Overview - 3D Postproduction: Stereoscopic Workflows and Techniques - page 22

these spatial relationships is the primary creative technique of 3D, known
as depth grading, which we discuss in Chapter 5.
1.5.3 Negative, Positive and Zero Parallax
Changing parallax (in terms of both horizontal separation and relative
position of L to R) over time results in moving the area of interest in a 3D
frame away from or toward the viewer. Distant objects have a maximum
parallax value; near objects also have a maximum parallax value. Exceeding
target specii cations for near and far parallax can generate discomfort and
discontinuity by exceeding capabilities of the human visual system.
Positive (far) parallax measures the horizontal separation of image pairs as they
appear as 3D frames to viewers behind the screen plane. In this scenario,
L is to the left of R in the 3D frame. The more to the left L is compared
to R, the deeper behind the screen (or screen plane) the object appears.
There is a point of maximum separation beyond which viewers may i nd
it uncomfortable to view elements in the scene. Assuming we start with a
scene playing behind the screen, as the distance between similar points in
L and R decreases, the scene element appears closer to the screen plane.
Negative (near) parallax measures the horizontal separation of image pairs
as they appear as 3D frames to viewers in front of the screen plane. In this
scenario, L is to the right of R in the 3D frame. The more to the right L is
compared to R, the further in front of the screen (or screen plane) the object
appears. There is a point of maximum separation beyond which viewers may
i nd it uncomfortable to view elements in the scene. Assuming we start with
a scene playing in front of the screen, as the distance between similar points
in L and R decreases, the scene element appears closer to the screen plane.
Zero parallax occurs when there is no horizontal separation between similar
points in L and R. In other words, when L and R overlap. This results in
the scene element appearing at the screen plane with no apparent depth.
1.6 Z-Space
1.6.1 XY-Space (2D)
In order to understand 3D space we need to take a quick look at 2D space.
2D imagery occurs in a realm dei ned by X (indicating width or the horizontal
dimension) and Y (indicating height or vertical dimension). Together these two
dimensions comprise an ini nite (2D) plane, the viewable portion of which
we call the 2D frame (Figure 1.10 ). One might call the 2D realm “XY-Space.”