AN AUTO-ADAPTIVE INFORMATION PRESERVATION FUSION METHOD FOR SAR AND MULTISPECRAL IMAGES

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1 AN AUTO-ADAPTIVE INFORMATION PRESERVATION FUSION METHOD FOR SAR AND MULTISPECRAL IMAGES H. Sun a, B. Pan b, Y. Chen a, *, J. L a, L. Deng a a College of Resources Scence & Technology, Beng Normal Unversty b School of Remote Sensng and Informaton Engneerng, Wuhan Unversty KEY WORDS:Fuson; SAR; Multspectral; Wavelet; Auto-adaptve ABSTRACT: Due to the specal magng model of SAR, ts mage ncludes certan specal nformaton that unavalable n multspectral mage, however, the nterpretaton of SAR mage becomes more dffcult than vsble mage. Therefore, the fuson of SAR mage and multspectral mage s prmarly for the purpose of mang better use of SAR mage nformaton. For ths applcaton, the autoadaptve nformaton preservaton fuson method based on wavelet transform was proposed. It automatcally adusts weght coeffcent accordng to the nformaton capacty of SAR mage and multspectral mage, and then auto-adaptvely adusts fuson result, avodng a sngle and ponderous fuson strategy for the whole mage. Experment results show that the novel method can better preserve SAR mage texture and structure nformaton, especally the unque nformaton n SAR mage, meanwhle, the novel method as well as better preserved spectral nformaton wth good results.. INTRODUCTION The remote sensng mages acqured by varous sngle sensors have some lmtaton and dfferences n the respect of geometry, spectrum and spatal resoluton. Therefore, t s necessary to fuse or synthesze the nformaton contaned n the mages of multple sensors, n order to enhance the dstngushng relablty and nterpretng ablty of the mages (L,200), and prepare for the classfcaton, target recognton and other more latter applcatons. The tradtonal processng method s that fusng multspectral mage of low spatal resoluton and panchromatc mage of hgh spatal resoluton and ganng result mage whch not only owng hgher spatal resoluton n panchromatc mage but also obtanng spectral nformaton n multspectral mage. As respect to applcaton, the processng method mght enhance the nterpretng ablty of orgnal mages. Nevertheless, dfferent applcatons requre the fused mages not ust eclectcally contan the detal nformaton n panchromatc mage and spectral nformaton n multspectral mage. As for the applcaton of feature extracton, mappng, the detal nformaton of mage s normally more mportant; whle as for the applcaton of classfcaton, t s normally more mportant to focusng on the accuracy and ntegraton of the spectral nformaton (Deng,2005). For the fuson of synthesze aperture radar (SAR) mage and multspectral mage, SAR mage s regarded as man body to contan the detal nformaton better, and the multspectral mage s used to supply addtonal spectral nformaton. As the magng mode of SAR s actve n mcrowave band, SAR can penetrate clouds, ran and snow, and bear the ablty of roundthe-cloc worng and n some sense penetratng earth s surface (Woodhouse, 2004). Therefore, SAR sensor mght acqure nformaton that unavalable for vsble or nfrared sensor. However, as the specal magng way SAR has, the nterpretaton of SAR mages s more dffcult than that of the vsble ones, t s necessary to fuse SAR mage and multspectral mage, n order to better utlze the specal nformaton SAR mage contaned. The tradtonal fuson methods ncludng Intensty-Hue- Saturaton (IHS) transform, Prncple Component Analyss (PCA), Wavelet Transform and Hgh Pass Flter (HPF), etc. And more and more advanced methods based on the tradtonal fuson methods have proposed (L,994). Most of these methods focus on preservng spectral nformaton or eclectcally preservng nformaton between spatal detal one and spectral one, whch resultng the fused mage usually only advance the vsble effect. But scarce methods regard panchromatc mage as man body, amng to mantanng as much as detal nformaton. In ths paper, from the aspect of applcaton-orented, we proposed a novel fuson method to synthesze SAR mage and multspectral mage. Ths novel method s based on wavelet transform, whch we called auto-adaptve nformaton preservaton fuson method. It not only better preserves spectral nformaton n multspectral mage, but also, more mportantly, preserves specal detal nformaton n SAR mage. 2. THE TRADITIONAL FUSION METHODS BASED ON WAVELET TRANSFORM An mage can gan (J+) components by J-level twodmenson dscrete wavelet transform, one of whch s lowfrequent approxmate component, noted as L, owng maor energy of orgnal mage, the other J ones, noted as H, H and H respectvely, are hgh-frequent detal 2 * Correspondng author. Emal address: cyh@res.cn 29

