Optical illusions

Author: Max R. Dürsteler

Here you will find my demonstrations of optical illusions in the form of videos or interactive web applications, with which you can determine the relevant parameters such as the speed of moving stimuli yourself. My HTML5 WebGL applications created with Unity3D run on all web browsers that support the HMTL5 and Web GL standards. Currently, these are Mozilla Firefox, Google Chrome and Safari. For the users of Android smartphones, I have created a first interactive Android application that they can download from the Google Playstore, iPhone and iPad users will find an interactive iOS application in the Apple iTunes App Store.

Freezing Rotation Illusion (Scheinbares Dreheinfrieren)

This illusion won first prize inThe Best Illusion ofthe Year Contest in Sarasota, Florida in 2006. The environment rotating back and forth deceives us that the rotational movement of an enclosed figure periodically freezes in time with the environment, while the figure actually rotates continuously around its sagittal axis.

https://youtu.be/IqFRIvRRWpI https://www.youtube.com/embed/bZbVTi3Ibmw

The video demonstrates:

  1. With the same (parallel) direction of rotation of the figure and the environment, the ratio of the mutual velocity amounts determines whether the figure periodically slows down or even stops or rotates as if glued to the environment.
  2. With different directions of rotation of the figure and the environment, the rotation of the figure seems to accelerate.
  3. While back and forth movements of the environment influence the perception of rotational movements of the figure, the reverse conclusion does not apply: Back and forth movements of the figure have no influence on the movement perception of the environment.

Presumably, these deceptions have to do with the fact that our visual system uses the normally stationary environment for image stabilization, similar to image stabilization in modern cameras. My interactive web application allows you to examine the critical parameters for these deceptions on your Mac or window computer or your smartphone itself.

Convince yourself, for example, that the plane does not rotate abruptly, but actually completely evenly by making the environment (“surround”) disappear (“Opacity” set to 0%). Play with the speeds of the plane and the environment to optimize the apparent “freezing” of the rotational movement with the same direction of rotation of the environment and the plane. or to “glue” the plane or other figure to the environment.

Click on the image below to launch my interactive WebGl application “Freezing Rotation Illusion” created with Unity3D.

Freezing Rotation WbGl Apllikation Image
Freezing rotation WbGl application.

If you are using an Android smartphone, you can download the interactive Android application“Apparent Rotation Freezing” from the Google Play Store. 

Freezing Rotation Illusion in 3D

Mixed reality is a new technology that makes it possible to create virtual, movable three-dimensional objects that can be placed somewhere in the real environment. To see these phantom images (holograms), a special vision device such as the Microsoft Hololens is needed. The Microsoft Hololens includes its own computer with a “holographic” version of Windows 10.

I created a 3D version of the Freezing Rotation Illusion: as a small model airplane (the “figure”) rotates continuously around its longitudinal axis. As “Environment I use a can-shaped ensemble of wedges surrounded by different colors and sizes. The ensemble moves back and forth.

If you have access to a Microsoft Hololens, you can download the “Freezing Rotation Illusion in 3D” application I created from the Microsoft App Store to your Hololens (click on the image below).

More information about the “Freezing Rotation Illusion” can be found in the following works:

Motion transparency

Motion transparency is the ability to perceive two independent movements in the same place of the retina. In a situation comparable to Utagawa’s woodcut

we can perceive both the movement of the rain in the foreground and the movements of a person on the bridge behind it at the same time.

Video Motion Transparency

On the video we see through a pattern of random points (comparable to the rain shower) a vertical sinus grid (comparable to a person in the rain). On the left section of the image, we can see the movement of the sinus grid behind the stationary point pattern just as well as the movements on the uncovered part of the grid. On the right section of the image, the dot pattern moves vertically without it dragging the physically stationary sine grid behind it in our perception: the part of the grid covered by the moving point pattern appears just as stationary as its uncovered part. The stimuli used here are both chiaroscuro stimuli. Does our perception of movement change when we replace the light-dark sinus grid with a red-green sinus grid with the smallest possible light-dark differences?

