In any discipline, nomenclature turns out to be of obvious importance. It’s crucial for all the people who are doing the thing to agree on the same set of definitions. Without that, it’s impossible to communicate – or it’s impossible to communicate without ambiguity. Part of my job is to work with filmmakers and studios to help them (and me) figure out how to make stereoscopic movies. This is an art that is being invented. Digital projection has opened the door for advances in stereoscopic content creation. Whereas in the past stereoscopic filmmaking was an erratic or a sporadic activity, or at best one that was practiced only by a handful of people typically for large format or theme park attractions, the door has now been opened for the conventional theatrical cinema, which has a greater activity level.
In my trips to studios and filmmakers, I have learned that people are using different terminology. Sometimes they are trying to say the same thing with different words, and sometimes they are trying to say different things with the same words. It’s confusing, and it doesn’t help develop a nascent art form to have such ambiguity or downright confusion. So here is my stab at a glossary that is based on the one that appeared in the CrystalEyes handbook that I put together when I was at StereoGraphics. I am hoping that in the weeks and months that follow the readers and I will have a chance to add to it. So if you have entries or ideas for concepts that need to be defined, send them to me.
I’ll provide a relatively benign example of what can go wrong. In this example of a lack of precision, everybody can more or less figure out what’s meant. It’s the matter of interaxial versus interpupillary or interocular. When doing stereoscopic cinematography, whether you are talking about virtual cameras in a computer-generated world or real cameras in the visual field, there needs to be some accepted term for the distance between the cameras; or more specifically the distance between the camera lenses; and to be even more specific, the distance between the camera lens axes. The interaxial is the distance between the optical axes of the camera heads’ lenses. I call them camera heads since a stereo rig (or camera) has two heads, and if we call the twin-lensed camera a “camera” we have failed to distinguish between the camera and the camera heads. It’s the distance between the lens axes that, to a large extent, determines the strength of the stereoscopic effect.
The distance between people’s eyes, depending on what field of medicine or science you’re in, is called the interpupillary or interocular distance. The distance between your eyes’ lens axes when your eyes are converged at infinity is a number that doesn’t change once you reach adulthood, and the spread is between 55 and 75 millimeters for the adult population. Typically, the interpupillary or interocular distance is given as an average of 65 millimeters for men. This has led some misguided workers to believe that stereoscopic movies should be shot with camera lenses whose axes are always the interpupillary separation.
People who are involved with the capture of stereoscopic images will use the term “interpupillary” or “interocular” when they mean interaxial. A camera lens, whether it’s a virtual or a real camera, has a lens axis that passes through the optical center of the lens and, if the lens is set up correctly, is orthogonal to the plane of the image sensor. The distance between the left and right axes is called the interaxial separation. So, if you’re talking about the distance between people’s eyes, please call it interpupillary or interocular and everybody will know what you’re talking about. If you’re talking about the distance between camera lenses, call it interaxial so we’re all speaking the same language. — LL
Accommodation. The focusing of the eyes – or more properly, the ability of the eyes’ lenses to change shape in order to focus.
Accommodation/Vergence relationship. The learned relationship established through early experience between the focusing of the eyes and verging of the eyes when looking at a particular object point in the visual world. Usually called the accommodation/ convergence relationship (or the convergence/ accommodation relationship.)
Anaglyph. Wavelength selection using complimentary colored images and color filters to filter or pass the appropriate perspective views to the appropriate eyes.
Autostereoscopic. Sometimes called “auto-stereo,” which can be confused with a car sound system.
Beamsplitter. Technically this is a couple of prisms cemented together with a semi-silvered layer to split a light beam into two halves. For the rig used for stereo-cinematography, a thin sheet of glass that is semi-silvered is used in the optical path. Such a device is more properly called a pellicule (or pellicle).
Binocular. Two eyes. The term “binocular stereopsis” (two-eyed solid seeing) is used in some psychology books for the depth sense more simply described as stereopsis.
Circular polarization. A form of polarized light in which the tip of the electric vector of the light ray moves through a corkscrew in space.
Conjugate points. See Corresponding points.
Conversion. Also known as synthesis or by the trade name Dimensionalization. A process by which a planar image is turned into a stereoscopic image.
Convergence. The inward rotation of the eyes, in the horizontal direction, producing fusion. The more general term is vergence, which includes inward and outward rotation. The term has also been used, confusingly, to describe the movement of left and right image fields or the rotation (toe-in) of camera heads.
Corresponding points. The image points of the left and right fields referring to the same point on the object. The distance between the corresponding points on the projection screen is defined as parallax. Also known as conjugate or homologous points.
