Check out this photo. Although it may look like a color picture upon first glance (and even more so if you squint or view it from a distance), it’s actually a black-and-white photo with thin color grid lines overlaid on it to trick your brain into filling in the missing color.
The viral image was created by artist and developer Øyvind Kolås, a prominent developer in the GIMP open-source image editor project. Kolås took a Creative Commons photo by Chuwa (Francis), converted it to black-and-white, and carefully overlaid red, orange, yellow, blue, and green grid lines over it.
“An over-saturated colored grid overlayed on a grayscale image causes the grayscale cells to be perceived as having color,” Kolås says.
Here’s another example Kolås created using the same technique:
“This is not the exactly same as the way JPEG compression works, since in JPEG compression the lower resolution color signal is present for in every reconstructed pixel, in this illusion the reconstruction is happening in our eyes/mind,” Kolås writes. “[B]ut it uses the same principle that Chroma Subsampling does, that luminance is a lot more important than the chroma for our visual perception.”
If you’re interested in playing with this concept yourself, Kolås is adding it to GIMP as an operation called color-assimilation-grid, and it’ll be available starting with the next GIMP-2.10 release.