Motion transparency of a chiaroscuro and a color stimulus

In the left section of the image, the red-green grid moves at the same speed as the light-dark grid in the previous video. However, the actual perceived movement appears to be much lower. In the part of the image section covered by the stationary dot pattern, one must attentively follow a color edge in order to perceive the movement of the grid at all. At even lower speeds of the color grid, it would appear to be standing still: “StandstillIllusion” . In the right part of the image, you have to carefully fix a color edge to perceive that it does not move with the points in the vertical; it appears as if the moving points aredraggingthe color grid with them: “Motion Capture Illusion

Such and similar observations and experiments led researchers of the visual system to the conclusion that the perception of movement in light-dark stimuli and in pure color stimuli takes place via various mechanisms: via an efficient in light-dark vision and a sluggish mechanism dependent on attention in pure color vision. In the video, bright yellow stripes appear in a second part to show how fast the grid moves when we have “first order” light-dark motion receptors for motion perception.

Motion transparency with a chiaroscuro and a color stimulus with blurred (top) or sharp color transitions (bottom)

The video shows that we perceive movement much better when it comes to color stimuli when the boundaries between the colors are sharp. Researchers have experimentally shown that the motion perception of color stimuli with sharp edges (spatial frequency > 2Hz) and/or higher speed occurs via a far more efficient mechanism than with color stimuli with fuzzy edges.

Similar deficits of motion perception as in color stimuli can be found in stereo stimuli.

To perceive the differences in depth in one of the so-called anaglyphic stereo videos shown here, you need red-blue glasses similar to those shown above with a red filter in front of the left eye and a blue filter in front of the right eye.

Motion transparency with a light-dark and a stereo stimulus

To create depth stimuli or stereo stimuli, you need some structured pattern like our random point pattern, which allows you to create horizontal disparities between left and right retinal image. Here, the disparities encode a vertical lattice whose depth changes according to a sine function.

If the dot pattern moves synchronously with the depth grid (right half-side of the two image sections), it is easy to see that the grid is moving in the left image section. In the right section of the image, dot patterns and depth grids remain fixed.

On the left half-side, the dot pattern is stationary (individual points move horizontally to code for the downward moving stereo sine grid). On the right side of the left half- side, the dot pattern moves up and down in the vertical. You have to fix one of the sine waves or follow it to see if and which direction the depth grid is moving.

Researchers who experiment with motion perception have experimentally found evidence that the same sluggish, attention-dependent mechanism by which we recognize the movement of blurred color stimuli is also used for the motion perception of pure depth stimuli.

Complex Motion Illusions

Complex movements are rotation, expansion or contraction, and shearing. While our chiaroscuro vision detects complex movements almost as well as straight-line movements, the following videos show that these abilities are completely lacking in pure depth or stereo vision and partially lacking in color vision.

Stereo standstill and carry-on illusions

In the stereo motion standstill deception,where the random point pattern does not move, apart from slight shifts along the horizontals, we do not perceive rotation, contraction/expansion or shear movements of the stereo figure during central fixation. If we follow a single height in the periphery of the figures due to depth differences, we can only guess the movements. Yellow brightness patterns demonstrate how the stereo figure changes.

In stereo motion-taking deception, a physically stationary stereo figure appears to move with a rotating or contracting/expanding or shearing dot pattern.

The complex movement of the stereo figure cannot be perceived even if a dynamic dot pattern(“flicker”)is used instead of a static dot pattern.   Here, a new dot pattern is created for each frame of the video.If the yellow lines appear in the flicker, which are supposed to indicate the actual movements, the stereo figure seems to move with the yellow lines.

Stereo rotation standstill and carry-on movement deceptions
Stererad from concave sectors

Explanations in English are displayed at the top of the video.

Stereo expansion/contraction standstill and carry-on deceptions
Stereo wheel made of concentric rings.
Stereo shear arrest and stereo shear portability Deceptions
Stereo wheel made of concentric rings.
Stereo spiral motion standstill and stereo spiral motion portability Deceptions

Complex color motion illusions

In order to demonstrate complex color movement illusions, the color stimuli should be isoluminant, i.e. have no differences in brightness, which requires an elaborate calibration of the color stimuli used.  Alternatively, chiaroscuro masks.B such as our random dot pattern were used in visual research to suppress remaining light-dark components in the color stimulus. Without this having been explicitly mentioned, it tests the transparency of movement between a light-dark stimulus (the mask) and a color stimulus.

In the case of the color motion standstill deception,where the random point pattern does not move, apart from slight shifts along the horizontal, we do not perceive rotation, contraction/expansion or shear movements of the color figure at central fixation. If we follow individual color edges, we can only guess the movements. Yellow brightness patterns demonstrate how the color figure should move. Especially with the rotation, one has the paradoxical impression that the yellow stamps glide over seemingly stationary color sectors without ever crossing them.

