Image processing apparatus

Abstract: In a processing system, video images of moving objects are processed to model the objects in a 3D computer model. video from multiple cameras is processed to separate objects from their shadows, and to test whether an object is made up of separate objects, which are then modelled separately. Each object is modelled using vertical planes whose bases approximate the object's ground footprint, using planes based on object surface planes identified in the image data, or using a single vertical plane. Pixel data from the video images is rendered onto the planes in the models. The video for rendering is selected based on the viewer's viewing direction, the camera viewing directions, and quality characteristics of the cameras and image data. If the viewer's viewing direction is close to vertical or a plane of an object, a schematic of the objects' positions is displayed. To account for image data from different cameras being used, successive images are tested for visual discontinuous, and are modified if necessary. Information indicating the accuracy/reliability of the rendered image is displayed.

Description: The present invention relates to the field of three-dimensional computer modelling, and in particular to the generation of representations of objects in a three-dimensional computer model using moving pictures such as video images. At present, the content of images produced from video, or other moving picture, data depends on the viewing characteristics of the camera which captured the data. More particularly, the position from which objects are seen by a viewer is determined by the viewing position and viewing direction of the camera with respect to the scene. As a solution to this constraint, interactive systems have been suggested in which video data is used to create a dynamic three-dimensional computer model of a scene, from which simulated views from any desired viewing direction can be created and displayed to a user. WO-A-96/31047 discloses immersive video, or television images of a real-world scene which are synthesized (i) on demand, (ii) in real time, (iii) as linked to any of a particular perspective on the scene, or an object or event in the scene, (iv) in accordance with user-specified parameters of presentation, including panoramic or magnified presentations, and/or (v) stereoscopically. The synthesis of virtual images is based on computerized video processing -- called "hypermosaicing" -- of multiple live video perspectives on the scene. Multiple video cameras each at a different spatial location produce multiple two-dimensional video images of the scene. A viewer/user specifies viewing criterion. A computer includes (i) a video data analyzer for detecting and for tracking scene objects and their locations, (ii) an environmental model builder combining multiple scene images to build a 3-D dynamic model recording scene objects and their instant spatial locations, (iii) a viewer criterion interpreter, and (iv) a visualizer for generating from the 3-D model in accordance with the viewing criterion one or more particular 2-D video image(s) of the scene. A video display receives and displays the synthesised 2-D video image(s). According to the present invention, there is provided a method of processing image data as set out in claim 1. The present invention also provides an apparatus for processing image data as set out in claim 11. The present invention also provides a storage medium storing instructions as set out in claim 18. The present invention also provides a signal conveying instructions as set out in claim 19. In an embodiment, image data relating to an object in a scene and image data relating to the shadow of the object is identified using image data from at least two cameras which have different views of the object. In an embodiment, image data of an object and its shadow from a first camera and image data of the object and its shadow from a second camera is transformed to a common modelling space, and a part of the image data relating to the object and a part of the image data relating to the shadow are identified on the basis of the transformed data. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically shows the components of an embodiment; Figure 2 schematically illustrates the collection of video data from a dynamic environment in an embodiment; Figure 3 shows, at a top level, the processing operations performed in an embodiment to process signals defining moving pictures, to create a three-dimensional computer model and to display images to a user from a desired viewing direction; Figure 4 shows the processing operations performed at step S3a or step S3b in Figure 3; Figure 5 shows the processing operations performed at step S6a or step S6b in Figure 3; Figure 6 shows the processing operations performed at step S32 in Figure 5; Figure 7 shows the processing operations performed at step S40 in Figure 5 (and at steps S106 and S112 in Figure 8); Figure 8 shows the processing operations performed at step S8 in Figure 3; Figures 9a, 9b and 9c schematically illustrate an example configuration of objects and cameras, and the images recorded by each camera for the illustrated configuration; Figure 10 shows the processing operations performed at step S10 in Figure 3; Figure 11 schematically illustrates bounding rectangles in 3D world space produced for the configuration of objects and cameras shown in Figure 9; Figure 12 shows the processing operations performed in a first embodiment at step S11a or step S11b in Figure 3; Figures 13a, 13b and 13c schematically illustrate the formation of a model of an object in accordance with steps S230 and S232 in Figure 12; Figure 14 shows the processing operations performed at step S16 in Figure 3; Figure 15 shows the processing operations performed at step S302 in Figure 14; Figure 16 shows the processing operations performed at step S304 or step S308 in Figure 14; Figure 17 shows the processing operations performed in a second embodiment at step S11a or step S11b in Figure 3; Figure 18 shows the processing operations performed in the second embodiment at step S250 in Figure 17; and Figure 19 shows the processing operations performed in the second embodiment at step S264 in Figure 18. First Embodiment Figure 1 is a block diagram showing the general arrangement of an image processing apparatus in a first embodiment. In the apparatus, there is provided a computer 2, which comprises a central processing unit (CPU) 4 connected to a memory 6 operable to store a program defining the operations to be performed by the CPU 4, and to store object and image data processed by CPU 4. Coupled to the memory 6 is a disk drive 8 which is operable to accept removable data storage media, such as a floppy disk 10, and to transfer data stored thereon to the memory 6. Operating instructions for the central processing unit 4 may be input to the memory 6 from a removable data storage medium using the disk drive 8. Image data to be processed by the CPU 4 may also be input to the computer 2 from a removable data storage medium using the disk drive 8. Alternatively, or in addition, image data to be processed may be input to memory 6 directly from a plurality of cameras (schematically illustrated as video cameras 12a and 12b in Figure 1) which, in this embodiment, have digital image data outputs. The image data may be stored in cameras 12a and 12b prior to input to memory 6, or may be transferred to memory 6 in real time as the data is gathered by the cameras. Image data may also be input from non-digital video cameras instead of digital cameras 12a and 12b. In this case, a digitiser (not shown) is used to digitise images taken by the camera and to produce digital image data therefrom for input to memory 6. Image data may also be downloaded into memory 6 via a connection (not shown) from a local or remote database which stores the image data. Coupled to an input port of CPU 4, there is a user-instruction input device 14, which may comprise, for example, a keyboard and/or a position-sensitive input device such as a mouse, a trackerball, etc. Also coupled to the CPU 4 is a memory buffer 16, which, in this embodiment, comprises three frame buffers each arranged to store image data relating to an image generated by the central processing unit 4, for example by providing one (or several) memory location(s) for a pixel of an image. The value stored in the frame buffer for each pixel defines the colour or intensity of that pixel in the image. Coupled to the frame buffers 16 is a display unit 18 for displaying image data stored in a frame buffer 16 in a conventional manner. Also coupled to the frame buffers 16 is a video tape recorder (VTR) 20 or other image recording device, such as a paper printer or 35mm film recorder. A mass storage device 22, such as a hard disk drive, having a high data storage capacity, is coupled to the memory 6 (typically via the CPU 4), and also to the frame buffers 16. The mass storage device 22 can receive data processed by the central processing unit 4 from the memory 6 or data from the frame buffers 16 which is to be displayed on display unit 18. Data processed by CPU 4 may also be exported from computer 2 by storing the data via disk drive 8 onto a removable storage device, or by transmitting the data as a signal, for example over a communication link such as the Internet, to a receiving apparatus. The CPU 4, memory 6, frame buffer 16, display unit 18 and the mass storage device 22 may form part of a commercially available complete system, such as a personal computer (PC). Operating instructions for causing the computer 2 to perform as an embodiment of the invention can be supplied commercially in the form of programs stored on floppy disk 10 or another data storage medium, or can be transmitted as a signal to computer 2, for example over a datalink (not shown), so that the receiving computer 2 becomes reconfigured into an apparatus embodying the invention. The operating instructions may also be input via user-input device 14. Figure 2 schematically illustrates the collection of image data for processing by the CPU 4 in an embodiment. By way of example, Figure 2 shows two vehicles, 30, 32 travelling along a road 34 towards a pedestrian crossing 36 at which two people 50, 52 are crossing. The road 34, crossing 36, and the movements of the vehicles, 30, 32 and people 50, 52 thereon are recorded by two video cameras 12a and 12b which, in this embodiment, are mounted at fixed viewing positions, have fixed viewing directions and have fixed zoom (magnification) settings. Cameras 12a and 12b are arranged so that their fields of view overlap for at least the portion of the road 34 and crossing 36 on which the vehicles 30, 32 and people 50, 52 may move. Figure 3 shows the image processing operations performed in this embodiment. In Figure 3, processing steps which are the same but which are performed separately on the data from video camera 12a and the data from video camera 12b are identified with the same reference number together with the letter "a" or "b" (referring to the camera 12a or the camera 12b respectively) depending upon the video data processed. Referring to Figure 3, at step S2, a three-dimensional computer model of the static background (that is, non-moving parts) against which objects will move is created by a user. Thus, with reference to the example shown in Figure 2, the road 34, crossing 36 and their surroundings are modelled. This is carried out in a conventional way, for example using a commercially available modelling package to model the background as separate objects, or using a modelling package such as Photomodeller by EOS Systems Inc. to facilitate the modelling of the background from images. At steps S3a and S3b, image parameters for the background scene of each camera are set. Figure 4 shows the processing steps performed at step S3a and step S3b in Figure 3. The processing is the same for each camera, and accordingly the processing for only camera 12a will be described. Referring