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al_check_inverse(3)                                        al_check_inverse(3)

NAME
       al_check_inverse - Allegro 5 API

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
       1e-7 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 1e-7 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:

       o trans - Transformation to check

       o tol - Tolerance

       Returns: 1 if the transformation is invertible, 0 otherwise

SEE ALSO
       al_invert_transform(3)

Allegro reference manual                                   al_check_inverse(3)