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Contents
1. RGB x 1 RGB x 0RGB x 3 with a being a coefficient controlling the contribution of the basis views which may be set to the same a to be defined elsewhere Forward warping can preserve well texture details and it can easily be implemented in hardware making real time rendering easier FIG 3 shows an inter mediate image obtained after forward warping Backward Searching and Propagation 0035 In the initial virtual view given by forward warp ing it is not uncommon to see many uncovered pixels which may be denoted as black holes These black holes are due to incomplete disparity map such as occlusions For each black hole pixel one may check its neighbor for a pixel that has been assigned a color value from the initial synthe sis The disparity of that pixel is then used for backward search on the images Unlike other similar disparity or depth searching algorithms that do exhaustive search on the entire disparity space the preferred system searches within a limited range within the disparity of the valid neighbors those with assigned color The search objective function is defined as F d A Disteotor Pan P 1 Distaisp dns d 4 min deldn A dn A where d is the disparity of a valid neighbor pixel and Pan is its color p p p2 are colors from two basis views corresponding to d Disty and Distsoior are two distance functions defined on disparity and color and is a w2. By applying the following homography transform to each of the projection matrices P PH 6 where I R 0 H HcHp He gt A o if foa one converts the cameras to canonical form as Pi KRU C1 Ki 710 nfo D wit P K2R3 1 Cy Cy Ri C2 C1 i e the first camera s center is the origin and camera 2 is related to camera 1 by rotation R and translation C 0041 One can specify the virtual view based on the canonical form Suppose the camera matrix for the virtual view is Po Ko Rolt Co 8 0042 One can use a to parameterize the path between basis views 1 and 2 Equation 8 then becomes Po Ko Ro L Co 9 For the camera intrinsic matrix the gradual change from view 1 to view 2 may be viewed as camera 1 changing its focus and principal points gradually to those of camera 2 if the two cameras are identical then this will not have any effect as desired Thus one may interpolate the intrinsic matrix and obtain K as Ko a 1 a K aK5 10 0043 For R a suppose Rr stil a1 where r s and t represent the x axis y axis and z axis respectively One may construct Ro a ro so to a as follows io 1 t a 1 a t at S 1 s 453 Fo Q s xto s xto So lo x7o 12 0044 The first step in equation 12 constructs the new z axis as the interpolation of two original z axes Then one interpolates a temporary y axis as s
3. Color segmentation and disparity maps of the monkey scene and the snoopy scene Top row original images Center row color based segmentation results shown as pseudo colors Botton row computed disparity maps FIG 2 Patent Application Publication May 17 2007 Sheet 3 of 7 US 2007 0109300 A1 Virtual view after forward warping with original two basis views on the top row FIG 3 A complete virtual view after the entire process FIG 4 Patent Application Publication May 17 2007 Sheet 4 of 7 US 2007 0109300 A1 Free Viewpoint TV VOL MENU CH VOL Virtual view specification A mockup illustrating the main idea FIG 5 Patent Application Publication May 17 2007 Sheet 5 of 7 US 2007 0109300 A1 Scene object Basis view 2 Virtual view Basis view 1 The virtual view as a function of the basis views through two parameters a and y which can be controlled by the left right and up down arrows of FIG 5 respectively FIG 6 US 2007 0109300 A1 Patent Application Publication May 17 2007 Sheet 6 of 7 tions t moving path i iven camera posi Simulated virtual viewpo The dots are the g FIG 7 Patent Application Publication May 17 2007 Sheet 7 of 7 US 2007 0109300 A1 Basis View 1 Viewpoint 30 Viewpoint 90 Basis View 2 Synthesized and basis views Left column monkey scene Right column Snoopy scene FIG 8 US 2007 0109300 Al VIRTUAL VIEW SPECIFICATI
4. R and t can be viewed as the relative rotation and translation matrix of camera 2 relative to 1 Now one has P K 1 0 Po K R t 19 and thus the corresponding fundamental matrices can be recovered This approach proved to be effective with mul tiple sets of data even if one has only an estimate in equation 16 without knowing the actual camera internal matrices 0051 Although it seems that one is going back to the calibrated case by estimating the essential matrix the scheme is totally different from true full calibration This is because one cannot expect to use the approximation of equation 16 for estimating the true rotation and translation that are needed for specifying the virtual as in the calibrated case However it is reasonable to use the approximation in the interpolation scheme as illustrated by equations 12 and 13 0052 A showing a simulated free viewpoint moving path by using data is shown in FIG 7 as paths with viewpoint moving from camera 67 to 74 over a parabola and continu ing to camera 80 following a piecewise linear curve As an example FIG 8 left shows the two basis views and three examples of synthesized images The results are shown in FIG 8 right The preferred approach is capable of purely working from uncalibrated views without using any pre calibration information rendering it as a viable approach for practical FTV 0053 The terms and expressions which have been employed in the for
