OBSTACLE INFLUENCED. “Instead of the configuration indifferent to obstacles, (x,y), a coordinate system describing straight lines would be (ξ,η). The notion of directions and angles imparted on passing particles would be that of the lines in figure 1 instead of lines of constant value of (x, y).”
https://link.springer.com/content/pdf/10.1007/JHEP07(2019)140.pdf