2 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng 2008 components concentratng detal nformaton of orgnal mage. We note orgnal mage f(x,y) as L 0, and the -level twodmenson dscrete wavelet decomposton of mage s shown as Fgure.. L H 2 H 2 H H 2 2 H 2 H H on wavelet transform stll eclectcally preserve spatal detal nformaton and spectral nformaton.. AUTO-ADAPTIVE INFORMATION PRESERVATION FUSION METHOD BASED ON WAVELET TRANSFORM The dea of the novel fuson method regards SAR mage as man body, amng to preservng nformaton of SAR mage better, and performng the fuson accordng to auto-adaptve nformaton preservaton. The algorthm s as follows: H 2 H Fgure. The -level two-dmenson dscrete wavelet destructon of mage In tradtonal fuson method based on wavelet transform, detal component of multspectral mage s substtuted by detal component of panchromatc mage n frequent doman, and then fused mage s obtaned by reconstructon of wavelet coeffcents of multspectral mage. Let P, X and F are panchromatc mage, multspectral mage, and fused mage, respectvely. The process of fuson based on wavelet transform s as follows:. After the strctly regstraton and other pre-processng of P and X mages, the J-level two dmenson dscrete wavelet decomposton are done for the two mages respectvely, P and the pyramd mages of P and X mages, noted as and X respectvely, are obtaned, contanng the correspondng low-frequent approxmate components and hgh-frequent detal components. 2. The detal component of X s substtuted by that of, and the low-frequent approxmate component of P X s preserved, then the fused pyramd mage obtaned. Therefore, components of X. F s F s conssts of the detal P and the approxmate component of. The fused mage F s obtaned by reconstructng and reverse transformng the fused pyramd mage F. In tradtonal fuson methods based on wavelet transform, due to the drectly abnegaton of low-frequent approxmate component of panchromatc mage, the detal nformaton of panchromatc mage lose n some sense. Fortunately, many advanced methods have proposed (L,994), but they nearly preserves nformaton of orgnal mages n the applcatonorented aspect. Consequently, tradtonal fuson methods based normal W ( x, y) W mn W ( x, y) = (4) W W max mn. A wndow of n n,(n=,5,,n) s defned. Let S s SAR mage, X s multspectral mage, and =,2,,K represents the band sequence of multspectral mage. The entropes of S and each band of X n the wndow are calculated. = () I H ( x, y) P lnp s s s = 0 = (2) I H ( x, y) P lnp = 0 where, = 0,,,I s dgtal number (DN), (x, y) s poston of the central-pxel n the wndow, P and P are s ratos of the pxel number to the total pxel number n the wndow for SAR mage and multspectral mage, respectvely. 2. The dscrepancy of nformaton capacty between SAR mage and the th band of multspectral mage s expressed by the weght W ( x, y ): W ( x, y) = H ( x, y)/ H ( x, y) () s. The wndow s employed to move and perform the same operaton of -2 steps, and the crculaton does not cease untl the entre mage s covered at all. And a new weght mage of the th band W s accomplshed. Hence, all of K weght mages correspondng to K bands of multspectral mage are ganed. 4. The normalzaton of weght mages s done to lmt W ( x, y ) between 0 and : normal where, W ( x, y ) s the normalzaton of W ( x, y ), W and max mn W (, ) W are the maxmum and mnmum of x y, respectvely. 5. The J-level two dmenson dscrete wavelet decomposton 0

