Patent Publication Number: US-8531504-B2

Title: System and method for 3D video stabilization by fusing orientation sensor readings and image alignment estimates

Description:
This application claims the benefit of U.S. Provisional Application No. 61/353,982, filed Jun. 11, 2010. 
     This application is also related to U.S. application Ser. No. 11/558,131, filed Nov. 9, 2006, and to U.S. application Ser. No. 12/416,040, filed Mar. 31, 2009. 
     The above three applications are incorporated herein by reference in their entirety. 
     BACKGROUND 
     Estimating the 3D orientation of a camera in a video sequence within a global frame of reference is a problem that may occur when addressing video stabilization in a virtual three-dimensional (3D) environment, as well as in navigation and other applications. This task requires the input of one or more orientation sensors (e.g., gyroscope, accelerometer, and/or compass) that may be attached to the camera to provide 3D orientation in a geographical frame of reference. However, high-frequency noise in the sensor readings may make it difficult to achieve the accurate orientation estimates that are required for visually stable presentation of a video sequence. This may be particularly true when the video is acquired with the camera as it undergoes high frequency orientation changes (i.e., jitter). Examples may include, for example, video shot from a moving car or while walking. Moreover, the quality of an orientation sensor can be a common problem in such contexts, especially for the low cost sensors available in consumer grade and cellphone cameras, leading to poor accuracy, especially in dynamic conditions. Typical values for angular root mean square (RMS) error may range from 0.5 to more than 2 degrees. Therefore such sensors may not measure camera jitter accurately, resulting in video sequences that may not show a stable scene when displayed in the context of a 3D environment. 
     On the other hand, image-based alignment has proven to be somewhat successful for image stabilization, providing accurate frame-to-frame orientation estimates. But image-based alignment may be prone to drifting over time due to error and bias accumulation and the lack of absolute orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         FIG. 1  is a block diagram illustrating the architecture of an embodiment. 
         FIG. 2  is a flow chart illustrating a rotation estimation and alignment process, according to an embodiment. 
         FIG. 3  is a flow chart illustrating the estimation of Euler angles, according to an embodiment. 
         FIG. 4  is a flow chart illustrating the use of quaternions in a fusing process, according to an embodiment. 
         FIG. 5  is a block diagram of an exemplary computer environment, in which a software or firmware embodiment may execute, according to an embodiment. 
     
    
    
     In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. 
     DETAILED DESCRIPTION 
     An embodiment is now described with reference to the figures, where like reference numbers indicate identical or functionally similar elements. Also in the figures, the leftmost digit of each reference number corresponds to the figure in which the reference number is first used. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the description. It will be apparent to a person skilled in the relevant art that this can also be employed in a variety of other systems and applications other than what is described herein. 
     Disclosed herein are methods and systems for generating estimates of the 3D orientation of a camera within a global frame of reference. Orientation estimates may be produced from an image-based alignment method. Other orientation estimates may be taken from one or more camera-mounted orientation sensors. The alignment-derived estimates may be input to a high pass filter. The orientation estimates from the orientation sensor may be processed and input to a low pass filter. The outputs of the high pass and low pass filters may be fused, producing a stabilized video sequence. 
     The overall architecture is shown in  FIG. 1 , according to an embodiment. A video camera  110  is shown. Camera  110  may output a digital video sequence  120 . A rotation estimation module  130  may estimate camera rotation by calculating a displacement between two successive frames. This may be performed for every pair of successive frames. The resultant displacements may be passed to image alignment module  140 . This module may generate an orientation time series that corresponds to the sequence of aligned images. This orientation time series may then be passed to a filter  150 . In the illustrated embodiment, this orientation time series is passed through a low pass filter, the output of which may be subtracted from the orientation time series. The net effect of this arrangement is to perform a high pass filtering operation on the orientation time series. In an alternative embodiment, a high pass filter may be used. 
     Video camera  110  may also include an orientation sensor (not shown). The orientation sensor may include one or more of an accelerometer, a compass, and a gyroscope, for example. The output from the orientation sensor is shown as output  160 . This output may then be processed by module  170  to produce a time series that reflects the changes in the orientation of camera  110  over time. This time series from module  170  may then be passed to low pass filter  180 . The outputs from the two filters may then be combined, or fused, to produce a stabilized 3-D camera orientation  185 . Stabilized orientation  185  can then be used to produce an output video sequence  195 . Note that in an embodiment, filters  150  and  180  and module  185  may be physically or logically combined in a sensor fusion module  190 . 
     As will be discussed further below, modules  130 ,  140 ,  150 ,  170 ,  180 , and  190  may be implemented in software, firmware, or hardware, or in some combination thereof. 
     A process for estimation of rotation and image alignment is illustrated in  FIG. 2 , according to an embodiment. At  210 , a Gaussian multi-resolution representation (MRR) of an input image may be calculated. Conceptually, such a representation may be viewed as a pyramid structure, wherein a first representation or pyramid layer may be a relatively coarse representation of the image, and each succeeding representation may be a finer representation of the image relative to the previous representation. This multi-resolution representation of an image may allow for a coarse-to-fine estimation strategy. In an embodiment, this multi-resolution representation of the input image may be computed using a binomial B 2  filter (¼, ½, ¼) for purposes of computational efficiency. 
     In the embodiment of  FIG. 2 , the sequence  220  through  240  may be performed for each level of the pyramid, beginning at the coarsest level. Generally, the process may be based on a gradient constraint, which assumes that the intensities between two images being aligned (or registered) are displaced on a pixel by pixel basis, while their intensity values are conserved. The gradient constraint may be stated as
 
