Chladni Pattern

Interference Pattern

I am referring to these patterns code by carlos ponce in open processing. I can’t seem to understand a bit of what is actually causing pattern. Dissecting it bit by bit but intuitive understanding is tricky.

Also the formula for chladni is different then what is essentially used in the chladni pattern code.

This is the general formula I found from internet :

asin(PInx)sin(PImy) + bsin(PImx)sin(PIny) = 0

But the formula used in the code is different than this. Any insight on how the calculation is carried out would greatly help to understand the code. Thanks:)