Texture rotation

In this section, we will examine the program that allows a texture created from a .png image file and loaded into the main window to be rotated with the left and right arrow keys.

When the program runs, in addition to the basic operations, the following operations occur:

1. At the beginning of the program, the global variables shown below are first defined:
   • SDL_Texture *texture: Texture variable
   • double angle: Angle variable
2. The init_vars() function assigns initial values ​​to global variables.
3. In the load_img() function, a clock image is loaded into the texture variable with the IMG_LoadTexture() function.
4. In the process_event() function, the left and right arrows keys, the angle variable value is increased or decreased by 5.
5. In the draw_screen() function, using SDL_RenderTextureRotated() function, rotates the source texture and copies it to the main window.

main.c file content will be as follows:

#include <stdio.h>
#include <SDL3/SDL.h>
#include <SDL3_image/SDL_image.h>

// Window width and height
#define WINDOW_WIDTH 640
#define WINDOW_HEIGHT 480

// Global variables
int is_running = false;        // Variable that controls the execution of the main loop of the program
SDL_Window *window = NULL;     // Main window variable
SDL_Texture *texture = NULL;   // Texture variable
SDL_Renderer *renderer = NULL; // Renderer variable
double angle;                  // Angle variable

// Function prototypes
int init_window(void);         // Create window and renderer
void init_vars(void);          // Initialize variables
void load_img(void);           // Load image to texture
void process_event(void);      // Process events
void update_screen();          // Update values
void draw_screen(void);        // Draw screen
void destroy_window(void);     // Destroy window

int main(int argc, char* argv[])
{
    // Create window and renderer
    is_running = init_window();

    // Initialize variables
    init_vars();

    // Load .png file to texture
    load_img();

    // Main loop
    while (is_running) {
       process_event(); // Processing SDL events (Here keyboard inputs)
       update_screen(); // Updating variables
       draw_screen();   // Drawing objects on the window (Rendering)
    }

    // Destroy renderer and SDL window
    destroy_window();

    return 0;
}

// Create window and renderer
int init_window(void)
{
    // Initialize the SDL library.
    if(SDL_Init(SDL_INIT_VIDEO) == false) {
       SDL_Log("SDL init error: %s\n", SDL_GetError());
       return false;
    }

    // Create a window and a 2D rendering context for the window.
    if(!SDL_CreateWindowAndRenderer("SDL3 window", WINDOW_WIDTH, WINDOW_HEIGHT, 0, &window, &renderer)) {
       return false;
    }

    return true;
}

// Initialization function that runs only once at the beginning of the program
void init_vars(void)
{
    angle = 0;
}

// Load image to texture
void load_img(void)
{
    // Loading image to texture
    texture = IMG_LoadTexture(renderer, "clock.png");

	if(texture == NULL) {
       SDL_Log("IMG_LoadTexture error: %s\n", SDL_GetError());
	}
}

// Function to control SDL events and process keyboard inputs
void process_event(void)
{
    SDL_Event event;

    // Creating a loop to process user inputs
    while (SDL_PollEvent(&event)) {
       switch (event.type) {
          case SDL_EVENT_QUIT: // Logout action by the user (x button at the top right of the window)
               is_running = false; // Terminates the execution of the program main loop.
               break;

          case SDL_EVENT_KEY_DOWN: // Key pressed.
               switch(event.key.key) {
                  case SDLK_LEFT: if(angle-5>=-360) angle -= 5; break;
                  case SDLK_RIGHT: if(angle+5<=360) angle += 5; break;

                  case SDLK_ESCAPE: is_running = false; break;
               }
               break;
       }
    }
}

// Updates objects in the main window
void update_screen(void)
{

}

// Render function used to draw game objects in the main window
void draw_screen(void)
{
    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255); // Set the color used for drawing operations.
    SDL_RenderClear(renderer); // Clear the current rendering target with the drawing color.

    SDL_FRect frect_dst = { (WINDOW_WIDTH-300)/2, (WINDOW_HEIGHT-300)/2, 300, 300 };
	SDL_RenderTextureRotated(renderer, texture, NULL, &frect_dst, angle, NULL, SDL_FLIP_NONE);

    // Loading all objects drawn one by one to the back buffer to the front buffer at once and loading them onto the screen
    // This process prevents each drawn object from being displayed on the screen one by one.
    SDL_RenderPresent(renderer); // Update on screen all operations performed since the previous call

    char cdizi[50];
    sprintf(cdizi, "SDL3 window - Angle: %.2f", angle);
    SDL_SetWindowTitle(window, cdizi);
}

// Destroy Renderer and SDL window, exit from SDL3
void destroy_window(void)
{
    SDL_DestroyTexture(texture);
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();
}