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Learning Paired-associate Images with An

Learning Paired-associate Images with An Unsupervised Deep Learning Architecture

Ti Wang and Daniel L.Silver

Jodrey School of Computer Science

Acadia University

Wolfville,NS,Canada B4P 2R6

danny.silver@acadiau.ca

Abstract

This paper presents an unsupervised multi-modal learning system that learns as-sociative representation from two input modalities,or channels,such that input on one channel will correctly generate the associated response at the other and vice versa .In this way,the system develops a kind of supervised classi?cation model meant to simulate aspects of human associative memory.The system uses a deep learning architecture (DLA)composed of two input/output channels formed from stacked Restricted Boltzmann Machines (RBM)and an associative memory net-work that combines the two channels.The DLA is trained on pairs of MNIST handwritten digit images to develop hierarchical features and associative repre-sentations that are able to reconstruct one image given its paired-associate.Ex-periments show that the multi-modal learning system generates models that are as accurate as back-propagation networks but with the advantage of a bi-directional network and unsupervised learning from either paired or non-paired training ex-amples.

1Introduction

Humans learn knowledge from the environment by data that is provided in several forms,or modal-ities ,such as audio and visual signals.Psychologists de?ne multi-modal learning as learning new knowledge from multiple sensory modalities [11].Researchers have shown that people’s under-standing of new concepts is enhanced with mixed-modality knowledge representations [10].The human brain has adapted to fuse associated sensory signals so as to learn more effectively and ef-?ciently.The long-term goal of this research is to develop a learning system that simulates aspects of the multi-modal learning ability of humans.In particular,we investigate unsupervised learning methods that can create a model capable of generalization and classi?cation from one input or output modality to another (eg.from visual to verbal).We are interested in how this can be done without resorting to any form of supervised learning that suffers from the need for labeled examples.Deep learning is a sub-area of machine learning,which typically uses Restricted Boltzmann Ma-chines (RBM),a type of stochastic associative arti?cial neural network (ANN),to develop a multi-layer generative models [6].Deep learning architectures,or DLA,provide an exciting new substrate upon which to explore new computational and representational models of how knowledge can be acquired,consolidated and used [1].Prior work has investigated the use of DLAs and unsupervised learning methods to develop models for a variety of purposes including auto-associative memory,pattern completion,and clustering as well as generalization and classi?cation [8].

This paper takes a ?rst step toward developing a multi-modal learning system by examining a DLA that is capable of learning paired-associate images at two input modalities (channels).The DLA must reconstruct the matching image at channel A when it observes it’s paired image at channel

a r X i v :1312.6171v 2 [c s .N E ] 10 J a n 2014

B,and vice versa .By doing so the system uses unsupervised learning to develop an associative memory model that performs a form of classi?cation from one channel to another.Additionally,this DLA can learn not only paired-associate examples,but also non-paired independent examples at each sensory modality.Experimentation shows quantitatively and qualitatively that the system generates models that accurately generates associated images as compared to models developed using traditional supervised back-propagation networks.

2Background

Arti?cial neural networks (ANN)are widely used to solve classi?cation problems such as image and speech recognition,however many do not work in the same fashion as the human nervous system.For example,back-propagation ANNs are good for modeling complex mapping relations between input and output data,but are not as good for reconstructing,or recalling a pattern.Humans have the ability to recover complete information from partial information;this is referred to as associative memory [4].When a child watches a tennis game,he or she learns the appearance of the tennis ball and the racket.Next time when the child sees a picture of a tennis ball,the child may recall an image of a racket and of the game.Associations are clearly a major part of learning about the world.

Associative ANNs are inspired by cognitive psychology and are designed to mimic the way that collections of biological neurons may store and recall associative memories [12].Geoffrey Hinton,University of Toronto,advocates using Boltzmann Machine associative networks to simulating hu-man brain structure.After a Boltzmann Machine has been trained on a set of patterns,it has the ability to reconstruct any one of those patterns from a partial or noisy pattern.However,learning is slow in large Boltzmann Machines because of the many weights in a fully connected network and the iterative sampling of node activities required for each weight update.

