SYNOPSIS

#include <allegro5/allegro.h>
int al_check_inverse(const ALLEGRO_TRANSFORM *trans, float tol)
DESCRIPTION
Checks if the transformation has an inverse using the supplied tolerance. Tolerance should be a small value between 0 and 1, with 1e7 being sufficient for most applications.
In this function tolerance specifies how close the determinant can be to 0 (if the determinant is 0, the transformation has no inverse). Thus the smaller the tolerance you specify, the "worse" transformations will pass this test. Using a tolerance of 1e7 will catch errors greater than 1/1000's of a pixel, but let smaller errors pass. That means that if you transformed a point by a transformation and then transformed it again by the inverse transformation that passed this check, the resultant point should less than 1/1000's of a pixel away from the original point.
Note that this check is superfluous most of the time if you never touched the transformation matrix values yourself. The only thing that would cause the transformation to not have an inverse is if you applied a 0 (or very small) scale to the transformation or you have a really large translation. As long as the scale is comfortably above 0, the transformation will be invertible.
Parameters:
 trans  Transformation to check
 tol  Tolerance
Returns: 1 if the transformation is invertible, 0 otherwise