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Multiview coding with geometry-based disparity prediction - Microsoft Corporation
Multiview coding with geometry-based disparity prediction
United States Patent Application 20080285654
Described herein is technology for, among other things, multiview coding with geometry-based disparity prediction. The geometry-based disparity prediction involves determining corresponding block pairs in a number of reconstructed images for an image being coded. The reconstructed images and the image represent different views of a scene at a point in time. Each corresponding block pair is projected on the image. This enables determination of disparity vector candidates. For each coding block of the image, a predicted disparity vector is determined based on the disparity vector candidates. Then, the predicted disparity vector may be utilized to obtain the bits to be encoded. The geometry-based disparity prediction reduces the number of encoded bits.
Lou, Jian-guang (Beijing, CN)
11/803972
375/E7.105
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20090135901 COMPLEXITY ADAPTIVE VIDEO ENCODING USING MULTIPLE REFERENCE FRAMES May, 2009 Au et al.
1. A method of geometry-based disparity prediction in multiview coding, said method comprising: for a coding image, determining corresponding block pairs between a first reconstructed image and a second reconstructed image, wherein said coding image, first reconstructed image, and second reconstructed image represent different views of a scene at a particular time; projecting said corresponding block pairs to said coding image to determine disparity vector candidates; and determining a predicted disparity vector for each coding block of said coding image based on said disparity vector candidates.
2. The method as recited in claim 1 wherein said determining corresponding block pairs comprises: partitioning said first reconstructed image into a plurality of partition blocks; for each partition block of said first reconstructed image, calculating a corresponding epipolar line in said second reconstructed image; and for each partition block of said first reconstructed image, searching within a search window based on said epipolar line for a corresponding block of said second reconstructed image that satisfies a criterion to form a corresponding block pair.
3. The method as recited in claim 2 wherein said searching comprises: searching outside said search window for said corresponding block of said second reconstructed image that satisfies said criterion to form said corresponding block pair.
4. The method as recited in claim 1 wherein said projecting said corresponding block pairs comprises: for each corresponding block pair, calculating 3D coordinates corresponding to said scene; for each corresponding block pair, calculating a projection position on said coding image of said 3D coordinates; for each corresponding block pair, calculating an offset between said projection position and a corresponding block pair position in said first reconstructed image, wherein said offset represents a disparity vector candidate.
5. The method as recited in claim 1 wherein said determining a predicted disparity vector for each coding block comprises: for each coding block of said coding image; determining quantity of disparity vector candidates in said coding block; for each coding block where said quantity is zero, setting said predicted disparity vector to zero for said coding block; for each coding block where said quantity is one, setting said predicted disparity vector to said disparity vector candidate for said coding block; for each coding block where said quantity is greater than one, calculating a quality measure of said disparity vector candidates of said coding block; for each coding block where said quality measure is less than a threshold, calculating an average disparity vector based on said disparity vector candidates and setting said predicted disparity vector to said average disparity vector for said coding block; and for each coding block where said quality measure is not less than said threshold, setting said predicted disparity vector to zero for said coding block.
6. The method as recited in claim 1 wherein said method further comprises: for each coding block, calculating a refined disparity vector based on said predicted disparity vector and calculating a disparity vector residual based on said refined disparity vector and said predicted disparity vector; for each coding block, encoding said disparity vector residual.
7. The method as recited in claim 6 wherein said method further comprises: for each coding block, decoding said disparity vector residual; for each coding block, calculating said refined disparity vector based on said predicted disparity vector and said disparity vector residual.
8. The method as recited in claim 1 wherein said coding image, first reconstructed image, and second reconstructed image are multiview video data.
9. The method as recited in claim 1 wherein said coding image, first reconstructed image, and second reconstructed image are multiview image data.
10. A multiview coding apparatus comprising: a corresponding block pairs determination unit operable to determine corresponding block pairs between a plurality of reconstructed images; a corresponding block pairs projection unit operable to project said corresponding block pairs to a coding image to determine disparity vector candidates, wherein said coding image and said reconstructed images represent different views of a scene at a particular time; and a predicted disparity vector determination unit operable to determine predicted disparity vectors using said disparity vector candidates.
11. The multiview coding apparatus as recited in claim 10 wherein said corresponding block pairs determination unit comprises: a partition unit operable to partition said reconstructed images into a plurality of partition blocks; an epipolar line calculation unit operable to calculate an epipolar line in said reconstructed images; and a block match search unit operable to search for block matches in said reconstructed images that satisfy a criterion to form said corresponding block pairs.
12. The multiview coding apparatus as recited in claim 10 wherein said corresponding block pairs projection unit comprises: a 3D coordinates calculation unit operable to calculate 3D coordinates corresponding to said scene; a projection calculation unit operable to calculate a projection position on said coding image of said 3D coordinates; and a disparity vector candidates calculation unit operable to calculate an offset between said projection position on said coding image and a corresponding block pair position in said reconstructed images, wherein said offset represents a disparity vector candidate.
13. The multiview coding apparatus as recited in claim 10 wherein said predicted disparity vector determination unit comprises: a disparity vector fusion unit operable to merge disparity vector candidates of a coding block of said coding image.
14. The multiview coding apparatus as recited in claim 10 further comprising: a refined disparity vector determination unit operable to calculate refined disparity vectors based on said predicted disparity vectors and to calculate disparity vector residuals based on said refined disparity vectors and said predicted disparity vectors; and an encoding unit operable to encode said disparity vector residuals for said coding image.