3 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng 2008 are done for SAR mage S, multspectral mage normalzed weght mage S, X and W W normal X and, and pyramd mage, respectvely, are obtaned. 6. In wavelet doman, pyramd mages S and X are fused accordng to the formula below, and fused pyramd mage F n wavelet doman s obtaned. F S (2, x, y) = (2, x, y) + W (2, x, y) X (2, x, y) + W (2, x, y) where, =,2,,J s pyramd level, 2 s resoluton, and F (2, x, y ) s the value of pyramd mage on -level at (x, y) poston. 7. By reconstructng pyramd mage F (5), fused mage of the th band F s ultmately obtaned. Hence, the whole fuson mage s ganed. As we can see from the algorthm, along wth movng of the wndow, the fuson mage mght auto-adaptvely fuse nformaton of the two mages accordng to the nformaton capactes of SAR mage and multspectral mage, more suffcently consderng local features of mages, but not perform the whole mage wth a sngle and ponderous way. 4. EXPERIMENT AND RESULTS ANALYSIS For the sae of valdatng the effectveness of the algorthm, we employed a RADARSAT fne model mage, wth 0m*0m resoluton, as the orgnal SAR mage, and a fused SPOT-5 mage, wth 2.5m*2.5m resoluton, as the orgnal multspectral mage. The fused SPOT-5 mage s syntheszed by the SPOT-5 multspectral bands wth 0m*0m resoluton and the SPOT-5 panchromatc band wth 2.5m*2.5m resoluton. As we all nown, the mage fuson usually occurs wth a hgh resoluton panchromatc mage and a low resoluton multspectral mage, yet t s opposte n ths paper. Both of these strateges are based on the fuson destnaton, n the usual stuaton, we want to acqure an mage holdng hgh spatal resoluton and fne spectral nformaton; whle n the applcaton-orented stuaton we assumed, we would le to gan an mage wth suffcent spectral nformaton and as more SAR nformaton as possble. So the orgnal mages are ust desgned for ths purpose. The comparson s done between the novel fuson method and other tradtonal fuson methods, such as Brovey (Zhang,2006), trangle IHS (Wang,2000), PCA (Ja,998), Gram-Schmdt (L,2004) and wavelet transform method n artcle (L,994). Wth respect to the fuson method based on wavelet transform n ths paper, both the normal wavelet transform method and the novel method adopted the sym4 wavelet base. The wndow sze n the novel method s 7*7 pxels. The mage sze s 024*024 pxels. The orgnal and result mages are shown n Fgure.2, where, Fgure2(a) s RADARSAT mage, Fgure2(b) s multspectral mage, Fgure2(c)-(h) are the fused mage of Brovey, trangle IHS, PCA, Gram-Schmdt, normal wavelet transform, and the novel method, respectvely. (a)sar mage (b) multspectral mage (c) Brovey fused mage (d) trangle IHS fused mage

4 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng 2008 (e) PCA fused mage (f) Gram-Schmdt fused mage (g) normal wavelet transform fused mage (h) the novel method fused mage Fgure. 2 SAR mage, multspectral mage, and the result mages of dfferent fuson methods 4. Subectve Qualtatve Analyss and Comparson As for vsual effect, the normal wavelet transform and the novel method have the advantages of preservng spectral nformaton over the other methods. Brovey s the smplest, but the fuson effect s the worst. Trangle IHS, PCA and Gram-Schmdt eep too much nformaton of SAR mage, but lose most of spectral nformaton. Besdes, the spectral nformaton dstrbutes unevenly, resultng the dstorton of the mage hue. In so far as the normal wavelet transform and the novel method, the former s not as good as the latter n preservng nformaton of the SAR mage, and also wth hue dstorton from the orgnal multspectral mage; whle the novel method not only preserves spectral nformaton better, but also auto-adaptvely eeps the texture feature of the SAR mage, so that the fused mage s smooth and natural n vsual. Furthermore, the novel method has the advantage of preservng nformaton of SAR mage. For nstance, the representatve obect crcled by the red rectangle n In statstcs, mean ˆμ and standard devaton ˆ σ are defned: ˆ σ n ˆ μ = x (6) n = (7) n 2 2 = ( x ˆ μ) n = Fgure(a) s the specal nformaton n the SAR mage, whch preserved fully n the novel method, whle relatvely poor for PCA or normal wavelet transform method. Comprehensvely, concernng vsual effect, the novel method s relatvely the best fuson method. 4.2 Obectve Quanttatve Analyss and Comparson Certan obectve quanttatve analyzng ndexes are utlzed to compare the fuson effects and nformaton preservaton abltes of dfferent fuson methods adopted n ths paper. We employed mean, standard devaton, entropy, correlaton coeffcent, average gradent and spectral dstorton to reflect mage lumnance, spatal detal nformaton preservaton, and spectral nformaton preservaton (Luclen, 997).. Mean and Standard Devaton (S. D.): where, n s the sample sze, x s the value of the th sample. Here, mean s the average gray value of all the pxels n the mage, and for the human vsual system, t s the average lumnance. So f the mean s moderate, the vsual effect s fne. Standard devaton represents the dscrete degree of the pxel value relatve to the mean. The larger the devaton s, the more dspersedly the gray grades dstrbute, and f the probablty of each gray grade equals, the nformaton capacty s the maxmum. 2

