Laser-point technique and associated image...

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Laser-point technique and associated image processing for free surface

measurement in breaking wave case

Romuald Boucheron1,*, Pierre Vonier1, Didier Fréchou1 1: DGA Hydrodynamics, Chaussée du Vexin 27105 Val-de-Reuil, France

* Correspondent author: [email protected]

Keywords: Free surface, Laser point, 2D Imaging

ABSTRACT

Water height measurements have been carried out on an anti-roll tank experiment. It consists in performing

seakeeping tests on a model scaled ship at rest in beam waves. A so-called antiroll tank partially filled with water is

disposed on the model and is devoted to limit its roll motion. Important movement created inside the tank can make

large pressure magnitude and cause high damage to the tank. Water height inside the tank is one of important

parameter of the experiment.

To reach such measurement, the experimental apparatus must be fitted in the ship on board with particular

constraints as light weight. A Laser based technique for the free surface height measurement is proposed. It

combines a Laser beam focused on water surface and a camera in 2D configuration. The developed technique

requires doping the water with white paint in order to create a diffused splotch at the surface. An adapted image

processing technique, using a local threshold, is also proposed in order to take advantage of the diffused light

splotch and to be as robust as possible even if breaking wave case occurs.

After briefly reviewing the different Laser based technique for free surface measurement, the technique is described.

Results on the passive roll damping system experiments are presented in a second part. Drawbacks, advantages and

way of improvements are then discussed just before concluding.

1. Introduction

Within the framework of naval hydrodynamic studies, DGA Hydrodynamics develops and uses

several devices for the measurement of free surface deformation. Seakeeping studies on naval

vessel which goals are to predict the ship behavior under waves imply to carry out experiments

of ship model in ocean basin where different sea states can be simulated. The knowledge of

incident wave height, wave direction and spreading and also waves diffracted by a vessel can be

viewed as fundamental parameters to be measured.

To perform these model tests DGA Hydrodynamics uses first a large towing tank (545m long,

15m large and 7m deep) called “Emile Barillon” equipped with towing carriage which can reach

a 10m.s-1 speed. This test facility is equipped with a wave generator able to simulate extreme sea


states (SS8-SS9) in similarity of different world area (among which Atlantic Ocean and

Mediterranean Sea States) for scale models in between 1/20th and 1/30th. A second experimental

facility is a wave tank more dedicated to seakeeping tests at zero ship speed, called “Cuve à

Houle” in which multidirectional waves can be generated thanks to a multiple hinge flap type

wavemaker. Its dimensions are 32m long, 10m large and adjustable deep from 0.4m up to 2.7m.

During time, measurement techniques for the free surface deformation estimate have evolved

trying to answer multiple aspects of the problem of free surface deformation estimate. Today,

this measurement of free surface is still a challenge. The problem is complex because multiple

kinds of deformation occur during tests: between the simplest case of a unidirectional regular

wave to the most complex as an irregular breaking waves, features are totally different which

implies adjustment of the measuring devices. The following section reviews the main measuring

techniques of a free surface deformation with Laser based techniques.

2. Short review of free surface deformation with Laser or imaging technique

Historically, free surface were measured with gauges as conductance or capacitance wave

probes. These intrusive wire probes are inefficient for wave heights larger than 0.7m due to

water flowing along the wire. Moreover, they are not useable with high carriage speed (typically

higher than 2m.s-1). In order to be less intrusive, servo mechanical wave probe were developed.

They use a metallic needle which vertical position is controlled to follow the water surface by

alternatively being in touch with the water and not being in touch. Its vertical motion is limited

in magnitude and velocity which represents the main limitation of this technique. Moreover,

ultrasonic wave probes have been developed on the principle of measuring the transit

propagation time of acoustic waves between the probe and the free surface, acting as a mirror.

These gauges are non-intrusive and main drawbacks of this technique are the limited sampling

frequency, limited height and wave camber.