15. The multiview coding apparatus as recited in claim 10 further comprising: a decoding unit operable to decode disparity vector residuals for said coding image; and a refined disparity vector determination unit operable to calculate refined disparity vectors based on said predicted disparity vectors and said disparity vector residuals.
16. The multiview coding apparatus as recited in claim 10 wherein said coding image and said reconstructed images are multiview video data.
17. The multiview coding apparatus as recited in claim 10 wherein said coding image and said reconstructed images are multiview image data.
18. A multiview coding apparatus operable in multiple coding modes comprising: a geometry-based disparity predictor which represents a first coding mode; a mode decision unit operable to select a coding mode from said multiple coding modes including said first mode.
19. The multiview coding apparatus as recited in claim 18 wherein said multiview coding apparatus is compatible with H.264/AVC specification.
20. The multiview coding apparatus as recited in claim 18 wherein said geometry-based disparity predictor comprises: a corresponding block pairs determination unit operable to determine corresponding block pairs between a plurality of reconstructed images; a corresponding block pairs projection unit operable to project said corresponding block pairs to a coding image to determine disparity vector candidates, wherein said coding image and said reconstructed images represent different views of a scene at a particular time; and a predicted disparity vector determination unit operable to determine predicted disparity vectors using said disparity vector candidates.
Advances in technology have ushered in various new user visual experiences based on captured video and/or captured images of a real world 3D (three-dimensional) scene, which may be static or dynamic. Some of these new user visual experiences are based on the concept of multiview. Multiview refers to the notion of enabling the user to watch the same scene from different viewing angles or perspectives. Movies, television broadcasting, home entertainment, sports venues, education, advertising, and real estate listings are some examples of areas that have employed multiview technology.
Multiview video, including freeview video, and multiview images are some examples of multiview technologies. In general, multiview video represents multiple video streams from synchronized video capture devices at different positions with respected to the captured scene. Similarly, a multiview image represents multiple image streams from synchronized image capture devices at different positions with respected to the captured scene. These video capture devices and image capture devices may be any one of numerous camera types.
Many techniques have been crafted or proposed to provide a multiview visual experience to the user that rivals the quality of single view technologies. Some of these techniques utilize existing solutions. Others utilize newly developed solutions. Still, others focus on using a hybrid solution. Different factors guide real world implementations of these techniques. However, each of these solutions has to be able to deal with the enormous amount of multiview data compare to single view data for a given application.
Thus, embodiments allow for greater compression when performing multiview coding. Embodiments reduce memory storage requirements for multiview data and reduce bandwidth requirements for transmitting multiview data. As result, real world applications of multiview data are more feasible and practical.
FIG. 2A illustrates a block diagram of a geometry-based disparity predictor in an encoding environment, in accordance with various embodiments.
FIG. 2B illustrates a block diagram of a geometry-based disparity predictor in a decoding environment, in accordance with various embodiments.
FIG. 3 illustrates multiview data, in accordance with various embodiments.
FIG. 4 illustrates multiview geometry, in accordance with various embodiments.
FIG. 5 illustrates a flowchart for geometry-based disparity prediction in multiview coding, in accordance with various embodiments.
FIG. 6 illustrates geometric relations utilized by geometry-based disparity prediction, in accordance with various embodiments.
FIG. 7 illustrates a flowchart for determining corresponding block pairs, in accordance with various embodiments.
FIG. 8 illustrates a flowchart for projecting corresponding block pairs to a coding image to determine disparity vector candidates, in accordance with various embodiments.
FIG. 9 illustrates a flowchart for determining a predicted disparity vector for each coding block of the coding image, in accordance with various embodiments.
FIG. 10 illustrates a flowchart for calculating refined disparity vectors, in accordance with various embodiments.
FIG. 11 illustrates a block diagram of an implementation of a geometry-based disparity predictor in an encoder environment compatible with the H.264/AVC specification, in accordance with various embodiments.
FIG. 12 illustrates a block diagram of an implementation of a geometry-based disparity predictor in a decoder environment compatible with the H.264/AVC specification, in accordance with various embodiments.
Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings. While the disclosure will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the disclosure to these embodiments. On the contrary, the disclosure is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the disclosure as defined by the claims. Furthermore, in the detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be obvious to one of ordinary skill in the art that the disclosure may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the disclosure.
Multiview data (e.g., multiview video and multiview images) significantly increase the number of bits that undergo encoding and decoding. Described herein is technology for, among other things, multiview coding with geometry-based disparity prediction. Geometric relations among different views of a scene are independent of the scene structure. These geometric relations depend on parameters associated with the capture device (e.g., camera). As a result, these parameters may be computed independently of the multiview coding and do need to undergo encoding/decoding.
The geometry-based disparity prediction involves determining corresponding block pairs in a number of reconstructed images for an image being coded. The reconstructed images and the image represent different views of a scene at a point in time. Each corresponding block pair is projected on the image. This enables determination of disparity vector candidates. For each coding block of the image, a predicted disparity vector is determined based on the disparity vector candidates. Then, the predicted disparity vector may be utilized to obtain the bits to be encoded. The geometry-based disparity prediction reduces the number of encoded bits.