to Figure 4, at step S22, a plurality of reference images of the static background are recorded using camera 12a. In this embodiment, ten frames of video are recorded. A plurality of reference images are recorded to take account of temporal changes in the lighting conditions of the background, noise, and unwanted movements within the "static" background (which could be caused by moving branches and leaves on trees etc, for example), as will be explained further below. At step S24, the transformation between image space (that is, an image recorded by camera 12a) and three-dimensional (3D) world space (that is, the space in which the three-dimensional computer model was created at step S2) is calculated. The transformation defines a mapping between the ground plane (the plane upon which the objects move) in image space and the ground plane in the 3D world space (3D computer model). This transformation is calculated because, in this embodiment, the absolute position of the camera, or the position of the camera relative to the scene being viewed, is not previously determined, and similarly the camera imaging parameters (focal length, size of the charge coupled device, zoom setting, etc) are not previously determined. The transformation enables a representation of an object to be created in the 3D computer model in a reliable and efficient way on the basis of the position and extents of the object in image space, as will be described later. To calculate the transformation at step S24, one of the images of the background recorded at step S22 is displayed to a user on display device 18, and the user designates, upon prompting by CPU 4, a plurality of points (in this embodiment, four points) in the image which lie on a plane on which objects in the scene will move and which also lie within the field of view of both camera 12a and camera 12b. Thus, referring to the example shown in Figure 2, corner points 38, 40, 42 and 44 defined by the pedestrian crossing markings may be designated (these lying on the road surface 34 on which the vehicles 30, 32 and people 50, 52 will move). The points in the three-dimensional computer model created at step S2 corresponding to the points identified in the video image are also defined by the user. For example, a view of the three-dimensional computer model for a predetermined viewing direction may be displayed to the user on display device 18 and the corresponding points designated using the input means 14. Using the positions of the points designated in the video image and the positions of the corresponding points designated in the three-dimensional computer model, CPU 4 then calculates the transformation between image space and 3D world space in a conventional manner, for example using the equation: where: n = 1....4; X n ,Y n are points in world space; x n ,y n are points in image space; and A-H are obtained by: This defines a transformation between the ground plane in image space and the ground plane in the 3D computer model (3D world space). When step S24 is performed for camera 12b in step S3b, the four reference points selected from the image recorded by camera 12b and the four reference points in the three-dimensional computer model are chosen to be the same as the reference points selected at step S24 for camera 12a. In this way, the scale of the 3D world space for cameras 12a and 12b is made the same. At step S26, CPU 4 calculates reference image pixel parameters for the static background. This is performed by calculating the mean grey level, µ, for each pixel from the plurality of images recorded at step S22. That is, the grey level for corresponding pixels in each ten frames is considered and the average taken. The variance, σ, of the determined mean is also calculated. A "window" for the grey level of each pixel is then set as µ ± (2σ + F) where F is an error factor set to take account of variables such as the gain of video camera 12a, and noise etc. In this embodiment, the total number of grey scale levels is 256, and the error factor F is set to 5 grey scale levels. The "window" set at step S26 for each pixel represents the spread of grey scale values which the pixel should take if it forms part of an image of the static background (the viewing position and direction of video camera 12a being constant so that the grey scale value of a pixel forming part of the background should only change in dependence upon lighting changes and errors due to noise). As will be described below, these "windows" are used to identify objects which are not part of the background (and hence cause the pixel values recorded by camera 12a to move outside the defined windows). Referring again to Figure 3, at steps S4a and S4b, images of "action", that is images in which there is movement of an object over the background (for example movement of vehicles 30, 32 on the road surface 34 and people 50, 52 on the pedestrian crossing 36), are recorded by video camera 12a and video camera 12b. The video frames recorded by cameras 12a and 12b are time-stamped to enable temporally corresponding frames to be used in subsequent processing. At steps S6a and S6b, CPU 4 processes time-synchronised images, that is, image data for an image recorded by camera 12a at step S4a and an image recorded at the same time by camera 12b at step S4b, to identify objects in the images which are not part of the "static background", that is, objects which are moving over the background or are stationary against the background. CPU 4 then projects these objects into the common world space (three-dimensional computer model) defined at step S2. Figure 5 shows the processing operations performed by CPU 4 at step S6a and step S6b. The processing is the same for each camera, and accordingly the processing for only camera 12a will be described. Referring to Figure 5, at step S30, CPU 4 compares the grey level of each pixel in the image data being processed with the grey scale "window" previously set at step S26 for the corresponding pixel in the image. Any pixel which has a grey level outside the predefined window for that pixel is considered potentially to be a "foreground" pixel, that is, a pixel which forms part of an object moving or stationary on the background. At step 530, CPU 4 therefore keeps a record of which pixels have grey scale levels outside the corresponding precalculated window. At step S32, CPU 4 processes the image data to remove noise. Such noise may have been introduced into the image data in a number of ways; for example by quantum effects if video camera 12a is a charged coupled device (CCD) camera, by data compression techniques used to compress the data from camera 12a, by a frame grabber used to capture frames of the video data for processing by CPU 4 etc, or may be noise which often occurs in image data near the boundaries of moving objects. Figure 6 shows the operations performed by CPU 4 in processing the image data to remove noise at step S32 in Figure 5. Referring to Figure 6, at step S50, CPU 4 applies a "shrinking" mask to the image data in a conventional manner, for example as described in "Computer and Robot Vision Volume 2" by R.M. Haralick and L.G. Shapiro, Addison-Wesley Publishing Company, 1993 ISBN 0-201-56943-4 (v.2), page 583. This operation involves applying a 3x3 pixel mask to the image data and counting the number of "foreground" pixels (identified at step S30) and the number of "background" pixels within each set of nine pixels defined by the mask. If the majority of pixels within the mask are background pixels, then the centre pixel is defined to be a background pixel (even if it was previously identified as a foreground pixel). No change is made if the majority of pixels within the mask are foreground pixels. This operation is repeated until the shrinking mask has been applied over the whole image data. At step S52, CPU 4 applies a "growing mask" to the image in a conventional manner, for example as described in "Computer and Robot Vision Volume 2" by R.M. Haralick and L.G. Shapiro, Addison-Wesley Publishing Company, 1993 ISBN 0-201-56943-4 (v.2), page 583. This operation is performed in the same way as step S50, with the exception that, if the majority of pixels within the mask are foreground pixels, then the centre pixel is defined to be a foreground pixel (even if it was previously identified as a background pixel) and no change is made if the majority of pixels within the mask are background pixels. The effect of step S52 is to return pixels which were erroneously set as background pixels by the shrinking mask operation in step S50 to foreground pixels. Referring again to Figure 5, at step S34, CPU 4 processes the data to identify clusters of foreground pixels. This is performed in a conventional manner for identifying clusters of pixels with the same characteristics by scanning the image data to identify a foreground pixel and then considering neighbouring pixels in an iterative manner to identify all connected foreground pixels. At step S36, CPU 4 considers the next cluster of foreground pixels identified at step S34 (this being the first cluster the first time step S36 is performed) and determines whether the number of pixels in the cluster is greater than 30. If the number of pixels is less than or equal to 30, the cluster is considered to represent noise since it forms a relatively small part of the overall image (768 pixels by 512 pixels in this embodiment). In this case, the cluster is not processed further. On the other hand, if the number of pixels in the cluster is greater than 30, then the cluster is considered to represent a foreground object and further processing is performed. At step S38, CPU 4 determines the extents of the cluster of pixels. In this embodiment, CPU 4 performs this operation by determining the bounding rectangle of the cluster within the two-dimensional image having sides parallel to the sides of the image. At step S40, CPU 4 projects the bounding rectangle determined at step S38 into the three-dimensional world space in which the computer model was formed at step S2 using the transformation calculated at step S24. This produces a single plane in the three-dimensional computer model at a position determined by the position of the object in the video image. In this embodiment, the plane in the three-dimensional computer model is defined to be vertical, and has its base on the surface within the 3D model defined by the points selected by the user at step S24 (since it is assumed that objects within the scene being viewed move on the corresponding real-world surface - the road surface 34 in the example of Figure 2). Figure 7 shows the operations performed by CPU 4 in transforming the bounding plane at step S40 in Figure 5. Referring to Figure 7, at step S62, CPU 4 projects the two corners of the bounding rectangle base from image space into three-dimensional world space by transforming the coordinates using the transformation previously calculated at step S24. Each corner of the bounding rectangle base is transformed to a point in the three-dimensional world space of the computer model which lies on the surface defined by the points previously selected at step S24. At step S64, CPU 4 calculates the width of the bounding rectangle in three-dimensional world space by determining the distance between the corners transformed at step S62. At step S66, CPU 4 calculates the height of the bounding rectangle in three-dimensional world space using the ratio of the width-to-height of the bounding rectangle in image space and the width in three-dimensional world space calculated at step S64 (that is, the aspect ratio of the bounding rectangle is kept the same in image space and three-dimensional world space). Referring again to Figure 5, at step S42 CPU 4 stores the position and size of the bounding rectangle in three-dimensional world space previously calculated at step S40, together with texture data for the bounding rectangle extracted from the bounding rectangle within the video image, and a "foreground mask", that is, a mask identifying which