3D RECONSTRUCTION METHODS, A SURVEY

Julius Butime

CEIT and TECNUN(University of Navarra), Manuel Lardizabal 15,20018 San Sebastian, Spain

Dr. Iñigo Gutierrez

TECNUN(University of Navarra), Manuel Lardizabal 15,20018 San Sebastian, Spain

Luis Galo Corzo

CEIT and TECNUN(University of Navarra), Manuel Lardizabal 15,20018 San Sebastian, Spain

Carlos Flores Espronceda

CEIT and TECNUN(University of Navarra), Manuel Lardizabal 15,20018 San Sebastian, Spain

Keywords: 3D Reconstruction, Triangulation, Holography, Stereoscopy, Conoscopic holography.

Abstract: 3D reconstruction technologies have evolved over the years. In this paper we try to highlight the evolution

of the scanning technologies. The idea of a survey came up with our decision to look at 3D reconstruction

methods. Little has been written about the methods in general, yet many developments have taken place in

this area. This survey will prove useful for those intending to embark on research in 3D reconstruction

technologies or are considering acquiring a 3D scanner. The survey takes a look at the major reconstruction

methods, which are; Laser triangulation, Stereoscopy, Conoscopic holography and Moiré Interferometry. A

review of the major producers of scanning technology for 3D reconstruction is also carried out.

1 INTRODUCTION

3D reconstruction methods are classified into

passive and active. Passive methods do not involve

interaction with the object, whereas active methods

use contact or a projection of some form of energy

onto the object. Our main in this paper is given to

the active methods that use the projection of a form

of energy onto the objects, light in our case. Active

methods involving contact with the object are being

phased out due to their slow reconstruction process

and the need for less contact with the object to avoid

them getting damaged (Curless, B). In this paper,

our main focus will be on the optical non contact

methods that offer faster reconstructions since they

are commonly applied in the manufacturing

industry. In the field, the main method used to

realise reconstructions is optical laser triangulation.

Section two deals with the various methods of

3D reconstruction, whereas section 3 analyses the

3D scanners on the market. We make comparisons

based on the technical specifications provided by the

manufacturers. The last section contains a summary

of all the results from the survey. Given the

importance of faster prototyping in modern industry,

one can easily appreciate the amount of time and

money that is saved when 3D scanning methods are

used. At present many of the leading manufacturing

industries have incorporated in their production lines

systems for 3D scanning. This has helped increase

their productivity and save on the time it takes for a

product to be released on the market. On the whole,

the benefit of these scanning systems is the

improved product quality, time to market and the

reduction of the overall production cost.

457

Butime J., Gutierrez D., Galo Corzo L. and Flores Espronceda C. (2006).

3D RECONSTRUCTION METHODS, A SURVEY.

In Proceedings of the First International Conference on Computer Vision Theory and Applications, pages 457-463

DOI: 10.5220/0001369704570463

 SciTePress

2 3D RECONSTRUCTION

METHODS

As has been mentioned in the previous section, here

we take a look at the broad range of 3D

reconstruction methods.

2.1 Laser triangulation

Laser triangulation is the most common method used

in commercial 3D scanners. The principle of

operation of Laser triangulation involves the

projection of a ray of light over an object in the form

of a point. If the object is to be captured by a

camera, only a bright spot should be detected.

Therefore knowing with precision the relative angle

of projection with respect to the base line, it is

possible to determine the position of the point in

space. The variants of laser triangulation are based

on the many ways of projecting and detecting the

light rays. In the case of a point source, the whole

scene has to be scanned both vertically and

horizontally to obtain the depth. If instead of

projecting a point, a line is projected, the depth of all

the points on the line can be obtained at the same

time. This explains why techniques based on the

projection of a line are much faster than the

projection of a single point. One may use various

methods to project the light onto the scene, each one

with its merits and demerits. The precision, the

presence of blind spots, where triangulation is made

impossible, and the speed of scanning the scene are

the principle factors to be taken into account when

choosing the kind of technique.

Principle of operation

Consider the figure shown below. A ray of light

originating from a laser diode is focussed on an

object at point P. The ray is observed using a camera

placed at an angle to the object. The separation of

the camera from the laser diode is known. Using

triangulation theory the distance of the object in the

z axis is recorded.

