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Posts Tagged ‘Visual Illusion’

Why Visual Illusions: Illusory Contours and Checkerboard Illusion

In Computer Vision, Paper Talk, Visual Illusion on September 16, 2013 at 6:33 pm

by Gooly (Li Yang Ku)

I talked about some visual illusions in my previous post but didn’t mention why they are important to computer vision and the pros of seeing visual illusions. In this post I am gonna talk about the advantage of having two of the most common known visual illusions, Illusory contours and checkerboard illusion.

Illusory Contours:

Kanizsa's Triangle

The Kanizsa’s triangle invented by Gaetano Kanizsa is a very good example of illusory contours. Even though the center upside down triangle doesn’t exist, you are forced to see it because of the clues given by the other parts. If you gradually cover up some of the circles and corners, at some point you would be able to see the pac man and the angle as individual objects and the illusory contours will disappear. This illusion is the side effect of how we perceive objects and shows that we see edges using various clues instead of just light differences. Because our eyes receive noisy real world inputs, illusory contour actually helps us fill in the missing contours caused by lighting, shading, or occlusion. It also explains why a bottom up vision system won’t work in many situations. In the paper “Hierarchical Bayesian inference in the visual cortex” written by Lee and Mumford, a Kanizsa’s square is used to test whether monkeys perceive illusory contours in V1. The result is positive but has a delayed response compared to V2. This suggests that information of illusory contours is possibly generated in V2 and back propagated to V1.

Checkerboard Illusion:

checker board illusion

This checkerboard illusion above is done by Edward H. Adelson. In the book “Perception as Bayesian Inference” Adelson wrote a chapter discussing how we perceive objects under different lighting conditions. In other words, how we achieve “lightness constancy”. The illusion above should be easily understandable. At first sight, In the left image square A on the checkerboard seems to be darker than square B although they actually have the same brightness. By breaking the 3D structure, the right images shows that the two squares indeed have the same brightness. We perceive A and B differently in the left image because our vision system is trying to achieve lightness invariant. In fact if the cylinder is removed square A will be darker than square B, therefore lightness constancy actually gives us the correct brightness when only constant lighting is presented. This allows us to recognize the same object even under large lighting changes, which I would argue is an important ability for survival. In the paper “Recovering reflectance and illumination in a world of painted polyhedra” by Sinha and Adelson, how we construct 3D structure from 2D drawing and shading are further discussed. Understanding object’s 3D structure is crucial in obtaining light constancy like the checkerboard illusion above. As in the image below, by removing certain types of junction clues, a 3D drawing can easily be seen flat. However, as mentioned in the paper, more complex global strategies are needed to cover all cases.

3D 2D Recovering  Reflectance  and  Illumination  in  a  World  of  Painted  Polyhedra by Pawan Sinha & Edward Adelson

I was gonna post this a few month ago but was delayed by my Los Angeles to Boston road trip (and numerous good bye parties), but I am now officially back to school in UMASS Amherst for a PhD program. Not totally settled down yet but enough to make a quick post.

Visual Illusion: Chronostasis and Saccadic Masking

In Computer Vision, Neural Science, Visual Illusion on June 26, 2013 at 9:54 pm

by Gooly (Li Yang Ku)

some visual art to attract your attention, has little to do with the post

I was always intrigued by visual illusions and am often surprised by how often we are fooled by our eyes. Some visual illusion is just as good as a good joke. One of my favorite illusion is the spinning dancer, which I can’t easily change the direction I interpret despite knowing it could be both. Understanding visual illusions is also crucial in Computer Vision because they are just side effects produced by the underlying algorithm that helps us see. A great vision algorithm should probably have the same visual illusion as humans do.

Spinning Dancer illusion

Chronostasis is a kind of visual illusion that occurs to you every moment without you noticing. To test it out, you need to find a clock that has a seconds hand; first focus your gaze on some where close so that you can still see the hand ticking from the side view but not too close, then shift your gaze to the seconds hand when it just moved. You’ll notice that the first tick seems to be longer than the other ticks after it.


This illusion is caused by Saccadic Masking, a mechanism that our brain uses to help us see the world without getting dizzy. Our eyes are constantly moving and our head also turns a lot. Saccadic masking shuts down the input when the scenes that shown to your eyes are blurry. So when you move your eyes, the brain has two choices, it can either keep the last image or show you the next stable image in the future. So now you might be yelling “HOW COULD THE BRAIN POSSIBLY SHOW YOU THE IMAGE IT HAVEN’T SEEN!” Yeah, that’s not possible. But remember that there is no clock ticking in your brain and time is just what you feel; so your brain can just freeze your internal clock and wait for the next image then fast forward your internal clock so it syncs back with the real world. And that’s what happened to you when you did that first gaze shift to the seconds hand.


To test out Saccadic Masking you can also find a mirror and stare at your pretty (or nerdy) eyes. First focus on your left eyes, then shift your gaze to your right eye. You won’t be able to see your own eyes saccade because of Saccadic Masking, but if you record yourself doing the same experiment with a smartphone’s forward facing camera, you would be able to see your eyes saccade clearly. (note that smart phone cameras have time delays, so don’t use them as a mirror for testing. It is highly recommended to be used as a mirror outside of the experiment though; it always shows a slightly younger you.)