Stereogram Tutorial

Julio M. Otuyama


Introduction

Stereograms are illusions of 3D surfaces. This tutorial shows how it works, with a simple "do it by yourself" approach. The random dot stereogram is very easy to understand, although not a trivial concept. Unlike holograms that require laser beams, stereograms can be created, printed and visualized with computers. But it may not be an easy task to see its 3D surfaces.

Stereo Pairs / Stereograms

Stereo Pairs / stereograms work because the brain can detect small differences between the images seen by each eye. Stereo pairs have been studied for a long time - Leonardo da Vinci almost discovered the concepts of stereo pair visualization. But only Wheatstone correctly described these concepts. Random dot stereograms is a very new discovery, developed by Bela Julezs, during 1970's. Random Dot Stereograms give us a deep impact because there is an invisible hidden surface.

Stereo pair

random dot stereogram
Hidden surface: square

Visualization

The visualization of a stereogram requires an unusual skill: each eye must be targeted to distinct places on the image. This is unusual to our brains. That is the reason some people hardly (or can not) see stereograms.

There are two ways to visualize stereograms:

The target of each eye (left and right) is shown:

regular visualization - no stereogram
divergent visualization of a stereogram
convergent visualization of a stereogram (cross)

Once one gets the divergent or convergent visualization, he/she has a clear view of 3D surfaces.

Helper to visualization:

Random Dot Stereogram

Our brain is able to perceive the 3D information of the environment. The main reason for that are small differences between each eye image. Both images are captured with a small difference (the distance between our eyes). This results in very small shifts on each retina projection image, hard to notice. But although small, those differences give us the 3D sensation.

Creating a Random Dot Stereogram

The easiest way to learn how a stereogram works is to create one. Next, the recipe to generate a random dot stereogram (adapted from Bela Julesz article). There is also a second approach, based on "ASCII Stereogram". With the ASCII Stereogram, it is easier to understand the disparity shifts. You can see both as stereograms, if you have skill enough.

Steps to generate a stereogram:
Procedure steps
Random Dot Stereogram
ASCII Stereogram

(a) Create a random dot image
ABCDEFGHI
JKLMNOPQR
STUVWXYZ0
123456789
0ZYXWVUTS
RQPONMLKJ
IHGFEDCBA
 
(b) Duplicate the image and put side by side
ABCDEFGHI ABCDEFGHI
JKLMNOPQR JKLMNOPQR
STUVWXYZ0 STUVWXYZ0
123456789 123456789
0ZYXWVUTS 0ZYXWVUTS
RQPONMLKJ RQPONMLKJ
IHGFEDCBA IHGFEDCBA
(c) Select a piece of one image
ABCDEFGHI ABCDEFGHI
JKLMNOPQR JKLMNOPQR
STUVWXYZ0 STUVWXYZ0
123456789 123456789
0ZYXWVUTS 0ZYXWVUTS
RQPONMLKJ RQPONMLKJ
IHGFEDCBA IHGFEDCBA
(d) Lateral shift of this piece of image
ABCDEFGHI ABCDEFGHI
JKLMNOPQR JKLMNOPQR
STUVWXYZ0 STVWXXYZ0
123456789 124566789
0ZYXWVUTS 0ZXWVVUTS
RQPONMLKJ RQPONMLKJ
IHGFEDCBA IHGFEDCBA
The stereogram is ready!
The hidden image is the piece shifted
ABCDEFGHI ABCDEFGHI
JKLMNOPQR JKLMNOPQR
STUVWXYZ0 STVWXXYZ0
123456789 124566789
0ZYXWVUTS 0ZXWVVUTS
RQPONMLKJ RQPONMLKJ
IHGFEDCBA IHGFEDCBA

Convergent/Divergent view and disparity

With a divergent view, the shifted square appears as floating (nearer than everything else). With a convergent view the shifted square appears as a hole (farther than everything else).
Disparity is the concept that explains why we get the 3D perception of the nearer and the farther distance. This concept can be explained with "ASCII Stereogram". Let's call disparity as the number of letters to to go from a letter in the left pair to its counterpart at the right pair. For instance, from the first letter "A" of the left pair takes 10 letters of disparity to reach its counterpart in the right pair:

ABCDEFGHI A
^123456789^

On the (b) step, all the letters have the same disparity (it is only a flat surface).

The shift on step (d) changes the disparity. For instance, the letter "V" now takes 9 letters of disparity to reach its counterpart in the right pair:

VWXYZ0 STV
^12345678^

With a divergent view this means that our eyes have to "move a little" to perceive this as the same letter by both eyes. The "movement" required is the right eye to move a little toward the left, just like we were seeing things nearer. That is why this letter is perceived as nearer than the background. As a matter of fact, our eyes don't move, but the image projected on the retina causes this effect, and our brain is able to detect this.
Now we can make a relationship of shifts and disparity:

  Left Pair Right Pair
Increase Disparity Shift to Left Shift to Right
Decrease Disparity Shift to Right Shift to Left

And a relationship of disparity and divergent/convergent view:

 

Smaller Disparity

Larger Disparity

Divergent View

Float Near

Float Far

Convergent View

Float Far

Float Near

The disparity causes inverse effects with divergent versus convergent views because of the cross in the convergent view.