of the pixels within the bounding rectangle correspond to foreground pixels. The extracted texture data effectively provides a texture map for the bounding rectangle in the 3D world space. At step S44, CPU 4 determines whether there is another cluster of foreground pixels identified at step S34 which has not yet been processed. Steps S36 to S44 are repeated until all clusters of foreground pixels for the video frame under consideration have been processed in the manner described above. At that stage, a three-dimensional computer model has been produced of the objects seen by camera 12a. In the model, a single planar surface (bounding rectangle) has been placed to represent the position of each moving object, and texture image data for these moving objects has been stored. This data therefore corresponds to a three-dimensional computer model of a single two-dimensional image (video frame) from camera 12a. (A corresponding three-dimensional computer model for the temporally-corresponding frame of image data recorded by camera 12b is produced at step S6b.) Referring again to Figure 3, at step S8, CPU 4 processes the three-dimensional computer models created at steps S6a and S6b to identify parts of objects in the three-dimensional computer models which correspond to shadows in the initial video images. Each object and its associated shadow are then stored as separate objects in the computer model. This processing will now be described. As described above, in this embodiment, each foreground object identified in a video image is assumed to touch the ground in the real-world at its lowest point, and the corresponding planar bounding rectangle for the object is placed in the 3D world space with its base on the ground surface within the 3D model. However, any shadows which an object casts will be identified as part of the object when steps S6a and S6b described above are performed because a shadow is attached to, and moves with, an object in an image. The bounding rectangle will therefore enclose both the shadow and the object, and hence if the shadow is beneath the object in the video image, the base of the bounding rectangle will be determined by the lowest point of the shadow. In this situation, the object will effectively stand on its shadow in the three-dimensional model. The processing performed by CPU 4 at step S8 addresses this problem, therefore. Figure 8 shows the processing operations performed by CPU 4 when carrying out the shadow processing at step S8 in Figure 3. Referring to Figure 8, at step S100, CPU 4 transforms each pixel of the image data for the next pair of corresponding objects (this being the first pair the first time step S100 is performed) from image space into the 3D world space. That is, CPU 4 maps each pixel of the image data stored at step S42 (Figure 5) for an object recorded by camera 12a from the image space of camera 12a to the 3D world space, and maps each pixel of the image data stored for the corresponding object recorded by camera 12b in the corresponding temporal frame from the image space of camera 12b to the 3D world space. In this embodiment, corresponding objects are identified from the coordinates of the corners of the bounding rectangle bases in the 3D world space - a given object will have corner coordinates within a predetermined distance of the corner coordinates of the corresponding object, and the mapping of the pixel of the image data is performed using the transformation previously defined for each camera at step S24. The mapping between the image space of each camera and the 3D world space of the computer-model calculated at step S24 is only valid for points which are on the real-world ground plane (the road surface in the example of Figure 2). Consequently, after this transformation has been applied at step S100, the transformed image data for the shadow of an object recorded by one camera will align in the 3D world space with the transformed image data for the shadow of the same object recorded by the other camera because the shadows lie on the real-world ground plane. On the other hand, the objects themselves are not on the real-world ground plane and, therefore, the transformed image data will not align in the 3D world space for the different cameras (this image data having being "incorrectly" mapped into the 3D world space when the mapping transformation is applied). Accordingly, also at step S100, CPU 4 compares the transformed image data for the corresponding objects. In this embodiment, the comparison is performed in a conventional manner, for example based on the method described in Chapter 16 of "Computer and Robot Vision Volume 2" by R.M. Haralick and L.G. Shapiro, Addison-Wesley Publishing Company, 1993, ISBN 0-201-56943-4 (v.2), by comparing the transformed pixel values on a pixel-by-pixel basis and identifying pixels as being the same if their grey scale values (and/or colour values) are within a predetermined amount of each other (in this embodiment 10 grey levels within a total of 256 levels). CPU 4 stores data defining the boundary between the aligned portion of the object data (the shadow) and the non-aligned portion (the object) for subsequent use (this boundary effectively representing the "footprint" of the object on the ground, that is, the outline of the points at which the object touches the ground). At step S102, CPU 4 extracts the portion of the corresponding objects which was identified at step S100 as being aligned. As noted above, this portion corresponds to the shadow of the object, and is stored by CPU 4 as a separate object on the ground plane of the 3D model for each camera. At step S104, CPU 4 considers the image data recorded by camera 12a which was identified in step S100 as being non-aligned with the transformed image data from the corresponding object recorded by camera 12b. CPU 4 determines the bounding rectangle of this image data in the image space of camera 12a (this step corresponding to step S38 in Figure 5 but this time for the object without its shadow). At step S106, CPU 4 projects the new bounding rectangle determined at step S104 from the image space of the first camera into the 3D world space. This step corresponds to, and is performed in the same way as, step S40 in Figure 5. At step S108, CPU 4 stores the position and size of the transformed bounding rectangle, together with the image data and associated "foreground mask" as object data for the 3D model of the first camera (this step corresponding to step S42 in Figure 5). The object data stored at step S102 (for the shadow) and the object data stored at step S108 (for the object without its shadow) replaces the composite object data previously stored for the object and its shadow at step S42. At steps S110 to S114, CPU 4 repeats steps S104 to S108 for the second camera (camera 12b). Again, the data stored at steps S110 and S114 for the shadow and the object as separate objects replaces the composite data previously stored at step S42 for the second camera. At step S116, CPU 4 determines whether there is another pair of corresponding objects in world space to be processed. Steps S100 to S116 are repeated until all corresponding objects have been processed in the manner described above. Referring again to Figure 3, at step S10, CPU 4 processes the object data for the different cameras to determine whether any object is actually made up of two objects which should be represented separately, and to perform appropriate correction. This processing is performed for objects, but not for shadows. By way of example, Figure 9a schematically shows one configuration of the people 50, 52 and the cameras 12a and 12b which may lead to two objects being represented as a single object in the 3D world space of camera 12a. In the example of Figure 9a, the viewing direction of camera 12a is such that the person 50 is directly behind the person 52. In this situation, as illustrated in Figure 9b, the people 50, 52 appear as one object in the image(s) recorded by camera 12a at the particular time(s) at which the people maintain this alignment. Accordingly, at step S6a, CPU 4 would identify the image data for the two people as a single foreground object, define a bounding rectangle surrounding this image data, and project the bounding rectangles into the 3D world space for camera 12a. On the other hand, as illustrated in Figure 9c, the image data recorded by camera 12b for the example configuration of the people shown in Figure 9a clearly shows the people as separate objects. At step S6b, therefore, CPU 4 would identify the people as separate foreground objects, define separate bounding rectangles, and project the separate bounding rectangles into the 3D world space for camera 12b. Figure 10 shows the processing operations performed by CPU 4 at step S10 in Figure 3. Referring to Figure 10, at step S200, CPU 4 compares the heights in the 3D world space of the bounding rectangle of the next object (this being the first object the first time step S200 is performed) from camera 12a and the bounding rectangle of the corresponding object from camera 12b. In this embodiment, corresponding objects are identified using the coordinates of the corners of the base of each bounding rectangle in the 3D world space, since these coordinates will be within a predetermined distance for corresponding objects. At step S202, CPU 4 determines whether the heights of the bounding rectangles are the same or within a predetermined error limit (in this embodiment 10%). Since, at step S24 (Figure 4) the same reference points are chosen to determine the mapping from the image space of each camera to the 3D world space, and since each camera has a constant zoom setting, the object data for each camera will have the same scale. Consequently, as illustrated in Figure 11, the bounding rectangle 100 generated from the image data of camera 12a and representing the composite object comprising the two people 50, 52 has a greater height in the 3D world space than the bounding rectangle 102 generated from the image data of camera 12b and representing the person 52. Accordingly, if it is determined at step S202 that the heights of the bounding rectangles are the same, then each bounding rectangle is determined to represent a single object, and processing proceeds to step S214. On the other hand, if it is determined that the heights are not the same, CPU 4 determines that the tallest bounding rectangle represents a composite object which should be split into its constituent objects, and processing proceeds to step S204. At step S204, CPU 4 splits the tallest bounding rectangle into a bottom portion (106 in Figure 11) which has the same height as the bounding rectangle 102 of the corresponding object and a top portion (108 in Figure 11) which comprises the remainder of the original tall bounding rectangle 100. The bottom portion 106 and its associated image data are stored as a single object, since this represents the single object (person 52) which is in front of the other object (person 50) in the composite object. CPU 4 then performs processing at steps S206 to S212 to represent the rear object (that is, the object furthest from the camera 12a) at the correct position in the 3D model, as described below. As step S206, CPU 4 projects the base corners of all of the bounding rectangles of the camera which produced the correct height bounding rectangle (102 in Figure 11) from world space to the image space of the camera (camera 12a) which produced the tall bounding rectangle (100 in Figure 11) split at step S204. This transformation is performed using the reverse transformation to that calculated at step S24 in Figure 4 (described above) for the camera which produced the tall bounding rectangle (camera 12a). At step S208, CPU 4 identifies which of the bounding rectangle bases projected at step S206 overlaps the "oversized" bounding rectangle which encloses the image data of the composite object in the image space of the camera which recorded the composite object. In this embodiment, this is done by comparing the