Right now the function used is based on ellipse fitting: https://github.com/DavidMercier/TriDiMap/blob/d35131e1d9c7c4d0b2de46a20488a934881df7bc/third_party_codes/fit_ellipse.m But why not using convex Hull function: https://towardsdatascience.com/clustering-using-convex-hulls-fddafeaa963c https://fr.mathworks.com/help/matlab/ref/convhull.html https://chitchatr.wordpress.com/2011/12/30/convex-hull-around-scatter-plot-in-r/ 
Right now the function used is based on ellipse fitting:
https://github.com/DavidMercier/TriDiMap/blob/d35131e1d9c7c4d0b2de46a20488a934881df7bc/third_party_codes/fit_ellipse.m
But why not using convex Hull function:
https://towardsdatascience.com/clustering-using-convex-hulls-fddafeaa963c
https://fr.mathworks.com/help/matlab/ref/convhull.html
https://chitchatr.wordpress.com/2011/12/30/convex-hull-around-scatter-plot-in-r/