sâmbătă, decembrie 18, 2010

Analiza unei iluzii optice

Sanse mari sa mai fi vazut aceasta iluzie optica. "Culoarea albastra" si verde sunt aceeasi (anume RGB 0,255,150). Ce nu ati vazut este acest studiu interesant:

So what we see above is the same greenish blue region, touching the green on top and the blue on bottom — with no particular seams, implying (correctly) that there is no actual difference in color and what is detected is merely an illusion.

And the story would end there, if not for DanKam.  DanKam’s an augmented reality filter for the color blind, designed to make clear what colors are what.  Presumably, DanKam should see right through this illusion.  After all, whatever neurological bugs cause the confusion effect in the brain, certainly were not implemented in my code.

You might notice that while we’re recovering the color, or chroma relatively accurately, we’re losing detail in perceived luminance. Basically, we’re losing edges.

It’s almost like the visual system sees dark vs. light at a different resolution than one color vs. another.

This, of course, is no new discovery. Since the early days of color television, color information has been sent with lower detail than the black and white it augmented. And all effective compressed image formats tend to operate in something called YUV, which splits the normal Red, Green, and Blue into Black vs. White, Red vs. Green, and Orange vs. Blue (which happen to be the signals sent over the optic nerve). Once this is done, the Black and White channels are transmitted at full size while the differential color channels are halved or even quartered in detail. Why do this? Because the human visual system doesn’t really notice. (The modes are called 4:2:2 or 4:1:1, if you’re curious.)

So, my theory is that these color artifacts aren’t the result of some complex analysis with pattern matching, but rather the normal downsampling of chroma that occurs in the visual system. Usually, such downsampling doesn’t cause such dramatic shifts in color, but of course the purpose of an optical illusion is to exploit the corner cases of what we do or do not see.

Tot articolul (cu si mai multe exemple): http://dankaminsky.com/2010/12/17/mindless-equals-blown/

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