Background: When you view 3d images in a viewer or with 3D glasses the optics of the viewer (or glasses) allow the left eye to see only the image intended for the left eye and the right only the image intended for it. With freeviewing both left and right images are always visible so you need to learn to point each eye at the correct image. But with some practice most people can learn to freeview parallel and crossed-eye images. Most people can learn to view both though most find one type easier to view.Parallel and crossed-eye images each have advantages (on computer screens) so many web pages, including mine, support both. With parallel viewing the images are spaced at roughly normal eye separation (2.5" or so). This limits the number of pixels available for the L and R images but allows the freeviewer to view with the eyes aimed more-or-less parallel as though staring at a far away scene. Crossed-eye images can have more pixels (up to half the screen horizontal resolution for each L and R). So the images can show much more detail. To view them the freeviewer's eyes are crossed as though viewing an image at approximately half the distance to the screen.
Parallel images can be made with more pixels but then the centers become further apart than the eyes. You can learn to freeview these but it is more difficult since diverging your eyes does not occur in viewing objects in the real 3d world. I won't try to teach you this here, but one hint is to move back from the overly wide pair so that the angle your eyes have to diverge is reduced. This is a handy skill for freeviewing old stereoscope prints (which have about 3.5" separation of the image centers).
Learning to Parallel Freeview: In parallel freeviewing the image for the left eye is on the left and the image for the right eye is on the right. You stare into the screen so that your eyes are aimed (more or less) parallel. As you stare into the screen you will get double vision as each eye sees the L & R images separately. When you are staring in at the right distance the middle two images will overlap and become 3d. The outer two images will remain and will still be 2d. A simulation of this effect is shown below (thanks to Boris S. for the idea).
Don't try to freeview the (above) image. It is just a simulation of what it looks like when you have the proper overlap of the double image. The outside images are shown blurred to simulate the effect you see when staring at the center 3D image (since your peripheral vision isn't as sharp as central vision). 
Now try to freeview the real stereo pair below.
Hints: There are several tricks that may help you as you learn. One is to turn up the room brightness until you can see your reflected face in the monitor. Looking at your reflection can help you achieve the proper overlap of the doubled images. Then you need to shift your focus onto the image without having it jump back to 2d (the tricky part :)
A second trick is to hold a stiff sheet of paper or cardboard vertically between your eyes and line it up with the center of the image you are trying to freeview. Removing the distraction of the extra images helps some people to learn to freeview.
Success: When the sample image is in 3d you will see the opening around the picture as the nearest thing (called the stereo window). The tree on the left is behind the stereo window but the closest object in the scene. The tree on the right is next further back. Then is the house and furthest is the sun. (Due to some pixel rounding in the PICT to JPEG conversion the house window is slightly closer than the house and the letters in "Parallel Viewing" aren't all exactly the same distance. If you can see all this, congratulations!)
Learning to Cross Eye Freeview: In cross eye freeviewing the image for the left eye is on the right and the image for the right eye is on the left. You stare at a point about 1/2 way to the screen so that your eye's gaze is crossing at the half way point. Thus the left eye sees the correct image which is on the right. As you cross your eyes the image will go double. When your eyes are crossed the correct amount the middle images will overlap and be in 3d. The two outer images will remain and will be in 2d.
Try to cross eye freeview view the image below.
Hints: There are several tricks that may help you as you learn. One is to move well back from the image you are trying to freeview. This will reduce the angle that your eyes need to converge at.
Another is to hold up a finger or pencil in front of the image about 1/3 to 1/2 way to the screen. Look at the pencil but concentrate on the doubled images on the screen. Move the pencil closer or further until they overlap to produce three images. The center one should be 3d, but out of focus. Then you need to shift your focus onto the center image without having it jump back to 2d (the tricky part :)
Another trick is to cut a square hole about the size of the image (the left side, say) in a sheet of paper. Hold this about half way between your eyes and the screen, centered between your eyes. When done properly it allows each eye to only see its own image. Removing the distracting extra images can help during the learning process.
Success: As with the parallel image, when the sample image is in 3d you will see the opening around the picture as the nearest thing (called the stereo window). The tree on the left is behind the stereo window but the closest object in the scene. The tree on the right is next further back. Then is the house and furthest is the sun. (Due to some pixel rounding in the PICT to JPEG conversion the house window is slightly closer than the house and the letters in "Cross Viewing" aren't all exactly the same distance. If you can see all this, congratulations!)
Caution : Freeviewing uses your eye muscles in ways you aren't (yet) used to. If you start to suffer eye strain or begin to get a headache take a break and try again later. It took me a long time to be able to view those random dots stereograms (aka SIRDS or Magic Eye images) but now I can freeview with the best of them.
Other Web Pages with Freeviewing Advice:
- Boris Starosta's Web Page (very good cross eye tutorial showing the three image effect and the blurring you see until you shift your focus to the screen).
- Duane Starcher's Juggling web page (cross eye tutorial).
- David Stuckey's parallel freeview tutorial.
- Ray Hannison's parallel and cross view tutorial.
Back to my 
Email: greg.erker@shaw.ca