2.1Restricted Boltzmann Machine

A Restricted Boltzmann Machine (RBM)is a variant of a BM that is meant to overcome long training times by limiting the number of connections in its network and using a modi?ed learning algorithm.RBMs have both visible and hidden layers of neurons just like BMs,however there are no intra-layer connections,so they can be characterized as a bipartite graph (see Figure 1)[8].When settling to equilibrium,neuron h j turns on with the probability p j =11+exp (?b j

? i w ij v i ),and neuron v i turns on with the probability p i =11+exp (?b i ? j w ij h j ).The states v i ,h j of neuron i and j keep changing with probabilities p i and p j .The system computes the activation energy E =? i b i v i ? j b j h j ? i j v i h j w ij where b i and b j are the bias terms for their respective nodes [9].The global energy E will be reduced more quickly in an RBM compared to a BM because of the reduced number of connections.The goal of training is to modify the weights of the network to establish low energy states that correspond with training patterns at the visible nodes.Similar input patterns will have energy states closer to each other,whereas two orthogonal patterns (e.g.patterns that share few common pixels)will have energy states more distant from each other.The method of weight update we use for this research is called Contrastive Divergence,or CD [8].The weights of the network are initialized to small random values.When training data x i is given to the visible neuron v i ,the RBM clamps the states of visible neurons and frees the states of hidden binary neuron h j (see Figure 1).Each weight w ij of the RBM is updated as per the following formula ?w ij =η(0?1),where ηis the learning rate,is the expectation over all possible pairs of visible and hidden node values,and the 0and 1superscripts indicate the expectation based on the training example and its reconstruction,respectively.This equation approximates the gradient of the log probability of a training example with respect to a weight.Weight w ij is updated until the global energy E reduces below a threshold.With probability p i ,neuron i will then reconstruct the input data x i .After training,the hidden layer weights of the RBM will have learned the feature distribution of the input space,that is w ij is equal to the probability of feature h j given input v i .

To test its ability to recall a pattern,the RBM is presented with all or some of the inputs x i of a test example at its visible units v i .These cause activations at each of the hidden units h j as described

Figure1:RBM Training Process Figure2:Stacking Multi-level RBMs above,and then the visible units are freed to generate new activations.If training has been successful, the reconstructed outputs at v i are close to the complete pattern of the original test example.

2.2Deep Learning Architectures

Humans tend to organize ideas and concepts hierarchically[5].Abstract concepts are learned and re-called through the composition of simpler concepts[1].This approach makes sense in a world where most objects are made from parts which are in turn composed of smaller features.For instance,a car is a combination of smaller parts like wheels and a frame.And a wheel is made up of smaller features like a tire and a rim.Neuroscience studies have con?rmed that this compositional structure can be seen in the human nervous system.The mammalian brain uses a deep learning architecture with multiple levels of abstraction corresponding to different areas of the neocortex[14].

Deep learning architectures,or DLA,is a sub-area of machine learning that places heavy emphasis on hierarchical composition and unsupervised learning methods.DLAs can be developed by stack-ing layers of RBMs one on top of another[8].They have been successfully used to develop models for recognizing hand-writing images of digits in a manner that simulates the human visual cortex[6] RBM-based DLA systems are capable of doing unsupervised clustering of unlabeled data based on a hierarchy of features.As shown in Figure2,the hidden layer of one RBM can be used as the input layer for a higher level RBM[1].The highest level features can be used to achieve classi?cation, if so desired.Subsequently,researchers feel that DLAs develop a hierarchy of features in a fashion similar to the mammalian brain.