As a result, greater compression of multiview data is achieved. Further, there are reductions in memory storage requirements for the multiview data and reductions in bandwidth requirements for transmitting the multiview data, making real world applications of multiview data more feasible and practical.
The following discussion will begin with a description of an example operating environment for various embodiments. Discussion will proceed to a description of a geometry-based disparity predictor. Discussion will then proceed to descriptions of multiview coding techniques using geometry-based disparity prediction.
With reference to FIG. 1, an exemplary system for implementing embodiments includes a general purpose computing system environment, such as computing system environment 100. In various embodiments, the computing system environment 100 may be a standalone coding device such as an encoder, a decoder, and an encoder/decoder. Also, the computing system environment 100 may be a system having a coding device. In its most basic configuration, computing system environment 100 typically includes at least one processing unit 102 and memory 104. Depending on the configuration and type of computing system environment 100, memory 104 may be volatile (such as RAM), non-volatile (such as ROM, flash memory, etc.) or some combination of the two. This most basic configuration is illustrated in FIG. 1 by dashed line 106. Additionally, computing system environment 100 may also have additional features/functionality. For example, computing system environment 100 may also include additional storage (removable and/or non-removable) including, but not limited to, magnetic or optical disks or tape. Such additional storage is illustrated in FIG. 1 by removable storage 108 and non-removable storage 110. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Memory 104, removable storage 108 and nonremovable storage 110 are all examples of computer storage media. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other storage devices, or any other medium which can be used to store the desired information and which can be accessed by computing system environment 100. Any such computer storage media may be part of computing system environment 100.
The computing system environment 100 may also include a number of audio/video inputs and outputs 118 for receiving and transmitting video content. These inputs and outputs may include, but are not limited to, coaxial, composite video, S-video, HDMI, DVI, VGA, component video, optical, and the like. It should be appreciated that since video content may be delivered over an Internet connection, a network interface may therefore also be considered an A/V input on which video content is received.
Geometry-Based Disparity Predictor
Multiview data, such as multiview video and multiview images, may be compressed by various coding schemes. A multiview coding scheme focuses on reducing the redundancy of multiview data captured by synchronized video/image capture devices (e.g., camera) at different positions with respect to the captured scene. For example, there is redundancy in video images or still images captured from different viewpoints (or views) at the same time. This inter-viewpoint redundancy may be minimized by inter-viewpoint disparity compensation.
Inter-viewpoint disparity compensation involves determining disparity vectors. A disparity vector represents a vector distance between two points on superimposed video images or still images from different viewpoints that correspond to the same scene point in the real world 3D scene. Due to numerous factors, it is more convenient to predict the disparity vectors using a disparity predictor. Then, the disparity vectors are derived from the predicted disparity vector to enable inter-viewpoint disparity compensation.
FIG. 2A illustrates a block diagram of a geometry-based disparity predictor 200 in an encoding environment 290A, in accordance with various embodiments. FIG. 2B illustrates a block diagram of a geometry-based disparity predictor 200 in a decoding environment 290B, in accordance with various embodiments.
Before describing in detail the geometry-based disparity predictor 200 of FIG. 2A and FIG. 2B, multiview data will be introduced by referring to FIG. 3, which illustrates multiview data 300, in accordance with various embodiments. It should be understood that multiview data 300 may be multiview video, multiview images, or any other multiview data type.
As depicted in FIG. 3, the horizontal axis represents TIME while the vertical axis represents VIEWPOINT. The multiview data 300 may represent multiview video or multiview images, depending on whether video or still images are captured from the different viewpoints. If video is being captured, the multiview data 300 may be multiview video, which may include m video streams captured from m viewpoints. The notation fi(j) may denote the jth video frame (or video image) of the ith view (or viewpoint). Each view may correspond to a separate video capturing device (e.g., video camera) at different positions with respected to the captured scene. A scene refers to a real world 3D (three-dimensional) scene. If still images are being captured, the multiview data 300 may be multiview images, where each multiview image may include m still images captured from m viewpoints at a point in time. The 1st multiview image and the nth multiview image are shown by broken line in FIG. 3. Further, the multiview video may be considered to be a sequence of multiview images.
Referring again to FIG. 2A, a block diagram of a geometry-based disparity predictor 200 in an encoding environment 290A is illustrated, in accordance with various embodiments. As seen in FIG. 2A, the geometry-based disparity predictor 200 generates predicted disparity vector(s) 245, which is/are utilized as described above with respect to inter-viewpoint disparity compensation. As will be described in detail below, geometric relations among different views of a scene are independent of the scene structure. These geometric relations depend on parameters associated with the capture device (e.g., video camera, still image camera, etc.). As a result, the geometry-based disparity predictor 200 uses various camera projection matrices 282, 284 and 286 representing parameters of the respective camera. Each camera projection matrix may be determined by a camera's intrinsic parameters (e.g., focal length) and its posture in the real world 3D (three-dimensional) space. Also, the fundamental matrix (FM) 280, which may be derived from camera projection matrices, is utilized. The camera projection matrices 282, 284 and 286 and the FM 280 may be computed independently of the geometry-based disparity predictor 200 through camera calibration.