 d   x ( p ) I   x ( x )+ d   y ( p ) I   y ( x )+Δ I ( p )=0  (1)
 
where I represents image intensity, d represents displacement, and ΔI(p)=I 2 (p)−I 1 (p), where I 2 (p) and I 1 (p) are the image intensities at pixel p.
 
     Each pixel in the image may contribute one constraint and, in general, two unknowns. However, it may be assumed that camera rotation jitter may be dominating the image motion over the camera translation so that the displacement between two images can be expressed as 
                 d   ⁡     (   p   )       =     (               x     2   x         x     2   z         -     x     1   x                       x     2   y         x     2   z         -     x     1   y               )       ,         
where x 1  is the location of pixel p in homogeneous image coordinates, x 2 =Px 1  and boldface P is a particular projective transform that depends on three parameters describing the 3D camera rotation and the two focal lengths of the images (assuming a simple diagonal camera calibration matrix):
 
                       x   2     =     Px   1       ,     P   =       (           f   1         0       0           0         f   1         0           0       0       1         )     ⁢     R   ⁡     (           1   /     f   2           0       0           0         1   /     f   2           0           0       0       1         )                   (   2   )               
where f 1  and f 2  are the respective focal lengths, and R is the 3D rotation matrix corresponding to the camera rotation. The rotation matrix may be parametrized using Euler angles ω=(ω x , ω y , ω z ) corresponding to an (x, y, z) convention. A small angle approximation may be used,
 
     
       
         
           
             
               
                 
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                               ω 
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                             ω 
                             y 
                           
                         
                       
                       
                         
                           
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                           1 
                         
                         
                           
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                               ω 
                               x 
                             
                           
                         
                       
                       
                         
                           
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                               ω 
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     When combining (1), (2), and (3), the following constraint may be obtained at each pixel: 
                         ω   x     [         -     I   x       ⁢     xy     f   2         -       I   y     (       f   1     +       y   2       f   2         )     +     Δ   ⁢           ⁢   I   ⁢     y     f   2           ]     +       ω   y     ⁡     [         I   x     (       f   1     +       x   2       f   2         )     +       I   y     ⁢     xy     f   2         -     Δ   ⁢           ⁢   I   ⁢     x     f   2           ]       +       ω   z     ⁡     [         f   ⁢           ⁢   1       f   ⁢           ⁢   2       ⁢     (         -     I   x       ⁢   y     +       I   y     ⁢   x       )       ]       +       (         f   ⁢           ⁢   1       f   ⁢           ⁢   2       -   1     )     ⁢     (         I   x     ⁢   x     +       I   y     ⁢   y       )       +     Δ   ⁢           ⁢   I       =   0           (   4   )               
Assuming that the focal lengths of both images are provided by the camera, this constraint is linear in the Euler angles vector ω.
 
