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How to View Stereograms If I had a dollar for every time someone told me they have never been able to see stereograms (”those things” is what they usually say), I could have retired years ago. Honest! And yet most people can see stereograms. It’s true. If your eyes are relatively in good condition, and you can perceive depth in everyday life, parallel parking for example, then you can probably see a 3D stereogram too. The biggest reason for people not being able to see a 3D stereogram, aside from performance anxiety, and sheer recalcitrance is they are looking in the wrong place. To put it another way, they are using the wrong focus. I’ll explain. We we read a book, look at a magazine or a photograph, our focus is on the page. This is fine for reading but not for looking at stereograms. When you view a 3D stereogram you have to focus a bit behind the image. This is not as hard as it sounds. The most sure fire method is to bring the image as close to your face (or bring your face as close to the image), until your nose is almost touching the image and everything is blurry. Then very, very slowly, move the image back, or move back from the image. Don’t try to focus on the image itself because if you do this you’ll never see it. Relax your eyes and don’t try to focus on anything. When you do this, and it may take several tries, you should start to get the feeling that you are seeing depth. If this happens, resist the temptation to focus on the image or you’ll lose it and have to start all over again. Just go with it and let your brain pop the hidden image. Nothing is more exciting than seeing your first hidden image 3D stereogram. And once you have seen one stereogram, the next, and the next, and the next one will get easier. I promise.
How Stereograms Work When you look at the two circles above. If you look directly at the circles you will only see two. If you relax your eyes, let them become lazy, the two circles should become three circles. I have no idea why this is but it is how your eyes focus when you look at a stereogram.
If you use the same vision to look at the three sets of five circles above, you will see three sets of six circles. Not only that but the white circles will appear as if they are coming forward and the dark gray circles will appear to be farther back than the other two sets of circles. Try it. Why is this? This one I have the answer for. It’s the spacing. Each of the three sets of circles are evenly spaced. The white circles are spaced closer together while the dark gray circles are spaced farther apart. This is stereograms work. I’ll elaborate on this in a moment.
Building on this principle, the illustration above should give the impression of two rows of three circles with the red circles being very far back and the pale yellow circles popping off the page. Once again this is done with spacing. The red circles are farther apart while the yellow circles are closer together.
How Stereograms are Made Hidden image stereograms are made from two images, a depth image, and a repeating pattern, which is similar to wallpaper. The depth image is a grayscale image that represents the depth of the hidden image. White comes forward, and black goes back, with 254 shades of gray in between white and black. The repeating pattern is used to hide the hidden image. The computer software interprets the depth information provided by the depth image and makes subtle changes to each of the repeating patterns. These changes are usually not apparent to the viewer but the subtle shifts in the patterns is what our eyes detect and interpret as depth. Remember how the spacing of the sets of circles effected the perception of depth, and made the white circles spaced closer together appear to come forward while the dark gray circles that were spaced farther apart appeared to be farther back? The stereogram software does a similar thing when it makes shifts in the pattern and this is what creates the illusion of 3D depth. If the stereogram is well crafted, you will not be aware of the pattern shifts.
Depth Images Hidden images in stereograms are weird. I say this because they go against our basic understanding of 3-dimensional objects. If we look at the red ball in the illustration above, we assume it is a sphere. Why? The answer is visual cues. In reality the red ball is a flat, two dimensional object. But because it has a highlight, a core shadow (the darker part of the ball), a reflected surface, and a shadow—all visual cues, we see this as a sphere with a 3-dimensional shape. But the hidden image in 3D stereograms has no visual cues. No color. No highlights. No shadows. None of the above. Yet, we we still perceive the hidden object as 3-dimensional. How can this be? I haven’t the slightest idea. I just create the images! It’s magic is my only explanation. When I create a depth image to instruct my software how to create this magical hidden image, I have to go about the process in an entirely different way. I have to create the hidden image from back to front, from dark to light. The sphere on the right is a good example. It’s as if I can slice the hidden object into 256 solid colored pieces. I have added outlines so you can see the individual slices. It looks sort of round but not at all like the red ball. This is what the software sees: A black background, which goes as far back as it can go, and a sphere that starts somewhere in the center of the 256 steps and which comes all the way to the front of the image.
In the stereogram above the top part of the circle is a sphere, the bottom half is the inside of the sphere. The depth image and repeating pattern are shown directly above. Because the pattern is very random, you should not be aware of the shifts. But the effect is magical.
Mapped Texture Stereograms - What You See Is What You See Hidden Image Stereograms such as the in and out spheres are good for 3D objects but not that good for text or more precise shapes. In this case a Mapped Image Stereogram is often the best solution. A Mapped Image Stereogram uses the same depth image, but creates a repeating pattern of an outlined or filled shapes that is the same as the hidden image. The difference is  you can actually see the hidden image, instead a random pattern, or in many cases, a combination of the two. But the results are a clear, sharp, well delineated 3D image as shown in the example below. In the Mapped Texture Stereogram shown above, you should be able to read each word separately as well as see the scissors. And you should perceive 3D depth. Do you? IN or OUT? Many people remark that the hidden image appears going away instead of coming forward. Kind of like it is indented into the background instead of popping out. This is because about 10% of stereogram viewers are looking at the stereogram with their eyes slightly crossed as opposed to looking through the image looking straight ahead with both eyes parallel. These people will see the image in the reverse of how it was designed. They will still see 3D, but they (or you, if this is the case), see things going into the background instead of coming forward. This is OK and I can design a stereogram that these people will see correctly, but the other 90% will see it wrong, or maybe not wrong, just not the way it was intended to be seen.
Diverging - Parallel 90%
Converging - Cross Eyes 10%
Look at the two images above. They both contain the words IN and OUT as shown in the depth image on the left. If you see the text coming forward in the stereogram on the left and going into the background on the stereogram on the right, you are in the 90% majority who see stereograms with the Parallel method. If you see the text on the left going into the background and the text on the right popping out, you are viewing the image with the cross eyes method along with about 10% of those who can see stereograms. What this means to you if you wish to use a custom stereogram, the image may not look right to you if you normally view it cross eyes, but it will look correct to most of your readers.
What Works and What Does Not Work in a Stereogram One frequently asked question is can you put a photograph or a likeness of a person in the hidden 3D image. The short answer is no. The long answer is perhaps, but it will be very expensive and you might not recognize the person. I can not just take a photograph and make it into a stereogram. The face has to be recreated in terms of a depth image, and requires the image to be painted from back to front. It can be done. But not by me. Simple images work best. More complex images are more difficult, if not impossible to see. Logos can work well if they are not overly complex. Another frequent request is to hide some text message in the image. A few words are possible but more than a few words becomes more difficult and produces fewer people who can read it. Over the years I’ve come to realize that every stereogram is different and a lot of trial and error is involved to get it right. My rule of thumb is simple is better. But I am always happy to discuss a project with you and to let you know if I think something will work or not. Go to the contact page and send me an e- mail or go to the eyeTricks site and submit a quote request.
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