# The Smartphone Phenakistoscope Experiment

Here in our bunker we’re always into finding new interesting ways of using the native features of mobile that don’t require any network connection.

For a period last year we were experimenting with ways of creating content that would be invisible to the naked eye but visible to a smartphone camera specifically.

After experiments with infra-red light (visible to any camera without an IR filter, which is quite a lot of them), an exploration of the iOS slow-motion feature led us to explore the peculiarities of smartphone video.

We found out that, at that time, a huge majority of smartphone video cameras have a frame rate of 30 fps (in layman’s terms, this means the camera is capturing a sequence of 30 images per second to create a moving picture).

This led us to ask whether we could make a moving image like Eadweard Muybridge’s original Phenakistocope* but that can only be seen when you view it through your smartphone camera. Sort of a bit like augmented reality in reverse.

*I always thought it was called a “zoetrope” but apparently this is the correct term.

First we got a vinyl record player and tried to work out how many ‘frames’ we would need around a circular platter if the turntable is spinning at 33 1/3.

Here’s a first random attempt.

As you can see, there’s not a lot happening here.

We then divided the disk into 36 segments (360 / 33 1/3 = 36), in a Powerpoint piechart, to see what would happen.

We have movement! We have animation! But it’s a bit of a blur. Could it be that getting a crisp animation is impossible, and that the smartphone camera struggles to cope with a fast moving image?

The next discovery was important. We found that shining a bright light onto the surface of the spinning disc allowed the camera to focus.

Then we needed to stabilise the moving image as much as possible. Mike (@mikemonteithdev), Movement developer and mathematics master, then worked out the following equation:

We have a rotating disk, with a radial velocity (ω).
A camera with frame-rate (p).
The disk is cut into frames (sector of a circle) with central angle (θ).
Important bit:
To get the camera capturing exactly one of our disk-frames per camera-frame, we must make sure that the angular displacement of our disk per camera-frame is equal to the sector angle of our disk-frames.
We express this mathematically as θ = ω / p.
The radial velocity can be calculated as ω = 2 π f.
Where f is the frequency (commonly expressed in revolutions per second or minute).
So a 45 rpm turntable has an angular velocity (ω) of 4.712 radians per second.
Therefore, for a camera recording at p = 30Hz: θ = 4.712 / 30 = 0.157 radians.
To find the number of frames in a full circle, N = 2 π / θ. So N = 40 frames.
We can simplify these equations to get rid of the angular terms, since N = 2 π p / ω, and ω = 2 π f. Therefore N = p / f.

To translate: if a turntable is travelling at 33 1/3 rpm, you need to put 54 frames around a spinning platter in order to make it the image move when viewed through a digital camera at 30 fps.

As you can see here — the outer ring of dots is now stabilised, near enough.

To animate, we got took a sprite sheet of Tim from Braid and created a 54 frame loop around a platter. Put it on the turntable, switched on a bright light and…

It works!

It’s important to note that what’s recorded here is also what you see in real-time through the camera. This isn’t just something that appears after you’ve recorded the video.

So what next? Well, we wanted to see how complex we could make an animation, and how much information we could put in a frame towards the middle of the spinning platter. That way we’d find out how near we could get to a full-on, hypnotic, psychedelic circle.

This level of animation was slightly outside of our comfort zone so we roped in our good friend Richard Mitchelson (@rich_mitch), illustrator and animator, to create a bespoke animation for us.

Check out Rich’s delightful bouncing balls.

For reasons we cannot fathom, the animation stops being captured by the shutter speed as you get nearer to the middle of the circle.

So there we have it. The great Smartphone Phenakiscope Experiment came to an end when Apple released the new iPhone 6 and 6 Plus…

Why? Well, the frame rate on these new iPhone models is 60 fps rather than the industry wide 30 fps, so the user gets a kind of double vision on their screen when they try and view our disc with 54 frames at 33 1/3 rpm. As Mike says, “the camera is recording frames twice as often, so it catches images in-between the frames that were designed for 30fps.

To create for 60 fps, you would either need to squeeze in double the number of frames, or double the speed of the disk. The problem is that at 60fps, you need to light the disk even brighter because the camera has half as much time to capture the frames. 30fps cameras would just miss out on every other frame.”

So it is possible to create an animation for both 60 fps and 30 fps cameras… you just need to light it brightly, and somehow fit 108 frames on a disc or spin a disc with 54 frames at 66 2/3 rpm. All a bit more tricky.

Anyway, it’s worth exploring further if you have the time or the inclination. This video here shows that the overall effect is pretty magic.

For commercial purposes, say, if one were to use this for a beer brand, you could place the spinning disc behind a beer bottle-shaped hole, for example, to create a video of a bottle with an animation inside of it that’s only viewable through a smartphone. It would be pretty special, and I imagine something that the audience would share, amazed, with their friends.

Anyway, if you’d like to try this at home, here is Rich Mitch’s lovely bouncing ball animation disc, which you can cut out, mount on card and try the experiment out for yourself.

Let us know how you get on…

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