projected base corner coordinates with the coordinates of the four corners of the oversized bounding rectangle. (In steps S208 and S210, it is only necessary to project and compare the coordinates of the corner points of the bases of the bounding rectangles because at least part of a projected base will lie behind the "oversized" bounding rectangle when the base belongs to the bounding rectangle enclosing the rear object in the composite object.) At step S210, the bounding rectangle base identified at step S208 is projected back into the 3D world space using the transformation previously calculated at step S24 for the camera which produced the oversized bounding rectangle (camera 12a). This re-projected bounding rectangle base represents the correct position in the 3D world space for the rear object from the composite image. Accordingly, CPU 4 repositions the top portion 108 of the bounding rectangle which was split off from the tall bounding rectangle at step S204 so that the base of the top portion runs along the re-projected base and the centre of the base of the top portion and the centre of the re-projected base are at the same position in 3D world space. At step S212, CPU 4 stores as a separate object in the object data for the camera which produced the initial oversized bounding rectangle (camera 12a) the repositioned top portion together with the image data therefor which was identified at step S204. After performing steps S200 to S212, CPU 4 has separated the composite object into its constituent objects and stored these as separate objects in the object data for the associated camera. This processing is effective in preventing sudden height changes in an object which, even though they may only last for a small number of image frames reproduced from the 3D computer model, have been found to be very noticeable. Referring again to Figure 9b, it will be seen that only part, and not all, of person 50, is above person 52 in the image data. Accordingly, when the top portion 108 of the bounding rectangle 100 is separated at step S204, the image data for only part of person 50 is separated. Steps S206 to S212 therefore produce a representation in which the image data for the object furthest from the camera (person 50) in the composite object merely corresponds to the top portion of the actual object. Further, the image data retained in the bottom portion 106 of rectangle 100 at step S204 for the object closest to the camera (person 52) may actually contain image data corresponding to the object furthest from the camera (if this lies in the bottom portion of the bounding rectangle). These "incorrect" representations have been found, however, to be relatively unnoticeable in images produced from the three-dimensional computer model, particularly because the composite objects only exist for a relatively small number of video frames (the alignment of the objects changing in the scene). At step S214 in Figure 10, CPU 4 determines whether there is another object in the 3D world space for the first camera. Steps S200 to S214 are repeated until all such objects have been processed as described above. Referring again to Figure 3, at steps S11a and S11b, CPU 4 produces representations (models) of each object in the 3D world space. More particularly, in step S11a, CPU 4 produces a representation in the object data for the first camera of each foreground object identified at step S6a. Similarly, in step S11b, CPU 4 produces 3D models for the 3D object data of the second camera. It should be noted that in steps S11a/S11b, CPU models the objects, but not their shadows. This is because the shadows have already been accurately modelled at step S8 (Figure 3). Figure 12 shows the processing operations performed by CPU 4 at steps S11a and S11b. The processing is the same for each camera, and accordingly the processing for only camera 12a performed in step S11a will be described. Referring to Figure 12, at step S230, CPU 4 reads the data previously stored at step S100 (Figure 8) defining the ground profile ("footprint") of the next object in the object data of camera 12a (that is, the contact profile of this object on the ground - the points at which the object touches the ground plane in the 3D world space). CPU 4 then approximates the "footprint" by fitting a plurality of straight lines to the shape of the boundary in a conventional manner, for example by applying a pixel mask to the boundary pixels to calculate a gradient value for each pixel and then applying a "t-statistic" test such as based on that described in Section 11.4 of "Computer and Robot Vision Volume 1" by R.M. Haralick and L.G. Shapiro, Addison-Wesley Publishing Company, 1992, ISBN 0-201-10877-1 (v.1). Step S230 is illustrated in Figure 13a and Figure 13b for an example ground boundary shape 150, representing the ground profile of vehicle 32. (Although the example boundary shape illustrated in Figure 13a already comprises substantially straight lines, curved footprints may also be approximated.) At step S232, CPU 4 uses the footprint approximation 160 defined at step S230 to define a model to represent the object in the 3D world space. In this embodiment, CPU 4 defines a plurality of planar surfaces 170, 172 (in this embodiment rectangles) in the 3D world space, with each respective rectangle having a base corresponding to one of the straight lines defined in steps S230, and vertical sides each having a height in the 3D world space corresponding to the height of the single bounding rectangle previously used as a representation of the object in the 3D world space (stored at step S42 in Figure 5 and subsequently modified if necessary at step S8 and/or step S10 in Figure 3). Step S232 is illustrated in Figure 13c. At step S234, CPU 4 maps each rectangle defined at step S232 from the 3D world space into the image space of camera 12a. This is performed by applying the inverse of the transformation previously calculated at step S24 (Figure 4) for camera 12a to the coordinates of the corners of the base of each rectangle, and assuming that the aspect ratio (that is, the ratio of height-to-width) of the transformed rectangle in the image space of camera 12a is the same as the aspect ratio of the rectangle in the 3D world space, thereby enabling the height of the transformed rectangle in image space to be determined. At step S236, CPU 4 extracts the pixel data which lies within each of the transformed rectangles in the image space of camera 12a. At step S238, CPU 4 stores as object data for camera 12a the position and size of the planar surfaces in the 3D world space (defined at step S232), the image data associated with each rectangle (defined at step S236) and the foreground mask for the image data (based on the data stored previously at step S42). At step S240, CPU 4 determines whether there is another object to be modelled for camera 12a. Steps S230 to S240 are repeated until all objects have been modelled as described above. Referring again to Figure 3, at steps S12a and S12b, CPU 4 stores the object data produced in steps S11a/S11b as time-stamped 3D object data for each camera. At step S14, CPU 4 determines whether there is another image recorded by cameras 12a and 12b to be processed. Steps S6a/S6b, S8, S10, S11a/S11b, S12a/S12b and S14 are repeated until all such images have been processed in the manner described above. At that stage, there is stored in memory 6 two time-stamped sets of 3D object data, one for each of the cameras 12a and 12b. At step S16, CPU 4 displays images to a user on display device 18 from any desired viewpoint selected by the user. The images displayed in this step by CPU 4 are simulated video images produced using the three-dimensional model object data previously created. Figure 14 shows the processing operations performed by CPU 4 in displaying the images at step S16. Referring to Figure 14, at step 5296 the direction from which the object is to be viewed is defined by the user using input device 14. At step S298, CPU 4 determines whether the viewing direction selected by the user is within a predetermined angle of a vertical viewing direction looking down on the objects. In this embodiment, CPU 4 determines whether the selected viewing direction is within ±15° of vertical. If it is determined at this step that the user has selected a viewing direction within such a cone having a vertical axis and a semi-angle of 15°, processing proceeds to step S300, in which CPU 4 renders data to the frame buffer 16 which schematically represents the positions of all of the objects on the ground plane. In this embodiment, this is carried out using the object data from a predetermined camera (for example camera 12a) and rendering a vertical view of the static background scene using the common 3D model created at step S2 together with a graphic, such as a cross, or other visual indicator at a position on the ground plane corresponding to the position of each object determined from positions of the objects in the object data for the chosen camera. Steps S298 and S300 comprise processing which enables a user to obtain an aerial view showing the positions of all objects. Such a view may be useful in a number of situations. For example, if the recorded video data related to an accident or crime, the image data produced at steps S298 and S300 may be used by the police gathering evidence to determine the relative positions of the vehicles and/or people at different times. Similarly, where the objects are people participating in a team sport, the image data produced in steps S298 and S300 may be used for training purposes to analyse the players' relative positions, etc. If it is determined at step S298 that the user-selected viewing direction is not within the predetermined angle, CPU 4 performs processing to render a realistic view of the objects to the user from the chosen viewing direction, as described below. At step S302, CPU 4 determines whether to use the object data produced from the image data of camera 12a (stored at step S12a) or the object data produced from the image data of camera 12b (stored at step S12b) to produce the image data for display to the user. Figure 15 shows the operations performed by CPU 4 in selecting the object data at step S302. Referring to Figure 15, at step S400, CPU 4 compares the viewing direction selected by the user at step S296 with the viewing direction (optical axis) of camera 12a and the viewing direction of camera 12b, and identifies the camera which has the viewing direction closest to the viewing direction selected by the user. At step S402, CPU 4 determines whether the viewing direction of the camera which was not identified at step 5400 (that is, the camera having the viewing direction which is furthest from the user-selected viewing direction) is within a predetermined angle (cone) of the viewing direction selected by the user. In this embodiment, CPU 4 determines whether the axis is within ±30° of the selected viewing direction. If the axis of the camera is outside this predetermined angle, the quality of the image produced by the camera for the viewing direction chosen by the user may be significantly inferior to that produced by the camera identified at step S400. Accordingly, in this case, processing proceeds to step S428, at which CPU 4 selects the object data from the camera having the closest viewing direction to that selected by the user. On the other hand, if it is determined at step S402 that the viewing direction of the other camera is within the predetermined angle, the object data for the camera whose viewing direction is furthest from the user-selected viewing direction may actually produce a better quality image for the chosen viewing direction under certain conditions. Accordingly, in this case, CPU 4 performs further tests at steps S404 to S426 to determine whether a better quality image may result from using the camera with the viewing direction furthest from that selected by the user than with the camera identified at step S400 whose viewing direction is closest to that