DLAs present a new way at looking at systems that learn.Deep architectures can be used as an auto-encoder to model high-dimensional data,such as images and audio[3].Bengio reports that deep architectures are more expressive than shallow ones by analyzing the depth-breadth trade-off of ar-chitecture representation[2].Perhaps most importantly,deep learning methods learn representative hierarchies directly from the data[1].This is in contrast to approaches such as convolutional net-works that use receptive?elds and modi?ed back-propagation methods that rely heavily on known topological characteristics of the input space[13].

3Multi-modal Learning Using an Unsupervised DLA

The objective of this research is to develop a learning system that can memorize and recall multi-channel data using an associative memory network.The learning system should be able to recall the pattern from the associative network on one sensory modality given data on another sensory modality.The long-term goal of our research is to create a system that can learn concepts using two or more sensory/motor modalities,such as audio,optical,and vocal(see Figure3).

3.1Learning Paired-Associate Images

Consider the problem of learning paired-associate images at two input modalities(channels).We propose to use a DLA network that,after training,will be able to generate a paired image on one channel when prompted with an image on another channel.The process is meant to simulate human sensory modalities and associative memory,and to provide insights into how classi?cation can be

Figure 3:Multi-modal data learning system Figure 4:Two channels DLA

done using an unsupervised learning approach.The learning system is composed of two major parts,a associative memory network and two associative sensory channel networks (see Figure 4).The sensory channel networks are designed for the recognition and reconstruction of sensory data.The associative memory network ties the sensory channel networks together and simulates the human associative memory.Both parts can be built using RBMs.

Because of its reduced representation,the recall capacity of an RBM is not as high as a fully-connected BM.We have determined that an RBM is unable to recall patterns when only half of the visible neurons are given correct pattern values [16].Thus when an RBM is used as the top associative memory network,additional steps are required after the CD algorithm has completed training.As per Hinton,the weights of the network require ?ne tuning [6].

To produce appropriate features at the top layer,the weights of the RBM model need to be ?ne-tuned.However,?ne-tuning the bi-directional weights of the RBM may destroy their ability to generate lower level features.To protect the accuracy of the generative model,it is necessary to untie the weights between the top layer of each channel and the associative memory network layer and create two sets of weights -recognition weights and generative weights (see Figure 5)[7,8].The recognition weights are used in the bottom-up pass which receives an input pattern and the generative weights are used in the top-down pass to reconstruct an output pattern.The generative weights are left as trained by the RBM.The recognition weights are ?ne-tuned using a back-?tting algorithm,such that the associative memory network can generate a relatively accurate full set of associative memory features with only input from one channel.

To ?ne-tune channel 1,the recognition weights w ij ,where i is a neuron in hidden layer 2and j is a neuron in hidden layer 3,are used as the initial weight values for a gradient descent regression over all paired patterns.For each training pattern,the posterior probabilities {p i }of hidden layer 2are used as the input attribute,and the posterior probabilities {p j }of hidden layer 3are used as the target output.A new set of posterior probabilities {p j }for hidden layer 3are computed

using p j =11+exp (? i w ij p i

),and the weights are updated using gradient descent to minimize

the error between {p j }and {p j }.In this way the recognition weights which pass the input signal from sensory channel 1to the associative memory network are ?ne-tuned to generate a full set of associative memory features which channel 2can use to generate the appropriate output.

With back-?tting,the multi-modal DLA should be able to achieve the learning goal that was previ-ously done with supervised learning by Srivastava [15].Without supervised learning between the two channels,the performance of the DLA is unlikely to exceed that of a traditional BP ANN ap-proach;however,we do expect it to do as well.The hierarchical feature learning of the sensory channels and the back-?tting of the recognition weights are expected to make up for the shortcom-ings of purely unsupervised learning approach that we are taking.

