Italics angle compensations

Hello everyone,

I'm having a hard time findind anything online regarding the angle-variations that some — most? — of the italics present (example below : Garamond).



So my question is: is there a rule or some research/guidance that could help understand how to calculate those angle variations ? If rule or calculation there is. 

I get that this is a way to compensate a kind of "spinning" effect — for instance the f could appear like rotating clockwise if not somehow backslanted — but I'd like to know more. 

Any help is more than welcome ! 
Thanks in advance and have a great day / evening depending on where you're reading this from !

Raphaël