selected by the user. At steps S404 to S414, CPU 4 compares characteristics of cameras 12a and 12b which will affect the quality of an image reproduced from the three-dimensional object data for the cameras, as will now be described. At step S404, CPU 4 compares the method used to transfer the image data from camera 12a to computer 2 and the method used to transfer the image data from camera 12b to computer 2. This information may be input by a user prior to processing or may be transmitted with the image data from the cameras. At step S406, CPU 4 determines whether the transfer methods are the same. If they are not, at step S408, CPU 4 selects the object data from the camera with the highest quality transfer method (that is, the transfer method which will typically introduce the smallest number of errors into the image data). In this embodiment, transfer of the image data via a cable and/or recording medium (for example if the image data is recorded on a recording medium in the camera which is subsequently transferred to computer 2) is considered to introduce fewer errors than if the image data is transferred by radio transmission. Accordingly, if one of the cameras transfers data by radio transmission, at step S408, CPU 4 selects the object data from the other camera. On the other hand, if it is determined at step S406 that the image data transfer methods are the same, processing proceeds to step S410, at which CPU 4 compares the resolutions of the cameras. In this embodiment, this is carried out by comparing the number of pixels in the images from the cameras. At step S412, CPU 4 determines whether the camera resolutions are the same. If the resolutions are not the same, processing proceeds to step S414, at which CPU 4 selects the object data from the camera with the highest resolution (the highest number of pixels). On the other hand, if it is determined at step S412 that the cameras have the same resolution, CPU 4 proceeds to compare characteristics of the image data produced by the cameras. More particularly, processing proceeds to step S416, at which CPU 4 compares the stability of the images produced by cameras 12a and 12b. In this embodiment, this is carried out by comparing the optical image stabilisation parameters calculated by each camera in dependence upon the amount of "shake" of the camera, and transmitted with the image data. At step S418, CPU 4 determines whether the stabilities compared at step S416 are the same. If the stabilities are not the same, at step S420, CPU 4 selects the object data from the camera with the highest image stability. On the other hand, if it is determined at step S418 that the image stabilities are the same, processing proceeds to step S422, at which CPU 4 compares the number of occluded objects within the object data from each camera. In this embodiment, this is carried out by comparing the number of bounding rectangles for each set of object data which were split when processing was performed at step S10 (each bounding rectangle that was split representing an occlusion). At step S424, CPU 4 determines whether the number of occluded objects is the same for each camera. If the number is not the same, then, at step S426, CPU 4 selects the object data from the camera which produces the smallest number of occlusions. On the other hand, if it is determined at step S424 that the number of occluded objects is the same, then processing proceeds to step S428, at which CPU 4 selects the object data from the camera with the viewing direction closest to that chosen by the user. This data is selected because CPU 4 has determined at steps S406, S412, S418 and S424 that the inherent camera characteristics and image data characteristics affecting image quality are the same for both cameras, and therefore determines that the best quality image will be produced using object data from the camera which is aligned most closely with the user's chosen viewing direction. Referring again to Figure 14, at step S304, CPU 4 renders the selected object data to a frame buffer 16. Figure 16 shows the operations performed by CPU 4 in rendering the object data at step S304. Referring to Figure 16, at step S500, the 3D world space of the object data selected at step S302 is transformed into a viewing space in dependence upon the viewing position and direction selected at step S296 (Figure 14). This transformation identifies a particular field of view, which will usually cover less than the whole modelling space. Accordingly, at step S502, CPU 4 performs a clipping process to remove surfaces, or parts thereof, which fall outside the field of view. Up to this stage, the object data processed by CPU 4 defines three-dimensional co-ordinate locations. At step S504, the vertices of the triangular surfaces making up the 3D computer model are projected to define a two-dimensional image. After projecting the image into two dimensions, it is necessary to identify the triangular surfaces which are "front-facing", that is facing the viewer, and those which are "back-facing", that is cannot be seen by the viewer. Therefore, at step S506, back-facing surfaces are identified and culled in a conventional manner. Thus, after step S506, vertices are defined in two dimensions identifying the triangular surfaces of visible polygons. At step S508, the two-dimensional data defining the surfaces is scan-converted by CPU 4 to produce pixel values. In this step, as well as rendering the surfaces representing the background in the image, the shadows (stored as separate objects) are rendered, and the surfaces defined in step S11a or step S11b to model each object are also rendered with the video texture data previously stored for those surfaces. Only foreground pixels within the surfaces are rendered with the stored video texture data, these pixels being defined by the stored "foreground masks". The other pixels are rendered with background texture data. The rendered data produced by step S508 represents a simulated video frame, in which the background is produced from the computer model created at step S2 and each object is represented by the model defined at step S11a/S11b, onto which the image data of the object extracted from the video image is projected. At step S510, the pixel values generated at step S508 are written to a frame buffer 16 on a surface-by-surface basis, thereby generating data for a complete two-dimensional image. Referring again to Figure 14, at step S306, CPU 4 determines whether the image to be displayed to the user for the current frame of data being processed and the image displayed to the user from the previous frame of data are derived from different cameras. That is, CPU 4 determines whether there is a camera change in the sequence of images to be viewed by the user. In this embodiment, CPU 4 determines whether this change has occurred by determining whether the object data selected at step S302 for the current frame is derived from a different camera than the object data selected at step S302 for the previous frame. If it is determined at step S306 that a camera change has occurred, at step S308, CPU 4 renders the object data of the other camera for the current frame to a frame buffer 16. This is performed in the same way as the rendering at step S304 (described above with reference to Figure 16) but the data is written to a different frame buffer 16 so as not to overwrite the data rendered at step S304. At step S310, CPU 4 compares the image data rendered at step S304 with the image data rendered at step S308. This comparison is carried out to determine the similarity of the images, and is performed using a conventional technique, for example as described in "Computer and Robot Vision Volume 2" by R.M. Haralick and L.G. Shapiro, Addison-Wesley Publishing Company, 1993, ISBN 0-201-56943-4 (v.2), pages 293-378. At step S312, CPU 4 determines whether the user will see a " jump" in the images if the image data rendered at step S304 is displayed (that is, whether the images will appear discontinuous). Such a jump (discontinuity) may occur because, in this embodiment, the models created for each object at step S11a/S11b are approximate, and because the position of each model in the 3D world-space may not have been determined with complete accuracy and may be different for the object data for each camera. In this embodiment, CPU 4 determines whether the images will be discontinuous by determining whether more than a predetermined number (e.g. 20%) of pixels failed the similarity test performed at step S310. If it is determined at step S312 that successive images will appear discontinuous, then, at step S314, CPU 4 combines the image data rendered at step S304 and the image data rendered at step S308 to produce an image which will not appear discontinuous to the user. In this embodiment, the image combination is performed using a conventional morphing technique, for example as described in "Digital Image Warping" by George Wolberg, IEEE Computer Society Press, ISBN 0-8186-8944-7, pages 222-240. At step S316, CPU 4 determines image quality information for the image data produced at step S304 or S314 for subsequent display to the user. More particularly, in this embodiment, CPU 4 determines the following information representing the standard of the image to be displayed, namely a value representing the degree of reliability (accuracy) of the image. This information may be useful to the user in a situation where the user is attempting to determine what happened in the real-world using the computer model (for example, the police may use recorded video images of a crime to create a computer model and then use the computer model to view the crime from different directions to gather evidence). Because each image generated from the computer model is a simulated image, the accuracy value determined at this step provides an indication of the reliability of the image. In this embodiment, CPU 4 allocates a value between 100% (when the viewing direction selected by the user is the same as the axis of the camera which produced the object data) and 0% (when the viewing direction is off-set from the camera axis by 30°). For off-set angles between 0° and ±30°, the percentage value allocated by CPU 4 varies linearly between 100% and 0%. For off-set angles greater than 30°, CPU 4 allocates a value of 0%. Further, in this embodiment, CPU 4 reduces the allocated percentage value by 10% (subject to a minimum limit of 0%) if an image produced by combining image data at step S314 is to be displayed to the user. CPU 4 also generates a graphic, such as an arrow, or other visual indicator for display to the user, to show which way the view direction can be changed to improve the quality score. At step S318, CPU 4 generates a signal defining the pixel values rendered at step S304 or step S314, and the quality information generated at step S316. The signal is used to generate an image of the objects on display unit 18 and/or is recorded, for example on a video tape in video tape recorder 20. The signal may also be transmitted to a remote receiver for display or recording. Further recordings may, of course, be made from a master recording. The quality information may be combined within the pixel values so that it is always visible in the image with the objects, or it may be selectively displayed and removed upon instruction by the user. At step S320, CPU 4 determines whether there is another time-stamped "frame" of three-dimensional object data previously created at steps S6a and S6b which has not yet been displayed to the user. Steps S296 to 5320 are repeated until all such frames of object data have been displayed in the manner described above, thereby displaying a sequence of simulated moving images to the user from the desired viewing direction. Of course, the user can change the viewing direction at any time during the display. In this embodiment, when step S306 is performed for frames subsequent to the first frame, CPU 4 determines that a camera change has occurred if the previous frame of image data was generated