# al_perspective_transform - Man Page

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)

## Referenced By

al_orthographic_transform(3), al_use_projection_transform(3).

Allegro reference manual