Three-Dimensional Data Acquisition - Computer Vision for Visual Effects - page 300

Three-Dimensional Data
Acquisition

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A key responsibility of a visual effects supervisor on a movie set is to collect three-
dimensional measurements of structures, since the set may be broken down quickly
after filming is complete. These measurements are critical for guiding the later inser-
tion of 3D computer-generated elements. In this chapter, we focus on the most
common tools and techniques for acquiring accurate 3D data.
Visual effects personnel use several of the same tools as professional surveyors
to acquire 3D measurements. For example, to acquire accurate distances to a small
set of 3D points, they may use a total station . The user centers the scene point to
be measured in the crosshairs of a telescope-like sight, and the two spherical angles
defining the heading are electronically measured with high accuracy . 1 Then an elec-
tronic distance measuring device uses the time of flight of an infrared or microwave
beam that reflects off of the scene point to accurately determine the distance to the
target. However, acquiring more than a few 3D distance measurements in this way is
tedious and time-consuming.
It's recently become common to automatically survey entire filming locations
using laser range-finding techniques, which we discuss in Section 8.1 . The result is a
cloud of hundreds of thousands of 3D points visible along lines of sight emanating
from the laser scanner. These techniques, collectively called Light Detection and
Ranging or LiDAR , are highly accurate and allow the scanning of objects tens to
hundreds of meters away.
For closer-range three-dimensional acquisition, the key technology is structured
light , which we describe in Section 8.2 . A stripe or pattern of visible light is projected
onto the surface of an object, and the deformation of this pattern as seen by a nearby
camera provides sufficient information todetermine the shape of the object's surface.
Structured light is commonly used for scanning actors' bodies and faces, as well as
movie props and models. A variety of patterns can be used for structured light, from
a single plane of laser light that casts a sharp stripe on the object to an array of
colored stripes projected from an off-the-shelf LCD or DLP projector. High-speed
projector-camera systems can even allow us to recover the real-time 3D motion of a
performer's face.
While highly accurate, LiDAR and structured light have the disadvantage that they
are active techniques — that is, visible light is projected into the scene. This can be
disorienting, especially for an actor suffering brightly colored, changing patterns of

1 This part of the device is called a theodolite or transit .

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