Continuing with FIG. 2A, the notations f-2th, f-1th, and fth denote viewpoints and relative order of the viewpoints. For example, f-1th viewpoint is between f-2th viewpoint and fth viewpoint. In an embodiment, the geometry-based disparity predictor 200 generates the predicted disparity vector(s) 245 for a fth viewpoint image (e.g., video image and still image) 250 based on a f-1th viewpoint image and a f-2th viewpoint image. Moreover, the f-1th viewpoint image and the f-2th viewpoint image have already undergone processing and encoding. Thus, the f-1th viewpoint image and the f-2th viewpoint image are reconstructed before being used by the geometry-based disparity predictor 200. Overall, the fth viewpoint image 250, f-1th viewpoint reconstructed image 252, and f-2th viewpoint reconstructed image 254 represent different images of a scene at a certain time. That is, the geometry-based disparity predictor 200 operates on images (e.g., video image and still image) captured from different viewpoints (or views) at the same time.
Also, each of the fth viewpoint image 250, f-1th viewpoint reconstructed image 252, and f-2th viewpoint reconstructed image 254 has a corresponding camera projection matrix for the camera positioned at the corresponding viewpoint. These camera projection matrices are fth viewpoint camera projection matrix 282, f-1th viewpoint camera projection matrix 284, and f-2th viewpoint camera projection matrix 286.
As depicted in FIG. 2A, the geometry-based disparity predictor 200 may have a corresponding block pairs determination unit 210, a corresponding block pairs projection unit 220, and a predicted disparity vector determination unit 230, in accordance with various embodiments. These components may be implemented in hardware, software, or a combination thereof.
The corresponding block pairs determination unit 210 is operable to determine corresponding block pairs between f-1th viewpoint reconstructed image 252 and f-2th viewpoint reconstructed image 254. That is, a block of f-1th viewpoint reconstructed image 252 corresponds to a block of f-2th viewpoint reconstructed image 254 if each block provides an image of the same portion of a scene from different views. These blocks are referred to as corresponding block pairs. In an embodiment, each block of corresponding block pairs is identified by its centroid coordinates. In accordance with various embodiments, corresponding block pairs determination unit 210 may include a partition unit 202, an epipolar line calculation unit 204, and a block match search unit 206. In an embodiment, the partition unit 202 is operable to partition the f-1th viewpoint reconstructed image 252 into a plurality of partition blocks. In an embodiment, each partition block is identified by its centroid coordinates. Next, the epipolar line calculation unit 204 is operable to calculate an epipolar line in the f-2th viewpoint reconstructed image 254 for each partition block of the f-1th viewpoint reconstructed image 252, in an embodiment. A detailed discussion of operation of the epipolar line calculation unit 204 is now presented.
To describe operation of the epipolar line calculation unit 204, multiview geometry and the epipolar line will be discussed while referring to FIG. 4. Multiview geometry is illustrated in FIG. 4, in accordance with various embodiments. Images representing different views of a scene at an instance of time are not unrelated. Several geometric relationships exist between two, three, or more images, where these geometric relationships are the subject matter of multiview geometry. Epipolar geometry focuses on the geometric relationships of two images representing different views of a scene at an instance of time.
Still referring to FIG. 4, the images I1 and I2 are different views of a scene at an instance of time, where C1 and C2 are the optical centers of first and second cameras (e.g., video camera, still camera, etc.), respectively. The point P is in a scene's 3D (three-dimensional) space. The notation P2 denotes the projection of point P on the image I2. According to epipolar geometry, the corresponding point P1 in the image I1 of the point P is constrained to lie on line l1. This line l1 denotes the epipolar line of P2. The epipolar constraint may be formulated as the equation (1):
{tilde over (P)}1T·F·{tilde over (P)}2={tilde over (P)}1T·l1=0 (1)
The notations {tilde over (P)}1 and {tilde over (P)}2 denote the homogeneous coordinates of P1 and P2. The notation T denotes transpose of a matrix. Moreover, F denotes the fundamental matrix (FM), which was introduced above in FIG. 2A and FIG. 2B as FM 280. The FM is a 3×3 matrix, which is determined by the intrinsic matrix and the relative position of the first and second cameras. If first and second cameras are calibrated, both intrinsic and extrinsic camera parameters are known. The FM may be readily calculated from camera projective matrices of the first and second cameras, where camera projective matrices were introduced above in FIG. 2A and FIG. 2B as camera projection matrices 282, 284 and 286. Therefore, from the equation (1), since F and the centroid coordinates of the partition block of the f-1th viewpoint reconstructed image 252 are available, the epipolar line calculation unit 204 (FIG. 2A and FIG. 2B) may compute the equation of epipolar line l1 in the f-2th viewpoint reconstructed image 254.
Continuing with FIG. 2A, the block match search unit 206 of the geometry-based disparity predictor 200 is operable to search for block matches between the partition blocks of the f-1th viewpoint reconstructed image 252 and blocks of the f-2th viewpoint reconstructed image 254 that satisfy a criterion to form corresponding block pairs, in an embodiment. The coordinates of the corresponding block pairs are noted. In an embodiment, each block of corresponding block pairs is identified by its centroid coordinates. In an embodiment, the criterion is a minimal mean-squared-error (MSE) criterion between the blocks. In an embodiment, the search is focused on a search window based on the epipolar line in the f-2th viewpoint reconstructed image 254. In an embodiment, a full search is performed on the f-2th viewpoint reconstructed image 254.
