If you’ve ever seen a “holographic” display, it was probably a persistence of vision (PoV) display unless it was a newer volumetric display. PoV displays work by spinning an array of LEDs very quickly and rely on our eyes’ natural “refresh rates” to create the illusion of a solid image. If you wanted to display a stable image, the LEDs would be timed to only flash on at the same point in the rotation. To create animations or moving text, the LEDs are additionally modulated like a conventional LED matrix display. But those are usually canned, pre-rendered animations. Redditor Evlmnkey took things to the next level with a PoV display that shows 3D video in real-time.
Evlmnkey built this amazing persistence of vision display as a project for their bachelor’s degree. Unlike most other PoV displays that show simple animations made from frames that were created ahead of time, this gathers 3D data and uses that to create a pseudo-3D hologram in real-time. The 3D data is captured by the depth camera in the Azure Kinect DK (Developer Kit), which is intended to replace the discontinued Kinect sensor that was originally developed for gaming on the Xbox 360. That provides a three-dimensional point cloud made from whatever the depth camera is pointed at. In their demonstration video, that point cloud is Evlmnkey’s body. The resolution isn’t particularly high, but it is enough to produce a clearly recognizable human.
That point cloud is processed on Evlmnkey’s computer and used to generate the frames of the video at 30 FPS. Those frames are then sent wirelessly to an ESP32, which controls the LEDs. Timing is incredibly important for a PoV display, because each LED has to be lit for just a tiny fraction of a second at the right time during the rotation. This display is monochrome, which means the brightness of the LEDs also need to be controlled in order to render the depth in the 3D video. This forced Evlmnkey to develop their own algorithms to control the LEDs, as existing libraries couldn’t keep up with the speed necessary. The LEDs seem to be SMD components soldered to a custom PCB, which spins on a motorized mount. The results here are fantastic and even surpass many of the professional PoV displays that we’ve seen in the past. We think it’s safe to assume that this project earned Evlmnkey an A+.