SDL-drawing-sample.c

#include <SDL.h>

int main()
{
    SDL_Init(SDL_INIT_VIDEO);
    
    const int w = 800, h = 600;
    SDL_Window *window = SDL_CreateWindow("Drawing-sample", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, w, h, 0);
    SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, 0);
    SDL_Event event;
    
    const int x0 = 0, y0 = h; // on macOS, the window's top-left position is (0, h)
    const int step = 1, delay = 1;
    int quit = 0, x = x0;
    while (!quit)
    {
        SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);     // white
        SDL_RenderClear(renderer);                                // background
        
        SDL_SetRenderDrawColor(renderer, 64, 64, 64, 255);        // dark gray
        SDL_RenderDrawLine(renderer, x0, y0, x0 + w, y0 + h);     // oblique line from top-left to bottom-right
        
        SDL_SetRenderDrawColor(renderer, 255, 128, 64, 255);      // orange
        SDL_RenderDrawLine(renderer, x0 + x, y0, x0 + x, y0 + h); // vertical line at current coordinate x
        
        SDL_RenderPresent(renderer);
        
        SDL_PollEvent(&event);
        switch (event.type)
        {
            case SDL_QUIT:
                quit = 1;
                break;
            case SDL_MOUSEBUTTONDOWN:
                x += step;
                if (x >= x0 + w) // when we reach right
                    x = x0;      // restart from left
                break;
        }
        
        SDL_Delay(delay);
    }
    
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();
    
    return 0;
}