Figure5:Untieing the weights Figure6:BP ANN

in Experiment1

3.2Impact of Learning Non-paired Patterns

Sensory data does not always come in pairs in real life.For example,one can see a cat meowing, see an image of a cat,or hear meowing without seeing a cat.In this case,the sound“meow”is the audio signal and the image of the cat is the visual signal.These two sensory channels can come together to allow paired-associate learning,but their individual channel representations can be learned and improved upon separately.We propose that learning each sensory modality with non-paired examples will help to improve the associative memories ability to generate the correct image on one channel when given its paired-associate on the other.It would be informative to have an experiment to test the impact on the multi-channel learning system by separately training the sensory channels with non-paired examples.

4Empirical Studies

Three empirical studies were carried out using two different data sets.The?rst and third experiments used paired images from the MNIST dataset of handwritten numeric digits.The second experiment used paired images from a synthetic dataset of numeric digits.In all experiments,?ve pairs of odd and even digits were associated with each:1-2,3-4,5-6,7-8,9-0.

4.1Experiment1

Objective:The objective of this experiment is to compare the unsupervised DLA with a supervised BP ANN approach to learning paired-associate images.Each learning system is trained such that when a handwritten digit image is provided,the system will generate its paired digit image. Material and Methods:This experiment uses a dataset of paired MNIST handwritten digits as the learning domain.The experiment is repeated four times with different training sets,validation sets and test sets.Each of these datasets contains1,000paired-associate examples that are randomly selected from the MNIST dataset.

A deep learning architecture of RBMs is used to develop an unsupervised learning model for the problem.The architecture is in accord with Figure4.Each channel network is composed of two RBM layers,each of which contains500hidden neurons.Hidden layers1and1’and then layers 2and2’will develop more abstract features of the original images[8].The associative top layer contains1,000neurons.The unsupervised DLA uses back-?tting to?ne-tune the weights of the associative top layer after the CD algorithm training is?nished.

When training the DLAs,the training process of each sensory channel stops when the maximum iteration of60is reached,and the associative memory network is trained to100iterations.Validation sets are used to monitor the back-?tting to avoid over-?tting.The odd digit part of a test example is used to test the reconstruction of its corresponding even digit image,and vice versa.

1→22→13→44→35→66→57→88→79→00→9Avg DLA95.2595.8882.6394.6392.3888.7590.579.7591.639390.74 BP ANNs98.072.583.7595.1390.3882.8891.1382.8889.092.588.82

Table1:Accuracy of test set reconstruction(%)

Figure7:Examples of reconstruction results with the DLA and BP ANNs

We developed two BP networks to learn the same paired-associate mapping.One network is trained to map odd digit images to even digits,the other vice versa.Both BP networks use the architecture shown in Figure6.The BP networks use the same training set,validation set and testing set as the DLA.The validation set is used to prevent the BP algorithm from over-?tting to the training set. The accuracy of reconstruction is measured by testing the output images using Hinton’s DLA hand-written digits classi?cation software.This software is known to classify MNIST dataset of handwrit-ten digits with only1.15%errors[8].One can pass the input images and the reconstructed images through Hinton’s classi?er to determine their digit category.The accuracy of the models is then based on the number of correctly paired images.

Results and Discussion:Using Hinton’s software,the reconstruction accuracy was checked on the testing set.The average results of four replications of the experiments are shown in Table1.On average,the unsupervised DLA(model1)generated images that were90.74%accurate,and the BP ANNs(model2)generated images that were88.82%accurate.One can see that the two models did equally well.This suggests that the unsupervised DLA models are able to achieve the same level of accuracy as the supervised BP approach.

Figure7shows examples of reconstructed images produced by the DLAs and the BP ANNs.One can see that the images generated by the DLAs are clearer than those generated by the BP ANNs. We suspect this because the DLA models are able to better differentiate features from noise.This will be investigated further in the next experiment.

4.2Experiment2

Objective:The objective of this experiment is to develop auto-associative models that can over-come noise injected into synthetic training examples.An unsupervised DLA with back-?tting and supervised BP ANNs will be developed from a noisy dataset,and the quality of their regenerated images will be compared.

