This document introduces 3D digitization technologies. It discusses acquiring visually rich 3D models through either modeling or sampling approaches. Modeling involves manually redrawing objects, while sampling uses semi-automatic processes like 3D scanning to photograph objects. Common 3D scanning devices and technologies are described, including laser scanners that use triangulation or time-of-flight measurements to sample surfaces. Raw scan data requires processing to transform redundant sampled points into a complete 3D model.

1. 1
Introduction to 3D
digitization
technologies
Roberto Scopigno
Visual Computing Lab.
CNR-ISTI
Pisa, Italy
R. Scopigno, 3D Digitization - HW 1
Overview
o Digitization for visual presentation: 3D
vs. enhanced 2D media
o 3D digitization technologies

2. 2
2
Acquiring Visually Rich 3D Models
Goal:
Build accurate digital models to
clone the reality (shape +
surface reflection properties)
Acquisition methodologies:
n Image-based Rendering
o Panoramic images (2D)
o RTI images (2D)
n Standard CAD modeling
(manual process)
n Approaches based on
Sampling
o 3D scanning (active)
o 3D from images (passive)
3
Modelling vs. Sampling
o Modelling
n Manual process
[“redraw”]
n Accuracy is unknown
n 3D model is usually
complete
o Sampling/scanning
n Semi-automatic process
[“photography”]
n Accuracy is known
n 3D model is usually
uncomplete (many
unsampled regions)
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3. 3
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3D scanning devices
Many different technologies, just two
examples:
o Laser or structured light,
Triangulation
n Small/medium scale artifacts (statues)
n Small/medium workspace 20x20 ->
100x100 cm, distance from artifact ~1 m
n High accuracy (>0.05 mm)
n High sampling density (0.2 mm)
n Fast (1 shot in ~1-2 sec)
o Laser, Time of flight
n Large scale (architectures)
n Wide workspace (many meters)
n Medium accuracy (~4-10 mm)
n Medium sampling density (10 mm)
n Slow (1 shot in ~20 min)
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Active Optical Technologies
o Using light is much faster than using
a physical probe
o Allows also scanning of soft or fragile
objects which would be threatened by
probing
o Three types of optical sensing:
n Point, similar to a physical
probe
o slow approach, lots of physical
movement by the sensor.
n Stripe
o faster: a band of many points
passes over the object at once
n Other patterns …

4. 4
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Stripe-based scanning
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Optical Technologies - Triangulation
How do we compute the 3D
coordinates of each sampled
point?
o By triangulation, known:
n emitting point of the
light source + direction
(illuminant or
emitter)
n the focus point of the
acquisition camera
(sensor)
n the center of the
imaged reflection on the
acquisition sensor plane
( P(a) )
Triangulation is an old, simple approach (Thales-Talete)
Issues: precision and price of the system

5. 5
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Output: range map
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Triangulation-based systems
An inherent limitation of the
triangulation approach:
non-visible regions
o Some surface regions can be
visible to the emitter and
not-visible to the receiver,
and vice-versa
o In all these regions we miss
sampled points è
integration of multiple scans

6. 6
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Scanning example
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Acquisition accuracy
o Depends on sweeping
approach …
o … on surface curvature
w.r.t. light direction …
o Laser syst.: the
reflected intensity can
be used as an estimate of
the accuracy of the
measure

7. 7
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Acquisition accuracy
o … on the surface shape nearby the sampled point
o … and on surface reflectance
[see Curless Levoy “…Space Time Analysis”, ’95]
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Optical Tech. – Time of Flight
Measure the time a light impulse needs to travel from the emitter to the
target point (and back)
n Source: emits a light pulse and starts a nanosecond watch
n Sensor: detects the reflected light, stops the watch
(roundtrip time)
n Distance = ½ time * lightspeed [e.g. 6.67 ns è 1 m ]
o Advantages: no triangulation, source and receiver can be on the
same axis è smaller footprint (wide distance measures), no shadow
effects
[Image by R. Lange et al, SPIE v.3823]

8. 8
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Optical– Time of Flight
o Optical signal:
n Pulsed light: easier to be detected, more complex to be
generated at high frequency (short pulses, fast rise and fall
times)
n Modulated light (sine waves, intensity): phase difference
between sent and received signal è distance (modulo
wavelenght)
n A combination of the previous (pulsed sine)
o Scanning:
n single spot measure
n range map, by rotating mirrors
or motorized 2 DOF head
[Image by Brian Curless,
Sig2000 CourseNotes]
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3D scanning – raw output data
For the user, same type of output data :
n Range map: 2D map of sampled 3D points
(640x480 -> 2M - 5M points)
n Can be managed as a point cloud or a
triangulated surface chunk

9. 9
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Why processing raw scanned data?
The acquisition of a single shot
(range map) is only a single step
in the 3D scanning process, since
it returns a partial & incomplete
representation
dal parziale
al totale
We need algorithms and software
tools for transforming redundandt
sampled data into a complete and
optimal 3D model

3D Scanning Pipeline
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10. 10
Note:
New approaches appeared that use many
redundant & overlapping images to
produce results similar to those produced
with active scanning devices
è
3D from images (passive methods)
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Questions?
o Contact:
Visual Computing Lab.
of ISTI - CNR
http://vcg.isti.cnr.it
r.scopigno@isti.cnr.it