Cylinder meshes and boundaries

Question

Maybe i am just missing something.

(Eevee, Blender 2.9) I have a cylinder and I wish to add 2 meshes to and cut a shape from the side.

I select cylinder object and then use Ctrl+R in edit mode to create an upper segment. I highlight all faces of the upper segment and apply one material - pink. I highlight all the remaining lower faces and apply second material - grey.

I add smooth shading, edge split and subsurface division (catmuli) - then the mesh material (pink) jumps and extends beyond the mesh bounds.

If i click "on cage" option in subsurf then the mesh material extends to the full size of material - but not to the original height of the mesh cut.

Then i add a rectangle mesh and perform a "knife project" on them to cut shape out of side of cylinder. Doing this displaces the entire material.

In the attached blend file there are 3 cylinders

Cylinder 1 - the original size of material applied to top segment, no modifiers

Cylinder 2 - the new size of pink material, after applying modifiers.

Cylinder 3 - the deformed material from the knife project

Why does this material displacement happen? Why the deform on knife project?

Thanks in advance. U.

Answer 1

This is caused by the Subdivision modifier.
It interpolates the spacing between edges, and the subdivided geometry is slightly moved.

The same thing is causing the distortion when adding cuts with knife-project. You're effectivelly adding loopcuts, which limits the stretching.

These are your options:

1. Add a crease to the edge
2. Make the geometry evenly spaced on the object
3. Add a loopcut on both sides of the edge
4. Split the edge using Edge split modifier