• Journal of applied mathematics & informatics
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• v.11 no.1_2
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• pp.305-316
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• 2003

Recently, an iterative algorithm for finding the interior eigenvalues of a definite matrix by CG-type method has been proposed. This method compares to the inverse power method. The given matrices A, and B are assumed to be large and sparse, and SPD( Symmetric Positive Definite) The CG scheme for the optimization of the Rayleigh quotient has been proven a very attractive and promising technique for large sparse eigenproblems for smallest eigenvalue. Also, it is very amenable to parallel computations, like the CG method for the linear systems. A proper choice of the preconditioner significantly improves the convergence of the CG scheme. But for parallel computations we need to find an efficient parallel preconditioner. Our candidates we ILU(0) in the wave-front order, ILU(0) in the multi-coloring order, Point-SSOR(Symmetric Successive Overrelaxation), and Multi-Color Block SSOR preconditioner. Wavefront order is a simple way to increase parallelism in the natural order, and Multi-coloring realizes a parallelism of order(N), where N is the order of the matrix. Another choice is the Multi-Color Block SSOR(Symmetric Successive OverRelaxation) preconditioning. Block SSOR is a symmetric preconditioner which is expected to minimize the interprocessor communication due to the blocking. We implemented the results on the CRAY-T3E with 128 nodes. The MPI (Message Passing Interface) library was adopted for the interprocessor communications. The test problem was drawn from the discretizations of partial differential equations by finite difference methods. The results show that for small number of processors Multi-Color ILU(0) has the best performance, while for large number of processors Multi-Color Block SSOR performs the best.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.11
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• pp.1243-1248
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• 2011

The purposes of this study were to develop a dynamic model of a human and to investigate the effect of a walker on an elderly subject's motions, such as sit-to-stand (STS) motion and normal gait, by using this model. A human model consisting of 15 segments and 14 joints was developed, embedded in $RecurDyn^{TM}$, and connected through a Simulink$^{(R)}$ interface with collected motion data. The model was validated by comparisons between joint kinematic results from inverse dynamics (Matlab$^{(R)}$-based in-house program) and from $RecurDyn^{TM}$ simulation during walking. The results indicate that the elderly walker induced a longer movement time in walking, such that the speed of joint flexion/extension was slower than that during a normal gait. The results showed that the muscle activities of parts of the ankle and hamstring were altered by use of the elderly walker. The technique used in this study could be very helpful in applications to biomechanical fields.

• Geophysics and Geophysical Exploration
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• v.10 no.2
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• pp.147-153
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• 2007

The conjugate gradient (CG) method is one of the most efficient algorithms for solving a linear system of equations. In addition to being used as a linear equation solver, it can be applied to a least-squares problem. When the CG method is applied to large-scale three-dimensional inversion of magnetotelluric data, two approaches have been pursued; one is the linear CG inversion in which each step of the Gauss-Newton iteration is incompletely solved using a truncated CG technique, and the other is referred to as the nonlinear CG inversion in which CG is directly applied to the minimization of objective functional for a nonlinear inverse problem. In each procedure we only need to compute the effect of the sensitivity matrix or its transpose multiplying an arbitrary vector, significantly reducing the computational requirements needed to do large-scale inversion.

• Journal of IKEEE
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• v.19 no.2
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• pp.219-224
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• 2015

Electrical impedance tomography is an imaging technique to reconstruct the internal conductivity distribution based on applied currents and measured voltages in a domain of interest. In this paper, a modified Gauss-Newton method is proposed for conductivity image reconstruction. In the proposed method, the dimension of the inverse term is reduced by replacing the number of elements with the number of measurement data in the conductivity updating equation of the conventional Gauss-Newton method. Therefore, the computation time is greatly reduced as compared to the conventional Gauss-Newton method. Moreover, the regularization parameter is selected by computing the minimum-maximum from the diagonal components of the Jacobian matrix at every iteration. The numerical experiments with several scenarios were carried out to evaluate the reconstruction performance of the proposed method.