from combined image data (produced at step S314). In this case, at step S308, CPU 4 renders a combined image for the current frame into a frame buffer (that is, an image produced by combining an image produced using object data from the first camera for the current frame with an image produced using object data from the second camera for the current frame). At step S310, CPU 4 then compares the image data rendered at step S304 for the selected camera with the combined image data rendered at step S308. At step S306, if the previous frame was a schematic view of the object positions generated at step S300, CPU 4 determines that there has not been a camera change (since the user would expect to see a "jump" in the images when switching between a schematic view of positions and a realistic view). Second Embodiment A second embodiment of the invention will now be described. This embodiment is the same as the first embodiment with the exception of the processing operations performed by CPU 4 at steps S11a/S11b to model the objects in the 3D world space. In the second embodiment, the modelling of the objects in the 3D world space is performed for each camera together (rather than separately as at steps S11a and S11b in the first embodiment). Figure 17 shows the processing operations performed by CPU 4 to model the objects in the 3D world space. As in the first embodiment, these operations are performed to model objects, but not shadows (which have already been modelled at step S8 in Figure 3). Referring to Figure 17, at step S250, CPU 4 considers the next object to be processed and identifies planes in the image space of each camera upon which points on the object lie. Figure 18 shows the processing operations performed by CPU 4 at step S250. Referring to Figure 18, at step S264, CPU 4 compares the image data for the object recorded by each camera to identify matching points therein. CPU 4 identifies which image data to compare by comparing the coordinates of the corners of the base of the bounding rectangle of the object in the object data for the first camera with the corners of the bases of the bounding rectangles of the objects in the object data from the second camera to determine which bounding rectangle in the object data for the second camera has base corners which lie within a predetermined distance of the base corners of the bounding rectangle in the object data for the first camera in the 3D world space. CPU 4 then compares the image data previously stored for the bounding rectangle in the object data of the first camera (step S6a in Figure 3 with subsequent modification in step S8 or S10 if necessary) and the image data previously stored for the bounding rectangle of the corresponding object in the object data of the second camera (step S6b in Figure 3 with subsequent modification in step S8 or S10 if necessary). In this embodiment, CPU 4 performs this comparison by comparing corner points identified in the image data from the first camera with corner points identified in the image data for the second camera. Figure 19 shows the processing operations performed by CPU 4 at step S264. Referring to Figure 19, at step S282, CPU 4 calculates a value for each pixel in the image data for the object from the first camera indicating the amount of "edge" and "corner" for that pixel. This is done, for example, by applying a conventional pixel mask to the image data and moving this so that each pixel is considered. Such a technique is described in "Computer and Robot Vision Volume 1" by R.M. Haralick and L.G. Shapiro, Section 8, Addison-Wesley Publishing Company, 1992, ISBN 0-201-10877-1 (V.1). At step S284, any pixel which has "edge" and "corner" values exceeding predetermined thresholds is identified as a strong corner in the image data for the first camera in a conventional manner. At step S286, CPU 4 performs the operation previously carried out at step S282 on the image data from the first camera on the image data from the second camera, and likewise identifies strong corners in the image data from the second camera at step S288 using the same technique previously performed at step S284. At step S290, CPU 4 compares each strong corner identified in the image data from the first camera at step S284 with every strong corner identified in the image data from the second camera at step S288 to produce a similarity measure for the corners in the image data from the first and second cameras. In this embodiment, this is carried out using an adaptive least squares correlation technique, for example as described in "Adaptive Least Squares Correlation: A Powerful Image Matching Technique" by A.W. Gruen in Photogrammetry Remote Sensing and Cartography, 1985, pages 175-187. At step S292, CPU 4 identifies and stores matching corner points. This is performed using a "relaxation" technique, as will now be described. Step S290 produces a similarity measure between each strong corner in the image data from the first camera and a plurality of strong corners in the image data from the second camera. At step S292, CPU 4 effectively arranges these values in a table array, for example listing all of the strong corners in the image data from the first camera in a column, all of the strong corners in the image data from the second camera in a row, and a similarity measure for each given pair of corners at the appropriate intersection in the table. In this way, the rows of the table array define the similarity measure between a given corner point in the image data from the first camera and each corner point in the image data from the second camera. Similarly, the columns in the array define the similarity measure between a given corner point in the image data from the second camera and each corner point in the image data from the first camera. CPU 4 then considers the first row of values, selects the highest similarity measure value in the row, and determines whether this value is also the highest value in the column in which the value lies. If the value is the highest in the row and column, this indicates that the corner point in the image data from the second camera is the best matching point for the point in the image data from the first camera and vice versa. In this case, CPU 4 sets all of the values in the row and column to zero (so that these values are not considered in further processing), and determines whether the highest similarity measure is above a predetermined threshold (in this embodiment, 0.1). If the similarity measure is above the threshold, CPU 4 stores the corner point in the image data from the first camera and the corresponding corner point in the image data from the second camera as matched points. If the similarity measure is not above the predetermined threshold, it is determined that, even though the points are the best matching points for each other, the degree of similarity is not sufficient to store the points as matching points. CPU 4 then repeats this processing for each row of the table array, until all of the rows have been considered. If it is determined that the highest similarity measure in a row is not also the highest for the column in which it lies, CPU 4 moves on to consider the next row. CPU 4 reconsiders each row in the table to repeat the processing above if matching points are identified the previous time all the rows were considered. CPU 4 continues to perform such iterations until no matching points are identified in an iteration. Referring again to Figure 18, at step S266, CPU 4 considers the next four pairs of matched points which were identified at step S264. At step S267, CPU 4 tests whether the four points in the image space of the first camera lie on a plane and whether the corresponding matched four points in the image space of the second camera also lie on a plane. If either set of four points does not lie on a plane, then the points selected at step 5266 are not considered further, and processing proceeds to step S276. On the other hand, if both sets of four points lie on a plane in the respective image spaces, then processing proceeds to step S268. By using four pairs of matched points at steps S268 and S267, rather than three pairs which is the minimum required to ensure that a plane can always be defined through the points in each image space, the number of "false" planes (that is, planes which do not correspond to a surface of the object) which are considered is reduced, thereby reducing processing requirements. At step S268, CPU 4 calculates the transformation between the plane in the image space of the first camera which contains the four points considered at step S266 and the plane in the image space of the second camera which contains the corresponding matched four points. This transformation is calculated in a conventional manner, for example using the equation: where: n = 1....4; x2 n ,y2 n are points in the image space of the second camera; x1 n ,y1 n are points in image space of the first camera; and J,K,L,M,N,P,Q,R are obtained by: At step S270, CPU 4 tests the transformation calculated at step S268 against each remaining pair of matched points (that is, pairs of matched points which were not used in calculating the transformation at step S268). That is, CPU 4 considers each remaining pair of points in turn, and uses the coordinates of the points to determine whether the calculated transformation is valid for those points (by determining whether the transformation calculated at step S268 holds for the coordinates of the points). At step S272, CPU 4 determines whether the number of pairs of points for which the transformation was determined to hold is greater than a predetermined number (in this embodiment eight). If it is determined that the transformation is valid for more than the predetermined number of pairs of points, CPU 4 determines that it has identified a plane of the object within the image data from the first camera and the second camera. Accordingly, processing proceeds to step S274, at which CPU 4 stores data defining the planes in the image data of the first camera and the image data of the second camera, together with the coordinates of the identified points which lie on the plane. On the other hand, if it is determined at step S272 that the transformation calculated at step S268 is not valid for the predetermined number of pairs of points, then CPU 4 determines that the points used in step S268 to calculate the transformation lie on a non-real plane, that is, a plane which does not actually form part of the surface of the object. Accordingly, the planes are disregarded. At step S276, CPU 4 determines whether there are another four pairs of matched points. Steps S266 to S276 are repeated until the number of remaining pairs of matched points which have not been considered is less than four. Referring again to Figure 17, at step S252, CPU 4 calculates the boundaries of the planes identified at step S250 and extracts the image data for the planes from the image data of the appropriate camera. CPU 4 carries out these operations by considering the points stored at step S274 (Figure 18) for all planes, and identifying points which lie on two or more planes, these points being points which lie on the boundary between planes. The processing performed by CPU 4 is arranged to generate such boundary points since the points matched in the image data from the first and second cameras at step S264 (Figure 18) are corner points. Having identified points lying on the boundaries between planes, CPU 4 defines the boundaries between the planes by connecting points identified for each boundary (these being points which lie on the same two planes) or, if there are three or more points which cannot be connected by a straight line, drawing a straight line between the points using a conventional "least squares" method. CPU 4 then determines the intersections of the defined boundaries, thereby defining the planar surfaces in the image space. (If part of a plane is not bounded by a defined boundary, the extent is defined by the identified pixel data of the object