2.1 Laser wave gauge

The principle of this technique is schematically explained in figure 1. A Laser beam is focused on

free surface to be measured. A camera is set up with an appropriate calibration to view the

measurement field. A simple point or gradient detection, which can be done actually in a quasi-

real-time configuration with current processor power, allows estimating the interface position

between air and water [7].


LASER

500 mW – 3 W

CCD Video

100 image / sec

Camera

sensor

H

t

Real-time

processing

Surge

air

w

ate

r

Fig. 1: Schematic principle of Laser wave gauge

2.2 Laser sheet technique

This technique is derived from the previous one, except that the Laser source is immerged. If we

materialize the free surface by a Laser sheet in a vertical plane (like explained by figure 2), with

an appropriated calibration (in air or water with the calibration target instead of Laser sheet) a

point on sensor reveals a 2D position of free surface in the measurement plane [6].

Camera

Camera

Calibration grid

Optical configuration for calibration

Optical configuration for measurement

Laser sheet

Fig. 2: Schematic principle of Laser sheet technique

2.3 Image inter-correlation technique


This technique tends to take advantage of a stereo sensor to measure directly the surface in 3D.

A stereo calibration is done, generally outward from the real measurement volume to get rid of

the motion of water. Figure 3 presents a schematic drawing of this technique which need the

materialization of the surface. In most of experimental cases, thin powder or merely natural

dusts are used. The pattern viewed by two cameras is recognized by inter-correlation algorithms

and allows estimating a whole 3D position [8].

Camera 1

Wave

Caméra 2

Image

processing


2.4 Pattern projection technique

The technique is similar to the previous one except that to ensure a good pattern contrast, the

water is slightly doped with white paint. An image is projected on the surface with an overhead

projector as explained by following figure 4.

Camera 2 Camera 1 Overhead

projector

Pattern image



2.5 Stereo-refraction technique


This technique uses an immersed referenced pattern, refracted by the free surface. With two

cameras, optimization of light propagation model can lead to an estimate on both position of free

surface (point P on figure 5) and curvature of the free surface at point [4].

2.6 Laser point scanning imaging technique

This technique forms part of stereo-imaging techniques family. The principle is to materialize the

surface by lighting a point with a Laser beam. Two cameras set up in stereovision configuration

acquire synchronized images. Each image presents, without surface particles, the light diffusion

of the beam in water domain as schematically explained by figure 6.

LLaasseerr DDrriivveerr

CCaammeerraa 11 CCaammeerraa 22

PPooiinntt ddeetteeccttiioonn

(u1,v1) (u2,v2)

PPooiinntt ddeetteeccttiioonn

(X,Y,Z)

TTrriiaanngguullaattiioonn

-- SStteerreeoo--ccaalliibbrraattiioonn

- (u1,v1)

- (u2,v2)

Fig. 6: Schematic principle of the beam scanning technique


The diffusion appears in each image as a beam with grey level gradient. The maximum is

supposed to be at the air/water interface. After processing point detection on each image of a

pair, a simple triangulation allows to calculate the 3D point coordinates [3,5]. The Laser beam is

moved over the surface to be measured.

2.7 Performances of the different techniques

Main advantages and drawbacks of all these techniques are summarized in the following table 1.

Technique 1D/2D type Advantages Drawbacks

Conductance /

Capacitance gauges 1D (Point) • Simple set up

• Intrusive

• Calibration drift

Servo-mechanical

gauge 1D (Point)

• Simple set up

• Quasi non-intrusive

• Magnitude range

• High wave height

Ultrasound

wavegauge 1D (Point)

• Non-intrusive

• Calibration simplicity

• Low sampling frequency

• Wave slope <4%

• 2 gauges not useable in restrained area

Laser wave gauge 1D (Point) • Non-intrusive


• 1D Measurement

• Laser safety

Laser sheet

technique 1.5D*

• Non-intrusive

• Temporal behaviour (quasi2D)

• Hard to load on

• Laser safety

Image inter-

correlation 2D

• Non-intrusive


• Optical adjustment simplicity

• Need surface particles

• Surface feed quality

• Light condition not easy to set up

Pattern projection 2D

• Non-intrusive



• Need doped water

• Pattern contrast quality

Stereo-refraction 2D

• Quasi non-intrusive

• Good accuracy for low wave

height

• Pattern to generate

• Calibration process

Laser point scanning 1.5D*

• Non-intrusive



• Non synchronous whole 2D field

• Laser safety

Tab. 1: Short summary of different techniques performances.

