Hans Demol (hd_lego) made this mosaic in the spirit of the work of M.C. Escher, who experimented with "'Regular Division of the Plane' (a composition form depicting irregular shapes or combinations of shapes that interlock completely to cover a surface or plane)".
You can see it better in the larger sizes, which is where I noticed that in some areas Hans used square pieces and in other areas used round pieces on different colored backgrounds to try to achieve a gradation of color. There are also transparent pieces used for the same purpose, as you can see here:
And here is one more close-up:
Thanks to LegoMyMamma for the tip.