NAME

       al_perspective_transform - Allegro 5 API

SYNOPSIS

              #include <allegro5/allegro.h>

              void al_perspective_transform(ALLEGRO_TRANSFORM *trans,
                 float left, float top, float n,
                 float right, float bottom, float f)

DESCRIPTION

       Like  al_orthographic_transform(3alleg5) but honors perspective.  If everything is at a z-
       position of -near it will look the same as with an orthographic transformation.

       To use a specific horizontal field of view you can use the relation:

              tan(hfov / 2) = (right - left) / 2 / near

       and therefore

              near = (right - left) / 2 / tan(hfov / 2)

       Example 1:

                 float w = 800, h = 450; // assume our display is 800 x 450
                 float fov = tan(90 * ALLEGRO_PI / 180 / 2); // 90 degree field of view

                 // Our projection goes from 0/0 to w/h with the near parameter set
                 // for a 90 degree horizontal viewing angle.
                 ALLEGRO_TRANSFORM projection;
                 al_identity_transform(&projection);
                 al_perspective_transform(&projection, 0, 0,
                  w / 2 / fov,
                  w, h,
                  2000);
                 al_use_projection_transform(&projection);

                 // Set the camera z to +400 (which is exactly the near distance)
                 ALLEGRO_TRANSFORM camera;
                 al_build_camera_transform(&camera, 0, 0, 400, 0, 0, 0, 0, 1, 0);
                 al_use_transform(&camera);

                 // This will draw two rectangles at the left and right edge of the
                 // screen and vertically centered. The x distance between them is 800
                 // units, but the camera transform moves them 400 along z, so with
                 // a 90° viewing angle both are visible.
                 al_draw_filled_rectangle(0, 200, 50, 250, red;
                 al_draw_filled_rectangle(750, 200, 800, 250, red);

       Example 2:

                 float w = 800, h = 450; // assume our display is 800 x 450
                 float fov = tan(90 * ALLEGRO_PI / 180 / 2); // 90 degree field of view
                 float aspect = h / w;
                 float zoom = 2; // enlarge x 2

                 // This projection is very different from the one in the first example.
                 // Here we map the left and right edge of the screen to -1 and +1. And
                 // the y axis goes from -1 at the bottom to +1 at the top, scaled by
                 // the aspect ratio. We also add a zoom parameter so we can control
                 // the visible portion of the scene independent of the field of view.
                 ALLEGRO_TRANSFORM projection;
                 al_identity_transform(&projection);
                 al_perspective_transform(&projection,
                    -1 / zoom, aspect / zoom,
                    1 / fov,
                    1 / zoom, -aspect / zoom,
                    2000);
                 al_use_projection_transform(&projection);

                 // Moves everything by -4 in the z direction.
                 ALLEGRO_TRANSFORM camera;
                 al_build_camera_transform(&camera, 0, 0, 4, 0, 0, 0, 0, 1, 0);
                 al_use_transform(&camera);

                 // At a z distance of 4 with a 90° hfov everything would be scaled
                 // down to 25%. However we also zoom 2-fold, so the final scaling is
                 // 50%. This rectangle therefore will appear at a size of 400 x 225
                 // pixel (assuming the display is 800 x 450).
                 al_draw_filled_rectangle(-1, -1, 1, 1, red);

SINCE

       5.1.3

SEE ALSO

       al_use_projection_transform(3alleg5), al_orthographic_transform(3alleg5)