5. view synthesis 0004 The essence of virtual view synthesis includes given a set of images or video acquired from different viewpoints to construct a new image that appears to be acquired from a different viewpoint This multiple image modification is also sometimes referred to as image based rendering IBR 0005 In the FTV application it is unlikely that the camera calibration information is likely to be available e g imagine shooting a movie with multiple cameras which need to be calibrated each time they are moved This renders IBR methods requiring full camera calibration generally inappli cable in most cases Moreover before virtual view synthesis the virtual view should to be specified Existing IBR tech niques use a variety of way to achieve this For example the virtual view specification may be straightforward when the entire setup is fully calibrated For example the virtual view specification may be based on the user s manual picking of some points including the projection of the virtual camera center None of these approaches is readily applicable to the FTV application with uncalibrated cameras where an ordi nary user needs an intuitive way of specifying some desired virtual viewpoints 0006 What is desirable is a framework for the rendering problem in FTV based on IBR The approach preferably includes multiple images from uncalibrated cameras as the input Further while a virtual view is synthesized mai
6. FIG 1 Instead of using the sum of squared difference SSD or sum of absolute difference SAD criteria as matching scores it is simpler to count the number of corresponding pixel pairs whose relative difference with respect to the absolute value is less than 0 2 i e R R3 R lt 0 2 similar for G and B and this number normalized by the number of pixels in the segment is used as the matching score denoted m d for any possible d and for j th segment in basis image i This measure was found to be robust to lighting condition 0031 In addition to using the matching score from the other basis image one may incorporate all the auxiliary images by computing the final matching score for a segment S in basis image i denoted as S with disparity d as m d max my d 1 where m d is the matching score of segment S in any other basis or auxiliary camera k Note that the d is for the basis views and searching in other auxiliary views is equivalent to checking which d is able to give arise to the most color consistency among the views whose relation is given in FIG 1 0032 Furthermore instead of deciding on a single d based on the above matching score one may use that score in the following iterative optimization procedure The basic technique is to update the matching score of each color segment based on its neighboring segments of similar color in order to enforce disparity smoothness Sid my d 2 Ho d
7. Note that s may not be perpendicular to the new z axis But with it one can May 17 2007 construct a new x axis r a with the new z axis and a temporary y axis Finally one constructs the new y axis as the cross product of the new z axis and x axis 0045 Finally one can construct the new camera center using linear interpolation Co 1 a C aC 13 0046 From equation 13 the new camera center is on the line connecting the two camera centers resulting in degeneracy for the epipolar constraint and thus one should not use it for virtual view synthesis see FIG 1 It is desirable to maintain the benefits derived from the constraint and thus want to avoid the degeneracy so that the funda mental matrix based method is still applicable Thus one should move the path away from the exact line between the two views This can be achieved by increasing slightly the y components of the virtual camera center computed from equation 13 In implementation by increasing decreasing the y component one can further achieve the effect of changing the viewpoint perpendicular to the first direction Suppose that C a x y z one gets a new C a as CU YY Zyl 0047 This entire process is illustrated in FIG 6 With the interpolated Po the corresponding fundamental matrices can be calculated and then used for virtual view synthesis Viewpoint Interpolation with Uncalibrated Image Capture 0048 Now the uncalibrated ca
8. ON AND SYNTHESIS IN FREE VIEWPOINT CROSS REFERENCE TO RELATED APPLICATIONS 0001 This application claims the benefit of Ser No 60 737 076 filed Nov 15 2005 BACKGROUND OF THE INVENTION 0002 The present invention relatives to determining a virtual viewpoint 0003 Television is likely the most important visual infor mation system in past decades and it has indeed become a commodity of modem human life With a conventional TV the viewer s viewpoint for a particular video is determined and fixed by that of the acquisition camera Recently a new technology has emerged free viewpoint television FTV which promises to bring a revolution to TV viewing The premise of FTV is to provide the viewer the freedom of choosing his her own viewpoint for watching the video by providing multiple video streams captured by a set of cameras In addition to home entertainment the FTV con cept can also be used in other related domains such as gaming and education The user chosen viewpoint s does not need to coincide with those of the acquisition cameras Accordingly the FTV is not merely a simple view change by switching cameras as possible with some DVD for a couple of preset views The FTV technology requires a whole spectrum of technologies ranging from acquisition hard ware coding technology bandwidth management tech niques standardization for interoperability etc One of the particular technologies to implement FTV is virtual
