High Spatial Frequency Portrait
There is a portrait in the checkerboard, although you might have to sit far back from your computer screen to see it. It might also appear when you scroll the page, possibly due to a saccade effect. The interference of the high-spatial frequency checkerboard with the low-frequency image was noticed by Akiyoshi Kitaoka, who has invented many wonderful optical illusions.
Making the image
This one was made from a portrait of Holly (also used in the halftone experiment), converted to monochrome and then composited with Imagemagick's
composite commands to mix it 90% checkerboard, 10% Holly:
convert -size 1024x1024 pattern:checkerboard -auto-level -level 0,100 cb.png convert -resize x1024 -crop 1024x1024+0+0 -monochrome holly.jpg holly.png composite -dissolve 90 cb.png holly.png out.png convert +append cb.png holly.png out.png tri.png