5 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng Entropy: The entropy of the mage s an mportant ndex to wegh the nformaton capacty, accordng to the Shannon s nformaton theory prncple, the defnton of entropy s: m H = P ln P (8) = where, M (, ), F(, ) are the pxel value of orgnal mage and fused mage n certan component, respectvely. Correspondngly, M (, ) and F (, ) are the means. M and N are the weght and heght of the mage. 4. Average Gradent (A. G.) Average gradent s able to accurately reflect the fne contrast of the mage. Generally, the larger the average gradent s, the clearer the mage s. where, P s the rato of the pxel number to the total pxel number n the mage, m s the number of grades. Snce the entropy represents the nformaton capacty, the larger the entropy s, the more the mage contans nformaton. Seen from formula (8), we now that the maxmum value of H occurs f and only f each P s equal. It means that the nformaton capacty tends to be the maxmum f each gray grade has almost the same account pxels. By comparson of the entropes of dfferent mages, we can udge the ablty of expressng detal nformaton.. Correlaton Coeffcent (C. C.) The correlaton coeffcent between the orgnal mage and the fused mage shows the smlarty n small sze structures between the orgnal and the fused mages. It should be as close as possble to (Luclen, 997). The correlaton coeffcent ρ s: M N 2 2 G = Δ xf ( x, y) +Δyf ( x, y) (0) MN = = where, Δ xf ( x, y) and Δ yf ( x, y) are the dfference of f ( x, y ) at x and y drecton n certan component, respectvely. 5. Average Spectral Dstorton (A. D.) The defnton of the spectral dstorton n the th component s: D = V V (),, n ρ = M N = = ( M(, ) M(, ))( F(, ) F(, )) M N M N 2 2 ( M(, ) M(, )) ( F(, ) F(, )) = = = = (9) where, V and V are, respectvely, the gray values of,, multspectral mage and fused mage n the th component at poston (,). As the spectral effect s expressed by the syntheszed of the multple bands, we employed the average spectral dstorton, noted as D, to llustrate t. The greater the D s, the larger the dstorton s. Image band Mean S.D. Entropy C.C wth multspectral C.C. wth SAR A.G. A. D Multspec tral SAR Brovey Trangle IHS PCA Gram Schmdt Normal wavelet transform The novel method

6 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng Table. The quanttatve analyss results of dfferent fuson methods Wth respect to mage lumnance, the mean of the novel method range between that of the multspectral mage and the SAR mage, whle the means of the other methods are generally low. Ths shows that the novel method well reflects the orgnal mage lumnance, whch concdes wth the subectve analyss result. In the aspect of spatal detal nformaton preservaton, ) Standard devaton: the trangle IHS result has the closest standard devaton to the orgnal mage, and n the next place the novel method; whle the Brovey method exorbtantly stretched the gray gradent, resultng the mage dstorton; the standard devatons of PCA result, Gram-Schmdt result and normal wavelet transform result are comparatvely smaller, as the dstrbutons of ther gray gradent are too concentrated. 