Artist James Nolan Gandy invents robot arms that sketch intricate mathematical shapes with pen and paper. When viewed in real time, the effect is impressive. But it becomes even more so when the videos are sped up in a timelapse. If you look closely in the video below, the illustration appears to materialize faster than the robot can put the design to paper. Gizmodo recently explained how the illusion works to make it look like parts of the sketch are forming before the machine has time to draw them.

The optical illusion isn’t an example of tricky image editing: It’s the result of something called the wagon wheel effect. You can observe this in a car wheel accelerating down the highway or in propeller blades lifting up a helicopter. If an object makes enough rotations per second, it can appear to slow down, move backwards, or even stand still.

This is especially apparent on film. Every “moving image” we see on a screen is an illusion caused by the brain filling in the gaps between a sequence of still images. In the case of the timelapse video below, the camera captured the right amount of images, in the right order, to depict the pen as moving more slowly than it did in real life. But unlike the pen, the drawing formed throughout the video isn't subject to the wagon-wheel effect, so it still appears to move at full speed. This difference makes it look like the sketch is drawing itself, no pen required.

Gandy frequently shares behind-the-scenes videos of his mechanical art on his Instagram page. You can check out some of his non-timelapse clips like the one below to better understand how his machines work, then visit his website to browse and purchase the art made by his 'bots.

And if you think his stuff is impressive, make sure to explore some of the incredible art robots have made in the past.

[h/t Gizmodo]