I am having so much fun with this thing.

I've implemented parts of a content pipeline for rendering a scene on the PC and streaming it to this display, but writing video streaming code is so much less fun than playing with voxels that it may take a while to finish.
Here, I've stored the animation uncompressed on the display itself, and am updating it as fast as the Pi's SD card can handle. (Not very fast.)

My target for this display is 600 rpm - lower than that and it's too flickery; higher than that and I can't refresh fast enough to get 400 voxels around the circumference without dropping to 1 bpc. I'm nudging 400 rpm here, and it's still pretty unfilmable and absolutely terrifying to be close to. I have to decide whether the overall approach is worthwhile enough to start spending money on aluminium and polycarbonate.

Slightly higher rpm, slightly longer shutter.

The other problem I have is that to sell the 3D effect I need to move the camera around a lot, so I'm going to have to put some effort into building a studio backdrop.

Incidentally, those models are from Cheello's voxel Doom: https://www.moddb.com/mods/doom-voxel-project/addons/voxel-doom - It's a lovely mod, and makes Doom feel more like my memories of playing it than the real thing does.

Latest flickery mess

I had a panel left over, and I thought I should have another stab at an oscillating display. I wanted to give it an undulating motion and came up with what seemed like a nice linkage, but the end result looks like it was designed by Trevithick.

It's a nice fluid motion on the panel, but overall it doesn't bring me joy.

Shiny! (Maybe too shiny. )

Guess I’m doing a cone next.

There must be at least 6D here.

I rewired the back of the panel to tidy up all the loose flappy cables. It now manages 600 rpm, which is not too flickery.

I mean, you should have seen it before.

In the continuing quest for higher rpm, I've moved the controller down below the screen and across the axis of rotation. It's a lot harder to get at if I need to rewire anything, but it does improve the balance.

It feels as though I'm endlessly rebuilding it, for diminishing improvements. But in the most recent rebuild I finally solved a mystery that has been bugging me. When the display had been running for a while, it would quite abruptly lose balance and start vibrating. After the last occurrence, it was never quite the same. On stripping it down I found this.

Pi 4 model A

I continue to fail to shoot footage of it that does it justice.

This feels like a good match of style and content.

I’m now suspicious of all the PLA parts. The little pit with the Pi in it is getting very warm.

kind of feels like it needs monsters?

Monsters.
https://youtu.be/yIfgjr-B8xI

I do like an ample window and natural light, but it makes it hard to see the leds. Hence this pirate astronaut.

Doom running at a larger scale. Easier to make out what's going on, harder to see what's shooting at you.
https://youtu.be/bRe1OSkeiQg

This display works by spinning a matrix display rapidly about a vertical axis, lighting up each LED as it passes through part of a 3D image. The way you update the displays has a big impact on the quality of the image.
In this gif, each dot represents a column of LEDs - we're looking at the device from above. Here the panel is treated like a 2D display which just happens to be moving. Each scan line is repeatedly visited in turn, sweeping out a set of slices where the image can be displayed.

If you turn the panels 90 degrees so their scan lines are now columns, you can do a bit better. The update doesn't have to be sequential - instead of stepping one column at a time, you can skip a few each time. As long as you pick a number which is coprime with the number of columns, and you wrap around once you go past the last one, you'll still visit each of them but spread out more evenly throughout the volume.

The real improvement comes when you adjust the update rate for each column to match the length of the track they have to sweep out. Instead of wrapping around when you reach the last column, you wrap around when your counter reaches the square of the number of columns, and you update the column corresponding to the integer square root of the counter. This gets rid of the bright dense region in the middle, and adds more updates out at the edges making them less sparse.

In practice it's complicated by the fact that these panels update two lines at once. Every time you update a column in the outer half, you're also updating one in the inner half. I couldn't find a simple procedural update strategy to spread these evenly, so I ended up generating a lookup table for it using simulated annealing.

First test of the new design, and already I’m happy. Quiet, high refresh rate, and doesn’t feel like it’s seconds away from embedding itself in my face.

Dynamic balancing using a tray of marbles and iPhone slowmo.

These guys.

In motion https://youtu.be/na7pvihXhYs

So many complaints about the framerate on that last video, so I decided to upload one with a shorter exposure and more flicker. But I still ended up keeping it below 30 fps, so I suspect the complaints will be about both framerate and flicker. Just have to hope the algorithm doesn’t go so large on this one. https://youtu.be/gBfclb9hXCI

New dome!
There are many advantages to moving to smaller panels in the new design (momentum etc), but the price drop going to a 300mm dome from 400mm would be justification enough.

There’s now an accelerometer in the base, synced to the rotation of the screen. I’m hoping this will help me get it balanced better. I don’t entirely understand the shape it’s producing, but the line is pointing in roughly the direction I think it should be pointing.

2D video of 3D projection of 4D object.

Original voxels

There’s a fair bit of planning involved in finding the true centre and height of these domes. When I come to make the cut it feels like cleaving the Cullinan diamond.

With the previous dome the cut had a somewhat hand made look to it, so I printed a thin piece of trim to slip over the edge and keep it neat. It was too big for the printer so I used TPU, printing it in a spiral and flexing it back in to the right diameter. An unexpected benefit was that it was way quieter with that isolating the dome from the base.
This time the cut went better, but I’m still going to give it a gasket for that reason.