Possible Issue about curvature calculation [levelset.py/line151]

[kakushiryou]

Hi Rocchi

Thank you for creating this project - it's been really helpful for me since I am a beginer in Level-Set method! 🙌

While working with the code in the function levelset.py/motion_under_curvature, I noticed an possible bug in calculating the curvature.

As far as I know, the curvature is calculated as follows:

The denominator should be the cube of the gradient of phi (||∇φ||³), but the current implementation uses squared (||∇φ||²) .
I don't know whether the φ is the signed distance function. If we assume the signed distance function always have ||∇φ|| = 1, it is OK. But in the simulation, ||∇φ|| = 1 is not always guaranteed because of the numerical discretization and calculation error.

So maybe it is better to change it to :
agrad2 = math.pow(px ** 2 + py ** 2, 1.5) + 0.001