• Journal of Korea Society of Industrial Information Systems
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• v.13 no.3
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• pp.110-116
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• 2008

In this paper, we proposed an improvement technique of image encryption using binary phase computer generated hologram(BPCGH) and multi exclusive-OR(XOR) operations. For the encryption process, a BPCGH that reconstructs the original image is designed, using an iterative algorithm, and the resulting hologram is regarded as the image to be encrypted. The BPCGH is encrypted through the exclusive-OR operation with the random generated phase key image. Then the encrypted image is divided into several slide images using XOR operations. So, the performance of encryption for the image is improved. For the decryption process, we cascade the encrypted slide images and phase key image and interfere with reference wave. Then decrypted hologram image is transformed into phase information. Finally, the original image is recovered by an inverse Fourier transformation of the phase information. If the slide images are changed, we can get various decrypted BPCGH images. In the proposed security system, without a random generated key image, the original image can not be recovered. And we recover another hologram pattern according to the slide images, so it can be used in the differentiated authorization system.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.11
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• pp.2085-2092
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• 2016

One of the most important properties of the IC substrate is that it should be uniform over large areas. Among the various physical approaches of wafer defect characterization, special attention is to be payed to the infrared techniques of inspection. In particular, a high spatial resolution, near infrared absorption method has been adopted to directly observe defects in semi-insulating GaAs. This technique, which relies on the mapping of infrared transmission, is both rapid and non-destructive. This method demonstrates in a direct way that the infrared images of GaAs crystals arise from defect absorption process. A new interpretation is presented for the observed reversal of contrast in the infrared absorption of nonuniformly distributed deep centers, related to EL2, in semi-insulating GaAs. The low temperature photoquenching experiment has demonstrated in a direct way that the contrast inverse images of GaAs wafers arise from both absorption and scattering mechanisms rather than charge re-distribution or local variation of bandgap.

• Proceedings of the Korea Water Resources Association Conference
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• 2011.05a
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• pp.357-357
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• 2011

폭설은 짧은 시간에 많은 양의 눈이 오는 기상현상으로, 자연재해에 속하며 각종 피해를 유발한다. 우리나라는 주로 산지가 많고 삼면이 바다로 둘러싸인 지형으로 폭설의 발생 및 영향은 국지적으로 차이가 많이난다. 이에 우리나라 폭설의 지역별 위험도를 이해하기 위해서는 체계적인 지역구분과 연구가 필요하다. 본 연구는 우리나라 폭설의 지역별 위험도를 분석하기 위해 지난 39년 겨울철동안(Oct-Apr, 1971-2010) 68개의 지상기상관측소에서 측정한 일별 최심신적설량을 이용하여 우리나라 대설지역을 바탕으로 크게 울릉도, 영동 해안북부(속초와 강릉), 서태백산맥(춘천, 홍천, 양평, 원주 등), 소백산맥북서부(부여, 보은, 대전 등), 남해안 (해남, 완도, 장흥 등)의 폭설지역으로 세분화하였다. 폭설지역을 기상청의 대설특보 기준에 따라 대설주의보(5cm이상)와 대설경보(20cm이상)가 발령됐던 기간으로 빈도분석을 시행하였다. 대설주의보의 발생일수는 총 5,712일로 전체 폭설 발생일의 68%를 차지한다. 이 중 대설경보의 발생일수는 총 1,976일로 전체 폭설 발생일의 23%를 차지한다. 울릉도에서는 대설주의보의 발령일수가 총 602회로 가장 눈이 많은 지역으로 나타났으며, 그 반대로 가장 눈이 적은 지역인 남해안은 3cm이상일 경우에 대설주의보를 발령하며, 이 지역에서의 대설주의보의 발령일수가 지역별 평균 21회였다. 영동북부해안에서는 대설주의보의 발령일수가 지역별 평균 180회, 서태백산맥에서의 대설주의보 발령일수가 지역별 평균 87회, 소백산맥북서부에서의 대설주의보 발령 일수가 지역별 평균 122회로 우리나라의 폭설지역에 속하는 것으로 나타났다. 각 관측소의 39년 겨울철 최심 신적설량을 대설주의보와 대설경보의 빈도수에 따른 지역별 위험 지역을 IDW (Inverse Distance Weighting) 기법을 이용하여 공간 분포도를 작성하였다. 본 연구의 결과는 폭설 위험 지역의 지역별 빈도 분석 결과를 위험 분포도로 도시화하여 매년 반복적으로 발생하는 폭설로 인한 피해 경감 및 사회 경제적 안정에 기여할 수 있을 것으로 사료된다.