2) Entropy: the entropy of the novel method s larger than that of any other method, and t s only the novel method result that has the larger entropy than the orgnal mage, whch shows that the novel method has the advantage of preservng nformaton over other methods. ) Average gradent: the average gradents of the PCA result, Gram-Schmdt result and normal wavelet transform result are comparatvely smaller, whch shows that these methods are less able to reflect the small detal nformaton; Although the average gradent of Brovey result s the largest, t s larger than that of both the multspectral mage and the SAR mage, not truly reflectng the detal nformaton; The average gradents of the novel method result and trangle IHS result are between that of the multspectral mage and the SAR mage, however, the average gradent of the novel method result s more closed to that of the SAR mage, whch means that the novel method excels trangle IHS n preservng detal nformaton of SAR mage, the very ntenton we desred. In the aspect of spectral nformaton preservaton, ) Correlaton coeffcent: the correlaton coeffcents of Brovey result, trangle IHS result, PCA result wth SAR mage are larger than the ones wth multspectral mage, respectvely, whle the stuatons of normal wavelet transform and the novel method are reverse. Other than Brovey method, trangle IHS and PCA are Component Substtutng methods (Pradhan,2005), therefore the correlaton coeffcents of ther result mages wth SAR mage are naturally larger, so t has no comparablty wth the novel method. As for the novel method and normal wavelet transform, the former has a larger correlaton coeffcent wth SAR mage than the latter, meanng that the novel method s the better one n preservng nformaton of SAR mage. 2) Spectral dstorton: the average spectral dstorton of the novel method result s 65., the smallest one, whle that of other method results are over 20, meanng the novel method lost the least spectral nformaton. Therefore, syntheszed the subect qualtatve and obectve quanttatve analyss, compare wth the tradtonal methods concerned n ths paper, the novel method not only has obvous advantage n preservng detal nformaton of SAR mage, but also s better n spectral nformaton preservaton and dstrbuton. 5. CONCLUSIONS From the applcaton-orented aspect, ths paper proposed autoadaptve nformaton preservaton fuson method based on wavelet transform, amng at better utlzng the nformaton of SAR mage n latter applcaton. It suffcently consders the local feature of the mage, auto-adaptve fused SAR mage and multspectral mage accordng to the local nformaton capacty, but not sngle and ponderous fused regulaton for the whole mage. The expermental result shows that the novel method s better than the tradtonal methods adopted n ths paper. The novel method not only better preserves spectral nformaton n multspectral mage, but also better preserves detal nformaton n SAR mage. Especally, the specal SAR nformaton s contaned n the novel method fused mage. However, the novel method stll has room for mprovement. ACKNOWLEDGMENTS Ths wor was supported by the Foundaton of Fo Yng Tong Educaton Foundaton (07), Natural Scence Foundaton of Chna (40670) and Beng Natural Scence Foundaton (407206). References from Journals: REFERENCES Deng L., Chen Y., L J., et al.,2005. Controllable remote sensng mage fuson method based on transform. Journal Infrared Mllmeter and Waves. 24(0), pp Ja Y.,998. Fuson of Landsat TM and SAR Images Based on Prncpal Component Analyss. Remote Sensng Technology and Applcaton. (0), pp L C., Lu L., Wang J., et al.,2004. Comparson of Two Methods of Fusng Remote Sensng Images wth Fdelty of Spectral Informaton. Journal of Image and Graphcs. 9(), pp Luclen W., Thlerry R. M. M.,997. Fuson of Satellte Images of dfferent spatal resolutons: assessng the qualty of resultng mages[j]. Photogrammetrc Engneerng & Remote Sensng. 6(6), pp Wang R., Q M., Wang H.,2000. Comparatve Study on the Method of IHS Transformaton for Image Fuson. Journal of The Pla Insttute of Surveyng and Mappng. 7(04), pp Zhang Y., Wu Q.,2006. Informaton Influence on QucBrd Images by Brovey Fuson and Wavelet Fuson. Remote Sensng Technology and Applcaton. 2 (0), pp References from Boos: 4

7 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng 2008 L B., Luo J.,200. Wavelet analyss and applcaton. Beng: Publshng House of Electroncs Industry. L H., Manunath B. S., Mtra S. K.,994. Multsensor Image Fuson Usng the Wavelet Transform. Woodhouse I. H.,2004. Introducton to mcrowave remote sensng. Taylor & Francs CRC Press. References from Other Lterature: Pradhan P.,2005. Multresoluton Based, Multsensor, Multspectral Image Fuson. Msssspp State, USA. 5

8 The Internatonal Archves of the Photogrammetry, Remote Sensng and Spatal Informaton Scences. Vol. XXXVII. Part B7. Beng