The laser diode is located on the x-axis, at a known

distance from the camera b. Assuming P is in the x,z

plane, the distance and projection angle of the laser

diode are established before hand.

Figure 1: Triangulation setup.

Our next main problem is to solve for the distance

from the object. Using similar triangles, the sine

rule is applied to the setup. A derivation of the

distance Zp is obtained can be found in the reference

(Klaus D. )

2.2 Stereoscopy

Stereo vision refers to the ability to deduce

information on the 3D structure and distance of a

scene from two or more images of taken from

different viewpoints. The name stereoscopy was

given to this method by Sir Charles Wheatstone

(Wheatstone, C). Stereo vision involves two

processes: the binocular fusion observed by the two

eyes, and the reconstruction of the three-dimensional

image. The pre-image of the matching points can be

found at the intersection of the rays passing through

these points and the associated pupil centres or

pinholes. In a stereo system, we look for

correspondences existent between the two images

i.e. which parts in both images are projections of the

same scene. Having obtained the correspondences,

the 3D structure is determined using epipolar

geometry. By estimating the disparity between two

images, the height of each point is evaluated. The

robustness of the process is ensured by modeling

and taking into account the geometric nature of the

elements observed.

VISAPP 2006 - MOTION, TRACKING AND STEREO VISION

458

Principle of Stereoscopy

Camera1

Camera2

Object as seen in

camera 2

Object as seen i

camera 1

Object

Figure 2: Stereo setup.

As was mentioned above, stereo vision involves the

matching of points observed in two images captured

by two cameras. This matching is done using

epipolar geometry. Epipolar geometry involves the

the observation of a single point in two images and

relating the coordinates in each of the images.

(Owen, R)

2.3 Holography

Holography is a technique by which a wave front

can be recorded and subsequently reconstructed in

the absence of the original wave front. The method

was proposed and demonstrated by Gabor in 1948

long before the laser came into existence.

Observation of this reconstructed wave front gives

exactly the same physical effect as the observation

of the original wave front. On illuminating the scene

after removing the object, a three dimensional image

is observed as though the object was still present.

Leith and Upatnieks were the pioneers in applying

holography to three dimensional imagery way back

in 1964. This depended largely on the availability of

the HeNe laser, which had an excellent temporal and

spatial coherence (Goodman, C).

Figure 3 shows the geometry used for recording

holograms of a three dimensional scene.

Figure 3: a) Capture of the hologram.

Figure 3: b) Observing the virtual image

Figure 3c) Observing the real image.

Coherent light illuminates the scene of interest. A

portion of the light is reflected from a mirror onto

3D RECONSTRUCTION METHODS, A SURVEY

459

the photographic plate. This light reflected serves to

form the 3D hologram of the object on the

photographic plate by interfering with the light rays

from the object. The reconstruction of the scene is

carried out as follows; there are two possible

configurations, one giving the virtual image and the

other giving the real image.

To obtain the virtual image, the object is

removed from the setup in figure 3a) and the

photographic plate is illuminated by the same light

source that was used while capturing. A virtual

image of the object results as can be seen in Figure

3b). Lastly, if the photographic plate is illuminated

from the opposite side i.e. different from the one that

was used to capture the hologram, a real image of

the object can be observed. These images both

virtual and real are used in the reconstruction of

objects in three dimensions (Goodman, C).

2.4 Conoscopic holography

Conoscopic holography is a relatively new non

contact method used to reconstruct objects in three

dimensions. It was discovered by Gabriel Sirat and

Demetri Psaltis in 1985 as a modification to

coherent holography for three dimensional data

recording and imaging (Sirat, G., Psaltis, D., 1985).

It is based on the propagation of light in anisotropic

crystals. The property that enables the crystal to split

the light rays into ordinary and extra ordinary rays is

referred to as birefringence.

Birefringence is a property of certain crystals,

which when a ray of light travels through their

different optical axes, travels at different velocities.

This behaviour is characteristic of anisotropic

crystals given that they posses varying indices of

refraction caused by the nature of the crystal lattice.

Having split the light ray into an extraordinary

and ordinary beam they are made to interfere thus

and give measurements of high precision.