Crosstalk. Incomplete isolation of the left and right image channels so that one leaks (leakage) or bleeds into the other. Looks like a double exposure. Crosstalk is a physical entity and can be objectively measured, whereas ghosting is a subjective term.
Depth range. A term that applies to stereoscopic images created with cameras. The limits are defined as the range of distances in camera space from the background point, producing maximum acceptable positive parallax, to the foreground point, producing maximum acceptable negative parallax. See parallax budget.
Disparity. The distance between conjugate points on overlaid retinae, sometimes called retinal disparity. The corresponding term for the display screen is parallax.
Extrastereoscopic cues. Those depth cues appreciated by a person using only one eye. Also called monocular cues. They include interposition, geometric perspective, motion parallax, aerial perspective, relative size, shading, and textural gradient.
Field-Sequential. In the context of cinema-stereoscopy, the rapid alternation of left and right perspective views projected on the screen.
Floating windows. Invented by Raymond and Nigel Spottiswoode, this is the use of printed vertical bands to create a surround to supplant the physical screen surround. The result is a so-called virtual window that is floating in space, to eliminate the screen edge cue conflicts and to extend the parallax budget of the projected image.
Fusion. The combination, by the mind, of the left and right images – seen by the left and right eyes – into a single image.
Ghosting. The perception of crosstalk is called ghosting.
HIT. Horizontal image translation. The horizontal shifting of the two image fields to change the value of the parallax of corresponding points. The term “convergence” has been confusingly used to denote this concept.
Homologous points. See Corresponding points.
Interaxial distance. Also interaxial separation. The distance between camera lenses’ axes. See t.
Interocular distance. See t.
Interpupillary distance. Also interpupillary or interocular separation. The distance between the eyes’ axes. See t.
Linear polarization. A form of polarized light in which the tip of the electric vector of the light ray remains confined to a plane.
Monocular cues. See Extrastereoscopic cues.
Multiplexing. The technique for placing the two images required for a stereoscopic display within an existing bandwidth.
Parallax. The distance between conjugate points. It may be measured with a ruler or, given the distance of an observer from the screen, in terms of angular measure. In the latter case the parallax angle directly provides information about disparity.
Parallax Budget. The range of parallax values, from maximum negative to maximum positive, that is within an acceptable range for comfortable viewing.
Planar. Flat. Two-dimensional. A planar image is one contained in a two-dimensional space, but not necessarily one which appears flat. It may have all the depth cues except stereopsis.
Plano-Stereoscopic. A stereoscopic projected image that is made up of two planar images.
Ramsdell Rig. See Beamsplitter.
Retinal disparity. See Disparity.
Rig. Dual camera heads in a properly engineered mounting used to shoot stereo movies.
Screen space. The region appearing to be within the screen or behind the surface of the screen. Images with positive parallax will appear to be in screen space. The boundary between screen and theater space is the plane of the screen and has zero parallax. See Theater space.
Selection device. The hardware used to present the appropriate image to the appropriate eye and to block the unwanted image. For 3-D movies the selection device is usually eyewear used in conjunction with a device at the projector, like a polarizing device.
Stereo. Short for stereoscopic. If you are trying to learn about multi-channel sound, you are in the wrong place.
Stereoplexing. Stereoscopic multiplexing. A means to incorporate information for the left and right perspective views into a single information channel without expansion of the bandwidth.
Stereopsis. The binocular depth sense – literally, “solid seeing.”
Stereoscope. A device for viewing plano-stereoscopic images. It is usually an optical device with twin viewing systems.
Stereoscopy. The art and science of creating images with the depth sense stereopsis.
Surround. The vertical and horizontal edges immediately adjacent to the screen.
t. In stereoscopy, t is used to denote the distance between the eyes, called the interpupillary or interocular distance. tc is used to denote the distance between stereoscopic camera heads’ lens axes and is called the interaxial.
Theater space. The region appearing to be in front of the screen or out into the audience. Can also be called audience space. Images with negative parallax will appear to be in theater space. The boundary between screen and theater space is the plane of the screen and has zero parallax. See Screen space.
Window. The stereo window corresponds to the screen surround unless floating windows are used.
ZPS. Zero parallax setting, or the means used to control screen parallax to place an object in the plane of the screen. ZPS may be controlled by HIT, or toe-in. We can refer to the plane of zero parallax, or the point of zero parallax (PZP) so achieved. Prior terminology says that left and right images are “converged” when in the plane of the screen. That term should be avoided because it may be confused with the convergence of the eyes, and because the word implies rotation of camera heads Such rotation produces geometric distortion and may be expedient in camera rigs, but is unforgivable in a CG virtual camera rig.