This is my Threaded Illusion Pencil Pot. You may be wondering where the illusion is - please enlarge (it does not work very well or at all with the smaller pictures) and look at the images that contain one pot. See how it looks bowed at the edges? The threads are actually exactly the same width up and down.

You may need to position the model carefully to see this if you have printed it, to ensure the shadow effect works at the right angle (or not, it might just pop) and that you are close enough.

Design

The illusion is achieved by use of shadow via the threads. A lot of so-called optical illusions actually occur in the brain, do you think this is one of those?

In the upper part of the model, the bow effect is real as the inner diameter of the thread gets smaller all the way to the bottom.

However, the the mid-point in the Z axis in the threaded portion of the model the outer threads begin to project larger shadows into the inner threads.

As the inner thread gets deeper, the effect of the shadow is increased, which gives the appearance of darker inner threads with a larger diameter, rather than the smaller one it actually has.

Printing

This has been designed to be easy to print with the bottom of the model on the print bed. The above models were printed at 0.16 layer height to keep the threads looking nice, and they look absolutely superb at this setting.

The blue pot above was printed in black/blue PLA-CF and the green one in normal matte PLA, both of which I prefer for the textured finish and less reflective properties than normal PLA or PLA+. I hope you enjoy this model!