The DEPTH Workflow - 3D Postproduction: Stereoscopic Workflows and Techniques - page 150

depth depending on the convergence/parallax and IA settings made on
set. Your production team may have elected to change depth over the
course of a shot.
5.6.2.2 Solution: Sideslipping (Horizontal Image Translation)
The technique of “sideslipping” 3D imagery to move the area of interest in
Z-Space is known as horizontal image translation , often abbreviated HIT . HIT
is jargon for “moving L and/or R to the right and/or left, relative to each
other.” Some 3D people call this technique convergence adjustment , although
strictly speaking one can only adjust convergence on-set during shooting
and only when shooting with converged imaging systems. Any depth
adjustments downstream of image capture are done by HIT/sideslipping/
convergence adjustment. Some productions shoot parallel then do a manual
or software-assisted depth pass in order to make viewable 3D dailies. Later,
in post, teams and i ne-tune depth using HIT. Teams shooting converged
may change their minds after production and can use HIT to i ne-tune
depth (within limits).
Moving the L and R frames horizontally to the right or left shifts where
the area of interest plays in Z-space when viewing a 3D scene. With both
parallel and converged 3D imagery, moving L and R relative to each other
horizontally changes the area of interest over the course of a shot so that it
appears in one of three possible locations (Figure 5.9) :
• in front of the screen plane (i.e., in negative Z-space)
• behind the screen plane (i.e., in positive Z-space)
• at the screen plane (i.e., in zero Z-Space)
The position where similar points on L and R images overlap exactly (the
place where there is no offset between L and R) is by dei nition the screen
plane. The amount of horizontal separation (parallax) is rel ected in a linear
numerical value corresponding to distance between similar points in L and R
within an image pair. As you would expect, this value increases or decreases
as we move L and R relative to each other. HIT is most commonly used on
a keyframed basis, with separation between L and R changing over time
to minimize extreme changes in depth. Of course, HIT can also be used to
creatively adjust (depth grade) a shot or sequence or to create smooth depth
transitions between shots.