*(1.5D denotes a 1D measurement which can be extended easily to 2D case with time for stationary measurement)


For a particular application, we can choose the best technique in connection with mechanical or

optical restraints. However, all these techniques fail to measure free surface height in breaking

wave cases.

3. Experimental set up

3.1 Breaking wave height measurement challenge

The problem of estimating the height of a breaking wave is a technical problem as well a

definition one. As schematically explained in figure 7, when a wave breaks down, we can easily

imagine a continuous water domain, an upper continuous air domain and between these two

perfectly defined domains, a two phases flow domain, composed by a mixture of water and air

(with bubbles and drops).

3 1 2 4 5

?

Fig. 7: The wave breaking height definition problem

The definition of the water height is then very difficult. For both points 1 and 5 of figure 7, there

is no difficulty to define what the height of free surface is. For the point 4, there are two

possibilities but function of what is the interest of an experiment, an agreement can be find. The

problem of surface height is not well defined for points 2 and 3 (when a mixture domain is

present on a vertical line, inside a water domain or upper the water domain).

Most of prior listed optical techniques fail in height estimating for case 2, 3 and 4 and the wire

probes give values which are difficult to relate with physical height. With a Laser beam (or

sheet), curvatures are highly modified and multiple reflections and refractions cause a

measurement impossibility.


3.2 The developed Laser-point technique

The above problem has been investigated in the particular case of an anti-roll tank experiment.

This kind of test is briefly described by figure 8. A ship (in yellow) is set perpendicular to wave

direction. The antiroll tank manufactured in Perpex to be transparent and partially filled with

water, is devoted to limit the roll motion of the ship.

Camera

(fixed on boat)

Wave

propagation

Model ship

Fig. 8: Schematic drawing of the experimental set up for the free surface measurement on the antiroll tank.

We have taken advantage of this experiment to test the Laser-point technique on water

movement inside the tank. In accordance with the schematic drawn of figure 9, a camera

(1280x720pix²@30Hz) is fixed on boat in order to facilitate the calibration linked to the tank.

Seven red Laser diodes (650nm@3mW each) are fixed above the plane measurement which gives

seven measuring points by image. The measurement plane is vertical and defined by each

vertical Laser beam like schematically drawn in figure 9. Camera optic is adjusted in order to be

able to catch the whole tank with the camera. Compared to the symmetry axis of the tank, the

camera is shifted up and slightly tilted downward (tilt angle around 18°) in order to be sure that

the optical path between the camera and the Laser impact on water surface will be in air and

Perpex, not through water (see figure 9(b)).

Measurement

plane

Camera

Water n ≈ 1.33

Perpex n ≈ 1.49

a)

Measurement

plane

Camera

Water n ≈ 1.33

Perpex n ≈ 1.49

Air n ≈ 1.00

Red Laser diodes

b)

Fig. 9: Schematic drawing of the optical arrangement of our test campaign with water in blue and Perpex in grey.


The calibration of such experimental set up is simply done by acquiring a picture of a calibration

target put in the Laser vertical plane, without water in the tank (indeed, the optical path of the

measured point in experiment go through air from the point to the sensor). Figure 10 presents

the calibration target located in the Laser beam plane with a superimposed calibration grid (in

green) showing the reconstructed points via calibration. Two extra lines of points have been

added to those used for calibration in order to appreciate the distortion due to camera optic and

due to Perpex dioptric effect.

