An effcent method to buld panoramc mage mosacs Pattern Recognton Letters vol. 4 003 Dae-Hyun Km Yong-In Yoon Jong-Soo Cho School of Electrcal Engneerng and Computer Scence Kyungpook Natonal Unv.
Abstract Proposed method Computng the projectve transformaton n overlapped areas of the two gven mages Usng four seed ponts /
Introducton A number of methods to buld mage mosacs Recordng an mage onto a long flmstrp Usng the panorama camera Usng lens wth a very large feld of vew Fsh-eye lens and parabolc mrror Image mosackng algorthm Takng regular pctures n order to cover the whole vewng space Algnng these mages and puttng them together Conventonal algorthm Usng geometrcal feature ponts and optmzaton to compute the projectve transformaton Consumng very tme because of the teratve computaton 3 /
Proposed method Usng four pars of seed ponts to compute the projectve transform n the overlapped areas of two mages Seed pont s the hghest textured pxel n the overlapped area of the reference mage Detecton of the correspondng pont Usng block matchng algorthm(bma) n the overlapped area of the target mage For more robust detecton of the correspondng pont Hstogram equalzaton of two overlapped mages before selectng seed ponts Addng the weght functon to BMA n order to mnmze mage dstorton due to camera rotaton 4 /
Flow chart Fg. 1. The proposed algorthm. 5 /
Computaton of the projectve transform Extractng overlapped areas Usng phase correlaton Algnng two mages that are shfted relatvely to each other Takng D fourer transform of each mage Computng the phase dfference at each frequency Takng the nverse fourer transform Estmatng overlapped areas of two gven mages Selectng seed ponts Dvdng overlapped area of the reference mage nto four subareas Central pxel of the block that kept the maxmum varance n each subarea 6 /
q σ = k j arg max = G MAX g g k [ ] σ h g k 0 3 M k j (1) σ k where varance M k mean value of the kth block n th subarea h g hstogram of the graylevel g maxmum graylevel G MAX Detectng correspondng ponts Hstogram equalzaton Comparng equally wthout the bas generatng due to the dfference of the perceved contrast and brghtness 7 /
8 / / More robust detecton of correspondences Usng weghted BMA Consderaton of the dstorton generatng due to camera rotaton j B j y x y x w k d j y d x I k j y x I d d E ) ( 1) ( ) ( ) ( + + + + + + + = () (3) 7 7 / j d j D d w j j j + = = where graylevel of (xy) n the kth mage dsplacement of the block dstance from the center of block maxmum dstance ) ( k y x I x d y d j d D
Buldng the panoramc mage mosac Transformaton of cylndrcal coordnates Estmatng the focal length from the projectve transformaton By Szelsk and Shum (1997) Warpng each perspectve mage nto cylndrcal coordnates World coordnate P( X Y Z) D cylndrcal screen coordnate ( θ v) θ = tan 1 where θ pannng angle v scanlne ( X / Z) v = Y / X + Z (4) 9 /
Because only the screen coordnate p( x y) Eq. (4) transform of screen coordnates Camera equatons x = f X / Y y = f Y / Z θ = tan 1 ( x / f ) v = y / x + f (5) Blendng warped mages Estmatng the dsplacement between two cylndrcally warped mages Usng phase correlaton Constructng panoramc mage Usng blnear weghtng functon 10 /
Expermentaton Expermental condtons and test mages 15x15 block and 3x3 search range (a) (b) (c) Fg.. Expermental mage pars. (d) 11 /
Estmatng PSNR n order to evaluate the performance of the proposed algorthm Accuracy of projectve transform Comparson by hstogram equalzaton Table 1. Means and standard devaton before and after hstogram equalzaton. 1 /
(a) (b) Fg. 3. Two overlapped mages and ther hstograms before (a) and after (b) hstogram equalzaton. 13 /
PSNR of the reconstructed mage mosacs before and after hstogram equalzaton Fg. 4. Performance comparson before and after hstogram equalzaton. 14 /
Weghted block matchng algorthm Fg. 5. Performance comparson between typcal BMA and weghted BMA. 15 /
Ghost mage phenomenon (a) Fg. 6. Mosackng mages of Fg. (b): (a) typcal BMA and (b) weghted BMA. (b) 16 /
Comparson by the mage mosackng algorthms Imad s algorthm Usng the geometrcal feature ponts Szelsk s algorthm Usng the Levenberg-Marquardt teratve optmzaton Proposed algorthm 17 /
(a) (b) (c) (d) Fg. 7. Mosackng mages: (a) mage mosac by human anmator (b) mage mosac by Szelsk s algorthm (c) mage mosac by the proposed algorthm and (d) PSNR of each algorthm. 18 /
Further analyss of the proposed algorthm Mnmzng the sum of the squared ntensty error E = [ ] ' ' I'( x y ) I( x y ) = where I( x y ) reference mage ' ' ' I ( x y ) warped target mage e ' (5) Table. Features for computng the projectve transform. Table 3. Quanttatve comparson between Szelsk s algorthm and the proposed algorthm. 19 /
(a) (b) Fg. 8. Reconstructed mage mosacs 0 /
(a) (b) Fg. 9. Reconstructed cylndrcal mage mosacs. 1 /
Concluson Proposed method An effcent method to buld a panoramc mage mosac wth four seed ponts Selecton of the hghest textured pxel n each subarea Detecton of correspondence Usng the weghted BMA n order to mnmze mage dstorton Future work Vew nterpolaton Generatng the novel scene n accordance wth the movement of the vewer s gazng drecton /