
The Remarkable “Curvature Blindness” Illusion - tomrod
http://blogs.discovermagazine.com/neuroskeptic/2017/12/08/curvature-blindness-illusion/
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macintux
I briefly thought this might be a discussion of Flat Earthers.

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Houshalter
Take a look at the individual segments zoomed in:
[https://i.imgur.com/H4y4XTH.png](https://i.imgur.com/H4y4XTH.png)

I think what's happening is the brain is processing the separate color
segments individually, instead of a continuous line. And at an individual
segment level, it's much harder to detect a curve on the bottom pieces than
the top. Notice the red line I drew. The bottom pieces fit much closer to a
true line than the top pieces.

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murkle
You can play with the colors here:
[https://www.geogebra.org/m/MTGuYMTC](https://www.geogebra.org/m/MTGuYMTC)

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bjterry
This is a very interesting visualization. If you adjust col2, you can see that
the illusion basically has a step function. If both line colors are darker
than the background, the two patterns look the same. If you slowly adjust col2
until it matches the background and then becomes just a hair lighter, they pop
into existence with the full illusion. There is no transition whatsoever
except insofar as you can't see the lines when they match the background
color.

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na85
How are you adjusting the colours? The page is not interactive on my device,
and I see no controls of any sort.

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coldtea
<Insert BS misconception of the illusion, and why you know better than the
researchers> here.

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rini17
"It's just image compression artifact" took the crown for me.

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philsnow
the zig-zags where the color change is at the peaks / troughs looks to me like
a wall with light coming from the right side. i wonder if that's what's going
on here, the brain is saying "i know that shape, it's a zig-zagging wall"

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MichailP
This seems not to be due to curvature but due to color distribution of lines.
I wonder if effect would still be visible if all lines had same color
distribution.

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hanoz
It's not supposed to be "due to curvature". It's blindness to curvature, due
to the colours.

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MichailP
But it is somewhat dishonest. You could color the lines in the same color as
the background and then you would have "illusion" of dissapearing lines,
right?

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mikeash
That’s not an illusion. You’d be seeing it accurately.

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microcolonel
Now you see why, in much simpler networks, with full observability, it is
trivial to generate the equivalent of psychovisual illusions.

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Retric
Interesting, it's a border dectecion issue, where a single color shows as a
curve when it bends sharply the entire way, but short segments approximate to
a line when they are mostly just lines with strong curves only at the end.

I suspect it would look very different when animated.

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jcreed
It does indeed look interesting when animated; not sure if this is which
parameter you had in mind to vary, but:
[https://jsfiddle.net/dvv0wd82/](https://jsfiddle.net/dvv0wd82/)

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knolan
The shift to zig-zag is particularly pronounced when I scan up and down the
animation.

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DrScump
In all curves in the image, the shaded area is either centered on the "crown"
of the wave or occupies the descending portion of the wave.

What is the visual effect if the _ascending_ portion of each wave is the
shaded portion?

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lubujackson
You know what this reminds me of? That simple graphical trick to give buttons
depth. Two sides have a dark border, two sides have a light border. This seems
to be a similar effect.

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mertd
"However, further experiments revealed that depth perception is not the
driving force behind the effect."

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deckar01
I read the paper, but wasn’t immediately convinced it wasn’t related to depth.
Once I looked at the illustration for the 3rd experiment [0] it was obviously
unrelated to depth.

[0]:
[https://www.ncbi.nlm.nih.gov/core/lw/2.0/html/tileshop_pmc/t...](https://www.ncbi.nlm.nih.gov/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=5703117_10.1177_2041669517742178-fig2.jpg)

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vmilner
I wonder if this is connected to the curious zigzags I see in part of my
vision field during a visual migraine.

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aisofteng
Looks to me like it's because of the average curvature per segment of a
constant color.

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curtis
_The “zig-zag” lines in the illusion are the ones in which the color of the
wavy line changes from dark grey to light grey at the ‘corners’ i.e. the peaks
and troughs of the curve._

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teilo
I don't find this at all "remarkable" because I don't think this has anything
to do with the curvature itself, but the relative contrast of the lines
against different backgrounds, and the, for lack of a better term, dithering
that occurs in the brain as it is interpreting the borders of contrast

If the lines were a solid color, instead of alternating bands, or if the
crossover happened somewhere other than a crest of a trough, this effect would
disappear.

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coldtea
> _I don 't find this at all "remarkable" because I don't think this has
> anything to do with the curvature itself, but the relative contrast of the
> lines against different backgrounds, and the, for lack of a better term,
> dithering that occurs in the brain as it is interpreting the borders of
> contrast_

That's because you didn't understand it.

For one, the "the relative contrast of the lines against different
backgrounds" is the same between the lines that appear curved and those that
appear "straight", because both are the same exact line (both curved) against
the same exact background part (the gray).

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teilo
No, you didn't understand me, sorry. I understood it just fine, but didn't
explain myself well enough.

The relative contrast at the point of curvature causes an aliasing effect in
the brain where the line colors are (depending on the color) added to or
subtracted from the gray value. In effect this causes an apparent sharpening
of the curve. However, on those lines where the transition takes place on the
straight portion of the wave, no such aliasing takes place because there is no
curvature.

It's like anti-aliasing in reverse.

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coldtea
A, fair enough then

