Why is blender treating vertices differently when applying subdivision surface modifier? I applied subdivision surface modifier to my model and afterwards, I found some annoying burrs on the surface. Taking a closer look revealed that these burrs are caused by vertices - the surface is kind of snapped to these vertices instead of going smoothly.

I have pretty just begun learning Blender so can neither explain or solve this problem. Please help.

P.S. Most vertices have surface goes smoothly under them.
To keep it simple:

If the faces normals are in the same orientation, they can be smoothly interpolated at the subdivision: their mean stays positive.



If they are not, their mean becomes null and so their is no interpolation possible and their mean (so the orientation) is null.



This is like to say the function is not derivable at the face junction.

So the junction stays a singularity and stays angular.