9. US 20070109300A1 a2 Patent Application Publication co Pub No US 2007 0109300 A1 as United States Li 43 Pub Date May 17 2007 54 VIRTUAL VIEW SPECIFICATION AND SYNTHESIS IN FREE VIEWPOINT 75 Inventor Baoxin Li Chandler AZ US Correspondence Address KEVIN L RUSSELL CHERNOFF VILHAUER MCCLUNG amp STENZEL LLP 1600 ODSTOWER 601 SW SECOND AVENUE PORTLAND OR 97204 US 73 Assignee Sharp Laboratories of America Inc Camas WA US 21 Appl No 11 462 327 22 Filed Aug 3 2006 Related U S Application Data 60 Provisional application No 60 737 076 filed on Nov 15 2005 Publication Classification 51 Int Cl G06T 15 20 2006 01 52 U S Chis strates crt on 345 427 345 419 57 ABSTRACT A system that receives a first video stream of a scene having a first viewpoint and a second video stream having a second viewpoint wherein camera calibration between the first viewpoint and the second viewpoint is unknown A viewer selects a viewer viewpoint generally between the first view point and the second viewpoint and the system synthesizes the viewer viewpoint based upon the first video stream and the second video stream Auxiliary camera k Basis camera 2 Patent Application Publication May 17 2007 Sheet 1 of 7 US 2007 0109300 A1 Auxiliary camera k Basis camera 2 Basis camera 1 Patent Application Publication May 17 2007 Sheet 2 of 7 US 2007 0109300 A1
10. after the previous steps this search is quite fast in practice 0037 It should be noted that there is no guarantee that all pixels can be covered by the above procedure For example the problem may be caused by a few isolated noisy pixels or maybe the scene is not covered by all the cameras A linear interpolation can handle the former situation while the latter situation can be alleviated by constraining the free viewpoint range which is already part of the preferred assumption i e the virtual view is always between two views and the cameras are strategically positioned Viewpoint Specification 0038 A complete virtual view obtained by following the preferred entire process is shown in FIG 4 An intuitive way for virtual view specification based on only uncalibrated views is desirable Essentially the technique provides a viewer with the capability of varying a virtual view gradu ally between any two chosen views The virtual view can thus be determined by for example conveniently pushing a button or similar until the desired viewpoint is shown similar to controlling color or contrast of a TV picture via a remote control button similarly a joystick on remote or a game console can be used for implementation 0039 A viewpoint can be specified by a translation vector and a rotation matrix with respect to any given view to determine its position and direction But it is unrealistic to ask a TV viewer to do this A
11. aid second viewpoint 6 The method of claim 1 further comprising forward warping from said first and second viewpoints to a virtual view based upon a disparity map 7 The method of claim 6 further comprising using a backward search based upon a third viewpoint to find a dominant and disparity consistent color 8 The method of claim 1 wherein a relationship between said first and second viewpoints is determined based upon a feature detector and a random sample consensus 9 The method of claim 6 wherein said disparity is based upon an epipolar constraint 10 The method of claim 1 wherein said viewer viewpoint is specified by a translation and a rotation 11 The method of claim 1 wherein said viewer viewpoint is selectable by the user from a plurality of potential view points
12. efer to the two user chosen views as the basis images The basis images are dynamically selected based on the user s choice and not specifically based upon specially positioned cameras 0022 The particular preferred approach to virtual view synthesis consists of the following steps 0023 1 Pair wise weak calibration of all views to sup port potentially any pair that a viewer may choose The calibration may exclude some views especially if one view is generally between a pair of other view 0024 2 Color segmentation based correspondence between the two basis views where other views are taken into consideration if desired 0025 3 Forward warping from basis views to the virtual view with a disparity map 0026 4 For unfilled pixels use an algorithm to do backward search on auxiliary views to find a dominant and disparity consistent color Virtual View Syntheses Via Weak Calibration 0027 The system may be based upon using n cameras in the system The basis views may be denoted as basis camera 1 and basis camera 2 The remaining views may be denotes as auxiliary cameras 3 to n Fundamental matrices between the basis and the auxiliary cameras are calculated with feature detector and the random sample consensus i e RANSAC algorithm denoted as F 3 F53 Fins Fon The fundamental matrix between the basis cameras is F Com putation of fundamental matrices need only be done once unless the cameras are moved The f
13. egoing specification are used therein as terms of description and not of limitation and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof it being recognized that the scope of the invention is defined and limited only by the claims which follow May 17 2007 1 A method for synthesizing a viewpoint comprising a receiving a first video stream of a scene having a first viewpoint b receiving a second video stream having a second viewpoint wherein camera calibration between said first viewpoint and said second viewpoint is unknown c a viewer selecting a viewer viewpoint generally between said first viewpoint and said second viewpoint and d synthesizing said viewer viewpoint based upon said first video stream and said second video stream 2 The method of claim 1 wherein said viewer selects said first viewpoint and said second viewpoint from a group of three or more video streams each of which has a different viewpoint 3 The method of claim 1 further comprising receiving a third video stream having a third viewpoint and a pair wise calibration is determined for each pair of said first second and third viewpoints 4 The method of claim 3 further comprising selectively excluding said calibration of one of said viewpoints 5 The method of claim 1 further comprising color based segmentation between said first viewpoint and s
