Graphics Reference
In-Depth Information
Figure 8.5.
A co-registered color camera can be used to texture-map the points in a LiDAR scan,
making it more understandable.
While LiDAR scanners are designed only to return distance measurements, many
systems include an integratedRGB camerawhose pixels are precisely alignedwith the
(θ
rays. This allows the raw 3D points to be texture mapped with color for a more
pleasing and understandable rendering, as illustrated in Figure
8.5
. Furthermore, the
color information can be extremely useful when registering multiple LiDAR scans, as
discussed in Section
8.4
. If a LiDAR sensor doesn't come with a co-registered camera,
a rigidly mounted auxiliary RGB camera can be calibrated against the LiDAR sensor
based on the resectioning algorithms discussed in Section
6.3.1
and an appropriate
calibration target.
,
φ)
8.1.1
Pulse-Based LiDAR
Pulse-based LiDAR
systems are based on a simple idea, as illustrated in Figure
8.6
.
A laser pulse is emitted along the given ray into space; it bounces off a surface in
the scene and the scanner measures the elapsed time
t
for the round trip. The pulse
traveled a distance 2
d
in this time, andwe use the constant speed of light
c
to compute
d
from the
time of flight
t
:
1
2
ct
d
=
(8.1)
To reduce noise in themeasurement, the times of flight of several pulses sent in quick
succession are often averaged. The lasers in pulse-based systems are generally eye
safe and have wavelengths of 500-1000nm (i.e., green, red, or, infrared light). On a
clear day, pulse-based LiDAR can receive returns fromover a kilometer away (though
this is rarely necessary for visual effects applications).
Suppose we want to ensure that the distancemeasurements are accurate to within
±
5mm; today's pulse-based LiDAR systems can achieve or exceed this accuracy.
Equation (
8.1
) implies stringent tolerances on the receiver electronics, since they
must be able to resolve time differences of 33 picoseconds or less. Furthermore, the
returned pulse may be very weak since its energy falls off with the inverse fourth
power of distance [
216
]. Pulse-based LiDAR systems are often somewhat expensive
for these reasons. The process of scanning a large scene with fine resolution in
θ
and
