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—A Guide to Stereograms How Stereograms Work
Humans see in stereo vision. The left eye and the right eye see slightly different angles of the same thing and our brain combines the two images into one vision that has form and depth. The two figures shown above (Hotei-Sama, one of the 7 Japanese gods of fortune) are seen from slightly different angles as might be seen with each eye. If you stare at the image for a little while, the two images should merge into one image, with the appearance of form and depth. The Stereopticon viewers that were popular in the late 1800s worked with this principal.
In stereo vision, the distance between objects is interpreted by our brain to represent space. In the above example, the moon is spaced farther apart than the figure and when viewed in stereo vision appears in back of the figure. The small spheres are spaced slightly closer together and thus appear to be floating in front of the figure.
Here is another example of the effects of spacing. The pair of circles on the top appear as though we are looking down a tube. The pair of circles on the bottom appear to be popping out towards the eye.
Stereograms hide portions of the image in a series of panels. The spacing of the image portions, determine whether the image comes forward or backwards. Stare at the example above. The white circles are spaced closer together and so they appear to come forward more than the dark gray circles which appear to be farther behind. Color and value play an important role as well as they provide a “visual cue” to the objects’ depth. Lighter objects appear to be closer to the viewer than the darker objects.
A realistically rendered sphere has a highlight and a core shadow and often a shadow to give it the appearance of depth and form. Depth map images, the images that tell the software how to create the hidden 3D image work in a very different manner.
The customized software that is used to create a Stereogram uses two images. The first is called a “Depth Map” (shown above) and tells the software how to create the hidden 3D image. White comes out the most while black goes the farthest back. In the illustration above, the sphere is divided into different planes ranging from white to black with 253 shades in between (for a total of 255 steps).
The kind of Stereograms we create have two main elements, the grayscale Depth Map image, and a pattern which the software uses to hide the image. The software interprets depth using the grayscale image and then makes subtle adjustments to the pattern that when viewed as a Stereogram create the impression of depth as you can see in the image below.
Here is the Stereogram created from the leafy pattern and the grayscale Depth Map image. The sphere appears to have depth and shape although there is no core shadow and no floor shadow.
Many times we will use floating objects such as the red spheres and boxes shown above to add an extra degree of depth and to help identify the hidden image. Floating objects can be logos, product photos, text, most anything really. So, when it is not always appropriate to use a complex logo or a line of text for the hidden image, the logo or text can often be used effectively as a floating object. Mapped Texture Stereograms Another new kind of stereogram that we have been working with recently is called a Mapped Texture stereogram. Unlike the hidden image stereograms in which the hidden image is completely hidden in the pattern, Mapped Texture Stereograms create depth using completely visible objects that are displayed overlapping five, six or seven or more times. This form a stereogram is perfect for complex logos or lines of text as demonstrated below.
The depth map image is the same for a Mapped Texture Stereogram as for a repeating pattern stereogram.
Unlike the repeating panel stereogram, however, Mapped Texture Stereograms use the actual images. If you look at the depth image and then the color texture image above, you can get an idea of how the final image will look. The scissors will be towards the back and the words will float in front on different levels.
Here is the resulting Mapped Texture Stereogram. The computer interprets the depth and makes subtle adjustments to the pattern resulting in depth. The not-so-hidden image when it pops is razor sharp, especially when printed or viewed on the screen at larger sizes. In or Out? About 90% of the people who can see stereograms, see the images coming forward (Diverging). The other 10% see the same image going backwards (Converging). The reason for this difference is this: Most people look at a stereogram using parallel vision—each eye looking straight ahead. This is the way we look at a distant object, a clock on the wall, for example, as opposed to looking at the computer monitor or something on a printed page. The 10% who see the images going inwards rather than popping out, are viewing the stereogram with crossed eyes as illustrated in the diagram below.
Stereograms can be created either diverging or converging. How to View Stereograms
Many people have seen a Stereogram but have never had much luck seeing the hidden image. This section is for you Stereogram-challenged persons. Most of us can see the hidden image in a Stereogram unless we have a serious problem with our eyesight. But sometimes the pressure to see the hidden image is too much and when the image does not immediately leap out at us, we panic and get performance anxiety and then no matter how hard we try, the image will not appear. The best advice for viewing Stereograms is RELAX. This is not a contest! Try to look beyond the Stereogram and focus your eyes at a distance behind the image. For example, if you are viewing these images on the screen, pretend you are focussing on something on the wall behind your monitor. Gradually, your eyes will focus on the image and your brain will pull it together. Another method is to put your nose almost on the image (don't get too close or you might put a nose print on your screen). Now pull back ever so slowly, again trying to focus on something behind the screen. As you slowly pull away from the screen, relax and let your eyes pick out the image and bring it into focus. Once you get the image to “pop”, stare at it for a bit longer. The longer you look at an image (well there are limits) the more depth you will see.
If the above suggested methods fail, try this. Stare at the two black circles above. Eventually your eyes will pull the two images together and it will look like there are three circles and you will have a feeling of space. This is what happens when you look at a Stereogram.
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. Simple images work best. More complex images are more difficult to see. This is a good opportunity, however, for using both a hidden 3D image along with floating objects. While creating a 3D portrait is very difficult, not impossible but difficult, a floating image can be almost anything you want it to be. Another frequent request is to hide some text message in the image. A few words are possible but more than a few words becomes extremely difficult, if not impossible to see. The best rule of thumb is simple works best.
Patterns are another important consideration. Random patterns such as the leaf patterned used previously work well because the adjustments to the pattern are subtle and not very noticeable. The example above uses the same Depth Map for the sphere but uses a black and white check pattern. In this case the distortion of the pattern is quite obvious. So, the most successful patterns are ones with a good random texture. Each Stereogram is different and takes a bit of time to fine tune the image and get the pattern and the Depth Map images to work to produce an image with a seemingly consistent pattern.
©2011 Custom-Stereograms.com
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In the later part of the 1800s, people amused themselves looking at 3D photographs and illustrations using a Stereopticon. Photographs were taken from two angles to simulate the way humans see. As the objects in the stereo pair of images were farther apart, they appeared to be in back of the objects that were closer together.
