Here I show some examples of visual illusions either as You Tube® videos or as interactive web applications.
Most of my interactive web applications were programmed using Microsoft Silverlight. Microsoft Silverlight applications run only on Windows PCs or MACs. They require the one-time installation of the Silverlight browser plugin. To run my Silverlight apps on a Windows PC Mozilla Firefox or a recent version of the Internet Explorer, on a MAC use either Safari or again Mozilla Firefox.
My recent web apps are pure HTML5-WebGL applications which no longer require the Installation of a browser plugin. I programmed them using Unity3D. Browser which support the HTML5 and WebGL standards are Mozilla Firefox and Google Chrome on Windows PCs and Safari, Mozilla Firefox and Google Chrome on MACs. Mobiles are only partly supported: I made an interactive iOS application demonstrating the “Freezing Rotation Illusion” which can be downloaded from the Apple’s iTunes store .
In 2006 this illusion was awarded the first price at “The Best Illusion of the Year Contest” in Sarasota, Fl.
Description of the illusion
- When a foreground object (center)is continuously rotating while its surround is rotating sinusoidally back and forth, the rotation of the foreground object or center is perceived as periodically slowing down.
- The slow-down occurs when the center rotates in the same direction as the surround.
- When the center and the surround rotate in opposite directions, the apparent rotation speed of the center is enhanced.
- Motion Freeze: The illusionary freezing of the center’s rotation is strongest, when the center’s rotational speed is about half of the surround’s speed.
- Merging with the Surround’s Rotation: If the speed of the center is close to speed of the swaying surround, a different percept reminiscent of motion capture may arise: the center sticks to the surround throughout the half-period, when center and surround are rotating in the same direction.
The motion capture effect is stronger with small translucent objects and eccentric fixation.
Center and surround effets
The roles of surround and center cannot be exchanged: an oscillating center does not alter the perceived speed of a continuously turning surround.
These illusions support the hypothesis that the visual system uses the normally stationary surround to stabilize the retinal image of a foreground object on a similar fashion like a modern camera.
Interactive Web Applications
When using an iPhone or an iPad, you may download the interactive iOS-Applikation “Freezing Rotation Illusion” from Apple’s iTunes app store.
The Freezing Rotation Illusion in 3D
The new technology of mixed realty facilitates to making o a three dimensional (3D) models, which yo can place at your will in your real environment. I was using the Microsoft Hololens to build a 3D model of the classical Freezing Rotation Illusion.
A small airplane is used a the foreground object in the center. There is a choice of different surroundings or backgrounds such as a tubular or disk-like structure made of wedges in different colors and sizes or a far-away surround made of a 360° x 180° panorama of the Massoala hall of the Zurich zoo.
Going to three dimensions new questions arise
- Which kind of surroundings (tubular and/or disk-like structure or far-away panorama) are required to obtain for a good illusion effect?
- For the percept of slowing-down and up and speeding up of the airplane to arise, do the airplane and its surround to be in at about the same depth?
- Have the centers of rotation of the surround and the airplane and the point the observer is looking at, to be superimposed on one line for the illusion to arise or can we look from the side as in the pictures below?
Here some references about the “Freezing Rotation Illusion” :
- Wertheim AH, Paffen CL. Centre-Surround relative motion and the freezing rotation illusion. Perception 2009:38(11):1610-20
- Dürsteler MR. The freeezing rotation illusion. Prog. Brain Res. 2008; 171:283-5
Motion transparency is the capacity to perceive two independent motions on the same retinal location. In a situation similar to the scene depicted in the woodcut by Utagawa we would be able
to perceive both the motion of the falling rain in the foreground and the motion of the walking humans on the bridge behind it.
Motion transparency of two bright-dark (luminance) stimuli
In the video we look through a pattern of black random dots (like the rain drops in Utagawa’s picture) onto a vertical sinusoidal grating (like a person walking in the rain). In the scene on the left, half of the moving grating is covered by a stationary dot pattern and half of it is not covered. Yet we perceive no difference of the motion of the coverd or the uncovered part. In the scene on the right, the sinusoidal grating is stationary and the dot pattern is moving. Despite the dots’ motion, we perceive the grating as stationary: the motion of the dots does not capture the motion of the underlying grating, i.e. we have perfect motion transparency for two luminance based stimuli. Will we get the same result, of one of the two stimuli is defined by color or depth differences only?
Motion Transparency of a Luminance and a Color Stimulus
In the scene on the left, a red-green sinusoidal grating is moving at the same speed as the luminance grating in the previous video. However, the color grating appears to move at a much lower speed. Where the moving color grating is covered by stationary dots, we barely perceive motion at all unless we follow attentively a red or a green color ridge. At slower speeds of the color grating we would perceive it as standing still: “Motion Standstill Illusion” (see video below).
In the scene on the right, the color grating is stationary and the dots are moving. In the parts of the physically stationary color grating covered by the moving dots one has attentively to fixate a red or a green color ridge to convince yourself that the color grating is not moving together with the dots. It seems the moving dots capture the color grating: “Motion Capture Illusion“.
Researcher exploring the visual systems concluded from similar observations and experiments as shown above, that there exist different mechanisms for the perception of luminance and color motion. The mechanisms to recognize color motion much less efficient than the mechanism for luminance motion. Attentive tracking is required to recognize pure color motion. In the videos, bright yellow markers are used to demonstrate the motion of the grating when the motion perception is supported by luminance based stimuli.
