Visual Illusions

[Ray]

I recently came across the following example of a strange illusion of perception whereby two equal shades of grey in an image appear very different shades. But Photoshop confirms they both have the same RGB values.

If one covers up the black and white seam that separates the two blocks, with one's hand or whatever, the two shades of grey suddenly appear the same.

What's also strange is, if one inverts the black and white seam that separates the blocks, the two shades of grey also appear the same. Any explanation, anyone?  (Sorry my inversion looks a bit messy.  )

[LesPalenik]

Could be also the viewing angle. On my monitor the lower picture looks slightly lighter - both the grays and blues.

[Ray]

Could be also the viewing angle. On my monitor the lower picture looks slightly lighter - both the grays and blues.

The blue background looks the same in both images on my monitor, but the lower block in the adjusted image looks just a slightly lighter shade of grey than the upper block, probably because it lacks the narrow shadow along the vertical edges, which the upper block has.

[Rajan Parrikar]

Seems like a variation of the Checker Shadow Illusion of Adelson.

https://en.m.wikipedia.org/wiki/Checker_shadow_illusion

[Bart_van_der_Wolf]

Seems like a variation of the Checker Shadow Illusion of Adelson.

https://en.m.wikipedia.org/wiki/Checker_shadow_illusion

Correct. Both show how we interpret shadow, direction of light, shape, and color/brightness. Something that is positioned in the shadow is presumed to look darker than it would in the light, so we think/expect/compensate it is lighter than it actually is. When the seam is rotated, the recognition of light direction and shadow is messed up, and we see it as it is, with identical brightness.

Fun stuff.

Cheers,
Bart