al_check_inverse(3)

💅 al_check_inverse(3) 💅

okay so this little icon is from the Library Functions Manual — specifically the Allegro 5 API — and she is here to answer one question: does this transform have an inverse? 💖

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🌸 NAME 🌸

al_check_inverse — Allegro 5 API. That's the whole name, bestie, and yes she's a function.

✨ SYNOPSIS ✨

#include <allegro5/allegro.h>

int al_check_inverse(const ALLEGRO_TRANSFORM *trans, float tol)

🦋 DESCRIPTION 🦋

So, al_check_inverse checks whether the transformation has an inverse, using the tolerance you hand it. The tolerance should be a small value between 0 and 1 — 1e-7 is sufficient for most applications, and that's lowkey the go-to vibe. ✨

In this function, tolerance says how close the determinant can be to 0. And bestie, if the determinant is 0, the transformation has no inverse. So 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 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 be less than 1/1000’s of a pixel away from the original point. Iconic behavior, honestly. 💅

Not me putting this on the record: 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. Periodt. 🫶

Parameters 🌈

• trans — transformation to check
• tol — tolerance

Returns: 1 if the transformation is invertible, 0 otherwise. That's the whole vibe check: 1 = slay, 0 = nope. 😭

Note: Allegro’s transformation inversion functions work correctly only with 2D transformations. Important caveat, bestie — don't forget it. 💀

👑 SEE ALSO 👑

• al_invert_transform(3) — the related read, go click it if you need the inversion itself. 🔥

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mwah 💋 — source/version: Allegro reference manual; date: the page doesn't print one, so no invented date bestie; page: al_check_inverse(3). ✨