Material and Methods:This experiment uses a synthetic dataset that contains?ve different sets of10x5paired images from Figure8.10%random noise was added to each template image to produce60instances of each category,or300in total.The?rst100of these images are used as a training set,the next100are used as a validation set,while the remaining100are used as a test set.

Figure8:Templates of the synthetic dataset

A DLA architecture,in accord with the previous experiment,is used to develop an unsupervised learning model.Each of the sensory channel layers contains50hidden neurons,and the associative

1→22→13→44→35→66→57→88→79→00→9Avg DLA0.0120.0710.0460.0290.0040.010.0080.00.040.0150.032 BP ANNs0.1620.2160.2090.0810.060.1150.1350.1650.110.1060.144

Table2:RMSE of test set reconstruction(out of1)

Figure9:Examples of reconstruction results with DLA and BP ANNs

top layer contains100neurons.The training process of the sensory channel networks stops when the maximum iteration of60is reached;the associative memory network trains for100iterations. As in Experiment1,two BP networks were developed to learn the same paired-associate mapping. Both BP networks used an architecture similar to that shown in Figure6with50neurons in layers 1and3and100neurons in layer2.The BP networks uses the same training set,validation set and test set as the DLA.

The accuracy of reconstruction was measured by comparing the similarity between the generated images and their corresponding template images for a set of test examples.The RMSE between the pixels of each reconstructed image and its corresponding template(without noise)was computed to give an average error over all examples(image pixels are normalized to the range[0,1]).

Results and Discussion:The RMSE of the reconstructed images for the test set is shown in Table2. The DLA with back-?tting out-performs the BP networks in generating the images in the presence of noise.Figure9shows examples of reconstructed images from the DLA and the BP ANNs.The generated images from the DLA are quite similar to the template images of Figure8,while there is signi?cant noise on the generated images from the BP network.DLAs attempt to probabilistically differentiate features from noises,whereas BP ANNs attempt to map input pixels to output pixels. Features are formed in BP networks,but they are for the purpose of mapping and not reconstruction of the original images.Hence a DLA is a better choice if the objective is to construct a noiseless category example as a form of classi?cation.

4.3Experiment3

Objective:The preceeding experiments used paired-associate examples to develop neural network models,however,sensory data does not always come in pairs in real life.The objective of this experiment,in accord with Section3.2,is to develop an associative learning system with both paired associative examples and independent non-paired examples.The experiment is designed to test if the performance of an associative learning system can be improved by separately training the sensory channels with non-paired examples.

Material and Methods:

This experiment uses the database of MNIST examples as in Experiment1.The experiment is repeated four times with different training sets,validation sets and test sets.For each repetition,four models are built using the same architecture but with different amounts of training examples.The ?rst model is built with100paired-associate examples.The second model is built with100paired-associate examples,and100non-paired examples of even digit images.The third model is built with 100paired-associate examples,100non-paired examples of even digit images,and100non-paired examples of odd digit images.The last model is built with200paired-associate examples.Figure10

Figure10:The number of examples

used to train four models

shows the number of paired and non-paired examples in each training set.All the odd digits images are used to train the odd channel and all the even digits are used to train the even channel,but only the paired-associate examples are used to develop the associative memory.

The four models use the same3-layered architecture,parameters,validation sets and test sets as in Experiment1.While doing back-?tting,validation sets are used to monitor over?tting.Test sets are used to examine the associative learning performance of the learning system.The odd digits are used to test the recall of even digits,and vice versa.The recalled images are classi?ed by Hinton’s classi?er to examine the accuracy of the models.

