A Short Course in Digital Photography
9. Stereo Photography

CONTENTS The First Stereo Photographs Taking Stereo Images Viewing Stereo Images Web 3D Viewers Making 3D Images Stereo Panoramas

Photographs are great at rendering the details in a scene but don't readily convey the impression of depth. Everything is rendered on a flat plane. Ever since photography was invented, people have been trying to correct this. There have been hundreds, if not thousands, of stereo patents filed and demonstration devices built. Sir Charles Wheatstone discovered the principles of stereoscopic vision in 1838, a year before photography was invented. 3-D or stereoscopic imagery uses two images of the same scene taken from slightly different viewpoints. Using one of many viewing technologies, the images are combined in your mind producing a third dimension—depth. 3D effects arise from the fact that each of your eyes sees from a slightly different perspective. To demonstrate this, hold your finger about a foot from your nose and close one eye, then reopen it and close the other. Your finger will appear to jump from side to side. This difference is due to parallax.

Stereo images displayed on the computer screen can great an illusion of depth. Image courtesy of StereoGraphics.

The First Stereo Photographs

As soon as Henry Fox Talbot's and Daguerre's photographic processes were introduced in 1839, people started making stereo views. In many respects, the results were as good as those we get today. The problem was that the process of making the photos was expensive and there weren't any special or widely available viewers.

The big breakthrough came in the 1850s with less expensive albumen prints and viewers—the first invented by David Brewster, and a later and less expensive one by Oliver Wendall Holmes. Views of foreign places, and other scenes of all kinds, became the rage in Victorian homes and millions were sold. You can easily find some of these views in almost any antique shop although some are very expensive and they are fast disappearing as collectors gather them up. These mass-produced stereo cards have two prints mounted side-by-side on a piece of cardboard. They are inserted into a holder on the viewer and you view them through an eyepiece to see them in 3D.

A Thomas Houseworth stereo of Yosemite.

A Holmes stereo viewer comes in a do-it-yourself kit. Images courtesy of Reel 3-D.

Taking Stereo Images

Almost all-stereo images start with a pair of photographs taken a few inches apart. The effect is to duplicate the spacing of our eyes that gives us stereo vision. Although it's possible to take a pair of stereo images with a single camera and lens, it's not the easiest way and the results are somewhat unpredictable. It's important that the lenses be the same focal length, exactly parallel, and offset by just the right amount—called the stereo base distance. For this reason, special cameras are usually used.

The Argus 3D Stereo Camera uses 35mm film. It has 2 matched 28mm lenses, 2 apertures (daylight and flash), and 1 shutter speed so everything is in focus from 2 1/2 feet to infinity. It also has a built-in pop-up flash. Image courtesy of Reel 3-D.

If you don't have a special camera, you can experiment using the one you do have. (Adapted from 3D-Web.)

1. Create a guide that helps you move the camera 2.5" between shots and also keep the lens aligned so it's parallel. Just take a flat board and put a small wooden rail on it. When you slide the camera along the rail from flush-left to flush right, the lens moves exactly 2.5 inches. You can even add hardware to mount the base to your tripod

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2. Select a scene where nothing is moving—not even clouds or leaves in the wind. Be sure to include something in the foreground to give you a stereo effect. (A shot of distant mountains won't have any.) Generally, a 35mm camera with a 35mm lens can handle a depth range from 2 meters (7 feet) to infinity. Photographs taken that only include objects in this depth range will view perfectly well in any viewing device. Here are some guidelines for other scenes:

• If a foreground object is as close as 5 feet, objects more than 20 feet away must be excluded.
• If a foreground object is as close as 3 feet, objects more than ten feet away must be excluded.

3. Use a level to keep the camera level between shots. 4. Set the camera to a small f-stop for maximum depth-of-field. 5. Use the same exposure for both pictures. An alternative approach is to buy two disposable cameras and tape them together. You might want to tape them to a board or tripod so they don't move or learn how to press both shutter releases together.

The problem with most stereo cameras is that they are based on old technology. There really isn't a high-quality modern 3D camera at a reasonable price. There are small firms that couple cameras together so you can take stereo images using modern equipment with autoexposure, auto focus, and so on. It's possible to couple two digital cameras in this same way. This eliminates the need to get film scanned to display it on the Web.

