_stars();
	}
	update_stars();
	draw_stars();

	/* Render everything. */
	SDL_RenderPresent(renderer);
}

/**
 * Main game loop.
 */
int main(void)
{
	SDL_Event event;
	srand48(time(NULL));

	/* Initialize. */
	if (SDL_Init(SDL_INIT_VIDEO) < 0)
		errx(1, "SDL could not initialize!: %s\n", SDL_GetError());

	window = SDL_CreateWindow("startrail",
		SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
		screen_width, screen_height,
		SDL_WINDOW_SHOWN|SDL_WINDOW_RESIZABLE);

	if (!window)
		errx(1, "Unable to create Window, %s\n", SDL_GetError());

	renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_PRESENTVSYNC);
	if (!renderer)
		errx(1, "Unable to create Renderer, %s\n", SDL_GetError());

	SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);

	while (1)
	{
		while (SDL_PollEvent(&event) != 0)
		{
			if (event.type == SDL_QUIT)
				goto quit;

			else if (event.type == SDL_WINDOWEVENT &&
				event.window.event == SDL_WINDOWEVENT_RESIZED)
			{
				IsWindowResized = 1;
				screen_width  = event.window.data1;
				screen_height = event.window.data2;
			}
		}
		animate();
    }

quit:
	if (renderer)
		SDL_DestroyRenderer(renderer);
	if (window)
		SDL_DestroyWindow(window);
	SDL_Quit();
}