Grid with 2d rotation example in SDL2

build.sh

#!/bin/sh

set -xe
gcc -o main main.c -lSDL2

main.c

#include <SDL2/SDL_events.h>
#include <SDL2/SDL_rect.h>
#include <SDL2/SDL_render.h>
#include <SDL2/SDL_stdinc.h>
#include <SDL2/SDL_surface.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>

#include <SDL2/SDL.h>

#ifdef DEBUG
#define debug(...) printf(__VA_ARGS__)
#else
#define debug(...)
#endif

#define error() do { printf("%s:%s:%d ERROR: %s\n", __FILE__, __func__, __LINE__, SDL_GetError()); abort(); } while (false)
#define cn(...) if ((__VA_ARGS__) == NULL) error();
#define cz(...) if ((__VA_ARGS__) < 0) error();

#define SCREEN_WIDTH 800
#define SCREEN_HEIGHT 800

#define LINE_SPACE 20
#define LINE_LENGHT 10

typedef struct {
	int x, y;
} vec2i;

// Angle in degrees
vec2i rotate(vec2i pos, vec2i orig, double angle)
{
	double radians = (3.14159265 / 180) * angle;

	int px = pos.x - orig.x;
	int py = pos.y - orig.y;

	vec2i new = {
		.x = px*SDL_cos(radians) - py*SDL_sin(radians),
		.y = px*SDL_sin(radians) + py*SDL_cos(radians),
	};

	new.x += orig.x;
	new.y += orig.y;

	return new;
}

int main()
{
	if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_EVENTS | SDL_INIT_TIMER) != 0)
	{
		printf("Unable to initialize SDL2: %s\n", SDL_GetError());
		return 1;
	}

	SDL_Window *window = SDL_CreateWindow("rotation", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
										  SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
	cn(window);

	SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
	cn(renderer);

	cz(SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND));

	SDL_Texture *texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_UNKNOWN, SDL_TEXTUREACCESS_TARGET, 128, 128);
	cn(texture);

	cz(SDL_SetTextureBlendMode(texture, SDL_BLENDMODE_BLEND));

	cz(SDL_SetRenderTarget(renderer, texture));

	cz(SDL_SetRenderDrawColor(renderer, 255, 255, 255, 150));
	cz(SDL_RenderClear(renderer));

	cz(SDL_SetRenderTarget(renderer, NULL));

	cz(SDL_SetRenderDrawColor(renderer, 18, 18, 18, 255));
	cz(SDL_RenderClear(renderer));

	cz(SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255));

	cz(SDL_RenderDrawLine(renderer, 0, SCREEN_HEIGHT / 2, SCREEN_WIDTH, SCREEN_HEIGHT / 2));
	cz(SDL_RenderDrawLine(renderer, SCREEN_WIDTH / 2, 0, SCREEN_WIDTH / 2, SCREEN_HEIGHT));

	cz(SDL_RenderDrawLine(renderer, 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT));
	cz(SDL_RenderDrawLine(renderer, 0, SCREEN_HEIGHT, SCREEN_WIDTH, 0));

	for (int i = 0; i < SCREEN_HEIGHT / LINE_SPACE; ++i)
	{
		cz(SDL_RenderDrawLine(renderer, SCREEN_WIDTH / 2 - LINE_LENGHT / 2, i * LINE_SPACE, SCREEN_WIDTH / 2 + LINE_LENGHT / 2, i * LINE_SPACE));
	}

	for (int i = 0; i < SCREEN_WIDTH / LINE_SPACE; ++i)
	{
		cz(SDL_RenderDrawLine(renderer, i * LINE_SPACE, SCREEN_HEIGHT / 2 - LINE_LENGHT / 2, i * LINE_SPACE, SCREEN_HEIGHT / 2 + LINE_LENGHT / 2));
	}

	double deg_angle = 45;
	SDL_Rect rect = {
		.x = SCREEN_WIDTH / 2 - 200,
		.y = SCREEN_HEIGHT / 2 - 25,
		.w = 400,
		.h  = 50,
	};

	cz(SDL_RenderCopyEx(renderer, texture, NULL, &rect, 0, NULL, SDL_FLIP_NONE));
	cz(SDL_RenderCopyEx(renderer, texture, NULL, &rect, deg_angle, NULL, SDL_FLIP_NONE));

	cz(SDL_SetRenderDrawColor(renderer, 0, 255, 0, 255));
	cz(SDL_RenderDrawLine(renderer, rect.x, rect.y + rect.h / 2, rect.x + rect.w, rect.y + rect.h / 2));

	vec2i orig = {SCREEN_WIDTH / 2, SCREEN_HEIGHT / 2};

	vec2i pos11 = {rect.x, rect.y + rect.h / 2};
	vec2i pos21 = {rect.x + rect.w, rect.y + rect.h / 2};

	vec2i pos12 = rotate(pos11, orig, deg_angle);
	vec2i pos22 = rotate(pos21, orig, deg_angle);

	cz(SDL_RenderDrawLine(renderer, pos12.x, pos12.y, pos22.x, pos22.y));
	printf("x1=%d, y1=%d, x2=%d, y2=%d\n", pos12.x, pos12.y, pos22.x, pos22.y);

	vec2i v11 = {rect.x, rect.y};
	vec2i v21 = {rect.x, rect.y + rect.h};
	vec2i v31 = {rect.x + rect.w, rect.y};
	vec2i v41 = {rect.x + rect.w, rect.y + rect.h};

	vec2i v12 = rotate(v11, orig, deg_angle);
	vec2i v22 = rotate(v21, orig, deg_angle);
	vec2i v32 = rotate(v31, orig, deg_angle);
	vec2i v42 = rotate(v41, orig, deg_angle);

	SDL_Rect bounding_rect = {
		.x = v22.x,
		.y = v12.y,
		.w = v32.x - v22.x,
		.h = v42.y - v12.y,
	};

	SDL_RenderDrawRect(renderer, &bounding_rect);

	SDL_RenderPresent(renderer);

	SDL_Event event;
	while (SDL_WaitEvent(&event))
	{
		if (event.type == SDL_QUIT) break;
		else if (event.type == SDL_MOUSEBUTTONDOWN)
		{
			int x = event.button.x;
			int y = event.button.y;
			printf("x=%d, y=%d\n", x, y);
		}
	}

	SDL_DestroyRenderer(renderer);
	SDL_DestroyWindow(window);
	SDL_Quit();

	return 0;
}