Above you see a “bird” moving over a background of random scrambles – nothing to write home about… Now click “move bird” (or press the space button). The bird will stop and merge with the background. With luck you might see a part of it, but usually it fades away until vanishing completely, immediatly re-appearing when starting its motion again.
Try it at various places – at most places the background is matched to the lines comprising the bird sothat we have near perfect camouflage.
When you have stopped the bird’s flight, and it has merged with the background, you can try to uncheck the “draw noise”. This will reveal that the bird is still there – just “invisible”.
The camouflage is broken by correlated motion, on of the gestalt principles, namely “common fate”. This is very similar to the running Dalmatian (“help me 2” there). The bird is made up of 21 penstrokes and the background is the same 21 penstrokes ordered randomly.
This is my re-implementation of the beautiful Hidden Bird demo on the ‘hidden’ former IllusionWorks pages by Al Seckel. That in turn was programmed by Akos Feher, who attributes the orginal drawing to David Regan.
2012-04: Source revealed! David Regan kindly responded to my enquiry, here goes:
Regan D (1986) Form from motion parallax and form from luminance: Vernier discrimination. Spatial Vision 1:305–318
Regan D (2000) Human Perception of Objects: Early Visual Processing of Spatial Form Defined by Luminance, Color, Texture, Motion, and Binocular Disparity. Sunderland, MA: Sinauer, 577 pp. [Here the bird is on the transparent bookmark and the background in the text.]