Signal Processing - September 2016 - 133

(a)

(b)

(c)

(d)

Figure 1. (a) A color image of Johannes Vermeer's Girl with the Red Hat (1665/1666). Andrew W. Mellon Collection, 1937.1.53, National Gallery of Art,

Washington, D.C., (b) infrared reflectance (2,100-2,400 nm), (c) X-radiograph, and (d) summation of the rotated X-radiograph and the intensity-inverted
and rotated infrared reflectance image. (Images and figure caption used with permission from [12].)

have more recently used a more convenient and purely imageobjects. Processing these data is also a nontrivial task since
based method, SfM, a photogrammetry stereo technique, to
gaps in the scanned area must be filled and the different scans
recover the shape of historical sites. In 2006, the 3-D strucmust be aligned and registered in X, Y, Z space. Also, further
ture of the Colosseum in Rome was generated from a large
processing to map color information on the acquired 3-D
collection of consumer photos taken at difmeshes is needed to produce a fully renferent viewpoints [16]. These photos were
dered result. The whole process was expenTo overcome some
gathered from an Internet-sharing website.
sive (US$2 million) and took 32 people
of the limitations of
The photo explorer uses image-based renyears (1997-2004) to plan, scan, and model
terrestrial and airborne
dering techniques to create smooth transiten statues. Thus, laser scanning poses
tion between different viewpoints, so the
many challenges that limit its widespread
laser scanning due to
user can comfortably and virtually tour
use. Beside academy, Aicon3D (http://www.
the high operational,
historic locations. SfM may also be used
aicon3d.com/) and Metis (http://metis-digiresearchers have more
to great effect on smaller moveable objects,
tal.com/) companies currently offer highrecently used a more
however, the depth accuracy of the phoresolution 3-D scanner for artworks, and
convenient and purely
togrammetry stereo method is limited to
Artmyn (http://www.artmyn.com) provides
image-based method,
only textured surfaces and fails on featureWeb solution for 3-D imaging, using the
less surfaces, hence, the depth resolution is
one of the oldest New Testament papyri
SfM, a photogrammetry
typically lower than the lateral resolution at
(Papyrus 66) as an example. Due to the limstereo technique,
each pixel.
itation of laser scanning, new methods are
to recover the shape
Another image-based method, photostill needed for quick and cost-effective
of historical sites.
metric stereo, recovers the 3-D shape of an
ways for digitally archiving art.
object by taking multiple images at fixed
Another effective approach to the imagview but varies lighting positions. Photometric stereo models
ing of extremely large structures has been airborne LiDAR
the image intensity as a function of surface normal, reflectance,
remote sensing techniques, which make it possible to record
and lighting/viewing angle. The surface normal is recovered
shapes on the extreme landscape scale. For instance, the 200
by solving an optimization problem, and the 3-D surface of
km2 area of the ancient Maya landscape at Caracol, Belize,
the object can be recovered by integrating the surface normal
was scanned with a resolution that could resolve structures of
across the field of view. Unlike photogrammetry, photometric
roughly 25-cm height [15]. The data obtained helped researchstereo works extremely well on textureless surfaces and can
ers understand that the ancient Maya could radically modify
produce high-resolution normal maps. Classic photometric stetheir landscape to create a sustainable urban environment. On
reo methods assume a point light source placed at infinitely far
the other hand, Google Earth (https://earth.google.com/) has
away from direction L, and assume the material is Lambertian
made 3-D buildings of cities (e.g., Chicago, Illinois) and 3-D
with albedo reflectivity of k, so that the reflected light intensihistorical sites (e.g., Rome's Colosseum) easily accessible to
ties at an object point with surface normal n becomes
the public, though in low resolution.
To overcome some of the limitations of terrestrial and airborne laser scanning due to the high operational, researchers
I = k (n : L) .
(2)
IEEE SIgnal ProcESSIng MagazInE

September 2016

133

http://www http://www.aicon3d.com/ http://metis-digi http://www.tal.com/ http://www http://www.artmyn.com https://earth.google.com/

Table of Contents for the Digital Edition of Signal Processing - September 2016