Question

Using Allegro 5, how do initialize a game in fullscreen mode so that it respects the format of the screen (widescreen 16:9 vs normal 3:4)

al_create_display (w, h)

Let’s you select any ratio you want. For example you can set 640×480, regardless of the screen size. But it will look weird on a widescreen monitor.
How do you know which ratio to use?

Answer 1

Hm, I can answer this as well – use al_get_monitor_info().

al_get_monitor_info(0, &info);
w = info.x2 - info.x1; /* Assume this is 1366 */
h = info.y2 - info.y1; /* Assume this is 768 */
al_create_display(w, h);

Now you can either render everything in a 640×480 rectangle centered within 1366×768 to make it appear pixel-perfect, or alternatively scale your graphics up by 768/480 and keep two black bars to the left and right. If you use OpenGL for rendering, both are very easy to do by simply altering the projection matrix.