Principle of operation

Consider a point source of light originating from

an opaque object as is shown in figure 4. The

conoscope comprises of the birefringent crystal,

usually made of calcite, sandwiched between two

circular polarizers. The recording of the holograms

is done on a CCD matrix. This ray travels from the

object towards the uniaxial crystal after penetrating

the first circular polarizer. On entering the uniaxial

crystal, the ray is divided in two, the ordinary and

extraordinary.

Figure 4: Conoscopic holography setup.

These rays are out of phase with each other and

travel at different velocities within the crystal. At the

other end of the crystal the ordinary and

extraordinary rays are made to interfere after going

through the second circular polarizer, which among

other things returns both rays to the same phase. The

resulting rays are made to interfere leading to either

constructive or destructive interference. The

resulting interference patterns are the conoscopic

holograms and these contain the distance

information that we are interested in. For a more

detailed explanation of the principle with a

mathematical foundation refer to (Sirat, G., 1992).

2.5 Moiré Interferometry

The term Moiré refers to an irregular wavy finish

usually produced on a fabric by pressing between

engraved rollers. In optics, it refers to a beat pattern

produced between two gratings of approximately

equal spacing. Moiré is a technique to study strains

and deformations of structural elements with very

high accuracy. It requires a highly stable

environment and has mainly been a laboratory tool.

Recently attempts have been made to develop a 3D

scanner that uses moiré interferometry (Dubowsky,

S). The method can be grouped under two major

classifications that depend on the optical

arrangement used: projection and shadow.

Projection moiré involves the projection of a

matching pair of gratings. The projection grating is

placed in front of the light source and the reference

grating is placed in front of the camera. The

projected beam light is amplitude modulated with

the pitch of the grating. When the beam falls on the

surface of the object, the phase of the spatial carrier

is modulated by the shape of the object surface. The

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460

Table 1: 3D Scanners’ parameters.

Company Type of

scanner

Scanning

technique

Working

range

(mm)

Speed

(points/sec

ond)

Accuracy

(mm)

Type of

surface

Konica

Minolta

Vivid 700

Triangulati

600-25,000 n/a 0.11 Diffuse

Optimet Conoline Conoscopic

holography

45 4,000 0.047

Diffuse to

shiny

Scanners

MMZ

Laser

Triangulati

50-150 n/a 0.1 Diffuse

Opton

Mini Moire

scanner

Moiré

interferome

try

15 150,000 0.025 Diffuse to

shiny

Roland

DGA

Pix30

Laser

Triangulati

0.2-406 n/a n/a Diffuse

3rdTech

Deltasphere

3000

Time of

Flight

300-

12,000

25,000 n/a Diffuse

instruments

CMMaster

Moiré

interferome

try

n/a 80,000 < 0.025 Diffuse to

Shiny

Nub 3d SIDIO Laser

Triangulati

700 0.08

Diffuse

Coherix Shapix Phase

shifting

interferome

try

450 17,476 <0.02 Diffuse to

shiny

Geodetic

systems

Vstars(E4X

)

Stereoscop

n/a n/a 0.008 Diffuse

Faro

Technologi

Faro laser

scanner

Interferome

try

120,000 120,000 0.025 Diffuse to

shiny

Brueckman

OptoTOP-

Structured

light

25-400 1,555,555 0.045-0.45

Diffuse

Perceptron Scanworks Laser

Triangulati

23-71 23,040 0.035

Diffuse

reference grating is phase shifted against the

projection grating.

Shadow moiré involves positioning a grating

close to an object and observing its shadow on the

object through the same grating. Moiré is considered

a very good method for 3D reconstructions because

it amplifies small errors, thus enabling their

detection. It requires less computer time, and so has

a great potential for rapid online registration and

inspection. The parameters of a moiré can easily be

changed. Moiré however has not been fully

exploited owing to the difficulties encountered in

designing and adjusting a system based on it.

In moiré interferometry, light is projected onto

an object’s surface through two equally spaced

fringes. The resulting patterns are viewed at an angle

different from that at which the fringes are projected.

The contour interval depends on the spacing of the

fringes projected on the surface and the projection

viewing angle. A detailed mathematical analysis of

the formation of the moiré fringes may be found in

(Creath, K., Wyant, J).