Returning to FIG. 2A, the corresponding block pairs projection unit 220 of the corresponding block pairs determination unit 210 is operable to project the corresponding block pairs to the fth viewpoint image 250, which is the image being encoded. This allows determination of disparity vector candidates, which may become predicted disparity vectors after further processing by the geometry-based disparity predictor 200. In accordance with various embodiments, the corresponding block pairs projection unit 220 may have a 3D coordinates calculation unit 222, a projection calculation unit 224, and a disparity vector candidates calculation unit 226. The 3D coordinates calculation unit 222 is operable to calculate each corresponding block pair's 3D coordinates in the real world scene, in an embodiment. A detailed discussion of operation of the 3D coordinates calculation unit 222 is now presented.
The projection of a point in a scene to a point in an image of a camera may be modeled by equation (2):
z[uv1]=M[xPyPzp1],(2)
The notation [xP yP zP 1]T denotes the homogeneous coordinates of a 3D point P in a scene. Also, the notation [u v 1]T denotes the homogeneous coordinates of the projection of the point P to a point in the image while the notation z denotes point P's depth. The notation M denotes a camera projection matrix, as illustrated by camera projection matrices 282, 284 and 286 of FIG. 2A and FIG. 2B. The camera projection matrix, which is a 3×4 matrix, may be determined by the camera's intrinsic parameters (e.g. focal length) and its posture in the real world 3D (three-dimensional) space.
Now, the projection of a 3D point P in a scene to a point P1 in a first image of a first camera and to a point P2 in a second image of a second camera may be modeled by equation (3) and equation (4):
Zc1[u1v11]=M1[xPyPzP1]=[m111m121m131m141m211m221m231m241m311m321m331m341][xPyPzP1](3)Zc2[u2v21]=M2[xPyPzP1]=[m112m122m132m142m212m222m232m242m312m322m332m342][xPyPzP1](4)
The notations M1 and M2 denote the camera projection matrices of first and second cameras. Since these camera projection matrices are known, they are entered into equation (3) and equation (4). Further, notations (u1, v1, 1) and (u2, v2, 1) denote the homogeneous coordinates of P1 and P2 in the respective images of the first and second cameras. The notation [xP yP zP 1]T denotes the homogeneous coordinates of the 3D point P in the scene. Additionally, the notations Zc1 and Zc2 denote point P's depth with respect to the first and second images, respectively.
With elimination of Zc1 and Zc2 and entry of known camera projection matrices M1 and M2 (e.g., f-1th viewpoint camera projection matrix 284 and f-2th viewpoint camera projection matrix 286), the equations (3) and (4) are converted to the following equations (5) and (6), respectively:
(u1m311−m111)xP+(u1m321−m121)yP+(u1m331−m131)zP=m141−u1m341
(v1m311−m211)xP+(v1m321−m221)yP+(v1m331−m231)zP=m241−v1m341 (5)
(u2m312−m112)xP+(u2m322−m122)yP+(u2m332−m132)zP=m142−u2m34 2
(v2m312−m212)xP+(v2m322−m222)yP+(v2m332−m232)zP=m24 2−v2m34 2 (6)
Here, [xP yP zP 1]T (the 3D spatial coordinates of P) is the solution of the above four linear equations. In an embodiment, if the two camera's projective matrices and the corresponding image points P1 and P2 are all known, a least-square method may be used to estimate the position of the 3D point P in a scene. In an embodiment, the 3D coordinates calculation unit 222 utilizes equations (5) and (6) and centroid coordinates of corresponding block pairs to calculate each corresponding block pair's 3D coordinates in the real world scene.
Again referring to FIG. 2A, the projection calculation unit 224 of the corresponding block pairs projection unit 220 is operable to calculate a projection position on the fth viewpoint image 250 of each calculated 3D coordinates, in an embodiment. The projection calculation unit 224 may utilize equation (2), the fth viewpoint camera projection matrix 282, and the calculated 3D coordinates to calculate projection positions on the fth viewpoint image 250. For equation (2), the notation z denotes the calculated 3D coordinates' depth with respect to the fth viewpoint image 250. Continuing, the disparity vector candidates calculation unit 226 is operable to calculate offsets between the projection positions on the fth viewpoint image 250 and corresponding block pair positions (based on centroid coordinates) in the f-1th viewpoint reconstructed image 252. These offsets represent disparity vector candidates.
The predicted disparity vector determination unit 230 of the geometry-based disparity predictor 200 is operable to determine predicted disparity vectors using the disparity vector candidates. In accordance with various embodiments, the predicted disparity vector determination unit 230 may have a disparity vector fusion unit 226. In an embodiment, the disparity vector fusion unit 226 is operable to merge disparity vector candidates of a coding block of the fth viewpoint image 250, which is the image being encoded.
Continuing with FIG. 2A, the encoding environment 290A may include a refined disparity vector determination unit 240 and an encoding unit 270, in accordance with various embodiments. The refined disparity vector determination unit 240 is operable to calculate refined disparity vectors based on the predicted disparity vectors 245, in an embodiment. The predicted disparity vectors 245 are not optimal disparity vectors from a coding efficiency perspective due to several factors, such as calibration noise, geometric estimation noise, and varying lighting conditions. Also the refined disparity vector determination unit 240 calculates disparity vector residuals 260 based on the refined disparity vectors and the predicted disparity vectors 245. Further, the encoding unit 270 encodes the disparity vector residuals 260 for the fth viewpoint image 250, which is the image being encoded.