• Proceedings of the Korea Water Resources Association Conference
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• 2019.05a
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• pp.101-101
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• 2019

전 지구적 기온상승으로 인한 기후변화는 사회적, 수문학적, 다양한 분야에 영향을 미친다. 또한 IPCC(Intergovernmental Panel on Climate Change)의 보고서에 따르면 미래에도 지속적으로 기온상승이 예상되며, 이러한 현상은 인류의 삶에 큰 영향을 미칠것으로 예상된다. 또한 수자원 및 관련 분야에서도 기온 상승에 따른 강수량, 강수의 주기 변동, 극한 기후사상의 심도(severity)와 빈도 변화에 따른 다양한 연구가 진행되고 있으며, 미래의 강우량과 온도를 예측하는 기후변화연구에서는 다양한 기후모형을 고려하여 분석한다. 하지만 모든 기후모형이 우리나라에 적합한 것은 아니므로 과거 기후를 모의한 결과를 토대로 성능이 뛰어난 모형의 결과에 더 높은 가중치를 주고 미래를 예측하는 연구가 활발히 진행되고 있다. 일반적으로 기후모형으로 GCM (General Circulation Model) 모의 결과가 이용되는데 우리나라에 대한 GCM 결과의 정확성을 분석하는 연구는 부족한 실정이다. 따라서 본 연구에서는 21개의 GCM을 대상으로 과거 모의 자료(1970년~2005년)를 실제 관측소에서 관측된 강수량과 비교하여 각 GCM들의 성능을 평가하고 이를 토대로, GCM들의 우선순위를 선정하였다. 또한 격자 기반 GCM 결과를 IDW (Inverse Distance Weighted) 방법을 사용하여 기상관측소로 지역적 상세화를 수행하였으며, GCM과 관측자료 사이의 편이를 보정하기 위해 6가지의 Quantile Mapping 방법과 Random Forest 기법을 사용하였다. 또한 편이 보정 기법 중 성능이 좋은 기법을 선택하여 관측소에 적용하였다. 편이 보정된 GCM 모의결과에 대한 성능을 토대로 우수한 GCM 순위를 도출하기 위해 다기준의사결정기법 중 하나인 TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution)를 이용하였다. 그리고 GCM의 전망기간인 2010년부터 2018년까지의 Machine learning 방법과 Quantile mapping의 기법을 비교 및 성능이 우수한 편이 보정 방법을 선택한 후 전망기간 동안의 GCM 성능의 우선순위를 선정하였다.

• Journal of the Korean Geophysical Society
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• v.5 no.1
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• pp.63-71
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• 2002

The integral equation method is a powerful tool for electromagnetic numerical modeling. But the difficulty of this technique is the size of their linear equations, which demands excessive memory and calculation time to invert. This limitation of the integral equation method becomes critical in inverse problem. To overcome this limitation, a lot of approximation and series methods, such as conventional Born, modifed Born and extended Born, were developed. But all the methods need volume integration of Green tensor, which is very time consuming. In electromagnetic theory, Green tensor rapidly decreases as the distance between source and field cell increases. Therefore, the source cell which are far away from the field cell does not make an effect on the electric field of the field cell. Consequently, by ignoring the effect of Green tensor due to far away source cells, computing time for electromagnetic numerical modeling can be reduced dramatically. Comparisons of this new method against a full integral equation, extended Born approximation and series code show that the method is accurate enough much less time consuming.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.23 no.1
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• pp.84-89
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• 2019

JPEG is a most widely used standard image compression technology. This research introduces the JPEG image compression algorithm and describes each step in the compression and decompression. Image compression is the application of data compression on digital images. The DCT (discrete cosine transform) is a technique for converting a time domain to a frequency domain. First, the image is divided into 8 by 8 pixel blocks. Second, working from top to bottom left to right, the DCT is applied to each block. Third, each block is compressed through quantization. Fourth, the matrix of compressed blocks that make up the image is stored in a greatly reduced amount of space. Finally if desired, the image is reconstructed through decompression, a process using IDCT (inverse discrete cosine transform). The purpose of this research is to review all the processes of image compression / decompression using the discrete cosine transform method.