2D C++ Game: Part 6 - Image Background Transparency, Game FPS, Events, Game Loop

// Allegro library
#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>

//Screen resolution
#define SCREEN_W 640
#define SCREEN_H 480

// Game fps
#define FPS 60

int main()
{
    al_init(); // Start Allegro
    ALLEGRO_DISPLAY *screen = NULL; // Main display
    ALLEGRO_EVENT_QUEUE *event_queue = NULL; // Event queue to maintain events
    ALLEGRO_TIMER *timer = NULL; // Timer to control game fps

    // Initializing screen, event queue and timer
    screen = al_create_display(SCREEN_W, SCREEN_H);
    event_queue = al_create_event_queue();
    timer = al_create_timer(1.0/FPS); // Timer set to tick at 60 frames per second

    // Register event sources to be handled in the event queue
    al_register_event_source(event_queue,al_get_display_event_source(screen));
    al_register_event_source(event_queue,al_get_timer_event_source(timer));

    al_init_image_addon(); // Initialize Allegro image addon
    al_start_timer(timer); // Start timer

    // ALLEGRO BITMAP 'image' to hold the image
    ALLEGRO_BITMAP *image = NULL;
    // Load bitmap
    image = al_load_bitmap("Pacman_Sprite.png");
    // Convert image background to transparent
    al_convert_mask_to_alpha(image,al_map_rgb(255, 0, 255));



    int x=0; // x coordinate of the image

    while(1) //Game Loop
    {
        ALLEGRO_EVENT event; // ALLEGRO_EVENT to hold events
        al_wait_for_event(event_queue,&event); // Wait for events to occur

        // Display handler
        // Draw if timer counter has incremented and there are no events to handle
        // and event queue is empty
        if(event.type == ALLEGRO_EVENT_TIMER && al_is_event_queue_empty(event_queue))
        {
            al_draw_bitmap(image, x, SCREEN_H/2, 0); // Draw image on the backbuffer
            al_flip_display(); // Display backbuffer on to the main screen
            al_clear_to_color(al_map_rgb(0, 0, 0)); // Clear display to black
        }

        //Event handlers
        if (event.type == ALLEGRO_EVENT_TIMER)
        {
            // Increment x coordinate of the image
            // When position of the image is more than the screen width, reset x coordinate to 0
            if(++x>=SCREEN_W)x=0;
        }else if(event.type==ALLEGRO_EVENT_DISPLAY_CLOSE)
        {
            break; // End game loop on closing display window
        }


    }// End of Game Loop

    al_destroy_display(screen); // Destroy display
    return 0;
}

Explanation:
▪ Image Background Transparency

    // ALLEGRO BITMAP 'image' to hold the image
    ALLEGRO_BITMAP *image = NULL;
    // Load bitmap
    image = al_load_bitmap("Pacman_Sprite.png");
    // Convert image background to transparent
    al_convert_mask_to_alpha(image,al_map_rgb(255, 0, 255));

Image background transparency is achieved with the Allegro routine al_convert_mask_to_alpha(ALLEGRO_BITMAP *bitmap, ALLEGRO_COLOR mask_color) which takes 2 parameters:

■ ALLEGRO_BITMAP structure which holds our image.
■ ALLEGRO_COLOR structure, in our case we gave al_map_rgb(255, 0, 255) which is the rgb value for magenta, which is the background color of our image.

▪ Game FPS

// Game fps
#define FPS 60
ALLEGRO_TIMER *timer = NULL; // Timer to control game fps
timer = al_create_timer(1.0/FPS); // Timer set to tick at 60 frames per second
al_register_event_source(event_queue,al_get_timer_event_source(timer));
al_start_timer(timer); // Start timer