Now referring to FIG. 2B, a block diagram of a geometry-based disparity predictor 200 in a decoding environment 290B is illustrated, in accordance with various embodiments. The discussion of the geometry-based disparity predictor 200 of FIG. 2A is equally applicable to the geometry-based disparity predictor 200 of FIG. 2B, except as noted below. The same reference numerals are used in FIGS. 2A and 2B to refer to corresponding components. In an embodiment, the decoding environment 290B may include a decoding unit 290 and a refined disparity vector determination unit 240. The fth viewpoint image 250 (FIG. 2A), which was encoded in accordance with the description of FIG. 2A, is now being decoded in FIG. 2B.
The decoding unit 290 is operable to decode disparity vector residuals 260 for the fth viewpoint image 250 (FIG. 2A), which is the image being decoded, in an embodiment. Further, in an embodiment, the refined disparity vector determination unit 240 is operable to calculate refined disparity vectors 275 based on the predicted disparity vectors 245 (determined as discussed with respect to FIG. 2A) and the disparity vector residuals 260. In accordance with various embodiments, the refined disparity vectors 275 are involved in decoding the fth viewpoint image 250 (FIG. 2A).
Multiview Coding Techniques Using Geometry-Based Disparity Prediction
The following discussion sets forth in detail the operation of geometry-based disparity prediction. With reference to FIGS. 5 and 7-10, flowcharts 500, 700, 800, 900, and 1000 each illustrate example steps used by various embodiments of geometry-based disparity prediction. Moreover, FIGS. 5 and 7-10 will make reference to FIG. 6. Flowcharts 500, 700, 800, 900, and 1000 include processes that, in various embodiments, are carried out by a processor under the control of computer-readable and computer-executable instructions. The computer-readable and computer-executable instructions reside, for example, in data storage features such as computer usable memory 104, removable storage 108, and/or non-removable storage 110 of FIG. 1. The computer-readable and computer-executable instructions are used to control or operate in conjunction with, for example, processing unit 102 of FIG. 1. Although specific steps are disclosed in flowcharts 500, 700, 800, 900, and 1000, such steps are examples. That is, embodiments are well suited to performing various other steps or variations of the steps recited in flowcharts 500, 700, 800, 900, and 1000. It is appreciated that the steps in flowcharts 500, 700, 800, 900, and 1000 may be performed in an order different than presented, and that not all of the steps in flowcharts 500, 700, 800, 900, and 1000 may be performed.
FIG. 5 illustrates a flowchart 500 for geometry-based disparity prediction in multiview coding, in accordance with various embodiments. Moreover, FIG. 6 illustrates geometric relations utilized by geometry-based disparity prediction in flowcharts 500, 700, 800, 900, and 1000, in accordance with various embodiments. As depicted in FIG. 6, the fth viewpoint image is the image being coded (e.g., encoded or decoded). In an embodiment, geometry-based disparity prediction generates the predicted disparity vector(s) for the fth viewpoint image (e.g., video image and still image) based on a f-1th viewpoint image and a f-2th viewpoint image. Moreover, the f-1th viewpoint image and the f-2th viewpoint image have already undergone processing and coding. Thus, the f-1th viewpoint image and the f-2th viewpoint image are reconstructed before being used by geometry-based disparity prediction. Further, the fth viewpoint image, f-1th viewpoint reconstructed image, and f-2th viewpoint reconstructed image represent different images of a scene at a certain time. The scene is a scene in real world 3D space.
Returning to FIG. 5, for fth viewpoint image (coding image), corresponding block pairs between f-1th viewpoint reconstructed image (first recon. image) and f-2th viewpoint reconstructed image (second recon. image) are determined (block 510). Further, the corresponding block pairs are projected to the fth viewpoint image to determine disparity vector candidates (block 520). A predicted disparity vector is determined for each coding block of the fth viewpoint image based on the disparity vector candidates (block 530). Each step of flowchart 500 will be discussed in detail below.
FIG. 7 illustrates a flowchart 700 for determining corresponding block pairs, in accordance with an embodiment. With reference to FIGS. 6 and 7, a block of f-1th viewpoint reconstructed image corresponds to a block of f-2th viewpoint reconstructed image if each block provides an image of the same portion of a scene from different views. These blocks are referred to as corresponding block pairs. In an embodiment, each block of corresponding block pairs is identified by its centroid coordinates. To find the corresponding block pairs, at least two images (e.g., f-1th viewpoint reconstructed image and f-2th viewpoint reconstructed image) representing the same scene from different viewpoints at a point in time and having already undergone processing and coding are selected, in an embodiment. The search for corresponding block pairs is carried out upon reconstructed images. This is done because the search for corresponding block pairs is repeated at the decoder environment and because reconstructed images are available at both the encoder and decoder environments.
In an embodiment, any two images (from multiview video/image and representing the same scene from different viewpoints at a point in time) that have already undergone processing and coding may be selected. In an embodiment, the two nearest neighboring images (e.g., f-1th viewpoint reconstructed image and f-2th viewpoint reconstructed image) are selected to improve accuracy of the search for corresponding block pairs. These two selected images (e.g., f-1th viewpoint reconstructed image and f-2th viewpoint reconstructed image) are less influenced by occlusion and thus provide predicted disparity vectors having greater accuracy.
