Watercolor Optical Illusion

This is an example of the watercolor effect. This effect was first demonstrated by Baingio Pinna in 1987.

Simply put The watercolor effect is perceived when a dark (e.g., purple) contour is flanked by a lighter chromatic contour (e.g., orange). Under these conditions, the lighter color will assimilate over the entire enclosed area.

What does that mean? Take a look at the below image. You see 9 distinct squares. The center of each square looks white but the outer area of each square has taken on a watercolor shade, pink, green, yellow, blue etc...

Tic-Tac-Toe Board
(c)Walt Anthony
Created from samples
provided by Akiyoshi Kitaoka

The truth is that aside from the squiggly lines the only color here is white. The outer ring and the inner square are pure white.

Prove It: You can prove this by using the eye dropper tool in either of the following free graphics programs Paint.Net or Gimp or you can use the eye dropper tool in the free Colorzilla extension for Firefox browser. When you use these tools you will find that RGB value of everywhere on this image that is not a squiggly line is R:255 G:255 B:255 or #FFFFFF.

Print out this image and slide it into a plastic document protector. Now use an earasable marker and you have a reusable optical illusion Tic-Tac-Toe board.

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Ponzo Optical Illusion

The Ponzo illusion is an optical illusion that was first demonstrated by the Italian psychologist Mario Ponzo (1882-1960) in 1913. He suggested that the human mind judges an object's size based on its background.

The typical example of this is the figure below. This is the typical railroad track scheme usually used to represent this illusion. The vertical lines appear to go off into the distance like train tracks. This gives us the impression that the line in the distance is larger then the line that appears to be nearer to the viewer.

Mario Ponzo
1913

Seeing this illusion against a great real photo backdrop can be amazing. Most of the time people pick train tracks. Below is an outstanding photo that presents this optical illusion slightly different...

The two red lines (or three if you move your mouse over the image) below are duplicates, they are the exact same size. Which image is more convincing the one with two lines or the one with three lines?

Ponzo Illusion
(c) 2006 Walt Anthony

Project: Create your own illusion. Here is a suitable image of train tracks (right click and select "save target as" or "save link as" depending on your browser). You can use Windows Picture and Fax viewer to print it out.

Now that you have it printed try and create your own illusion.

Use 2 Popsicle sticks painted red or just draw red lines with a marker.
Does changing the color change the effect?
Does using two different colors alter the illusion?
Try fat and skinny lines. Does the thickness of the lines affect this illusion?
Vary the distance between the lines, does this change the effect?
Is a three dimensional Popsicle stick as effective as 2 dimensional red line?
Does the distance from which you view the illusion change the illusion?

What can you conclude, if anything, about how we perceive things from this project?

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Hering Optical Illusion

This optical illusion was discovered by the German physiologist Ewald Hering in 1861. The two blue lines are both straight and parallel to each other. If you are like most folks you may see things differently. The lines in fact may look as if they are bowed outward.

Ewald Hering
1861

The distortion is produced by the lined pattern on the background, that simulates a perspective design, and creates a false impression of depth. If you pass your mouse over the image the background lines will be removed and you can see for yourself that the blues lines are definitely straight.

Project: Create your own illusion. Select a sheet of paper and place a dot in the center of the sheet. Now draw 10 lines through the center, mimicking as close as you can the background of the above illusion. Now on a sheet of clear document protector draw your two vertical lines.

Increase the amount of background lines to 18. Does that affect the illusion?
Does changing the color of the background change the effect?
Does changing the color of the background lines change the effect?
Does using two different colors for the vertical lines alter the illusion?
Try fat lines. Try skinny lines. Does the thickness of the lines affect this illusion?
Vary the distance between the vertical lines, does this change the effect?
Does the distance from which you view the illusion change the illusion?

What can you conclude, if anything, about how we perceive things from this project?

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Checker Shadow Optical Illusion

This is the Checker Shadow Optical Illusion. It was developed by Edward H. Adelson of the Perceptual Science Group at MIT. The interesting thing about the below optical illusion is that square A is the exact same shade of gray as square B.

One way to identify a color is by it's Red, Green, Blue values or RGB for short. When written for 24 bit truecolor R:0 G:0 B:0 is black and at the other end of the spectrum R:255 G:255 B:255 is white. The other 16 million or so colors fall somewhere in between. Squares A and B below each have the same RGB value of R:120 G:120 B:120.

So now that I said it is true you have all obviously accepted the fact that square A and square B are indeed the exact same shade of gray, right? I mean if it is on the internet it must be true. Would it be easier if you could prove it to yourself and any skeptics you might run into (like Mom and Dad or that annoying sibling). Continue on and we'll get around to proving my statements correct.

Project: Proving square "A" is the same shade of gray as square "B".

There are a few ways you can prove that the 2 squares are the same shade.

Before we continue, right click on the image and open it in a new window. Now you have a giant image to work with.

1) You can use a graphics program like Photoshop, Paint.Net, Gimp or the Colorzilla extension for Firefox browser.

My choice is Colorzilla w/Firefox. Using the eyedropper tool you can determine
that the RGB values of the grays in both square A and square B are 120-120-120.
Not good enough for you, heh? Still not ready to trust that the computer is correct or you don't have an eyedropper tool? Either way you can move on to step 2 or 3 below.

2) Cut out a cardboard mask.

By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two holes created in the right spots will work as a mask for a computer screen or as a mask for a the printed illusion. Holding up this mask to the image on the screen or printed paper should be enough to convince you. But if you were like my daughter nothing but this next step would do.


3) Print the image and cut out the squares.

This is another way to isolate the patches from their surrounding context. Cut out each square along the edges. Remove them. Hold them side by side. Overlap the cut out
squares. Yup they're the same shade of gray. No denying it now, is there?

Please note that I have heard that some printers have "enhancement" processing that increases the contrast of edges. This can cause the printed squares to have slightly different values of gray. I haven't run into one of these printers yet where the overlapped squares didn't look identical, but your mileage may vary.

WHY you ask? You might have come to terms with the fact that the two squares are the same shade of gray, but how does this optical illusion work? The creator of the illusion has an explanation here.

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