principle_curvature returns PD1, PV1 as minimal curvature direction and value, PD2, PV2 as maximum curvature direction and value.
The order is not the same as described in the docs(https://libigl.github.io/libigl-python-bindings/igl_docs/#principal_curvature) and the comments in the header file:

Here's the code (using python bindings) and screenshot to demonstrate this bug:

v, f = igl.cylinder(20, 20)
# pd1, pv1: maximal
# pd2, pv2: minimal
pd1, pd2, pv1, pv2 = igl.principal_curvature(v, f)
print(np.all(pv1<=pv2)) # printing true means pv2 is actually maximal curvature value

p = meshplot.plot(v, f, shading={"wireframe": False}, return_plot=True)
avg = igl.avg_edge_length(v, f) / 2
# red segments should be parallel to the maximal curvature direction
p.add_lines(v + pd1 * avg, v - pd1 * avg, shading={"line_color": "red"})
# blue segments should be parallel to the minimal curvature direction
p.add_lines(v + pd2 * avg, v - pd2 * avg, shading={"line_color": "blue"})

The red lines are actually parallel to the minimal curvature direction.

I wrote unit test to simply make sure that max curvature is no less than min curvature.

I also fix the corresponding tutorial.

Checklist

• All changes meet libigl style-guidelines.
• Adds new .cpp file.
• Adds corresponding unit test.
• This is a minor change.