David Grenewetzki rigged two Kodak DC20 cameras to produce digital stereo pairs. Believe it or not he does this from remote control airplanes. There are lots more details on his Web site. Courtesy of David Grenewetzki

The Reel 3-D twin camera bar lets you mount two 35mm cameras side by side. The rig can then be tripod mounted or handheld. Image courtesy of Reel 3-D.

This beautiful mahogany and brass 5 by 7 inch stereo camera is much like those used in the late 1800s to take stereo images. Courtesy of Mottweiler Design

Viewing Stereo Images

There are several methods that can be used to create stereoscopic images for viewing on a PC or web page. The most common images are:

• Cross-eye view images
• Parallel view images
• Anaglyph images
• Page flipped images viewed with shutter glasses
• Line alternate images viewed with shutter glasses and head mounted displays
• Squished Side by Side images viewed on lenticular displays
• Polarized images viewed with polarized glasses

Stereo Pairs

Stereo pairs can be easily viewed with a stereoscope or other viewer. But with practice, you can also view them unaided by such devices. All you have to do it look at them in the right way. When you finally see the depth information you have "fused" the images. When practicing these unaided techniques, take a break if you experience eye fatigue or discomfort. Keep in mind that not everyone can do these techniques.

• A pair of images designed for cross-eye viewing is arranged side-by-side with the image for the left eye on the right, and the one for the right eye on the left. To view them in 3D, you stare at them with your eyes crossed and this takes some practice. To begin, hold your head level and stare at a point between the two images and slowly cross your eyes. The images should merge so a third image appears in the middle in 3D. The original images will still be seen on either side, so ignore them and concentrate on the middle one.

You should see the solid circle floating above the outline circle.If the solid circle is below the outline circle, you have used the parallel free-vision fusion technique.

If you can't get the effect, hold your index finger half way between your eyes and the computer screen so both images are still visible. While staring at your fingertip, slowly bring it closer and farther from the screen. At some point, the images should merge into a single 3D image.

• A pair of images designed for parallel viewing (also called relaxed viewing or free viewing) is also arranged side-by-side. However, the image for the left eye is on the left, and the one for the right eye is on the right. To view the images, place your face close to the screen and then slowly back away while staring "through" the images to an imaginary point past them and behind the screen. When there appear to be three images, concentrate on the middle one until it becomes 3D. If you first see four images, concentrate harder on the imaginary point behind the screen until they become three images. It may help if you rotate your head slightly to the right or left while keeping it level.

If you have trouble bringing stereo prints into view, you can use a modern print viewer. There are lots of variations to choose from. Image courtesy of Reel 3-D.

Anaglyph Images

Du Hauron, a French scientist patented the anaglyph method of stereoscopic photography in 1891. Anaglyphs, like other technologies, use a pair of images taken from slightly different vantage points. These two images are then color corrected and superimposed slightly out of register so one image is offset slightly from the other. When viewed through a pair of glasses with different colored lenses, the image appears in 3D. The glasses are usually red and blue, but they can also be other combinations depending on how the image was coded and the color of the glasses used to view them. Normally the red lens covers the left eye and the blue or green lens the right eye, but this can vary.

Here are the familiar and low-cost red & blue anaglyph glasses. Image courtesy of Reel 3-D.

In most cases the original images are converted to grayscale images which are then coded with red and blue colors that are balanced with the red/blue glasses. These are sometimes called "pure anaglyphs." Some images can be displayed in full color but most won't work very well.

The Mars Pathfinder site even offers 3D movies of the rover exploring the surface of the planet. http://mars.sgi.com/vrml/qtvr_stereo.html

Shutter Glasses

An increasingly popular way to view 3D images on the screen is using shutter glasses. These glasses have high-speed electronic shutters that open and close in sync with the images on the monitor. Liquid Crystals are used for the shutters because an electronic signal can make the crystal turn instantly from transparent to opaque.

• When the left image is on the screen the left shutter is open and the right shutter is closed which allows the image to be viewed by your left eye only.
• When the right image is on the screen the right shutter is open and the left shutter is closed.

