What to do & see
On the right you will see a ‘bird’ flying above a background consisting of random lines. Now press the button “Bird stop”: the bird doesn't just stop flying, but slowly fades away until it vanishes completely!
Try it at various places – the background is so well matched to the lines comprising the bird that we have nearly perfect camouflage.
When you have stopped the bird’s flight, and it has merged into the background, you can try the button “Noise off”. This will reveal that the bird is still there – just invisible.
The camouflage is broken by correlated motion. This is very similar to the running Dalmatian (“help me 2” there). The bird is made up of 21 ordered 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 (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.]