al_transpose_transform - Man Page

#include <allegro5/allegro.h>

void al_transpose_transform(ALLEGRO_TRANSFORM *trans)

Transposes the matrix of the given transform. This can be used for inversing a rotation transform. For example:

al_build_camera_transform(camera, 0, 0, 0, x, y, z, xu, yu, zu)
al_copy_transform(inverse, camera)
al_transpose_transform(camera)
// Now "inverse" will be a transformation rotating in the opposite
// direction from "camera". Note that this only works if the camera
// position is 0/0/0 as in the example.

5.2.5