3D Production Terms

Happened across this amazing collection of terms (on Definition Magazine compiled by Phil Streather) that get tossed around during a stereoscopic production. Confidingly, I have not known the true definition of some, so I’m posting this cheat-sheet for myself and others to see.

BIOLOGY

BINOCULAR VISION
A vision system that utilises a pair of forward looking eyes. Humans have this, enabling them to judge distance.
BINOCULAR DISPARITY
The difference between the view from the left and right eyes.
STEREOPSIS
The ability to perceive in a third dimension, to have a depth sense. A by-product of Binocular Disparity.
HUMAN INTEROCULAR
The interpupiliary distance in humans; approximately 2” in children and 2.5” in adults.

ACQUISITION

CAMERA INTEROCULAR
The distance between the centres of the two lenses used in any 3-D filming set up (sometimes referred to as Interaxial).
STEREO PAIR OF CAMERAS
Two vertically aligned cameras, in either a Side by Side Rig or a Mirror/Beam-splitter Rig.
SIDE BY SIDE RIG
Two cameras placed side by side, able to achieve (at least) a minimum distance between the centres of the two lenses of around 2.5” -the average adult human Interocular.
MIRROR OR BEAM-SPLITTER RIG
Two cameras one looking forward through a 45º half silvered mirror and the other looking up or down for 50pc of the reflected light from the same mirror. Required if Interoculars of less than the approx. 2.5” of the Side by Side Rigs are needed. Mirror Rigs can go down to zero Interocular by moving one of the cameras on a motorised platform. Normally a stop of light is lost in the process.
STEREOGRAPHER
Can also be the DoP. Person with mathematical or intuitive knowledge of the 3-D process. In particular, the interactive relationship between Convergence and Interocular. Most often Stereographers use a spreadsheet to determine where an object will appear on the Z Axis given the focal length of lens, distance of object from camera, amount of Camera Convergence, amount of Camera Interocular and the theatre geometry.

There’s more.  Click to go on:

APPROACHES TO 3-D FILMING

Z AXIS
The Y Axis is up and down, the X Axis is side to side and the Z Axis is forward and back.
CAMERA CONVERGENCE
The toeing in of one or both cameras in a Stereo Pair so that the optical axes of the cameras align somewhere on the Z Axis; at the point of interest, in front of the point of interest, or behind the point of interest. The more you converge on a Point of Interest the further away on the Z Axis that object moves.
CONVERGENCE POINT
Where the axes of the toed in cameras align on the Z Axis.
CONVERGING LENSES
A configuration where one lens is offset with respect to the film plane.
DYNAMIC (CAMERA) CONVERGENCE
Camera Convergence taking place during a shot. This necessitates a Side by Side or Beam Splitter Rig with the appropriate motorisation.
SHOOTING PARALLEL
Acquiring stereo images without Camera Convergence. The axes of the cameras remain parallel to each other, at whatever Interocular is chosen.
ORTHOSTEREO
It is the default methodology for films originating in the IMAX 3D format (it is however a choice, not obligatory). Orthostereo re-creates in both depth and scale what it was like in front of the camera. It is achieved by shooting with the cameras parallel, with wide angle lenses (matching as close as possible the viewer’s field of view of the screen) and a 2.5” interocular.
SHOOTING CONVERGED
There are 2 ways to shoot Converged. Either by Camera Convergence or by using Converging Lenses, such as can be found on the IMAX Solido twin strip 15perf 65mm camera or the Gemini twin strip 8erf 35mm camera.

3D PROJECTION AND VIEWER PERCEPTION

•STEREO WINDOW
The edge or boundary that surrounds the three-dimensional image. It also describes the style of 3D film making that is used for screens significantly smaller than IMAX.
SCREEN PARALLAX
The on screen horizontal distance between corresponding image points.
THE SCREEN PLANE OR WINDOW
Where objects appear as if they are on the physical surface of the display screen, as in 2D. There is zero Parallax at the Screen Plane as the left and right eye images are exactly overlaid.
EYE CONVERGENCE
The toeing in of eyes in order that the brain perceives a single image, rather than two separate images, when the optical axes of the eyes align on any given object.
POINT OF CONVERGENCE
The intersection of the two images. At this point there is Zero Parallax and the subject at the Point of Convergence appears on the Screen Plane.
ZERO PARALLAX
Where the left and right eye images are exactly overlaid.
FUSING THE IMAGES
The act of Converging the Eyes on the left and right eyes displayed on the screen and the brain successfully the interpreting the two images as a single entity.
NEGATIVE PARALLAX
On screen the left eye image is to the right of the right eye image. Whilst the eyes are focus on the Screen Plane the eyeballs have to Converge in front of the Screen Plane in order to Fuse the Images into a single entity. The resulting images appear as if they are in front of the Screen Plane and out in the theatre space.
POSITIVE PARALLAX
On screen the left eye image is to the left of the right eye image. Whilst the eyes focus on the Screen Plane the eyeballs either converge, are parallel or diverge behind the Screen Plane, depending on the amount of Positive Parallax and the viewer distance from the screen. The resulting images appear as if they are behind the Screen Plane.
DIVERGENCE
If Positive Parallax exceeds the eye Interocular then the viewer will outwardly rotate their eyes in order to Fuse the Images into a single entity. The images appear behind the Screen Plane.
DEPTH BUDGET
The combined values of the Positive and Negative Parallax. Often given as a % of screen width.
DISLOCATION OF ACCOMMODATION AND CONVERGENCE
In the real world we Converge our eyes at the point at which we focus or Accommodate. In a 3-D film we always focus on the Screen Plane BUT Converge either in front of the Screen Plane for Negative Parallax “out of the screen effects” or at the Screen Plane for the Convergence Point. The eyes either Converge, are parallel or Diverge behind the Screen Plane for positive parallax ‘depth’ effects (dependent on viewer distance from the screen).

