`\pi` Approximation Day Art Posters

▲ 2019 `\pi` has hundreds of digits, hundreds of languages and a special kids' edition.

▲ 2018 `\pi` day

▲ 2017 `\pi` day

▲ 2016 `\pi` approximation day

▲ 2016 `\pi` day

▲ 2015 `\pi` day

▲ 2014 `\pi` approx day

▲ 2014 `\pi` day

▲ 2013 `\pi` day

▲ Circular `\pi` art

The never-repeating digits of `\pi` can be approximated by 22/7 = 3.142857 to within 0.04%. These pages artistically and mathematically explore rational approximations to `\pi`. This 22/7 ratio is celebrated each year on July 22nd. If you like hand waving or back-of-envelope mathematics, this day is for you: `\pi` approximation day!

Want more math + art? Discover the Accidental Similarity Number. Find humor in my poster of the first 2,000 4s of `\pi`.

There are two kinds of `\pi` Approximation Day posters.

The first uses the Archimedean spiral for its design, which I've used before for other numerical art. The second packs warped circles, whose ratio of circumference to average diameter is `22/7` into what I call `\pi`-approximate circular packing.

As you probably know, the ratio of the circumference of a circle to its diameter is `\pi`. $$ C / d = \pi $$

For `\pi` approximation day, let's ask what would happen if $$ C / d = 22/7 $$

where now `C` is the circumference of some shape other than a circle. What could this shape be?

A good place to start is to think about an ellipse. I've done this before in the 22/7 Universe article, in which I considered an ellipse with a major axis of `r+\delta` and a minor axis of `r` and solved for `\delta` such that the circumference of the ellipse divided by `2 r` would be `22/7`. Doing so means numerically solving the equation $$ \frac{C(r,r+\delta)}{2r} = 22/7 $$

where `r + \delta` is the major axis, `r` is the minor axis and `C(r,r+\delta)` is the circumference of the ellipse. Substituting the expression for the circumference, $$ 4(r+\delta) \int_0^{\pi/2} \sqrt { 1 - \left(1-\frac{r}{(r+\delta)^2}\right)\sin^2 \theta } d \theta = 2 r \frac{22}{7}$$

If we set `r=1` and solve it turns out that only a very minor deformation is required and `\delta = 0.0008`. You can verify this at Wolfram Alpha.

I wanted to make some art based on the shape of the this ellipse, but a deformation of 0.08% is not perceptible. So I came up with a slightly different approach to how I define the original circumference-to-diameter ratio.

Instead of treating the diameter as `r` and using `r + \delta` as the major axis, I now define the diameter as twice the average radius, or `2r + \delta`. This means that the equation to solve is $$ \frac{C(r,r+\delta)}{2r+\delta} = 22/7 $$

As before, setting `r=1` and substituting the expression for the circumference of an ellipse, we get $$ 4(1+\delta) \int_0^{\pi/2} \sqrt { 1 - \left(1-\frac{1}{(1+\delta)^2}\right)\sin^2 \theta } d \theta = (2+\delta) \frac{22}{7}$$

and solving this for `\delta` find $$ \delta = 0.083599769... $$

You can verify this at Wolfram Alpha.

This is a more useable approach since an 8% warping of a circle can be easily perceived.

▲ The ratio of the circumference of a circle, `C(r)`, to its dimameter, `2r`, is `\pi`. If we warp the circle by 8%, the corresponding ratio, if we use twice the average radius as the diameter, is 22/7. This deformation can be easily identified.

Below is matrix of perfect circles along side the 8% deformed circles.

▲ A matrix of perfect circles and ones which have been stretched by 8% along one axis and then randomly rotated. The deformed circles embody the `\pi` approximation of 22/7.

The art posters are based on a packing of these deformed circles.

▲ Warped circles, packed.

▲ Even more warped circles, packed.

By superimposing perfect circles on the warped circles, fun patterns appear.

▲ Superposition of perfect and warped circles, packed.

perfect vs approximate packing

If you pack perfect circles perfectly, the area occupied by the circles is `\pi/4 = 78.5%`.

What is the area occupied by perfect packing of warped and randomly rotated (like in the posters) circles?

color scheme

To motivate choice of colors, I chose images with a 1970's feel.

▲ Images used for color schemes. The colors of each image were grouped into clusters—8 for the first two images and 6 for the third—to obtain proportions of representative colors.

Using my color summarizer, I analyzed each image for its representative colors. Using these colors and their proportions, I colored the perfect and warped circles.

▲ Packed warped circles colored in proportion to color schemes derived from the images above.

For each poster of these color schemes, two poster versions are available. In one, the perfect cirlces are shown with warped circles as a clip mask. In the other, warped circles are shown, clipped by perfect circles.