In the last posts I have shown kaleidoscopes that make repeating images in Euclidean, spherical and hyperbolic spaces. They are decorations of regular tilings. But what about semi-regular tilings? Could we decorate them too using mirrors? This would give us new designs. With real physical mirrors we probably can’t do it. On the other hand, with computer generated mirror symmetries this becomes possible.

Let us take as an example the regular tessellation of hexagons shown in the image below. Drawing lines between the mid points of adjacent sides we get a semi-regular tessellation of triangles and hexagons. Each triangle is made up of three corners cut off from the hexagons. A decoration of the regular tessellation arises from a triangular kaleidoscope with angles of 30°, 60° and 90°. It is shown in red lines in the image and uses the part of an input image that fits into this triangle. Mirror reflections cover the entire plane with copies of this image part. This emphasizes the regular tessellation of hexagons because there are mirror symmetries at the borders of the hexagons. To get a decoration of the semi-regular tiling we need additional mirror symmetries at the borders between the triangles and the hexagons. We note that one of these border lines cuts the red triangle in two parts and we only use the pixels of the input image that lie inside the larger part, which is coloured yellow. We cover the other rose-coloured part of the triangle doing a mirror image at the border of the triangle. Thus we get an image with additional mirror symmetries that emphasize the semi-regular tessellation. In this sense we have made a decoration of the semi-regular tiling.

We can use the method discussed above to get semi-regular tessellations of the hyperbolic plane. Using a kaleidoscope with a right-angled triangle characterized by the numbers (k 2 n) we get a decoration of a regular tessellation with regular polygons that have k corners. Each corner point is shared by n polygons. To create a decoration of a semi-regular tessellation we again use reflection at additional lines. They go through the corner of the triangle which has the right angle, cross the opposite side at a right angle and are straight lines in hyperbolic space. In the Poincaré disc representation they are circles that are perpendicular to the boundary of the disc. These lines define the semi-regular tessellation. It is made of regular polygons. Half of them have n corners and the other half k corners. A corner point is shared by 4 polygons. Using reflections at these additional lines we get a decoration of the semi-regular tessellation. The image below shows an example.