Results and Discussion:

The performance(averaged over four repetitions)of the four models at recalling even digits from odd digits,odd digits from even digits,and the average of them are shown in Figure11;the error bars represent the95%con?dence over the repeated studies.The mean accuracy increases marginally (the error bars show that the improvements are not signi?cant)from model1to model3,which means that using non-paired examples to better develop one of the channels representation may im-prove the overall performance of an associative learning system.We conjecture that this is because both the recognition weights and the generative weights of this channel are optimized.Improving the recognition weight performance of the odd digits channel will provide better features to the asso-ciative memory network to generate the corresponding even digits.Better generative weights for the odd digits channel will generate more accurate odd digits when even digits are provided.In general, this result suggests that improving one of the sensory channel networks of a multi-channel learning system which contains more than two channels will improve any recall that involves that channel. It is also important to note that the reconstruction accuracy clearly increases from model3to model 4.This demonstrates that using more paired-associate examples to develop the associative memory network can improve the performance of the system over the equivalent number of non-paired ex-amples.In a system with three or more channels we conjecture that paired-associate examples for any two channels will be of bene?t to the entire associative memory network.

5Conclusion

This paper presents recent work on an unsupervised multi-modal learning system that can develop an associative memory structure that combines two input/output channels.Our long-term goal is to develop learning systems that are able to learn conceptual representations from multiple sensory input and/or motor output modalities in a manner similar to humans.

We have demonstrated an unsupervised deep learning architecture(DLA)that can reconstruct an image of a MNIST handwritten digit from another paired handwritten digit.The system develops

a kind of supervised classi?cation model meant to simulate aspects of human associative mem-

ory.The DLA is formed with stacked Restricted Boltzmann Machines(RBM)and trained with the Contrastive Divergence(CD)algorithm.The RBM associative memory network that ties the input/output channels together requires re?nement using a back-?tting technique to increase the recall accuracy when only50%of its visible neurons are available from one channel.Experimenta-tion shows quantitatively(using an independent classi?cation method)and qualitatively(by viewing the generated images)that the system develops models that are able to reconstruct accurate paired images as compared to supervised back-propagation network models and have the advantage of unsupervised learning from either paired or non-paired training examples.

In future work,different types of sensory data will be used to train the multi-modal learning sys-tem,such as audio signals.Furthermore,we are interested in knowledge transfer in DLAs using unsupervised methods for learning new tasks and new modalities.

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运用线性规划对运输问题研究

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星巴克咖啡连锁店客户满意度测评报告

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2、测评指标设定 该模型主要由6种变量组成,即顾客期望、顾客对质量的感知、顾客对价值的感知、顾客满意度、顾客抱怨、顾客忠诚。其中,顾客期望、顾客对质量的感知、顾客对价值的感知决定着顾客满意程度,是系统的输入变量;顾客满意度、顾客抱怨、顾客忠诚是结果变量。 顾客满意度指数测评指标体系分为四个层次: 第一层次:总的测评目标“顾客满意度指数”,为一级指标; 第二层次:顾客满意度指数模型中的6大要素,如下所示; 顾客对星巴克的期望 顾客对星巴克质量的感知 顾客对星巴克价值的感知 顾客对星巴克的满意度 顾客对星巴克的抱怨 顾客对星巴克的忠诚 为二级指标 第三层次:由二级指标具体展开而得到的指标,为三级指标; 第四层次:三级指标具体展开为问卷上的问题,形成四级指标。 测评体系中的一级和二级指标适用于所有的产品和服务,实际上我们要研究的是三级和四级指标。见下表: 顾客满意度指数测评的二、三级指标