14. eight coefficient The combination of the differences of color and the disparity is intended for the smoothness of both texture color and depth In reality F d is set as the minimum one obtained from all the valid neighbor pixels A new disparity will be accepted only when the resulting F d is below a predetermined value If the search fails after all possible d is tested on all valid neighbors the corresponding pixel is left May 17 2007 empty until propagation is reached from other pixels Oth erwise it is assigned a color based on the blending method of equation 3 and is denoted as valid A new search then continues for other black hole pixels 0036 Even after the search and propagation processes there may still be black holes left when the points cannot be seen in both basis cameras To address this the same search and propagation method as described above may be used but with p p i 1 2 This means that one may assume that the pixel may be for example occluded in either or both of views and thus both of them are excluded But one may be able to obtain the information from other views Since there is no information for any preference for any of the auxiliary views a dominant color found from the views is taken to fill the black holes While it may appear to be computationally expensive to search in multiple images if the number of views n is large considering that the number of uncovered pixels is relatively small
15. eras used in a FTV program are located strate US 2007 0109300 Al gically so that the most potentially interesting viewpoint should lie among the given views For the convenience of a viewer this can be simplified to the following the virtual view is defined as one between any two or more user chosen views from the given multiple ones two or more The choice of the two views can be quite intuitive and transparent in practice for example a viewer may feel that view 1 is too far to the left than desired while view 2 is too far to the right than desired then the desired virtual view should be somewhere generally between view 1 and view 2 0020 Thus the system may solve the following two aspects to support the FTV application 1 given the multiple video streams from uncalibrated cameras and any two or more user chosen views synthesize a virtual view gener ally between the two or more views and 2 provide the viewer an intuitive way of specifying the virtual viewpoint in relation to the given available views 0021 As defined above one may have a set of video streams with two that are the closest to the user s desired viewpoint In an uncalibrated system the notion of closest may not be well defined and accordingly the user may select the pair of views It is desirable to make maximum use of the two specified views although other views user selected or not can likewise be used For identification purposes one may r
16. nly from two principal views chosen by a viewer other views may also employed to improve the quality Starting with two May 17 2007 optimal user chosen views also contributes to the reduc tion in the number of required views In addition a technique for specifying the virtual view in uncalibrated cameras is desirable and thus providing a practical solution to view specification in the FTV application without requiring either full camera calibration or complicated user interaction both of which are all impractical for FTV BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 0007 0008 FIG 2 illustrates color segmentation and disparity maps FIG 1 illustrates a camera layout 0009 FIG 3 illustrates virtual view after forward warp ing 0010 FIG 4 virtual view after processing 0011 FIG 5 illustrates an interface 0012 FIG 6 illustrates virtual view as a function of the basis views 0013 FIG 7 illustrates simulated virtual viewpoint 0014 FIG 8 illustrates synthesized and basis views 0015 The foregoing and other objectives features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 0016 The preferred embodiment to the rendering solu tion should not merely involve mathematical rendering techniques bu
17. practical method is to start with a real view and let the viewer move to a desired viewpoint in reference to that view This relative viewpoint moving in an interactive manner is much more convenient for the user Thus the system should permit interpreting continuous vir tual views from one view to another The interpolation can be controlled by a single parameter a When a 0 the basis view 1 is the current view and with a increasing to 1 the viewpoint changes gradually to another view 2 A mockup user interface is illustrated in FIG 5 for an illustration where the left right arrow buttons control the viewpoint change from two underlying basis views and the result is shown immediately on the screen as visual feedback to the viewer The system may also display the two basis views on the screen as well The up down arrow buttons can add variability of the views along a path between the two basis views as explained later US 2007 0109300 Al Viewpoint Interpolation with Calibrated Image Capture 0040 We begin with the calibrated case as it is instruc tive although the ultimate goal is to deal with the uncali brated case The preferred interface is similar to that shown in FIG 5 to support intuitive virtual view specification Suppose one has two camera matrices for the two basis views respectively P K R C P K R3 I C 5 For this case one is typically only concerned with only relative relationship between the two views
