Question on the SCF visualization image in Readme

Hi, 

Thank you for this nice work, and the valuable proposition of utilizing SCF to encode the object geometry distribution in space.

I have a question regarding the image of SCF visualization in Readme -> https://github.com/zzilch/NIFT/blob/main/imgs/SCF.png
Could you please let me know what does the x-axis and y-axis indicate? 

Thanks!