What Is Stereoscopic 3D? Meaning, and How It Works

The whole idea in one object: a card with two nearly identical photographs, and a viewer that shows one to each eye. Photo: Ben Franske, CC BY-SA 4.0, via Wikimedia Commons.

What does stereoscopic mean?

The Oxford Advanced American Dictionary gives the word two senses. Of a picture or photograph: "that is made so that you see the objects in it with length, width, and depth when you use a special machine", with "three-D" as a synonym. Of sight: "able to see objects with length, width, and depth, as humans do" [2].

Both senses rest on the same thing: two views of one scene. A primer on stereoscopic 3D written by two senior editors at the broadcaster BSkyB defines stereoscopic as "Concerned with, or relating to, seeing space three-dimensionally as a result of binocular disparity" [3]. Binocular disparity is the difference between what the left eye and the right eye see, caused by the horizontal distance between them [1]. The brain fuses the two offset views into one picture with depth, a process known as stereopsis [13].

The word comes from Ancient Greek: stereós, "firm, solid", and skopéō, "to look, to see" [1]. The noun is stereoscopy.

Is stereoscopic the same as 3D?

No. Every stereoscopic picture is called 3D, but much of what is called 3D is not stereoscopic.

The BSkyB primer puts the split in two lines. "3D in film/television traditionally refers to computer-generated images (CG or CGI) using 3D models." Stereoscopic 3D, which the primer shortens to S3D, is the kind that creates the illusion of depth "when viewed on appropriate displays" [3].

So an animated film or a video game built from 3D models is "3D" in the first sense even on an ordinary screen, where both eyes see the same flat picture. What makes 3D images stereoscopic is a separate view for each eye: a game in 3D graphics on an ordinary screen gives the illusion of depth "only by monocular cues", while a stereoscopic game adds binocular depth [14].

A hologram is not stereoscopic 3D either. Presenting two flat images is "distinctly different from displaying an image in three full dimensions" with a volumetric display or by recreating a light field [1]; see how a hologram differs from 3D.

How do two flat pictures make depth?

Your eyes sit about 6 cm apart, so each sees the scene from a slightly different angle, and the brain combines the two views to judge distance [3]. A stereoscopic picture copies the arrangement: two cameras record the scene from two positions about as far apart as the eyes, and on playback the left image is shown only to the left eye and the right image only to the right [3].

You can see the raw material yourself. Hold a finger about 3 inches from your face and close one eye, then the other. The finger jumps from side to side, and, as John Daro, who has worked on the post-production of 3D films, puts it, "You also see “around” your finger more in one eye vs the other" [4]. That offset is what the brain turns into depth.

What puts an object in front of the screen or behind it

On a 3D screen, where an object seems to sit depends on how far apart its left-eye and right-eye images are drawn. The trade calls that separation parallax [3]. How much of it a screen can take depends on the screen's width: 3D parallax and screen size.

Drag the slider to see it happen.

Why you still sense depth with one eye closed

Binocular disparity is one cue among several. Wikipedia's list of the brain's depth cues puts it first; further down are occlusion ("the overlapping of one object by another"), the visual angle of "an object of known size", linear perspective, and haze [1]. All but the first three cues on the list, the article notes, "exist in traditional two-dimensional images, such as paintings, photographs, and television" [1].

Does stereoscopic 3D work if you see with one eye?

The sources define stereoscopic depth as something made from two eyes' pictures. Stereo vision is, in the BSkyB primer's words, "a result of good binocular vision, wherein the separate images from two eyes are successfully combined into one 3D image in the brain" [3]. For the other cues, see why you still sense depth with one eye closed.

Our guide to 3D without glasses has a section on one working eye, a squint or a lazy eye, which quotes an optometry professor.

Ways to give each eye its own picture

Every stereoscopic system has the same job: get the left picture to the left eye and the right picture to the right eye [1].

• Anaglyph. The two images are tinted, typically red and cyan, and laid over each other; coloured filters sort them [3]. See anaglyph 3D.
• Polarized and active shutter glasses. Polarized: each image is polarized differently and both are shown together; each lens blocks the other eye's image. Active shutter: the screen alternates left and right images; LCD lenses black out each eye in time with it [3]. See how 3D glasses work.
• Headset. Two small displays with magnifying lenses, one for each eye [1]. See movies in VR.
• Glasses-free (autostereoscopic) screens. Optics in the display, not worn by the viewer, send a different image to each eye [1]. See 3D without glasses.

Shot in stereo or converted to 3D: what is the difference?

A stereoscopic film gets its two pictures in one of two ways. Shot in stereo, two matched cameras record both eyes' views on set [3]. Converted, the film is shot in 2D and the 3D is made afterwards, in post-production [5].