In the color motion-taking deception, a physically stationary color figure seems to move with a rotating or contracting/expanding or shearing dot pattern.

The complex movement of the color figure cannot be perceived even if a dynamic dot pattern(“flicker”)is used instead of a static dot pattern.   Here, a new dot pattern is created for each frame of the video.If the yellow lines appear in the flicker, which are supposed to indicate the actual movements, the color figure with the yellow marks now seems to move along.

Color rotation standstill and rotation portability Deceptions
Color expansion/contraction standstill and carry-on deceptions
Colour-shear movement standstill and carry-on deceptions
Color spiral motion standstill and carrying deceptions

More videos on this topic can be found here:

If you are interested in the movement deceptions shown here, I refer you to the following article, in which you will also find further references:

Here is an example ofan interactive application created with Unity:

Stereo Rotation Standstill and Related Illusion Web-App: Click on the image to start the interactive HTML5-WebGL program.

Instructions for operation: make sure that the wheel sectors that stand out for this reason actually rotate by ticking the “Markers” box. Yellow markers appear, which rotate physically synchronously with the wheel sectors. One has the paradoxical impression that the brand wreath rotates over a seemingly stationary sector wheel without the brands ever crossing the sector boundaries.

In the drop-down list at the top right, switch e.B. from “Sectors” to “Rings” to see stereo expansion/contraction etc. The menu on the left is adjusted according to the adjustable parameter.

Below is the interactive WebGL application “Color Rotation Standstill Illusions” created in Unity3. Click on it to start the program.

Color Complex Motion Illusion Web App: Click on the image to launch the interactive HTML5 WebGL program.

Instructions for operation:

Please first take off the red-blue glasses, the illusion only works if the differences in brightness between red and green are close to zero. If necessary, you can adjust the colors to your monitor/light room projector in the menu. Again, you will find the new Unity3d WebGL application above, which is compatible with most new Internet browsers.

When the application starts, there seems to be nothing to move, although the color wheel rotates at a speed of 12°/sec. Tick the “Markers” box and you will experience your miracle: yellow color squares rotate cheerfully in circles, while the red sectors in between still do not seem to move (although the yellow squares never cross them). With the button “Change Mode” the speed settings are changed so that after the color standstill illusion you see the color take-away illusion the dot pattern rotates and the physically stationary color sectors seem to rotate along. After clicking again, they see a “normal” rotation when dot patterns and colors move synchronously. In the drop-down list at the top left, you can study other complex movements by choosing the appropriate color figure, e.B. when choosing “rings” expansion/contraction.

Colour stimuli with a sharp transition between the colours: standstill and deadweight deceptions only in the subliminal range

With the interactive application for complex color movements, you can find color stimuli with sharp color boundaries in the selection list (e.B. “BW Sectors”). Convince yourself that you now clearly perceive the rotation of the sharply limited color sectors at a rotation speed of 12°/sec. If you reduce the rotation speed to about 4°/s, you will only perceive the rotation if you actively follow a color limit in the periphery, i.e. You can still perceive the local translation movements, but not the rotation of the whole figure (subliminal area for rotations).

Standstill and carry-on deceptions during complex movements of contrast-modulated stimuli.

For the generation of contrast-modulated stimuli, similar to the generation of stereo stimuli, a structured pattern such as .B a random point pattern is used, the contrast of which is modulated according to the figure to be represented.

At a rotational speed of 12°/sec with a stationary point pattern, we can see the rotation of the sector wheel quite well.Movements of contrast-modulated stimuli aresecond-order motionstimuli;  the contrast is defined as the difference between the brightness of two points along the direction of movement.

At rotational speeds of less than 6°/sec, it becomes difficult to perceive the rotation of the whole wheel while still recognizing the local displacements of sector edges.  The stimulus thresholds for translation are lower than the stimulus thresholds for complex movements.

The perception of complex movements of contrast-modulated stimuli is as good as the perception of complex movements of chiaroscuro or sharply limited color stimuli behind a mask of random points; this is in clear contrast to the deficits shown in the perception of complex movements in stereo and blurred pure color stimuli.

Publications of Max R. Dürsteler

Imprint: Dr. Max R. Dürsteler, Burgweg 46, 8008 Zurich.