Count the circles - The Coffer Illusion

Most people view this as a set of door panels. It may take quite a while to convince oneself that there are actually 16 circles in the image...

(Reposted from 2009 and 2012)

Twelve brown balls

Each one of them is the same shade of brown.  Zoom in for confirmation.

The same image with the colored lines blacked out.

"Perpetual diamond" optical illusion

YouTube link.

The diamond in the video IS NOT MOVING.

The key to the illusion is the thin perimeter of the diamond -

The Perpetual Diamond produces motion continuously and unambiguously in one direction despite never physically changing location. The phenomenon consists of a steady, mid-luminance diamond bordered by four thin edge strips and a surrounding background field. The direction of motion is determined by the relative phases of the luminance modulation between the edge strips and the background. Because the motion is generated entirely by changing contrast signals between the edge strips and background, the stimulus is a valuable tool for tests of spatial contrast, temporal contrast, contrast gain, and color contrast. We demonstrate that observers see motion even when the edge strips subtend only seconds of arc on the retina (which is less than the frequently reported 10 minutes of arc) and that perceived motion is due entirely to changes in the difference in contrast phase modulation, independent from the luminance phase.

For a full explanation, go to Neatorama and follow the links there - one of which is an interactive demo.

Look at these HORIZONTAL blue bars

Perfectly horizontal.  Really.

Based on the classic "cafe wall" optical illusion.

If you like this, note that the TYWKIWDBI category of optical illusions currently has 68 entries.

Via Boing Boing.

(Reposted from 2017 because I still find it hard to believe...)

Forced perspective

As used in the filming of the movie Elf.  Via.

Mephisto spiral illusion

YouTube link.

It looks like 2 interlocking wire spirals. In your hands, the two spirals seem to wind together or wind apart, completely effortlessly. However when you hand the Mephisto Spiral over to someone else, they find that they cannot replicate the action – the two wire spirals are completely rigid.

Alternatively, by simply moving your hands in one direction, you can make the two spirals appear to unwind, yet however many times you repeat the action, the two spirals never come apart.

From another discussion:

For those who still don't get it: the guy is only moving his hands along the rod. He's also rotating the rod at the same time so that the part of the rod that's "coming out" of his hand is also at the same angle, to create the illusion that he's firmly holding the "rods" with his fingers.

Two perfect circles

Top image hat tip to Alex Santoso, who found this excellent optical illusion on Twitter and posted it at Neatorama in 2012.  Reposted to add the bottom one, via.  The two illusions are based on the same principles; I believe the lower one was just modified to bring the inner circle closer to the outer one.

All the circles are the same color

Via.

An example of "Troxler fading"

Fix your gaze on the center of this image (the black pixel) and stare at it for about 20 seconds.  The colors will disappear.

Troxler's fading has been attributed to the adaptation of neurons vital for perceiving stimuli in the visual system. It is part of the general principle in sensory systems that unvarying stimuli soon disappear from our awareness. For example, if a small piece of paper is dropped on the inside of one's forearm, it is felt for a short period of time. Soon, however, the sensation fades away.

An even more dramatic example is in the video at Digg.

A mechanical model of Cardano's Hypocycloid

YouTube link.

I've seen animations of hypocycloids before, but not a working model.  Most interesting.

Found at The Awesomer, via Neatorama.

Looks like phocomelia ("T. rex arms")...

... but it's just an optical illusion.  The mother has normal arms.  Look closely.

Phocomelia.

Via.

Optical illusion

The two photos are identical - even though it appears that the tracks are more vertical in the right photo and more slanted in the left one.

Similar in concept to the Leaning Tower of Pisa illusion.

Via within the crainium.  

Reposted from 2009 to add this even more dramatic pairing:

Remember, the two photos are exactly identical.

Almost as awesome as this one.  If you enjoy these, note you can access this blog's 62 posts about optical illusions from the categories section in the right sidebar.

All the horizontal lines are identical

They just look different from one another.  Some look to be continuous sine-wave curves, while others resemble staggered split-rail fences.  But they all have the same shape.  The optical illusion arises from the way they are colored.

The illusion was developed by psychology professor Kohske Takahashi of Chukyo University.  Via Neatorama.

Auditory illusions in movies

YouTube link.

I thought I had blogged the Shepard tone before, but I can't find a prior post.  It's certainly explained lucidly in this video.

The tops of these two tables are the same shape

They are both parallelograms with identical lengths, widths, and angles.  I enhanced the illusion by referring to them as tables, but they really are identical.

You won't believe me.  You'll need to view the graphic at Digg, or read about the Shepard tabletop illusion.

There are NO red pixels in this picture

Your brain sees the objects being bathed in blue light, so it compensates by adding a red that is not there.  The fact that the objects are strawberries accentuates the illusion, but is not a necessary feature.

Further discussion and explanation (and a photo with the "red" sampled to show that it is actually grey) at the source article in Vice's Motherboard.

A mind-bending optical illusion

YouTube link.

This illusion won second place in the annual competition for the Best Illusion of the Year.

As you watch it, keep in mind that this is a competition for optical illusions, NOT for sleight-of-hand or video tricks or other stage "magic."  That's a real mirror, showing the reflected cylinders and the hands of the operator.  The entire illusion is based on the neurophysiology of optics, and the human mind's interpretation of what the eye sees.

If you can't figure it out When you give up, the basis for the illusion is explained in this video.

"Metamers" - images that look the same, but are different

YouTube link.

Our illusions show the opposite: two images that are different but look the same -- those are called "metamers"; -- and two images that are identical but look different -- we call those "anti-metamers." Our main illusion mixes the two: it shows three images, two of which match with a third one mismatching. Viewers see one image as odd, but it's one of the two identical images they see as different, an illusion we call "false pop out."

Count the black dots

Image saved in the pre-blogging era without attribution.

This hat is as wide as it is tall

The width of the brim is the same as the height of the stovepipe top.

You don't believe me.  You're reaching for a measuring ruler.  I'll wait...

Source:  2006, but I don't know where.