Mosaicing of acoustic camera images

Mosaicing of acoustic camera images K.Kim,N.Neretti and N.Intrator Abstract:An algorithm for image registration and mosaicing on underwater sonar image sequences characterised by a high noise level,inhomogeneous illumination and low frame rate is presented.Imaging geometry of acoustic cameras is signi?cantly different from that of pinhole cameras.For a planar surface viewed through a pinhole camera undergoing translational and rotational motion,registration can be obtained via a projective transformation.For an acoustic camera,it is shown that,under the same conditions,an af?ne transformation is a good approximation.A novel image fusion method,which maximises the signal-to-noise ratio of the mosaic image is proposed.The full procedure includes illumination correction,feature based transformation estimation,and image fusion for mosaicing. 1Introduction The acquisition of underwater images is performed in noisy environments with low visibility.For optical images in those environments,often natural light is not available, and even if arti?cial light is applied,the visible range is limited. For this reason,sonar systems are widely used to obtain images of seabed or other underwater objects. An acoustic camera is a novel device that can produce a real time underwater image sequence.Detailed imaging methods of acoustic cameras can be found in[1].Acoustic cameras provide extremely high resolution(for a sonar)and rapid refresh rates[1].Despite those merits of acoustic cameras over other sonar systems,it still has shortcomings compared to normal optical cameras: (i)Limitation of sight range:Unlike optical cameras which have a2-D array of photosensors,acoustic cameras have a 1-D transducer array.2-D representation is obtained from the temporal sequence of the transducer array.For this reason,it can collect information from a limited range. (ii)Low signal-to-noise ratio(SNR):The size of the transducers is comparable to the wavelength of ultrasonic waves,so the intensity of a pixel depends not only on the amplitude,but also on the phase difference of the re?ected signal.This is the reason for the Rician distribution of the ultrasound image noise.In addition,there is often a background ultrasound noise in underwater environments. It follows that the SNR is signi?cantly lower than in optical images. (iii)Low resolution with respect to optical images:owing to the limitation in the transducer size,the number of transducers that can be packed in an array is physically restricted,and so is the number of pixels in the horizontal axis.For example,a mine reacquisition and identi?cation sonar(MIRIS)has64transducers[1].(iv)Inhomogeneous insoni?cation:The unique geometry of an acoustic camera requires the sonar device to be aligned parallel to the surface of interest,so that the whole surface falls within the vertical?eld of view of the acoustic camera [1].This alignment is not always trivial,and the misalign-ment often makes dark areas in acoustic camera images. The above limitations can be addressed by image mosai-cing,which is broadly used to build a wider view image [2–4],or to estimate the motion of a vehicle[5,6].For ordinary images,mosaicing is also used for image enhancement such as denoising,deblurring,or super-resolution[7,8]. There has been extensive research on image mosaicing, and its applications[9–13].However,standard methods for image registration[14,15]are not directly applicable to acoustic camera images,because of the discrepancy of image quality,inhomogeneous insoni?cation pro?le,and different geometry.Marks et al.have described a mosaicing algorithm of the ocean?oor taken with an optical camera [2].Rzhanov et al.have also described a mosaicing algorithm of underwater optical images resulting in high resolution seabed maps[3].Both of them deal with a similar problem of illumination,but use different methods:image matching by edge detection and Fourier based matching, which are not directly related to our work.In addition,since their mosaicing algorithms are not intended for image quality enhancement,we need to come up with a different mosaicing algorithm. In this paper,we describe a mosaicing algorithm for a sequence of acoustic camera images.We show that an af?ne transformation is appropriate for images taken from an acoustic camera undergoing translational and rotational motion.We propose a method to register acoustic camera images from a video sequence using a feature matching algorithm.Based on the parameters of image registration,a mosaic image is built.During the mosaicing,the image quality is enhanced in terms of SNR and resolution. 2Properties of acoustic camera images Sonar image acquisition includes several steps,insoni?ca-tion,scattering,and detection of the returning signal.In this Section,we describe physical aspects of images acquired from acoustic lens sonar systems,or acoustic cameras. q IEE,2005 IEE Proceedings online no.20045015 doi:10.1049/ip-rsn:20045015 The authors are with the Institute for Brain and Neural Systems,Brown University,Box1843Providence RI02912,USA E-mail:kio@https://www.wendangku.net/doc/7917294886.html, Paper?rst received21st May2004and in revised form22nd April2005

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