3-D EFFECTS

HYPERSTEREO
By using camera Interoculars larger than the average human Interocular of 2.5” the resulting images display objects that appear smaller than they are in reality.
HYPOSTEREO
By using camera Interoculars smaller than the average human Interocular of 2.5” the resulting images display objects that appear larger than they are in reality.
MINIATURISATION
The on screen effect of using Hyperstereo is that objects appear smaller than they do in life. Theoretically this is a linear effect. Shooting with a Interocular of 5” will make an object look about half its normal size.
GIGANTISM
The on screen effect of using Hypostereo is that objects appear larger than they do in life. Theoretically this is a linear effect. Shooting with a Interocular of 1.25” will make an object look about twice its normal size.
RE-CONVERGING IN POST
The shifting of the Screen Parallax. Moving the left eye left decreases the Negative Parallax so objects don’t come so far off the screen. Within the Positive Parallax there is a corresponding movement of the right eye to the right and so care must be taken not to induce the problem of Wall-eye. As well as re-positioning objects on the Z Axis Re-converging in Post is also a tool for smoothing the edits in terms of the 3-D. At each cut the Screen Parallax can be pushed/pulled towards the Screen Plane, having shifted from either Negative or Positive just before the cut and just after the cut. Thus, from cut to cut the eyes are both temporarily Converging on the same point (reducing eyestrain and fatigue) before returning to whatever Parallax the next shot requires.

PROBLEMS

VERTICAL MISALIGNMENT
Where the horizontal lines of the two images are misaligned and one is higher than the other when exactly overlaid.
IMAGE SIZE DIFFERENCES
If the prime lenses are not exactly matched or zoom lenses do not give the same image size for a given focal length then the Image Size Differences will need to be normalised in post.
KEYSTONING
Vertical Misalignment at the edges of one or both eyes as a result of excessive Camera Convergence. A squared grid shot at an angle becomes a grid of trapezoids or Keystones. When two such images are overlaid Vertical Misalignment occurs except in the middle of the frame. With the limited amount of Camera Convergence used in most instances the Keystoning is not enough to result in uncomfortable viewing.
BREAKING THE FRAME
If an object has Negative Parallax and is bisected by the edge of the frame then that object is Breaking the Frame and there is a visual/brain conflict. How can something be both in front of the Screen Plane (as suggested by the Negative Parallax) yet at the same time behind the Screen Plane, suggested by the fact that the subject is only partly visible? In life the right eye sees MORE through the left hand side of a window than the left eye. If a part of an object Breaking the left of Frame is in front of the Screen Window (with Negative Parallax) then the right eye is seeing LESS than the left eye because in Negative Parallax the left eye image is the right of the right eye image. This is not how we see in life and results in confusion and loss of the 3D illusion.
FLOATING WINDOW
A device to deal with objects that are Breaking the Frame. A Floating Window (applied in post) places a thin, partially see-through, mask on the left of the left eye image and the right of the right eye image in order to disguise the conflict on which ever side the object is breaking frame (and simultaneously if there are objects breaking frame both left and right). This makes the images easier to watch.
CARDBOARD CUT-OUT EFFECT
Where objects at the same distance from the camera/viewer look like flat planes with stereo space between them. The Cardboard Cut-out Effect is a result of using long lenses at the normal Interocular. The use of wider Interoculars can eliminate the effect by giving objects more roundness; but this has to be executed with care as one can end up with too much parallax.
WALL-EYE
Wall-eye effect is caused by excessive Positive Parallax in which the eyes are asked to have Divergence greater than approx 1 degree per eye (dependent on viewer distance from the screen).
UNFUSABLE IMAGES
Images with either excessive Positive or Negative Parallax so that the eyes are unable to either Converge on or Diverge on and Fuse them as a Single Image.
GHOSTING
On screen image bleed from one eye to the other eye. This occurs particularly when shooting in high contrast situations or shooting objects of a dark or light colour in front of their opposite coloured backgrounds.
SORTING IT OUT IN POST
The Producer’s fall back position, maybe!

© Phil Streather

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