Try it (MS Windows 3.x/95/98/me/NT/2000/XP/2003/Vista/7/8/10)!

This lab can be done on your computer in a few moments. No extra software is required.

This description is based on MS Windows / IE, but it's easy to adapt to Apple Mac, Linux, etc.

Steps to generate a Stereogram:
Save this image on your computer:

Open the image with an image edition tool (Paint, Paintbrush, Photoshop, etc)

Select a small piece of one image with the selection tool (usually the dotted square)

 

Shift the selection (lateral shift only)

The resulting image is a random dot stereogram made by you!
Hint: you can select any kind of shape, even letters (yes, your name!). But the shape must be large enough for our brain to detect it.

SIRDS - Single Image Random Dot Stereogram

The Random Dot Stereogram is a scientific proof (see below), but what made this concept widely known was the illustrations called SIRDS (books like Magic Eye, CG Stereogram, etc). To create a SIRDS, you have to put not two, but a repeating pattern of random dots, and make shifts to get the desired effect. Instead of black and white random dot patterns, you can use a colored pattern to fill the repetition. To create complex surfaces there are computer algorithms that copy the pattern side by side based on grayscale disparity maps.

Grayscale disparity maps

The easiest way to create stereograms with complex surfaces is to use grayscale disparity maps. The grayscale disparity map is a representation of some surface with each gray tone associated with a distance. For instance, you could create a background black disparity map, with a floating square near, represented by a white square. To create a random dot stereogram based on this disparity map, you have to copy (or copy and shift) the random dot pattern based on the corresponding coordinate content color in the disparity map. When the color is black, just copy the pixel to the other image. If the color is white, copy the pixel to the other image and shift it. The figure below illustrates this procedure. This is almost the same random dot stereogram presented previously.

Grayscale disparity map (only one cycle)

 

The disparity map associates gray tones to distances:

  • black - background distance
  • gray and white - floating squares at different distances

Copy the pixel based on the corresponding coordinate content color of disparity map:

  • copy pixel with no shift if the disparity map content is black
  • copy pixel with small shift if the disparity map content is gray
  • copy pixel with large shift if the disparity map content is white

Result: Random Dot Stereogram

  • black: background
  • gray square: small disparity
  • white square: large disparity

Creating a SIRDS

Now we can create a generic SIRDS with a (not so) complex surface. There are many cycles to compose the SIRDS because one cycle requires the pattern from the previous cycle. That is the reason SIRDS requires computer programs to generate. It is a difficult task to copy the pattern based on the disparity map, and this copy requires data only available after the computation of each cycle. Sometimes there are holes with no pattern after cycle copy, that requires artificial filling with extra patterns. Sometimes the same pattern is copied many times at very close pixels, creating an (undesirable) visual clue of the hidden surface. As a matter of fact, a computer program doesn't need to implement the cycles, just copy a non-transparent pixel to another pixel as many times as is required. The cycle is just a consequence of the procedure.

This procedure shows a black (no shift) and white (shift) copy. With a general grayscale disparity map, the shift can be small or big, based on how black or white the gray tone is. Usually the image is processed with higher resolution than the original, to achieve a sub pixel shift accuracy.

Grayscale disparity map
(many cycles)

Disparity Map

 

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Result: SIRDS

Someone could try to move the whole white selection with just one shift, avoiding the cycles. It wouldn't work because the disparity of the background and the whole selection is the same.

There is another approach to generate SIRDS, with an algorithm based on ray tracing concepts. The result is almost the same, with no visible differences, but it takes a lot more computer resources. Yes, I did implement a ray tracing algorithm to create stereograms - because at that time there was no information of how stereograms worked, and I had no idea who developed those illustrations (there was no stereogram books yet; I could only get a copy from an anonymous copy (of a copy) of a one page stereogram illustration). Everybody was amazed with that illusion. I was learning C programming language during my under graduation, so the ray tracing stereogram program was a good exercise. Later I found information about Bela Julesz and random dot stereograms.

An interesting illusion

The random dot stereogram is described in the "Foundations of Cyclopean Perception" article of Bela Julesz. This article is not about entertainment, but a scientific procedure to show how our brain works. In this article Bela Julesz shows that with the random dot stereogram:

So, this shows that:

Stereogram Gallery

Images: Flag of Brazil, Wine Glass, Rings, USP, Pigeons, Birds, Half Sphere, Helicoid

Flash animation: Hanabi Stereogram

QR Stereo Code

HTML5: Generate SIRDS online

External Links

Foundations of Cyclopean Perception - Bela Julesz

Book Eye, Brain and Vision (EBV) from David Hubel. That is a good introduction to human vision from a neuro-scientist. It is a scientific book, but it is very easy to read.

Stereogram, Autostereogram, Christopher Tyler


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