within the overall bounding rectangle for the object, as determined by the "foreground mask" stored previously at step S42, for example.) After defining the boundaries of each plane in the image space of each camera to define planar surfaces of finite extent, CPU 4 extracts and stores the pixel data lying within each planar surface. At step S254, CPU 4 uses the surfaces defined in the image spaces of the cameras to model the object in the 3D world space for each camera. To do this, CPU 4 calculates a position for the planar model in the 3D world space for each camera, and transforms the planar model defined in the image space of a camera into the 3D world space for the camera. More particularly, in this embodiment, CPU 4 applies the transformation from the image space of the first camera to the 3D world space previously calculated at step S24 to the coordinates of the corners of the base of each plane identified in the image data for the first camera at step S252. Similarly, CPU 4 applies the transformation between the image space of the second camera and the 3D world space previously calculated at step S24 to the coordinates of the corners of the base of each plane in the image data for the second camera determined at step S252. Since the transformations previously calculated at step S24 are only valid for points which lie on the ground plane in the image space and the 3D world space, the corner points for planes which touch the ground plane will align (to within a tolerance distance) in the 3D world space when transformed from the image space of the first camera and the image space of the second camera, whereas the corner points of other planes which do not touch the ground plane will not align. CPU 4 therefore identifies which corner points align, and, for these points, places a vertical planar surface in the 3D world space of each camera having the same aspect ratio as the planar surface in the image space of the appropriate camera (that is, the corresponding planar surface having corner points on the ground). CPU 4 then defines planar surfaces in the 3D world space for each camera having the same interconnections and aspect ratios as the surfaces identified in image space for the appropriate camera at step S252. At step S256, CPU 4 stores as object data for each camera the data defining the planar surfaces in the 3D world space, the image data extracted for each surface, and the foreground "mask" for each surface. At step S258, CPU 4 determines whether there is another object to be modelled. Steps S250 to S258 are repeated until all object have been modelled in the manner described above. The rendering steps in this embodiment are performed in the same way as those in the first embodiment, with the image data stored at step S256 being rendered onto the appropriate surface. The modelling of objects in the 3D world space described above for the second embodiment is particularly effective for objects made up of a number of planes, such as vehicles 30, 32 in the example of Figure 2 etc, and the processing operations performed in this embodiment model the tops of objects more accurately than the modelling operations in the first embodiment. Third Embodiment In the first embodiment, the processing operations performed to model objects in the 3D world space at steps S11a/S11b are particularly effective where the object being modelled touches the ground over a wide area (e.g. vehicles 30, 32 in the example of Figure 2). In the second embodiment, the modelling technique is particularly effective where the object is made up of a number of planes. However, not all objects have these characteristics (for example the people 50, 52 in the example of Figure 2) and, while they can be modelled using the technique of the first or second embodiment, these techniques may introduce unnecessary processing operations. The third embodiment is the same as the first embodiment with the exception of the processing operations performed at steps S11a/S11b to model the objects in the 3D world space. In the third embodiment, CPU 4 models the objects at steps S11a/S11b using the vertical planes in the 3D world space produced at steps S6a and S6b, and subsequently modified at steps S8 and S10. Each object is therefore modelled as a single vertical plane in the 3D world space of each camera (one plane per camera). As before, shadows are not modelled at steps S11a/S11b. Various modifications are possible to the embodiments described above. Referring again to Figure 3, in the embodiments above, steps S6a and S6b (in which image data is processed to identify foreground objects and to create object data therefrom) are performed after all images have been recorded at steps S4a and S4b. Similarly, step S16 (in which images are displayed) is performed after steps S4, S6a/S6b, S8, S10, S11a/S11b, S12a/S12b and S14 have been completed. However, these steps may be performed so as to allow real-time display of images to a user from a desired viewing direction. That is, steps S6a/S6b, S8, S10, S11a/S11b, S12a/S12b, S14 and S16 could be performed on one frame of video data while the next frame of data is being recorded by the video cameras. This real-time operation is possible since the processing requirements of steps S6a/S6b, S8, S10 and S11a/S11b are not particularly onerous on CPU 4, and could be carried out within 1/30th of a second, this being the time between the recording of video frames. In the embodiments above, foreground objects are identified at steps S6a/S6b on the basis of grey scale values. However, in addition, or instead, it is possible to set windows for colour and/infra-red values and to identify foreground objects using these image characteristics. In the embodiments above, at step S8, shadows are identified by mapping (transforming) image data for corresponding foreground objects onto the ground plane in the 3D computer model defined at step S2 and comparing the transformed data to identify the boundary between aligned portions (shadow) and non-aligned portions (object). The identified boundary (ground "footprint") is subsequently used in the first embodiment to model the object (steps S11a/S11b). However, the mapping and comparison need not take place on the ground plane in the 3D computer model. More particularly, shadows may be identified and the ground "footprint" determined for object modelling by mapping image data for corresponding foreground objects onto a surface in any modelling space by applying a transformation which defines a mapping from the ground plane of the image data of the respective camera to the surface in the modelling space, and comparing the transformed data. Models (representations) of the object and its shadow would then be generated in the 3D computer model in dependence upon the results. Similarly, at step S10, instead of comparing the heights of the bounding rectangles of corresponding objects in the 3D computer model to determine if an object is a composite object, the bounding rectangles may be transformed from the image data of each camera to any common modelling space and the heights compared in that modelling space. In the embodiments above, CPU 4 performs processing at steps S298 and S300 (Figure 14) to display an aerial representation of the objects' positions if the user selects a viewing direction which is close to the vertical. Further processing on the image (video pixel) data stored as object data for each object in the 3D world space could be carried out to determine the colour of the objects and to indicate this colour in the aerial representation of positions. Such processing could be particularly useful where the objects are people taking part in a team sport, in which case the colours of the top of each player could be identified from the object data and a visual indication presented in the aerial view to show to which team each player belonged. Further, the apparatus may be arranged to enable the user to selectively enable and disable the processing operations performed by CPU 4 at steps S298 and S300. That is, the user could instruct CPU 4 not to show an aerial view for any viewing directions, and instead always to render a realistic view of the objects from the chosen viewing direction. This enable/disable feature may be useful in embodiments in which the tops of objects are modelled, for example the second embodiment. In the embodiments above, when selecting which set of object data to use to generate a frame of image data at step 5302 (Figure 14), CPU 4 prioritises the inherent camera characteristics and the image data error characteristics which affect image quality in the order (Figure 15) of image data transfer method between camera and processor, camera resolution, image stability, and number of occluded objects in the image. However, different priorities can be used. Also, not all of the camera and image data characteristics described above need to be used. For example, it is possible to omit one or more of the four tests (defined respectively by steps S404-S408, S410-S414, S416-S420 and S422-S426). Further, other camera characteristics (such as shutter speed, which may be transmitted with the image data from each camera - the camera with the fastest shutter speed being selected because this indicates more favourable lighting conditions) and the image data characteristic of whether the image is colour or black and white (a camera producing colour image data being selected in preference to a camera producing black and white image data) may be taken into consideration. In the embodiments above, step S302 is performed to select the data to be used to create an image for display to the user after each object has been modelled at steps S11a/S11b using image data from each camera. However, if the user has defined a viewing direction, step S302 may be performed to select data before objects are modelled at steps S11a/S11b. Thus, for example, a camera would be selected as described above, and then the object(s) modelled in the 3D computer model using the image data from the selected camera. Such processing could prevent unnecessary modelling of objects (since models that will not be used for the image are not created), thereby saving processing time. In the embodiments above, at step S310, CPU 4 compares the rendered images using a differencing technique. However other techniques such as Hausdorff distance described in "Tracking Non-Rigid Objects in Complex Scenes" by Daniel Huttenlocher et al, Proc. 4th International Conference on Computer Vision, 1993, Berlin, IEEE Computer Society Press ISBN 0-8186-3870-2 may be used. Similarly, other image combination techniques (such as averaging) may be used at step S314. In the embodiments above, steps S304 to S314 (Figure 14) are carried out with respect to image data for a whole frame. However, the steps can be carried out to compare image data on an object-by-object basis to determine whether objects will appear discontinuous on an individual basis. This may result in the generated frame of image data being made up of image data from different cameras (different objects may use image data from different cameras). In the embodiments above, at step S416 (Figure 15) image stability is determined using data from sensors mounted on the camera or its stand. Alternatively, image stability may be determined by processing the received image data itself. For example, the image data may be processed using an optic flow technique, or using a technique such as that described in "Virtual Bellows: Constructing High Quality Stills From Video" by S. Mann and R.W. Picard, MIT Media Laboratory Perceptual Computing Section Technical Report No. 259, appears, Proc. First IEEE Int. Conf. on Image Proc., Austin TX, November 1994. In the embodiments above, two cameras are used to produce the input image data. However, the processing operations performed in the embodiments can equally be performed for a greater number of cameras. Indeed, a greater number of cameras may assist a number of the processing operations. For example the shadow processing