     At  220 , each iteration may begin by gathering constraints from a sampling of pixels from a first input image. The locations from which the constraints are formed may be chosen using a rectangular sampling grid in the frame of reference of the first input image, according to an embodiment. Given these pixels and their constraints, a vector ω may be estimated for each pixel. The process for estimating these angles, according to an embodiment, will be discussed in greater detail below. 
     Given the resulting estimations of the Euler angles, at  230  a rotation matrix R may be determined according to (3) above. After this matrix is determined, at  240  the projective transform P may be calculated according to (2) above. With each iteration, the transform P may be combined with the transform P that resulted from the previous iteration, i.e., from the previous resolution level. 
     At  250 , the displacement d(p) may be calculated as the estimated interframe camera rotation. At  260 , the input frame and its succeeding frame may be aligned according to the estimated camera rotation. In an embodiment, bilinear interpolation may be used to obtain the displaced intensity values of the succeeding image at the identified pixel locations. 
     In an embodiment, it may be desirable to avoid problems caused by sudden changes in exposure. Such problems are sometimes introduced by the auto-exposure feature of cameras. To avoid such problems, the images may be pre-processed to equalize their mean and standard deviation prior to the alignment. 
       FIG. 3  illustrates the estimation of Euler angles ( 220  above) in greater detail. At  310 , a constraint of the form of equation (4) may be created for each sampled pixel at the given resolution level. This results in an equation for each sampled pixel. The resulting set of equations represents an over-determined system of equations that are each linear in a). At  320 , this system of equations may be solved. In the illustrated embodiment, the system may be solved using an M-estimator with a Tukey function. 
       FIG. 4  illustrates the fusing of orientation information derived from the orientation sensor(s) with aligned image information. At  410 , a time series representing the aligned image information may be modeled as rotational quaternions q i (t). At  420 , a time series representing the orientation information derived from the orientation sensor(s) may be modeled as quaternions q s (t). The appropriate filtering may be performed on the quaternions q s (t) and q i (t) in calculation of fused quaternions q f (t), as indicated in equation 5 below. 
                       q   f     ⁡     (   t   )       =           (       q   s     *   g     )     ⁢     (   t   )       +       (       q   i     *     (     δ   -   g     )       )     ⁢     (   t   )                    (       q   s     *   g     )     ⁢     (   t   )       +       (       q   i     *     (     δ   -   g     )       )     ⁢     (   t   )                        (   5   )               
Here, q i (t), q s (t), and q f (t) are the aligned image, orientation, and fused quaternions respectively. g(t) is a low pass filter; the convolutional operator * denotes convolving each of the components of the quaternion with the convolutional kernel; and ∥ is the quaternion norm.
 
     Note that in different embodiments, different low pass filters may be used. A particular low pass filter may be chosen based on the particular sensor characteristics, for example. In an embodiment, a Gaussian low pass filter with a standard deviation of σ=0.5 s may be used, for example. 
     Moreover, in an embodiment, convolution may be implemented using a discrete convolution mask with the number of taps equal to
 
2└σ f   r ┘+1
 
where f r  is the frame rate of the video which may be equal to the sampling frequency of the orientation time series.
 
     In an embodiment, after filtering and adding the quaternions from both sources, the resulting quaternions may not represent proper 3D rotations and may be re-normalized to the unit norm. 
     One or more features disclosed herein may be implemented in hardware, software, firmware, and combinations thereof, including discrete and integrated circuit logic, application specific integrated circuit (ASIC) logic, and microcontrollers, and may be implemented as part of a domain-specific integrated circuit package, or a combination of integrated circuit packages. The term software, as used herein, refers to a computer program product including a non-transitory computer readable medium having computer program logic stored therein to cause a computer system to perform one or more features and/or combinations of features disclosed herein. 
     A software or firmware embodiment of the processing described herein is illustrated in  FIG. 5 . In this figure, system  500  may include a processor  520  and a body of memory  510  that may include one or more computer readable media that may store computer program logic  540 . Memory  510  may be implemented as a hard disk and drive, a removable media such as a compact disk, a read-only memory (ROM) or random access memory (RAM) device, for example, or some combination thereof. Processor  520  and memory  510  may be in communication using any of several technologies known to one of ordinary skill in the art, such as a bus. Computer program logic  540  contained in memory  510  may be read and executed by processor  520 . One or more I/O ports and/or I/O devices, shown collectively as I/O  530 , may also be connected to processor  520  and memory  510 . 
     Computer program logic  540  may include alignment processing logic  550 . This logic may be responsible for performing the processing illustrated in  FIGS. 2 and 3 , for example. Logic  550  may therefore include, for example, logic for the computation of Gaussian multi-resolution representations of input images. Logic  550  may also include logic for the estimation of Euler angles, the determination of a rotation matrix R, and the determination of a projective transform P. Logic  550  may also include logic for the calculation of the displacement d(x), resulting in an aligned image time series. 
     Computer program logic  540  may also comprise orientation processing logic  560 . Logic  560  may be responsible for processing the output of an orientation sensor, and generating a corresponding orientation time series. 
     Computer program logic  540  may also comprise fusion logic  570 . Logic  570  may be responsible for performing the filtering of the orientation time series and the aligned image time series, modeling this information as quaternions, and calculating the fused quaternions. 
     Methods and systems are disclosed herein with the aid of functional building blocks illustrating the functions, features, and relationships thereof. At least some of the boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed. 
     While various embodiments are disclosed herein, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made therein without departing from the spirit and scope of the methods and systems disclosed herein. Thus, the breadth and scope of the claims should not be limited by any of the exemplary embodiments disclosed herein.