18. se is considered i e how we can achieve similar results from only the fundamental matrices Given a fundamental matrix F the correspond ing canonical camera matrices are Pi 1 0 Pa leLFiote v he 14 where e is the epipole on image 2 with F e 0 V can be any 3 vector and is a non zero scalar Note that the reconstructed P is up to a projective transformation Appar ently a randomly chosen v cannot be expected to result in a reasonable virtual view if the fundamental matrix is based on a P defined by such a v It is desirable to obtain the P s from an approximately estimated essential matrix First the essential matrix by a simple approximation scheme is esti mated The essential matrix has the form Ep K FiK 15 0049 For unknown camera matrices K although auto calibration can recover the focal length at the expense of tedious computation it is not a practical option for the FTV application unless the information is obtained at the acqui sition stage As an approximation one sets the parameters of the camera matrix based on the image width w and height h f w h 2 Px W 2 py h 2 16 So K becomes f 0 pK d7 0 f Py DS n 0050 Further one assumes that both cameras have simi lar configuration and use the same K to get the essential US 2007 0109300 Al matrix E An essential matrix can be decomposed into a skew symmetric matrix and rotation matrix as E t hR 18 where
19. t also be modeled in such a manner to reflect a perspective on how the FTV application should configure the entire system including how ideally cameras should be positioned and how a user should interact with the rendering system 0017 In most cases multiple synchronized views of the same scene are captured by a set of fixed but otherwise un calibrated cameras In practice moving cameras pose no theoretical problem if the weak calibration is done for every frame Practically it may be assumed that the cameras are fixed at least for a video shot and thus the weak calibration is needed only for each shot In most cases multiple video streams are available to a viewer The viewer specifies a virtual viewpoint and requests that the system generates a virtual video corresponding to that viewpoint 0018 In a typical IBR approach since no explicit 3D reconstruction and re projection is typically performed in general the same physical point may have a different color in the virtual view than from any of the given views even without considering occlusion The differences among dif ferent views can range from little to dramatic depending on the viewing angles the illumination and reflection models etc Therefore the IBR approach should preferably include a limitation that the virtual views should not be too far from the given views otherwise unrealistic color may entail 0019 With this consideration one may further assume that the cam
20. undamental matrices between the basis and the virtual views are denoted as F and F respectively 0028 With fundamental matrices determined for any point x in camera 1 its corresponding point in camera 2 x is constrained via the fundamental matrix by x F x 0 which can be used to facilitate the search for the disparity d May 17 2007 A third corresponding point in an auxiliary camera k is denoted by x which is determined from x F x 0 and x F x 0 Once the correspondence between x and x is determined a virtual view pixel x can be determined by forward mapping where x satisfies both x F x 0 and x F x 0 These relationships are illustrated in FIG 1 Segmentation Based Correspondence 0029 Even with the epipolar constraint described above it is still desirable to search along an epipolar line for the disparity for a given point x To establish the correspondence between x and x one may first use graph cut based seg mentation to segment each of the basis views For all pixels within each segment one may assume that they have the same disparity i e on the same front parallel plane Over segmentation is favored for more accurate modeling and each segment is limited to be no wider and higher than 15 pixels which is a reasonable value for a traditional NTSC TV frame with pixel resolution of 720x480 0030 Each segment may be warped to another image by the epipolar constraint described above also see
21. y dge d A d A Si do C i E AA dEldmin 4max sg d ae where is the set of neighbor segments with similar color defined by Euclidian color distance under a pre determined threshold B is the inhibition constant set to 2 for compu US 2007 0109300 Al tational simplicity controlling the convergence speed and k the iteration index The system may use the following stopping criteria at any iteration k if for any d S exceeds the threshold the updating process for this segment will stop at next iteration the entire procedure will terminate until it converges i e no segments need to be updated The technique typically converges after 10 iterations and thus we fix the number of iteration to 10 0033 The above procedure is performed for both basis views and the disparity map is further verified by left right consistency check and only those segments with consistent results are used for synthesizing the virtual view thus some segments may not be used resulting in an incomplete disparity map In FIG 2 two examples are shown of the color segmentation results together with the resultant dis parity map Forward Warping 0034 Using the verified disparity map and the two basis views an initial estimate of the virtual view can be synthe sized by forward warping For a pixel x in basis view 1 and x in basis view 2 their corresponding pixel on the virtual view will be x whose color is computed as
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