Extra

lines

Fig. 10: Calibration target and reconstructed calibration grid.

3.3 Image processing technique

To increase the light efficiency of the Laser on camera sensor, water was doped with white paint

(see figure 11 for an example of acquisition image by camera).

Laser diodes Tank Perpex wall

Points revealing free surface height

Fig. 11: Typical acquisition image of the anti-rolling tank experiment.


Notice that recently, the colorant adjunction in water has been used for model tests as an

adjustable-density filter to measurement of height with success [1]. In our case, the colorant

addition has two major effects: the first is to increase efficiency of Laser beam diffuse reflection

which is the intended effect. The second is a high scattering in water which causes a spread red

splotch around the impact location. This latter effect could be appreciated on the acquisition

image presented by figure 11. The scattering effect has been used in the image processing to

follow the Laser impact on water surface even if breaking wave happened. The image processing

is described in different steps as shown in figure 12 and described hereafter.

+ +

a b c d e

Fig. 12: The different image processing steps.

On first image of an acquisition, we select manually the area by clicking close to the red splotch

to feed image processing algorithm. Then, the image red channel is chosen and a rectangular

area (described in green on fig. 12b) is selected to compute its grey level histogram (fig. 12c). A

local threshold is used to compute binarization image (fig.12d). This threshold is the one which

makes the first largest splotch of a given size (the choice of this size is discussed later). To

compute this threshold, we decrease step by step the grey level threshold of a binarization

process and compute the size of greater splotch in the rectangular area. As soon as the given size

is reached, we use this threshold to compute the binary image. The barycentre is then computed

to get the position of this point in the real plane. The fact to make binarization as previously

described presents, on our experiments, robustness about signal loosing even with breaking

wave case.

The next part will give experimental results on the anti-roll trial and discussion about the

breaking wave height measurement.


4. Results

4.1 Results from the anti-roll tank

The anti-roll tank is fixed on the ship model like the measurement camera. With wave generated

in the towing tank “Cuve à Houle”, the boat is rolling that induces fluid motion in the anti-roll

tank. In such experiments, the movement of water volume in the tank is crucial, especially

because the efficiency of the anti-roll tank is linked to the height of the bore traveling from a side

to the other. Moreover, it’s important to predict the pressure peak due to impact on the tank

side. Because the motion of the water volume is mainly 2D, the measurement plane has been

located in the central area of the tank (see figure 9b) the water height over the main axis of the

tank is one of the parameter to be measured. Figure 13 presents an example of results obtained

for a side measuring point during time.

Fig. 13: Results obtained inside the tank for the 6th (from the left) measurement point in black.

Generated wave is scaled (for better viewing) and plotted in red.

The experiment starts before the wave arriving at boat location. The wave is a regular wave

(sinusoidal shape) with a controlled period of 1.8s. Camera sampling frequency is fixed at 30Hz.

The temporal behavior of the water height can be observed over duration of around 1 minute.

The wave is well defined revealing a quasi-steady process. The Fourier transform of water height

is given by the figure 13 on the right. A peak at 0.55Hz (corresponding to the period of 1.8s) can

be observed. Upper modes appear in the tank with much smaller magnitude than main period.

These modes present in the tank are not measurable in wave signal plotted in red. Phase


statistics can then be calculated with image processing. To perform this kind of treatment, the

main frequency must be known with high precision. Synchronous detection on temporal signal

is performed and allows such determination. Using this frequency, phase slot distribution is

done. It consists in making different image sets whose phase acquisition time is included in slot

thresholds, like schematically explained by figure 14.

time

n slots

acquisition time of 1 image

1 period

Fig. 14: Phase slotting distribution for statistics calculation

Then, for a given phase (that is a given slot), images are processed with statistics tools as for

example Shannon entropy / probability image processing techniques [2]. Performing such

processes leads to phased entropy images. The principle of the Shannon entropy images has

been also improved by doing the process with two different Look-Up Tables (LUT). The

principle is to combine an entropy process with a LUT of n slots with another entropy process

defined for a LUT of n+1 slots. Entropy is averaged between the two processes which avoids the

threshold effect on both Shannon entropy images and probability images, and allows a more

refined probability image.

