Optical Illusion - Shades of Difference

This picture illustrates the same illusion,
http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html?gray

The real issue here is that our eyes and brains work to figure out the MATERIAL properties of what we see, not the exact red/green/blue numbers of the pixels (our eyes aren't simply photometers). The rgb values will change with lighting, and our brains are very good at overcoming such lighting effects and extracting the underlying material's color. When you cover the contrast edge with a pencil, your eyes perceive the different shades on either side as different levels of illumination, and your brain decides that the same material color is present on both sides.

@whitcwa: BTW, if you like the illusion you posted, there's a whole bunch of them at Michael Bach's site: (warning: your eyes will hurt)

http://www.michaelbach.de/ot/index.html

Alejo

so cool, I saw a TED conference presentation that showed a similar optical illusion.

AlejoHauser, the same colour IS present on both sides. The only variation in colour (actually intensity) is present at the lines. Lemme see if I can do an ascii art illustration...

¦\
_________________ ¦ \________________________
\ ¦
\¦

If you cover the area where the intensity changes it destroys the illusion that the colour is different on both sides of the line.

Damn, ascii art didnt work at all. Sorry

Dear lady,
The same colour is not present. Would it be the same colour, you would be able to take out the 2 middle colours and see no difference. The minimal difference in colous causes the brain to perceive the difference that is there as a difference in light or shadow. Therein lies the illusion.

I assure you they are the same. I have found the wikipedia article which explains it:
http://en.wikipedia.org/wiki/Cornsweet_illusion

This one is not based on the Cornsweet-principle. This is just graduations like I had to make when I had to learn to mix colours in paint-restauring class.

You are right, I was wrong. I found an instructables page showing how to make it:
http://www.instructables.com/id/Shady-Optical-Illusion/

I still think this is witchcraft!