Focusing on FIG. 7, the f-1th viewpoint reconstructed image is partitioned into a plurality of partition blocks (block 710). In an embodiment, the partition blocks are rectangular blocks. For example, FIG. 6 shows, in the f-1th viewpoint reconstructed image, a partition block Bf-1(xf-1,yf-1) with coordinates (xf-1,yf-1). In an embodiment, the coordinates (xf-1,yf-1) are centroid coordinates of the partition block.
Continuing with FIG. 7 at block 720, for each partition block of the f-1th viewpoint reconstructed image, a corresponding epipolar line is calculated in the f-2th viewpoint reconstructed image. In an embodiment, a partition block's epipolar line is calculated using equation (1) discussed above. With equation (1), the fundamental matrix F, and the centroid coordinates of the partition block Bf-1(xf-1,yf-1), the epipolar line (FIG. 6) in the f-2th viewpoint reconstructed image may be computed.
At block 730, for each partition block of the f-1th viewpoint reconstructed image, a search is performed within a search window (FIG. 6) based on the epipolar line (FIG. 6) for a corresponding block of the f-2th viewpoint reconstructed image that satisfies a criterion to form a corresponding block pair, in an embodiment. Searching along the epipolar line (FIG. 6) reduces the block mismatching probability. In an embodiment, the criterion is a minimal mean-squared-error (MSE) criterion between the blocks. In an embodiment, a full search (including the search window (FIG. 6)) is performed on the f-2th viewpoint reconstructed image. FIG. 6 shows the corresponding block Bf-2(xf-1+Δxf-2,yf-1+Δyf-2) in f-2th viewpoint reconstructed image of partition block Bf-2(xf-1, yf-1), where (Δxf-2,Δyf-2) denotes the block offset from (xf-1,yf-1). Thus, Bf-1,(xf-1,yf-1) and Bf-2(xf-1+Δxf-2,yf-1+Δyf-2) are a corresponding block pair.
FIG. 8 illustrates a flowchart 800 for projecting corresponding block pairs to the fth viewpoint image (coding image) to determine disparity vector candidates, in accordance with an embodiment. For each corresponding block pair, 3D coordinates in the scene (real world 3D space) are calculated (block 810). Using equations (5) and (6), the camera projection matrices Mf-2 and Mf-1 (e.g., f-1th viewpoint camera projection matrix and f-2th viewpoint camera projection matrix), and the blocks' coordinates (xf-1+Δxf-2,yf-1+Δyf-2) and (xf-1,yf-1), the corresponding 3D coordinates may be computed by a least-square method, in an embodiment.
At block 820, for each corresponding block pair, a projection position on the fth viewpoint image of the calculated 3D coordinates is calculated. Using equation (2), the camera projection matrix Mf (e.g., fth viewpoint camera projection matrix), and the calculated 3D coordinates, the projection position on the fth viewpoint image may be calculated, in an embodiment. FIG. 6 shows the projection position of corresponding block pair Bf-1(xf-1,yf-1) and Bf-2(xf-1+Δxf-2,yf-1+Δyf-2) is B′f.
Continuing at block 830 of FIG. 8, for each corresponding block pair, an offset is calculated between the projection position on the fth viewpoint image and a corresponding block pair position in the f-1th viewpoint reconstructed image. This offset represents a disparity vector candidate. In an embodiment, the offset between the corresponding block pair's position in the f-1th viewpoint reconstructed image and the projection position in fth viewpoint image may be calculated with equation (7) as follows:
vf(xf-1,yf-1)=(xf(Bf-1)−xf-1,yf(Bf-1)−yf-1), (7)
The notations xf(Bf-1) and yf (Bf-1) denote the projected coordinates of the partition block Bf-1(xf-1, yf-1) of the f-1th viewpoint reconstructed image in the fth viewpoint image.
FIG. 9 illustrates a flowchart 900 for determining a predicted disparity vector for each coding block of the fth viewpoint image (coding image), in accordance with an embodiment. A coding block represents an image portion being coded. For each coding block, the predicted disparity vector {tilde over (V)}f(i,j) is based on the disparity vector candidates in the coding block. In an embodiment, fusion utilized to merge similar disparity vector candidates into one predicted disparity vector {tilde over (V)}f(i,j). In one embodiment, the predicted disparity vector {tilde over (V)}f(i,j) is calculated according to the quantity N of disparity vector candidates located in a coding block.
For each coding block of the fth viewpoint image, the quantity (N) of disparity vector candidates in the coding block is determined (block 910).
If N=0, the predicted disparity vector {tilde over (V)}f(i,j) is set to zero for the coding block (blocks 920 and 930). This indicates the predicted disparity vector {tilde over (V)}f(i,j) for the coding block is unpredictable.
If N=1, the predicted disparity vector {tilde over (V)}f(i,j) is set to disparity vector candidate for the coding block (blocks 940 and 950).
If N>1, a quality measure (QM) of the disparity vector candidates of the coding block is calculated for each coding block (block 960). In an embodiment, the quality measure (QM) performed on the disparity vector candidates is calculated with equation (8):
1n-1∑i=1n∑j=1,j≠in(xi-xj+yi-yj)<ThresDV.(8)
The notation (x1,y1) denotes the coordinates of the ith disparity vector candidate. Also, the notation ThresDV denotes a threshold. The threshold may be fixed. In an embodiment, ThresDV equals 4.0. Other values are possible.
If QM<ThresDV, an average disparity vector based on the disparity vector candidates is calculated (block 970 and 980). The predicted disparity vector {tilde over (V)}f(i,j) is set to the average disparity vector for the coding block (block 995).