3D RECONSTRUCTION METHODS, A SURVEY

461

3 COMPARISON OF 3D

SCANNERS ON THE MARKET

The manufacturers of 3D scanning equipment can be

grouped in three categories; those that provide the

hardware, those that write the software and those

that implement both systems. We looked at the

leading manufactures of 3D scanners and this

enabled us to make a comparison of their various

products. Given that these use different technologies

these results are not easily compared since we have

to identify uniform parameters for comparison in

each of the methods. Owing to the variations in

software available on the market, the survey has

been limited to the comparison of the hardware and

its performance. Definitely their performance

depends greatly on the software, but that will be

dealt with in another article.

The providers of scanners that we have been able

to identify, that use each of the various technologies

for 3D reconstruction are diverse. A look is taken at

those who provide the datasheets on their products.

Using these, a comparison of similar parameters is

carried out to come up with unbiased conclusions.

The scanners are commercial implying that we

have had to rely on the information provided by their

manufacturers, as it is close to impossible to have all

scanners in the university laboratory considering

their high cost. The parameters taken into

consideration were; the operating range, accuracy,

speed of capture, and the types of surfaces. Each

scanner will be compared with a series of cameras in

the same range. This will be followed by a

comparison of with the results from other scanning

techniques. The survey looked at products from the

following companies: 3D Scanners (MMZ), Konica

Minolta (VIVID 700), Optimet technologies

(Conoline), Opton formerly EOIS (Moiré scanners),

Roland (Pix 30) among others.

Table 1. shows these parameters in several

commercial scanners. Judging from the results

presented, several observations about the various

technologies can be made. Laser triangulation is the

commonest method used in 3D scanners, as seen in

the table most scanners use triangulation. The

reasons that are put forward for its popularity are;

it’s easy setup, its low cost as compared to other

methods, its speed which enables real time scans and

reconstructions on the production line. Laser

triangulation is limited when very high precision and

accuracy are required. It is also marred by speckle

and inability to scan reflective surfaces.

Stereo is passive and does not emit any radio or

light energy. Recording on site can be done very fast

and as such it can be used on-line. However, the

matching of the points in the two images captured by

the cameras in stereo is tedious. Whereas they can

adjust for component size, stereo cannot easily

accommodate free form surfaces. In addition, the

requirement for one of the two cameras to be fixed

at a specific angle restricts the inspection region and

presents difficulty in inspecting the entire

component. The scene has to be rigid to begin

reconstruction which makes it difficult for on-line

implementation.

Moiré is considered a very good method for 3D

reconstructions because it amplifies small errors,

thus enabling their detection. It requires less

computer time than other methods like laser

triangulation during capture, once it has been

properly setup, and so has a great potential for rapid

on-line registration and inspection. The parameters

of a moiré can easily be changed. Moiré limitations

can be cited in the difficulty in its design and in

adjusting the setup to capture data. Its very high

precision makes the acquisition costs extremely

prohibitive (Dubowsky , S).

Holography performs much better than laser

triangulation when it comes to precision and

accuracy. Readings of up to several microns can be

taken. However it is limited as far as speed is

concerned. A lot of precaution has to be taken when

recording the holograms since lengthy exposure

requires a high level of stability in order to obtain

good results in the processing. The range for

recording the holograms also has to be well chosen

in order to get a good reconstruction. Holography

has not been fully taken on in industry inspite of

being very accurate, given the complication in

adjustments that have to be made while taking

readings.

Conoscopic holography being a modification to

holography sorts out the speed problem. It features a

high precision in its readings and reconstructions of

up to 47 microns using an objective of focal length

100mm. The accuracy and range depend on the

objectives used on the conoline. Conoscopic

holography has clearly enabled holography to be

applied to the manufacturing industry. Of the

benefits we are able to note using the conoline is the

ability to measure several surfaces and reach an

angle of incidence up to 85º, which not many

methods are capable of doing. Conoscopic

holography could still benefit from an increase in its

speed of capture in order to be adapted to more real

time applications. (Optimet)

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4 CONCLUSIONS

Having looked at the various methods of 3D

reconstruction, we hereby are able to reach a

decision on which technology to be used in our 3D

scanner. The pros and cons of each and every

method have been expounded, in order to serve as an

aid to all those wishing to implement scanning

processes for both experimental and industrial

purposes.

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