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(3) 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(3), al_orthographic_transform(3)