---
name: "al_perspective_transform"
section: "3"
platform: "linux"
package: "allegro"
version: "5.2.11.3-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/al_perspective_transform"
---

# al\_perspective\_transform(3)

Allegro 5 API

## 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)](/al_orthographic_transform/3.md) 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_use_projection_transform/3.md), [al\_orthographic\_transform(3)](/al_orthographic_transform/3.md)