How to put an image as background (Allegro 5) - allegro5

Question

I am new to Allegro (5) and I am making my first game. I need to put an image as background and a little square that moves in front of it.
In my code, on each frame the image is drawn at the (0;0) coordinates and then the square is drawn at (something;something), and, I think, it should appear over the image because it was drawn after it, but it doesn't.
Can someone explein me how to do this? Thanks
PS : Looking around in the Internet I found that this could have something to do with blitting, could someone explain me what that operation is?

Answer 1

I'll post the code to just draw the background on display. And I'll explain it little by little
#include <stdio.h>
#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>
#define BACKGROUND_FILE "background.png"
int main(void){
ALLEGRO_DISPLAY *display = NULL;
ALLEGRO_BITMAP *background=NULL;
if(!al_init()) {
fprintf(stderr, "failed to initialize allegro!\n");
return -1;
}
al_init_image_addon();//should check for errors
display = al_create_display(640, 480);
if(!display) {
fprintf(stderr, "failed to create display!\n");
return -1;
}
background=al_load_bitmap(BACKGROUND_FILE);
if(!background)
{
fprintf(stderr, "failed to load background bitmap!\n");
return -1;
}
al_draw_bitmap(background,0,0,0);
al_flip_display();
al_rest(2.0);
al_destroy_display(display);
al_destroy_bitmap(background);
al_uninstall_system();
return 0;
}
This part starts up allegro, al_init starts up the allegro system, and al_init_image_addon lets us use bitmaps which is how allegro manages images:
if(!al_init()) {
fprintf(stderr, "failed to initialize allegro!\n");
return -1;
}
al_init_image_addon();//should check for errors
Here we create a display and check if it was successfully created:
display = al_create_display(640, 480);
if(!display) {
fprintf(stderr, "failed to create display!\n");
return -1;
}
Here we load an image, you just simply put in the file name and allegro will load it for you returning a ALLEGRO_BITMAP * (NULL if unsuccessful). After calling al_load_bitmap we check if the bitmap was successfully loaded.
background=al_load_bitmap(BACKGROUND_FILE);
if(!background)
{
fprintf(stderr, "failed to load background bitmap!\n");
return -1;
}
Allegro draws to a backbuffer, what this means is that it won't directly draw to the display. Instead, it will draw to a copy of the display(backbuffer) and once you flip the backbuffer(al_flip_display()), this copy (which you were drawing on top of) will show up all at once.
This can be see seen here:
al_draw_bitmap(background,0,0,0);
al_flip_display();
If you're going to be initializing lots of allegro stuff, you might want to initialize all together like for example :
int allegro_startup(void)
{
if(al_init())
{
if(al_init_primitives_addon())
{
if(al_install_keyboard())
{
if(al_install_mouse())
{
if(al_init_image_addon())
{
al_init_font_addon(); //Void
if(al_init_ttf_addon())
{
if(al_install_audio())
{
if(al_init_acodec_addon())
{
if(al_reserve_samples(1))
{
return AL_STARTUP_SUCCESS;
}
else
fprintf(stderr,"ERROR: Failed to reserve samples:(\n");
//al_shutdown_acodec_addon(); Does not exist
}
else
fprintf(stderr,"ERROR: Failed to initialize acodec addon\n");
al_uninstall_audio();
}
else
fprintf(stderr,"ERROR: Failed to install audio\n");
al_shutdown_ttf_addon();
}
else
fprintf(stderr,"ERROR: Failed to initialize ttf addon\n");
al_shutdown_font_addon();
al_shutdown_image_addon();
}
else
fprintf(stderr,"ERROR: Failed to initialize image addon\n");
al_uninstall_mouse();
}
else
fprintf(stderr,"ERROR: Failed to install mouse\n");
al_uninstall_keyboard();
}
else
fprintf(stderr,"ERROR: Failed to load primitives addon\n");
al_shutdown_primitives_addon();
}
else
fprintf(stderr,"ERROR: Failed to install keyboard\n");
al_uninstall_system();
}
else
fprintf(stderr,"ERROR: Failed to initialize allegro system\n");
return AL_STARTUP_ERROR;
}
void allegro_shut_down(ALLEGRO_DISPLAY *display,ALLEGRO_EVENT_QUEUE *event_queue)
{
al_destroy_display(display);
al_destroy_event_queue(event_queue);
al_uninstall_audio();
al_shutdown_ttf_addon();
al_shutdown_font_addon();
al_shutdown_image_addon();
al_uninstall_mouse();
al_uninstall_keyboard();
al_shutdown_primitives_addon();
al_uninstall_system();
}
Here are a lot of tutorials which are a lot more clearer and a little broader subjects if you're interested.