Comparision between SSIM and MAD Image Quality Assessment Algorithms
A natural practice to convert, in a monotonous way, data in $[0\,\infty)$ to $[0\,1)$ is an increasing function, like those called sigmoid functions. They behave like $x\mapsto x$ close to $0$, and flatten to $1$ when $x\to \infty$. Classical examples are:


$$  x\mapsto \frac{x}{1+x} $$
$$  x\mapsto \frac{x}{\sqrt{1+x^2}} $$
$$  x\mapsto \frac{2}{\pi}\arctan{x} $$


There are many more versions, since they are used a lot in statsitics, articial neural networks, etc.