An anamorphic image is one that appears normal only when seen from some particular perspective or when viewed through something such as a mirror.
This rainbow is the shape you would expect it to be until you place it at a precise distance from a cylindrical mirror. It is then reflected as a striped rectangle.The height of the rectangle is related to the angle from which you view it.
My rainbow, which is about 29 cm wide, was made for a ‘mini pillar’ as part of the Mirror Pillar project which takes place in 2018/2019. You may find other uses for a rainbow. The pattern includes a grid for creating mini-pillar images and there are many other resources on the official site.