## waves – a browser app for creating quasiperiodic wallpapers

I have made a browser app that lets you create quasiperiodic wallpapers. You find it at http://geometricolor.ch/waves.html . It uses a symmetric superposition of waves as proposed by Frank Farris and presented by Erica Klarreich in the Quantamagazin in “How to Make Impossible Wallpaper“. Christoph Pöppe is writing about this idea in a forthcoming article for the Spektrum der Wissenschaft. To make the app easy to use, I have only included a few of the possibilities presented in this blog. To make your own experiments and to modify the app you can use my Github repository https://github.com/PeterStampfli/images, in particular the current commit f260803. The code of other commits might be broken.

The app obtains the colour of a pixel of the symmetric image from its coordinates (x,y), which are transformed to new coordinates (X,Y). The app then takes the colour of an input image at the position (X,Y). Instead of using an input image, the app can directly show the structure of the waves by using the X-coordinate value for the green colour component and the Y-coordinate for the blue and red components of the pixel. For the transformations I used the most basic results from the general discussion of the post “Improved symmetric sum“. Note that the transformations are different for rotational symmetry of odd or even order.

For 5-fold rotational symmetry and rotational symmetry of higher odd order the app calculates the (X,Y) coordinates using and where m is an odd number, the order of the rotational symmetry. Only for m=3 we get periodic images.

Obviously, at the origin where (x,y)=(0,0) we get the transformed coordinates (X,Y)=(m,0). At the origin we have trivially a m-fold rotational symmetry. Going away from the origin the different waves first get out of sync, but sufficiently far away they get in phase again. This is similar to a Moiré pattern, see my earlier post “Quasiperiodic designs and moiré patterns“. Thus there are many points where X≈m and Y≈0. Around these points we get approximate m-fold rotational symmetry. They are distributed quasiperiodically. You can see this if you choose the 5-fold rotational symmetry in the app. This becomes more clear if you also switch on the additional tiling, which is the Penrose rhombus tiling for m=5. Note that the corners of the rhombs lie at points with approximate local 5-fold symmetry. Unfortunately, 7 and higher odd order rotational symmetries have more interfering waves and they get in phase again only at very long distances. Thus, the resulting images seem to be rather chaotic and it is nearly impossible to see their quasiperiodic structure. Thus I choose not to include them in the app.

For 8-fold and higher rotational symmetries of even order 2m, where m is an integer,  the app uses a similar equation for the transformed X-coordinate as for odd order: Note that the cosine function is symmetric, cos(-x)=cos(x), which makes that we get a 2m-fold rotational symmetry from only m waves. But we cannot simply replace the cosine by a sine to get Y(x,y) because the sine function is antisymmetric, sin(-x)=-sin(x). This would destroy the symmetry. Instead, I choose the slightly more complicated equation which too has 2m-fold rotational symmetry. Note that these waves are rotated with respect to the waves of X(x,y) by an angle of π/2m. The app uses these packages of symmetric waves to create quasiperiodic images of 8, 10 and 12-fold rotational symmetry. In all three cases you can see basic motives repeating with slight variations throughout the plane. There are many points with approximate local rotational symmetry. This becomes more clear if you choose to display the matching tilings. For 8-fold rotational symmetry the app displays a patch of the Ammann-Beenker tiling, for 10-fold rotational symmetry it shows only a rosette around the origin and for 12-fold rotational symmetry you see a tiling of rhombs, triangles and squares resulting from an iteration method presented earlier. Images with rotational symmetry of higher order again have a rather chaotic appearance and it is difficult to see that they are quasiperiodic. Thus they are not included in the app. However, you might want to do your own experiments with its code.

For rotational symmetries of even order 2m the app offers an additional two-color symmetry: Rotating (x,y) by an angle of π/m inverses the colours of the image. It uses the method presented in the post “Improved two-color symmetry upon rotation“. An additional function U(x,y) modifies the color. The output image pixel has the original colour of the input image if U is positive; and if U is negative then the colour is inverted. The equation for U is different for even or odd values of m as shown in “two-color rotational symmetry“. For odd m the app uses and for even m You can easily verify that rotating (x,y) by π/m inverts the sign of U. The transformation to (X,Y) uses the absolute value of U(x,y) for X(x,y) and the same form for Y(x,y) as for simple rotational symmetry without colour change.

You are welcome to do your own experiments with http://geometricolor.ch/waves.html. If you have any questions you can reach me at pestampf@gmail.com.

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