Because mostly grey lines appear in the periphery, the black dots don’t appear. The image is a variation on the Hermann grid illusion, a diagram of black squares separated by white lines.

A dotted black line followed a character's (usually Billy's or Jeffy's) carefree peregrinations through the privet-lined, strangely carless, possibly post-Raptured streets of his neighborhood.

Learning how to draw dotted lines in GIMP can add interest to your work. How to draw a Dotted Line in GIMP. All artwork is made up of lines, and lines are the basic building block of artwork.