If this process happens fast enough, your brain thinks it is seeing a true stereoscopic image. If this shuttering speed is not fast enough, you can still see a stereoscopic image, but you may also see some flickering.

CrystalEyes shutter glasses use a wireless connection and deliver high-resolution stereoscopic 3D images. Many common software applications used in mechanical CAD, molecular modeling, GIS/mapping and medical imaging support StereoGraphics' CrystalEyes on all major UNIX platforms and Windows NT workstations.Image courtesy of StereoGraphics.

Shutter glasses connect to your video card, parallel port or serial port with wires or with a wireless infrared transmitter. There are three approaches to rapidly alternating the displayed images while coordinating the shuttering of the LCS glasses. All display images in high-resolution and full-color.

• Page flipping rapidly switches the monitor between the right and left images. Special purpose video boards that support high-speed page flipping are available. These video adapters quickly alternate between frame buffers that each contains an entire image. The problem with this approach is that you may experience some flicker.

• Sync-doubling, used by a company called Neotek, displays the left image on the top half of the display and the right image on the bottom half. But you don't see them this way because a hardware device inserts an extra vertical-sync, or "new frame" signal after the computer has displayed the top half. The result is that you see an image that looks like it is page-flipped. In addition, the extra synch signal has the effect of doubling the refresh rate so each eye sees a normal frame rate.

• Line alternate images use a monitor's or head mounted display's ability to interlace images. Interlacing was used on early computers (and TVs) to create images while using little bandwidth. The screen was divided into scan lines. The image was then painted on it using first the odd lines. The scanning beam then returned to the top of the screen and filled in the even lines. When stereoscopic images are interlaced, the odd and even scan lines are used to display the left and right images. On the first pass, the image for one eye is displayed on odd lines. On the next pass, the second image is displayed on even lines. Many video cards have a built-in interlaced video mode that supports this technology.

Because computer display manufacturers dropped interlace mode in the 1980's, these devices require special 'device drivers' supplied by the LCD glasses supplier. There is also a drawback in that each left or right eye view is only made up of either the odd or even lines. This results in only half of the screen being used for each image and a 50% decrease in brightness.

StereoGraphics SimulEyes are lower-cost shutter glasses designed for viewing 3D multimedia and games.Image courtesy of StereoGraphics

Autostereo Images

The problem with most 3D viewing systems is that they require some form of eyewear. However, there are systems that dispense with these by using various techniques to guide the right and left images into the correct eye. These autostereoscopic displays are expensive and display the images in a format that is squashed side-by-side. One big problem with these systems is that you can only view the images from a specific angle. The 3D image isn't seen if you are not positioned correctly.

Pulfrich Images

Strange as it may sound, if video is shot with the camera moving to the left or right, or if an object is spinning, it can be viewed in 3D. To do so, you cover one eye with a dark filter and leave the other eye uncovered. This effect, known as the Pulfrich effect was used for an episode of "3rd Rock from the Sun" in May of 1997. You can purchase a copy of this episode at www.3rdrock.com and view it with one lens of a pair of sunglasses held over your right eye.

Polarized Images

A final way to display images pairs is to display them on the screen one after the other with different polarizations. The first image is displayed with vertical polarization and the second with horizontal. When wearing a pair of glasses with matching polarizations, each eye only sees the image that matches its polarization.

• There are LCD panels that fit over a monitor and polarize the view in sync with the frame rate and allow the use of standard polarized glasses.
• If your system (monitor or presentation panel) has an Active Matrix LCD display, a Vrex micropolarizing filter polarizes every other pixel row into a left and right view. A group can view the displayed or projected image with standard polarized glasses.

Similar in appearance to an anti-glare screen mounted on the front of a computer monitor, the ZScreen enables on-screen images to be displayed with realistic depth, making objects appear to have presence in the user’s physical environment. This allows scientific and technical professionals to better see and understand complex interactions between a wide variety of 3D elements, from molecules containing thousands of atoms, to the design and layout of an automobile’s drive train and suspension.Stereographics ZScreen. Image courtesy of StereoGraphics.

Web 3D Viewers

The interest in stereo has been given a big boost by computer games but it's now being widely displayed on the Web. Some forms of display don't require any special provisions, but others require a plug-in for your browser or Java capability.

