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        riddles >> easy >> Shadow on a Checkerboard
        
(Message started by: william wu on Jan 26^th, 2004, 10:00am)

Title: Shadow on a Checkerboard
Post by william wu on Jan 26^th, 2004, 10:00am

Check out the image below.

Squares A and B are the same color! Believe it!

(no not the letters A and B -- the squares themselves)

Image relayed to me by my friend Scott Heise; original source, ebaumsworld.com

Title: Re: Shadow on a Checkerboard
Post by towr on Jan 26^th, 2004, 10:28am

I think Thud allready posted it once..
Though I suppose since +/- 60 of his posts disappeared this might have been one of them..

[e]here's the thread:Which Knight is Which? (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_easy;action=display;num=1064743648)
not identical but quite similar..[/e]

Title: Re: Shadow on a Checkerboard
Post by william wu on Jan 29^th, 2004, 8:51am

Ah. Sorry didn't see that one a priori.

Title: Re: Shadow on a Checkerboard
Post by John_Gaughan on Jan 29^th, 2004, 9:06am

Does anyone know why the human eye perceives these colors in this way? (hint -- [hide]it has to do with what the colors are next to in the image[/hide])

Title: Re: Shadow on a Checkerboard
Post by towr on Jan 29^th, 2004, 10:24am

on 01/29/04 at 09:06:27, John_Gaughan wrote:

Does anyone know why the human eye perceives these colors in this way?

Evolution.. :P

It's hard to pin it down to just one thing, really.. For instance it also makes sense from a pattern completion perspective. You can also put it down to our brain compensating for the shadow we perceive. And there are other contributing processes.. Our brain works in mysterious ways. But's it's all evolution approved, it works. (At least well enough to not get us killed before we reproduce)

Title: Re: Shadow on a Checkerboard
Post by John_Gaughan on Jan 29^th, 2004, 11:03am

I always thought it was because our eye perceives color relatively, not absolutely. Based on the relative color of the surrounding squares, they appear differently, despite being the same. It's the same idea as night vision. In the absense of a direct light source (just low ambient light) our eyes are more sensitive to differences in luminesence. Some things appear brighter than they would in full lighting, relatively to other objects or colors.

There is a better and more scientific explanation for this but I cannot find it offhand.

Title: Re: Shadow on a Checkerboard
Post by towr on Jan 29^th, 2004, 3:10pm

Well, yes, that might well be part of it, but it was obvious you were hinting at that :P.

There is a specific reason that cylinder is there though, it makes our brain think "hey there's a shadow being cast by yonder cylinder" (it's the older part of our brain ;))
If you remove the cylinder I predict the effect is significantly lessened.
The pattern adds to the effect as well, "black, white, black... white!" It's harder to remove this effect without changing the 'relative color perception' effect, but it would also lessen the overall effect.

Title: Re: Shadow on a Checkerboard
Post by Sameer on Jan 29^th, 2004, 3:28pm

This basic limitation is used in design of television!!!! And solved the problem of showing color pictures on black and white televisions too!!!
To understand it take a basic course in B&W television design and Color TV Design!!!

Title: Re: Shadow on a Checkerboard
Post by towr on Jan 29^th, 2004, 3:32pm

Here's a picture which leaves only a contrasting square, but no more of the checkerboard, or the cylinder. There's still a slight difference in perception, but much less pronounce imo..

(it's still a less then perfect hack of the image.. the 'random' fill seperating both side could have been better..)