Motion Transparency using a luminance and a color stimulus with either a smooth (above) or a sharp transition between colors (below).
The video demonstrates that the speed threshold to perceive color translation behind a luminance pattern is higher for stimuli with smooth than for stimuli with sharp transitions between the colors.
Similar deficits as demonstrated for the perception of color motion have been found for the perception of depth motion,
In order to perceive depth differences in the 3D-videos shown below, one needs anaglyphic glassed similar to the one shown above. Put the cyan filter before your right eye and the red filter in front of your left eye.
Motion Transparency with a Luminance and a Depth (Stereo) Stimulus
When using different horizontal disparities of corresponding points to create a percept of depth differences, one needs a structured texture pattern like the random dot pattern used here. The horizontal disparities in the textures presented to the left and the right eye encode a vertical grating whose depth varies according a sinusoidal function.
When the random dot texture is moving/standing still in synchrony with the encoded depth grating one perceives well a translating depth grating in left scene (right half side)and a stationary grating in the right scene (right half side). However, the motion of the texture and the depth grating can be varied independently. On the left half side of the left scene, only the depth grating is moving up and down, while the random dots are stationary. On has to track one of the depth ridges attentively to perceive its motion. On the left half side of the right scene, the random dots are moving up or down while the depth grating is stationary. On has again to fixate attentively on the stereo ridge to recognize the fact that it is not moving with the dots. Scientists exploring stereo vision have shown experimental evidence that the same tracking mechanism used to perceive the motion of pure color stimuli without sharp borders may be used to perceive pure depth motion.
Complex motions are rotation, expansion/contraction (scaling) or shearing. While we perceive complex motions of dark-bright stimuli nearly as well as straight (translational) motions, the videos below demonstrate that our visual system fails to recognize complex motion of pure depth and low spatial frequency color stimuli.
Stereo wheel made out of concave depth sectors. Explanations are shown on the top border of the video.
Stereo Scaling Standstill and Capture Illusions
Stereo wheel made out of concentric rings.
Stereo Shearing Standstill and Capture Illusions
Stereo wheel made out of depth sectors or concentric rings. Shearing is taken as contraction along the horizontal and expansion along the vertical axis or vice versa.
Stereo Spiraling Motion Standstill and Capture Illusions
Color Expansion/Contraction Standstill and Capture Illusions
Color Shearing Standstill and Capture Illusions
Color Spiraling Motion Standstill and Capture Illusions
More video about standstill and capture illusions can be found at:
If you want to read more about the motions illusions shown here, have a look at the following article which also contains references to relevant experimental work.
Here I show my interactive HTML5 WebGL application made using Unity3D:
Tips: Convince your-self that the wheel made out of concave sectors raising from the ground is in fact turning by checking the checkmark “markers”. Yellow lines appear within the sectors’ ridges. While the lines are perceived as readily rotating, the sectors appear as standing still. – Change the selection in left upper selection list from “sectors” to “rings” to see stereo expansion/contraction illusions. Note that the available parameters change according to this choice.
Tips how to use this app: when the applications starts, noting seems to move. Yet if you perceive motion of the color sectors, their colors are probably not isoluminant. Either you may have forgotten to take off our anaglyphic glasses or the color calibration of your display device may differ from usual settings. In the menu on the left allows you to change color settings. Color standstill and captures illusion are only perceived with minimal brightness differences between colors.
Check the checkmark “Markers” and get a paradoxical experience of yellow color blobs to go round in circles while the red sectors between them appear still to by stationary (albeit the yellow blobs never cross them).
Clicking on the button “Change Mode” changes the speed parameters in such a way that now the “Color Capture Illusion” is demonstrated (the random dots are turning around and the physically stationary color sectors are perceived as rotating with them). After clicking the button again, a “normal” rotation is shown where dots and colors are rotating in synchrony.
The selection list on the left top allows you to study other color standstill or capture illusions: choose for example “rings” to demonstrate contraction/expansion color illusions.
Color stimuli with a sharp transition between different colors: standstill and capture illusions can only be demonstrated with subthreshold speeds
Using the top-left selection list in the interactive web application for color complex motion illusions allows you to choose color stimuli with a sharp transition between the color ( try for instance “BW Sectors”). Convince yourself that with the default rotational speed of 12°/sec you perceive the rotation of the sharply delimited color sectors below the stationary dots clearly. The color sectors appear to standing still when their rotational speed is to decreased to about 4°/sec. Following the border between colors at the periphery allows you to perceive that the color sectors are still moving; i.e. at this slow speed you still perceive local translational motion but not the global rotation of the color sectors.
In order to produce contrast modulated stimuli one has to use a structured pattern like a random dot pattern in a similar fashion as to produce stereo stimuli. The contrast of the structured pattern is modulated according to the figure to be encoded.
Complex motion speed threshold for contrast-modulated stimuli are in a similar range as complex motion speed thresholds for luminance defined stimuli or sharply delimited color stimuli behind a random dot luminance mask. These findings demonstrate a clear difference to the perceptual deficits in pure depth vision or low-spatial frequency color vision where the speed threshold for complex motions is never reached.
Impressum: Dr. Max R. Dürsteler, Burgweg 46, 8008 Zürich.