Instagram Lux effect

Question

Instagram added a Lux button recently, which allows auto-contrasting / leveling for the pictures you take.

I have a bunch of pictures that I need to autolevel in a simi

Answer 1

For those who haven't solved a linear equation in a very long time (such as I) here the required parameters from Adi Shavit's answer in a formula (in pseudo code) - I renamed p5 and p95 to p1 and p2 (as 5 is just a suggestion):

p_div := float(p1) / p2;
b := -255 * p_div / (1 - p_div);
a := (255 - b) / p2;

I don't know if it can be done easier, but you can check that:

a * p1 + b  -> 0
a * p2 + b -> 255