If QM≧ThresDV, the predicted disparity vector {tilde over (V)}f(i,j) is set to zero for the coding block (blocks 970 and 990). This indicates the predicted disparity vector {tilde over (V)}f(i,j) for the coding block is unpredictable.
FIG. 10 illustrates a flowchart 1000 for calculating refined disparity vectors, in accordance with an embodiment. In an embodiment, the calculation of a refined disparity vector involves a refinement process performed in a square area centered at the corresponding predicted disparity vector {tilde over (V)}f(i,j). In an embodiment, sub-pixel accuracy compensation may be performed. The refinement process is performed because the predicted disparity vector {tilde over (V)}f(i,j) is usually not the optimal disparity vector from a coding efficiency perspective due to factors such as calibration noise, geometric estimation noise, and varying lighting conditions. This refinement process is similar to the conventional motion estimation in which the median predicted motion vector MV is often used as the search starting point (or center of the search area).
At block 1010, the predicted disparity vector {tilde over (V)}f(i,j) for each coding block is determined, as discussed in detail above.
If encoding is being performed, a refined disparity vector based on the predicted disparity vector {tilde over (V)}f(i,j) is calculated for each coding block (blocks 1020 and 1030). Moreover, a disparity vector residual based on the refined disparity vector and the predicted disparity vector {tilde over (V)}f(i,j) is calculated (blocks 1020 and 1030). Further, the disparity vector residual is encoded for each coding block (block 1040).
If decoding is being performed, the disparity vector residual is decoded for each coding block (blocks 1020 and 1050). Further, a refined disparity vector based on the predicted disparity vector {tilde over (V)}f(i,j) and the disparity vector residual is calculated for each coding block (block 1060).
Referring again to FIG. 6, block Bf represents the real corresponding block of block Bf-1 in the fth viewpoint image. Moreover, the refined disparity vector (DV) is shown in FIG. 6.
FIG. 11 illustrates a block diagram of an implementation of a geometry-based disparity predictor 200 in an encoder environment 1100 compatible with the H.264/AVC specification, in accordance with various embodiments. It is possible to use other coding specifications.
As depicted in FIG. 11, the encoder environment 1100 includes a geometry-based disparity predictor 200, a refined disparity vector determination unit 240, a mode decision unit 299, and components for executing an encoding scheme compatible with the H.264/AVC specification, in an embodiment. The encoder environment 1100 may operate in any of a plurality of coding modes, in an embodiment. The geometry-based disparity predictor 200 and the refined disparity vector determination unit 240 operate as described in detail with respect to FIG. 2A, in an embodiment. The mode decision unit 299 is operable to select a coding mode from the coding modes for operation of the encoder environment 1100, in an embodiment.
In an embodiment, the coding modes include a GP (geometric prediction) mode. In an embodiment, the coding modes also include INTER mode and INTRA mode. The INTER mode and INTRA mode are based on the H.264/AVC specification. In the GP mode, the geometry-based disparity predictor 200 and the refined disparity vector determination unit 240 are active and operate as described in detail with respect to FIG. 2A, in an embodiment.
In an embodiment, the first image is encoded as an I-frame using the INTRA mode. The second image is encoded as a P-frame using both the INTER and INTRA modes. Starting from the third image, all the three modes may be used. In an embodiment, the mode decision unit 299 decides the optimal coding mode that provides the best compression. After this decision, a one-bit overhead is generated for each coding block to signal whether the GP mode is used if the coding block is not coded in the INTRA mode. In the GP mode, disparity compensation (use geometry-based disparity predictor 200) is then performed to generate disparity vector residual signals for encoding.
FIG. 12 illustrates a block diagram of an implementation of a geometry-based disparity predictor 200 in a decoder environment 1200 compatible with the H.264/AVC specification, in accordance with various embodiments. It is possible to use other coding specifications. In an embodiment, the decoder environment 1200 includes a geometry-based disparity predictor 200, a mode decision unit 299, and components for executing a decoding scheme compatible with the H.264/AVC specification. The decoder environment 1200 may operate in any of a plurality of coding modes as described above with respect to the encoder environment 1200, in an embodiment. The geometry-based disparity predictor 200 operates as described in detail with respect to FIG. 2A and FIG. 2B, in an embodiment. The mode decision unit 299 is operable to select a coding mode from the coding modes for operation of the decoder environment 1200, in an embodiment.
As depicted in FIG. 12, in the GP mode the geometry-based disparity predictor 200 repeats its operation to get the same predicted disparity vector {tilde over (V)}f(i,j) as that at the encoder environment 1100. According to the overhead bit of each coding block, the mode decision unit 299 decides whether the GP mode is used for coding the current coding block, in an embodiment. The actual disparity vector is obtained by adding a residual vector onto the predicted disparity vector. In an embodiment, this predicted disparity vector may be generated by the geometry-based disparity predictor 200 or by a median predictor based on the H.264/AVC specification, depending on the coding mode. Then, after the disparity compensation, the image may be reconstructed at the decoder environment 1200.
Geometry-based disparity prediction reduces the number of encoded bits. As a result, greater compression of multiview data is achieved. Further, there are reductions in memory storage requirements for the multiview data and reductions in bandwidth requirements for transmitting the multiview data, making real world applications of multiview data more feasible and practical.
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