Software can also convert a video that was shot with one camera into stereoscopic 3D. Owl3D, the publisher of this guide, makes such a program. The Owl3D desktop app, for Windows and Mac, converts 2D videos and photos into stereoscopic 3D using AI, and the processing runs on your own computer. It is an AI conversion, not the studio's 3D version, and not footage shot with two cameras. It is not instant: you give it a video file you have the right to use, and it takes time.

From Owl3DOwl3D converterA desktop app for Windows and Mac that turns 2D videos and photos into 3D.

Where is stereoscopic 3D used besides films?

Stereoscopic pictures have mostly been used to entertain: stereo cards, 3D films, 3D television and video games [1]. Today a phone takes them too: 3D photo apps. They also do work:

What does the "Stereoscopic 3D" setting do in a game?

A "Stereoscopic 3D" option makes the game produce two views of the scene, one for each eye, so that a 3D stereoscopic display, a 3D projector or TV with its glasses, or a VR headset can show it with depth [1][6][14]. An ordinary monitor with nothing to wear cannot deliver one view to each eye [1], so leave it off there.

Games make the two views in two main ways. Dual rendering draws both images: the best effect, at double the graphics requirements. 2D + depth builds the second view from one rendered image, with an upper limit on parallax [14]. On PCs the option has worked through NVIDIA 3D Vision and similar active stereo 3D systems such as AMD HD3D and TriDef, with compatible hardware and active shutter glasses; on consoles each game needs its own built-in support [14]. For what works on a PC today, see five ways to play PC games in 3D.

Try it: a stereo pair you can see without a device

Free-viewing means fusing a side-by-side pair with your eyes alone. There are two ways, parallel and cross-eyed [1]. The pair below is laid out for the cross-eyed way: the left-eye picture is on the right and the right-eye picture is on the left [1].

1. Hold up a fingertip. Place it just below the line where the two pictures meet [1].
2. Bring it toward your eyes. Move it slowly, keeping your eyes on the fingertip [1].
3. Watch the pictures behind it. At a certain distance a fused, three-dimensional picture should seem to hover just above the finger [1].

Some people take a long time to learn this, and some cannot do it at all, the author of a free-viewing tutorial writes [7]. For both methods in more detail, see free-viewing.

To make a pair like this from two photos of your own or a 3D camera's file, lined up automatically, use our Stereo Photo Maker; it also saves red-cyan and wiggle versions.

A short history of stereoscopic pictures

Stereoscopic pictures are a year older than photography [8].

• 1838. Charles Wheatstone described stereoscopic vision "in a paper read to the Royal Society on 21 June 1838", and built a stereoscope with two mirrors to show pairs of line drawings as one image in relief [8].
• 1839. Photography arrived a year later and turned the stereoscope into "a popular parlour entertainment" [8].
• 1939. View-Master was introduced; Wikipedia describes it as "still in production" [1].

How does a stereoscope work?

A stereoscope shows each eye only its own half of a stereo pair, like the card in the photo at the top of this page. A typical one has a lens for each eye that makes the picture look larger and farther away, and usually shifts it sideways so the two halves seem to merge into one; a divider usually stops each eye from seeing the other half [12]. A View-Master and a VR headset are stereoscopes too [12].

Frequently asked questions

What does stereoscopic 3D mean, and how does it work?

Stereoscopic 3D shows each eye a slightly different picture of the same scene, and the brain fuses the two into one picture with depth, a process called stereopsis [13]. The offset between the pictures copies what your own eyes see from about 6 cm apart [3]. Glasses, a headset or a glasses-free screen gives each eye only its own picture [1].

What does stereoscopic mean in games and movies?

The same thing in both: each eye is shown its own view, so the picture has depth [1][14]. A 3D film gets its two views from two cameras or from conversion afterwards [3][5]; a stereoscopic game renders them itself [14].

My game has a "Stereoscopic 3D" setting. What is it, and should I turn it on?

It makes the game produce a left-eye and a right-eye view of every scene [6][14]. Turn it on only with a 3D display and its glasses, a glasses-free 3D screen or a VR headset; on an ordinary monitor it gives no depth [1]. See the setting.

Is side-by-side (SBS) the same as stereoscopic 3D?

Side-by-side is a way of packing a stereoscopic picture into one frame, not a different kind of 3D [3]; see 3D video formats.

What are the drawbacks of stereoscopic 3D?

Two that reference works name. Your eyes focus on the screen while they converge on objects that seem nearer or farther, which Wikipedia calls the vergence-accommodation conflict [1][3]. And some of one eye's image can leak into the other, called crosstalk or ghosting [1][3]. If 3D gives you headaches, see what to try at a 3D showing.