An example of entropy images provided by this new processing images technique applied to red

channel is given by figure 15. Four images are shown: from the top to the bottom, we show an

instantaneous image (in color, not the red channel), the phased mean image (in color), the

phased entropy image and the phased probability image (both in grey level). Green crosses

plotted on each images reveal the mean tracking position of each phase: this mean position is

processed by the average of all positions lying in the slot. The mean image shows high diffused

red splotch, revealing that the diffusion of Laser in water and also the water height varies during

time whereas the entropy image not. The tracked point is found to be in the black area of

entropy images and precisely above the center of this black area. Due to the fact that the Laser

points impact points are viewed from above, the diffusion is more important below the real


Laser impact point that above. It explains the localization of the tracked point and gives

confidence on measurement values obtained by Laser point technique.

Moreover, if a double light spot occurs, for example, due to two water droplet on the Laser beam

path (see instantaneous image of figure 15), the technique is not disturbed by the second spot

and track the principal diffuse splotch, at the free surface (blue circle of figure 15).

Instantaneous image

Mean image

Entropy image

Probability image

Fig. 15: Instantaneous image and the corresponding phased mean, phased entropy and phased probability images. Green crosses

indicate the mean position of tracked point for the phase (exactly the same on all images, including the instantaneous one).

4.2 Influence of threshold parameter on height estimation

The technique is based on light diffusion in water. The threshold parameter is of great

importance on height estimation, especially when no break-down wave occurs. Such influence

can be estimated at the beginning of the experiment, when the water surface is at rest. Figure 16

presents the same image processing with three different thresholds (10 in black, 20 in blue and 90

in red). The water height estimation is similar for a 10 and 20 threshold values. When the

threshold is very high, the diffuse splotch in image processing grows at water surface. The

splotch barycenter lies no longer in measurement plane and a bias occurs. It could be noticed

that the water height estimation during high wave gradient moment does not depend on


threshold value but on calm moment yes. For example, in the trough of the waves, the red line is

often lower than other curves: it indicates that the diffused splotch is important.

Small threshold

Large threshold

bias

Fig. 16: Threshold effect on water height estimation.

In order to have efficiency measurement device, a short study on bias effect due to threshold

parameter has been conducted. Considering that no bias occurs for small threshold with free

surface at rest, we can express the measured bias in pixel for larger threshold. Results, given in

figure 17, show that the bias increases as expected with threshold and can reach not

inconsiderable values of 3 and more pixels. This behavior imposes to limit the value of threshold:

the best threshold to use in our application is then a compromise between the fact to increase the

threshold not to lose the point during the process and to decrease it to have good estimation. An

error of one pixel (average dimension of around 0.8mm in our experiment) is tolerated in height

which leads to a threshold around 25 pixels. This value can be linked to the characteristic

dimension of the splotch L=√(π R²) measured on entropy images, which is 33 pixels for the

typical splotch.

Fig. 17: Estimated bias against threshold for three different measuring points.


4.3 Experimental problems

The technique above presented seems to give good results with acceptable accuracy for our

experiment. However, different kinds of problems have been encountered. They are presented in

the following figure 18.

a b c d e f g h i j

Fig. 18: Zoology of different encountered problems.

Image processing must be as robust as possible to huge light saturation (18a), loose of point due

to water masking (18f) or model masking (18i), apparent double points due to droplets (18b,

18c), great diffusion spread (18e, 18h), apparent line due to particular reflections (18d), small

droplet diffusion (18g) or water height definition (18j). It appears that a quality indicator of the

process could be defined as the real splotch size effectively used compared to the threshold.