DepthCharge

The VRex DepthCharge plug-in allows you to view a variety of stereo image formats on the Web. The plug-in works on Windows 95/NT, using version 3 or higher of either Netscape or Internet Explorer. It supports the following viewing technologies:

• Cross-eye view images
• Parallel view images
• Anaglyph images using red/green glasses
• Page flipped images with shutter glasses
• Line alternate images for shutter glasses and head mounted displays
• Squished side by side images for lenticular displays

The VRex DepthCharge plug-in allows you to view a variety of stereo image formats on the Web. You'll see this logo on many sites that support the plug-in.

When you are browsing the Web with DepthCharge installed, you'll find monoscopic or flat view DepthCharge images that you can view just like any other image on the Web. When you point to one of these DepthCharge images, your mouse pointer will take the shape of 3D glasses. (Note that some sites display thumbnails that you must first click to display the DepthCharge image.) To display the image in 3D, just click it. To return to the normal browser view, click it again. To specify which view to display it in, right-click the image to display a pop-up menu.

SimWeb3D

SimWeb3D is a Java plug-in for your Windows Web browser that works with SimulEyes™ glasses.

Making 3D Images

In the old days, stereo photographs were mounted side by side on a card or printed on top of one slightly out of register. Today, this work can be done on the computer using a variety of software.

3D Stereo Image Factory™ by SOFTreat

Stereo 3D images can be viewed in 3D Stereoscopic Image Factory™ in many ways. On the low-tech side this includes free viewing as parallel or crossed side by side pairs or with red/blue glasses as anaglyphs. On the Hi-tech side there are Liquid Crystal Shutter Glasses which operate with images prepared for interlacing, page flipping, or sync doubling. Work in your favorite 3D format and explore all the other types too.

Photoshop

You can create anaglyph images with Photoshop.

1. Start with a stereo pair.

2. Remove the red component from the right image.

3. Remove the green and blue components from the left image

4. Superimpose the two images

StereoVR

StereoVR allows you to create your own stereoscopic 3D images and animations. It includes a modeler to make your own 3D mesh wireframes, an extensive library of objects, lighting, colors and textures to render in full color 3D, and an animator to propel your creations right from the computer monitor.

VRex STEREO IPAS

3D Studio users can render stereo 3D images with Vrex's STEREO IPAS plug-in module. This software runs in the 3D Studio keyframer to automatically position two cameras in your already existing or new 3D scenes so the correct left and right perspective views are computed and rendered. You then multiplex the two image files with any VRex S-MUX program.

MUX-IT

MUX-IT is the VRex program that combines left and right images into a 3D image using the Spatially Multiplexed Imaging (SMI) technique. Pixel rows of the left image are interleaved with pixel rows of the right image to produce a single stereo image for display through the patented µPol optical system.

S-MUX

VRex's most popular stereo multiplexing software, S-MUX provides a familiar interface to easily select and multiplex your graphics into Spatially Multiplexed Images (SMI) for viewing on any VRex 3D display system. Just click on the graphics files for your left and right perspective views, click on the Multiplex icon and you're done! For special 3D graphics applications you'll appreciate the full range of features in S-MUX including real-time editing, parallax adjustment, image scaling, batch animation mode, and more. Available for DOS, Windows and Macintosh.

Stereo Panoramas

The best of all worlds are stereoscopic panoramas. There are a couple of tools available to capture such images.

With PanDC, you can create stereographic images that approach 120° in each half of the stereo pair. To achieve a stereo effect, a series on 15 or more images is taken with the camera moved to the right between exposures of each pair. The distance that you move the camera, called the stereo base distance, depends partly on the scene (you have to experiment). To see this one, first click it to enlarge it. Image courtesy of Orphan Technologies.

By mounting two cameras side-by-side as you take a series of pictures for a panorama, you capture a series of stereo pairs. By stitching those from each camera together into it's own panorama, you create a panoramic stereo pair.

The Kaidan QPST-1 allows you to mount two 35mm cameras side by side for creating stereoscopic panoramas. It's not usable separately, but is an add-on to Kaidan's QP-4 and QP-6 panoramic brackets. Image courtesy of Kaidan.