Robert Kooima

There are many methods for displaying strereoscopic 3D imagery on normal displays. Anaglyphic (red-blue) stereo is perhaps the most well-known, but it requires colored glasses and doesn’t handle color imagery well. Random-dot autostereograms enjoyed a brief spike of popularity in the mid 90s, but many people (like me) have trouble seeing the image through the dots, and the 3D content may have only structure, with no color or shading allowed. Wall-eyed and crossed-eye autostereograms (same wiki article) are related techniques that do away with the random dots, but require horizontal repetition in the scene.

Of all of the existing standard-display stereoscopic techniques, wall-eyed and crossed-eye stereo image pairs are the most flexible and reliable. Most people find crossing their eyes to be much easier than wall-eyeing, since we cross our eyes automatically anytime we focus on an object near us. For this reason, I tend to focus upon cross-eyed stereo pairs as a preferred method of presenting stereo imagery on the web.

But despite being the easiest stereo technique to view, many people still have trouble with cross-eye image pairs. It is a skill that gets easier with practice, and some might need a little push to get started. This page exists to offer advice and explanation which will hopefully help some of those who continue to be baffled.

Here is a cross-eye stereo pair of the famous teapot.

When you focus on this image normally, you see a pair of teapots. They are similar, but not identical.

Now, if you cross your eyes you’ll begin to see everything doubled. You’ll see two of each of your hands, two of your computer, and two of the image on your display. You’ll see four teapots.

If you have trouble crossing your eyes intentionally, position your finger half way between your face and your screen. Look at your finger, and in the background you will see the four teapots.

In this image, the pair of teapots that your left eye sees are drawn in red, and the pair of teapots that your right eye sees are drawn in blue.

If the two pairs of teapots are not vertically aligned, then your head is tilted with respect to your screen. This can happen if you’re not sitting directly in front of the screen. Tilt your head until all four teapots are lined up vertically.

Once they’re aligned vertically, adjust the crossing of your eyes until the two teapots in the middle overlap one another.

If you’re using your finger to help you cross your eyes, then move your finger forward and back. While continuing to look at your finger, you will see the pairs of teapots shift horizontally. Adjust the position of your finger until the two middle teapots overlap. Once they are aligned, attempt to shift your focus from your finger to the teapot.

With these two teapots aligned, something in your mind will snap. It will flip on like a switch. What was once a baffling jumble of double vision will suddenly merge into clear 3D image. Once your mind has latched on to this 3D image, you will find it very easy to maintain the illusion.

The solid 3D teapot will be flanked on each side by another teapot afterimage. As you concentrate on the middle teapot, these ghost teapots will become easy to ignore. If they bother you, then you may block these flanking images with your hands, and the 3D illusion will remain.

Here is the stereo pair again. Take a look.

The size and resolution of your display should be such that the distance between the two teapot images is roughly the same as the distance between your eyes. It doesn’t really matter much if this isn’t the case, but it is optimal. It is also more intuitive to understand what’s going on. Here’s a view from above:

Your eyes are crossed. Rather than converging on the screen, as they normally do, they are converging on a point half of the distance to the screen. When you position your finger at that point, you trick your eyes into merging the teapot images on the screen.

The teapot image is an easy one. It displays a single, isolated object against a plain background. This isolated object gives the viewer a well-defined reference to apply the merging described above. Not all stereo pairs have such a useful feature, but most images have some contrast that can be focused upon.

As an example, here is a stereo pair of the constellation Orion and the Hyades cluster. Focus on the large red star Betelgeuse at Orion’s shoulder. Merge that feature, and you will find his club poking out at you. After you’ve settled in to the image, examine the detail in the Hyades cluster at the lower right, and the subtle variation in the carpet of background stars.

Click the image for a larger version. While the larger image will show more detail, it will be more difficult to view, as it will require you to cross your eyes farther.

To find other examples to practice upon, search Google Images for “cross eye stereo pair”.