Figure 19 presents a typical evolution of this size effectively used against time for a threshold of

30 pixels.

Fig. 19: Link between effective splotch size and large diffusion spread.


We can see a variation around 30-40 pixels and huge gaps when the splotch spreads over water

surface. This effect is due to standardization of grey level around the Laser impact which makes

impossible to reach small values of threshold for binarization. Table 2 resumes the different

problems encountered and present an estimate of quality indicator reliability.

Kind of problem Example Technique

robustness

Is effective size

indicator relevant?

Signal consequences

Loose Bias

Saturation a + Yes No Yes

Double point b

c

++

- No No

No

Huge

Particular reflections d ++ Yes No Yes

Diffusion spread e/g/h + Yes No Yes

Point loose f/i - Yes Possible Yes

Water height def. j - No No Refer to § 3.1

Tab. 2: Robustness of the presented technique to different problems encountered.

4.4 Technique improvements

The present experiment, using Laser and camera dedicated to general public, has not been

optimized for the developed technique. As a consequence, some improvement can be easily

done. Because the image processing technique uses only the red channel, a main improvement

can be the use of red optical filter tuned to Laser wavelength to increase image contrast. Another

improvement could be the use of Laser with smaller beam waist to increase the Laser point

impact precision and to be able to reduce the process threshold. Finally, an improvement of

image processing which has not been tested but could probably easily limit the bias in large

diffusion case, would be the use of weighting function using red channel intensity of each pixel.

5. Conclusion

The developed technique presented in this workshop paper appears as a promising technique

thanks to good results obtained in the anti-roll tank experiment. The technique is merely

adjustable for a lot of optical configurations, which is one of most important point in the future.

Increasing Laser light efficiency on camera sensor (with for example red filter accorded to Laser

wavelength) could be an important improvement in the future for high sampling applications.


However, two major drawbacks of the presented technique will remain: the first is the need of

doping water with white colorant, which cannot be done for all applications (wave

measurements in large basin for example). The second drawback is that the technique is a local

measurement and cannot simply be extrapolated for 2D surface measurement. Fortunately, the

possibility to use several Laser points (as in the presented application) will reduce cost of spatial

sampling. The proposed technique could be of great interest for the estimation of water height in

breaking wave cases and will supplement the diversity of free measurement surface techniques.

Further planed works aim to adapt the technique in different applications of experiments as the

stem wave measurement along the hull with a movable measuring device.

Acknowledgment

Authors are indebted to Pierre Roux de Reilhac and Olivier Perelman for fruitful discussions,

their help and advices.

6. References

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[2] Boucheron R, Blaisot J B (2008) Experimental study of diesel sprays in near field :

experimental setting up and jet morphology. Int. Diesel Engine Conf. Rouen 28th May France.

[3] Boucheron R, Perelman O, Gomit G et al (2014) Free surface measurement by optical

techniques. 14th Journées Hydrodynamiques, Val-de-Reuil France.

[4] Gomit G, Chatellier L, Calluaud D. David L (2013) Free surface measurement by stereo-

refraction. Exp. In Fluids 54(6):1-11.

[5] Gomit G, Chatellier L, et al (2015) Large‑scale free surface measurement for the analysis of

ship waves in a towing tank Exp. In Fluids. doi:10.1007/s003480152054

[6] Perelman O, Wu C H, Boucheron R, Fréchou D (2011) 3D wave fields measurements

technique in model basin: application on ship wave measurement. 2nd advanced Model

Measurement Technology for the EU Maritime Industry, University of Newcastle.

[7] Richon J B, Reeves M, Darquier M and Fréchou D (2009) Development of a Laser wavegauge

for dynamic wave height measurements in the B600 towing tank. AMT09, Ecole Centrale Nantes,

France.

[8] Wanek J M, Wu C H (2006) Automated trinocular stereo imaging system for three

dimensional surface wave measurement. Ocean Engineering 33:727-747.

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