1 Answer
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I guess there are at least 2 ways of doing it: either you do it "for real" with topology, or you simulate it in the material with Bump or Normal maps
Topology:
- Create a cylinder with as many vertices as you want
- Select the face loop
- Select > Checker Deselect so that only one of two faces is selected
- Keep the faces selected and Alt+E > Extrude > Individual Faces
- Scale down these extrusions except on the Z axis with S and Shift+Z
Material:
- Create a surface with a series of crenels
- Bake this surface so that you have a Normal image
- Use this Normal for you whole ring (on my screenshot it's very quick made, sorry)