Tips and tuts

Ostrich Egg Globe or Da Vinci Globe
From Wikipedia, the free encyclopedia
Da Vinci Globe on Sketchfab
…and a simple and laborious video tutorial in french :

Penrose Triangle, Reutersvard Rectangle, Impossible Arch. axonometric camera
Eevee

Select the face and inset it by pressing [i] and drag
Select the vertices to uncross
Press [s] and drag them
Optionally correct them with Loop tools

With two cubes, I modeled three simples shapes : one for the head (connector), the other for the inner part of the head (from a face) and the last one for the body (I kept a third of it and applied a mirror modifier in Z and Y).
… then applied a bunch of modifiers (subdivision surface, mirror, boolean, bevel, weighted normals…)
Note : to apply an image texture, delete each uv map of the sources of the boolean opérations (the « Brushes Boolean) »
…and finally rendered it with Cycles (...)

Model
a photoscanned pine cone from Sketchfab Render engineWorld
No lamp and a dark background Material
Overview  :
A layer weight node as a mix factor between a transparence shader and an emission shader — to reveal the contour of each seed scale and the central stem  :
A camera data node to show the z-depth with colors  :
the provided normal map to the layer weight node  :
the normal map’s strength from 0 to 10  :
Salut (...)

Maybe more accurate than the node tree from Chocofur, a slightly adapted version from the Lance Phan’s one Add three nodes to a material with [Ctrl-A] : Fresnel, Invert, Multiply (see the first image below) Select those nodes and group them with [Alt-g] Connect them with the Group Input node and the Group Output node (see the second image below) Create the Roughness input with the Fac of the Invert node to have a slider from 0 to 1 (set the Default value to 0,5 — see the third image) then (...)

To detect the edges of a model, Pointiness (Geometry node) is difficult to setup and need at least bevel or subdivision of the model. I did not find a correct math node or a mixRGB to use correctly the bevel node.
…so I used the AO node !
Simple cube with boolean modifier and a bevel node.

… and random orientation too
https://www.youtube.com/watch?v=i4Z_WGZLpm0