Allegro Programming Tutorial, by Richard G Baldwin

Save a square portion of the background image

Listing 4 calls the blit function to

Copy and save a square area of the background image
Surrounding the location at which the ball will be drawn when it first appears on the screen.

Each side of the square is two pixels larger than the diameter of the ball:

To allow for the possibility that the ball isn't perfectly centered in the square.

Listing 4. Save a square portion of the background image.

blit(buffer,smallBuffer, x-radius-1,y-radius-1, 0,0, radius*2+2,radius*2+2);