performed by CPU 4 at step S8 (Figure 3) may be improved. This is because, with only two cameras, one camera may not have a complete view of the shadow of an object, and accordingly the aligned image data portions in the 3D world space extracted and stored at step S102 (Figure 8) may not correspond to the complete shadow. This problem is likely to be reduced as the number of cameras increases since the possibility of at least two cameras having a view of the complete shadow will also increase. If required, the extracted complete shadow could then be added to the object data belonging to cameras in which the complete shadow did not actually appear in their image data. In the embodiments above, the viewing direction of the cameras is fixed. However, the viewing direction may be varied, and conventional direction and angle sensors may be placed on the camera or its stand to provide information defining the viewing direction of each camera for each recorded frame of image data. Processing would then be carried out to project the image data to a fixed virtual image plane in a conventional manner, for example as described in "Statistical Background Models for Tracking with a Camera" by Simon Rowe and Andrew Blake, British Machine Vision Conference 1995. In the embodiments above, foreground objects are identified in an image by comparing the value of each pixel with a value set on the basis of a plurality of images recorded of the background only (steps S3a/S3b, S4a/S4b and S6a/S6b). However, conventional optic flow techniques may be used to identify foreground objects. Such optic flow techniques may be used, for example, when the viewing direction of the camera changes. In the embodiments above, the zoom (magnification) of each camera is fixed. However, the zoom may be varied. In this case, for example, the transformation between the image space of the camera and the 3D world space calculated at step S24 (Figure 4) would be calculated for the same four reference points for each zoom setting. This calculation could be performed prior to recording frames of image data at steps S4a/S4b and the zoom setting for the camera for each frame could be transmitted from the camera to the computer 2, allowing the correct transformation to be selected for the image data for a frame. Alternatively, the transformations could be calculated in real-time as the zoom setting of the camera changes (in which case, the four reference points used for the calibration would be chosen so as always to be visible within the field of view of the camera and also to be easily recognisable by conventional image recognition techniques to allow them to be identified by CPU 4 from the image data to be used to calculate the transformation). In the embodiments above, CPU 4 and memory 6 form part of computer 2 which is separate to display unit 18 and cameras 12a and 12b. The processing described in the above embodiments could, however, be carried out within one or more cameras or a display device by providing the appropriate processor and memories in the camera or display device. In the first embodiment above, at steps S11a/S11b (Figure 3), CPU 4 defines the model to be used for an object in the 3D world space by fitting straight lines to the ground "footprint" of the object and defining vertical planar surfaces having the straight lines as bases. In addition, CPU 4 may carry out processing to define a top to the object by connecting the tops of the vertical planar surf aces. Further, CPU 4 may use the ground "footprint" to define the 3D model in different ways. For example, CPU 4 may represent the ground "footprint" using a spline curve (such as a Bezier curve) and may represent the object as a vertical curved surface in the 3D world space which has a horizontal cross-section corresponding to the defined spline curve. In the second embodiment above, at step S254 (Figure 17) CPU 4 defines a plane in the 3D world space of a camera for every plane which is determined to touch the ground in the image space of the camera. However, to allow the model of the object to be defined in 3D world space, it is sufficient to define a plane in the 3D world space for only one plane which touches the ground in image space (since the position of a single plane in 3D world space will fix the positions of all the planes in the model). In the second embodiment above, at step S264 (Figure 18), CPU 4 identifies and compares corner points in the image data. In addition, or instead, minimum, maximum, or saddle points in the colour or intensity values of the image data may be identified and compared. For example, techniques described in "Computer and Robot Vision Volume 1" by Haralick and Shapiro, Chapter 8, Addison-Wesley Publishing Company, ISBN 0-210-10877-1 (V.1) for detecting such points may be employed. The detected points may be matched using an adaptive least squares correlation as described previously. In the embodiments above, at step S24 (Figure 4) a transformation is calculated for unknown camera position and parameters which maps the ground plane from image space to world space. However, one or more of the cameras could be arranged in known relationship to the scene to be viewed and with known camera parameters, allowing a different transformation to be used. In the embodiments above, at steps S296 to S300 (Figure 14), the viewing direction selected by the user is determined and, if this direction is within a predetermined angle of the vertical, then processing is carried out to display a schematic image of the position(s) of the object(s). In addition, or instead, processing may be carried out to determine if the viewing direction is parallel to or within a predetermined angle of, a vertical planar surface making up the model of the object and, if it is, to render image data for the display of a schematic image of the position of the object in some way as at step S300 above. This would have particular advantage when the object is modelled using a single vertical planar surface, as in the third embodiment, since the object itself would then not appear in a true image if the viewing direction is such that the planar surface is viewed edge-on, and therefore a schematic image of object position may be more useful to the user. Of course, by determining whether the viewing direction lies within a predetermined range of angles relative to a planar surface of an object, cases can be identified where the object is to be viewed top-edge-on (or at an angle close thereto) and hence this test can be used instead of the test at step S298 above of determining the viewing direction relative to a predetermined range of angles fixed with respect to the vertical direction.

Claims: A method of processing image data defining a plurality of sequences of images, each from a respective camera, of an object moving in a scene to produce signals defining a representation of the object in a three-dimensional computer model, the method comprising: processing image data from a first of the cameras to identify image data relating to the object in the scene; processing image data from a second of the cameras to identify image data relating to the object in the scene; applying a transformation to the identified image data from the first camera which defines a mapping from the ground plane in the space of the image data of the first camera to a surface in a modelling space; applying a transformation to the identified image data from the second camera which defines a mapping from the ground plane in the space of the image data of the second camera to the surface in the modelling space; comparing the transformed image data from the first and second cameras on the surface in the modelling space; determining which part of the image data represents shadow in dependence upon the comparison results; and generating a representation of at least the object in the three-dimensional model. A method according to claim 1, further comprising the step of generating a representation of the shadow in the three-dimensional model. A method according to claim 1 or claim 2, wherein the surface in the modelling space is the ground plane in the three-dimensional model. A method according to any preceding claim, wherein it is determined that aligned parts of the transformed image data represent shadow. A method according to any preceding claim, further comprising the step of generating image data by rendering an image of the three-dimensional computer model in which texture data based on the processed image data is rendered onto the representation of the object. A method according to claim 5, wherein the image data rendered onto the representation is determined in dependence upon the comparison results. A method according to claim 5 or claim 6, further comprising the step of generating a signal conveying the image data. A method according to claim 7, further comprising the step of recording the signal. A method according to any of claims 5 to 8, further comprising the step of displaying an image of the object using the generated image data. A method according to any of claims 5 to 9, further comprising the step of making a recording of the image data either directly or indirectly. Apparatus for processing image data defining a plurality of sequences of images, each from a respective camera, of an object moving in a scene to produce signals defining a representation of the object in a three-dimensional computer model, the apparatus comprising: means for processing image data from a first of the cameras to identify image data relating to the object in the scene; means for processing image data from a second of the cameras to identify image data relating to the object in the scene; means for applying a transformation to the identified image data from the first camera which defines a mapping from the ground plane in the space of the image data of the first camera to a surface in a modelling space; means for applying a transformation to the identified image data from the second camera which defines a mapping from the ground plane in the space of the image data of the second camera to the surface in the modelling space; means for comparing the transformed image data from the first and second cameras on the surface in the modelling space; means for determining which part of the image data represents shadow in dependence upon the comparison results; and means for generating a representation of at least the object in the three-dimensional model. Apparatus according to claim 11, further comprising means for generating a representation of the shadow in the three-dimensional model. Apparatus according to claim 11 or claim 12, wherein the surface in the modelling space is the ground plane in the three-dimensional model. Apparatus according to any of claims 11 to 13, wherein the apparatus is operable to determine that aligned parts of the transformed image data represent shadow. Apparatus according to any of claims 11 to 14, further comprising means for generating image data by rendering an image of the three-dimensional computer model in which texture data based on the processed image data is rendered onto the representation of the object. Apparatus according to claim 15, wherein the apparatus is operable to determine the image data to render onto the representation in dependence upon the comparison results. Apparatus according to claim 15 or claim 16, further comprising means for displaying an image of the object using the generated image data. A storage medium storing instructions for programming a programmable processing apparatus to become operable to perform a method according to at least one of claims 1 to 6. A signal conveying instructions for programming a programmable processing apparatus to become operable to perform a method according to at least one of claims 1 to 6.

Inventors: DAVISON ALLAN JOSEPH, ROWE SIMON MICHAEL, DAVISON, ALLAN JOSEPH, ROWE, SIMON MICHAEL
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1999
Main classification: G06T17
IPC codes: H04N1, H04N13, H04N5, G06T7, G06T17, G06T1, G06T15