The area of this image is partitioned into 4096 concentric rings of 4096 pixels each.
Colours are assigned to rings based on their luminance, so that inner rings have colours with higher luminance than outer rings.
Within each ring, colours are then assigned to pixels based on hue versus angle of pixel with respect to the centre of the image.
The formula used to calculate luminance is:
L = 0.2126 * R + 0.7152 * G + 0.0722 * B
The coefficients have different values and this results in different shades of colour being more or less prominent in the image depending on their distance from the centre. It also results in radial (as well as circular) variations in hue and an overall spiral-like shape.
The setting of 0 degrees reference hue at -90 degrees results in the yellow, magenta and cyan shades taking more vertical directions, from the centre going towards the bottom centre of the image.
Some distortion occurs close to the corners, where rings turn into sections of rings. This is noticeable for instance in the shape of the dark green shade close to the bottom left corner.
As per all seemingly smooth omnichromatic images, on a closer look this image is also affected by dithering.
Image Licence: Public Domain (CC0).
