A first look at the FPGA architecture

In order to synchronize the LEDs and create a persistence of vision image, we use a CycloneV SOC.

Here is a first idea of the architecture we might use :

The ARM processor fills a buffer with the next image to be displayed, that is then swapped with the second buffer. It can pull this image from the SD card, or from another source such as the text printing function we plan on adding.

The full buffer is then accessed by a Direct Memory Access (DMA) IP that then sends the relevant cylinder (32 * 128 pixels) to each LED band controller that is then stored in their memory.

The sync module uses the signal from the hall effect sensor to compute the current rotation speed and deduce the approximate angular position of the rotating part(current_angle).

This current_angle combined with the pcb_angle parameter (that gives the angular position of the PCB in the configuration) allows each LED band controller to read a in the relevant addresses of their memory and flash the LEDs with the color corresponding to their position.

This first draft gives us an idea of the architecture we are going to use, however a lot of choices have still to be made, such as the type of buses to use.

Ah, yes. Stereoscopic vision.

This is a follow-up to my previous article, in which we explored the choice of PCB placement in order to get an optimal visibility.

My previous analysis only covered the case of a punctual point of view. However, most people have two eyes, which provide a double point of view and could increase our coverage of space. It is to be pointed out that a point of space seen by one eye does not provide depth information to the brain, however let us assume that our image perception is smart enough to overcome this limit.

In the following simulation, we used a typical eye distance of 6 cm. To understand the different configurations, please refer to this article.

As you can see, the dispositions that seem to benefit the most of stereoscopic vision are modulo forests, particularly the ones where the repartition of PCBs seems the most uniform. The parallax effect is all the stongest when PCBs hiding each other are far away in the axis of vision. Modulo forests with k = 3 and 11 seem to be good candidates as the front portion of the cylinder is (almost) complete in both cases.

Choosing a shape

As stated in this previous post, we have to compare different PCB dispositions to avoid blind spots. We soon came to realization that we would not be able to get rid of blind spots with our design, but it is possible to study the different ideas we came up with in order to mitigate this issue.

That’s why we created a python program that can simulate :

  • blind spots created by PCBs hiding each other
  • the variations of brightness due to the fact that LEDs can be seen at an angle (which is what created the dark center zone in CyL3D, which we are trying to get rid of)

Stairs : an iteration over CyL3D’s design

To avoid the issue of a dark zone due to all LEDs facing the observer at an angle in the center zone, an idea was to make sure that all LEDs were not facing the same direction. There came the stairs idea:

Stairs design. The PCBs have LEDs on one face.

After modelling this idea in python we get:

As you can see, there are a lot of deadzones, particularly in the central viewing axis.

The radial revolution?

Imagine a configuration with one vertical PCB with LEDs on both sides, facing the radial direction. Wait, don’t. I modeled it so you don’t have to :

This creates a 2D cylindrical POV display, with dark zones on the sides, which is in our opinion much better than a center dark zone. To create a 3D POV display, we “only” need to add concentric cylinders. The question is now: how serious is the problem of PCBs hiding each other and how to avoid it.

Spiraling down the rabbit hole

The idea of spirals seems natural. PCBs can laid out with an angular offset when compared to the adjacent PCB. For balancing reason, we alternate the PCBs with a central symmetry in order to create a double spiral.

The major issue is that the adjacent PCBs tend to hide each other in large portions of space, due to the fact that they are aligned and closed to each other. We can also notice that the resulting shape is far from being symmetric (which is expected with a spiral).

Some arithmetics

In order to shake things up a little more, we can keep the idea of a constant angular offset, but with a twist.

Let us take a circle split in n angular positions. If you choose a starting point and then jump from position to positions by skipping k positions, you will reach all positions if, and only if, n and k are coprime. This gives us a more evenly spread disposition.

For the sake of convenience, and because it is a nice name, we shall name this configuration “modulo forest”.

Deadzones tend to be much more spread out, which seems more acceptable. Further investigation has to be done in order to choose a definitive shape.