Abstract:
The invention provides a simple method and apparatus for tracking image recording device motion using a light field. The invention locates the image recording device&#39;s position and orientation in each frame very precisely by checking the radiance seen along lines captured in previous frames. The invention provides an interactive system that provides the operator with feedback, to capture a sequence of frames that sufficiently cover the light field, to provide sufficient data for reconstruction of three-dimensional structures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an apparatus and method for image-based rendering. In particular, the invention relates to an apparatus and method for tracking camera motion using a light field in order to provide data for later reconstruction of a three-dimensional structure or environment. 
     2. Description of Related Art 
     The computer graphics industry has become increasingly interested in methods by which three-dimensional objects or environments can be represented using a large collection of two-dimensional images. This is referred to as image-based rendering. One way in which to represent an object or environment is to use light fields. 
     A light field is any representation of a three-dimensional scene by the radiance induced on the set of incident lines in three-dimensional free space R 3 . This includes, but is not limited to, representations which sample and store this radiance as a collection of two-dimensional images and representations which sample and store this radiance in a data structure representing lines in three-dimensional free space R 3 . 
     For example, If the two-dimensional images of the object or environment are sampled densely at a two-dimensional set P of camera positions, the radiance seen along all the lines passing through P has been sampled. These radiances can be stored, for example, in a four-dimensional array where each element corresponds to a line in three-dimensional free space R 3 . Any image of the object from a three-dimensional set of positions can be reconstructed by collecting the appropriate lines from this array. Such a representation is referred to as a light field. 
     A light field is best understood with reference to FIG.  1  and the following explanation. FIG. 1 shows a light slab representation of a light field. Consider the set of lines passing through two parallel planes P 1  and P 2  in the three-dimensional free space R 3 . Each pair of points p 1  of the plane P 1  and P 2  of the plane P 2  defines a unique line. Each line, except for those parallel to P 1  and P 2 , defines a unique pair of the points p 1  and P 2 . So the set of all lines in the three-dimensional free space R 3  is a four-dimensional space, which can be parameterized by the four coordinates (u,v,s,t) required to specify the points p 1  and p 2 . 
     The lines through a specific point p in R 3  form a two-dimensional subset, which is a plane under this parameterization. An image is a rectangular subset of this “plane of lines” with p as the focal point. space R 3  is a four-dimensional space, which can be parameterized by the four coordinates (u,v,s,t) required to specify the points p 1  and p 2 . 
     The lines through a specific point p in R 3  form a two-dimensional subset, which is a plane under this parameterization. An image is a rectangular subset of this “plane of lines” with p as the focal point. 
     Light fields such as these can be used to reconstruct images by collecting a subset of the four-dimensional space of lines which contain a lot of images of an object. This collection is done by pointing an image recording device at the object and physically scanning the image recording device across a two-dimensional square. A two-dimensional set of lines is collected at each image recording device position. The lines in all the images are parameterized using two parallel planes (one containing the moving camera and the other in front of the object) and stored as a four-dimensional array. The radiance seen along each line in the four-dimensional array is addressed by the line coordinates. An image of the object with the focal point anywhere in the three-dimensional vicinity of the two planes can be extracted by collecting the radiance of each of its lines from this array. In this way, images can be reconstructed in real time. 
     These light fields can be compressed to a reasonable size and accessed quickly enough to make them useful for real-time rendering on a high-end machine. However, the capturing of these light fields has been limited to mechanically scanning a camera over a two-dimensional plane using such devices as a computer controlled camera gantry. Such a device is disclosed in Levoy et al., “Light Field Rendering”,Computer Graphics Proceedings, SIGGRAPH &#39;96, p. 528, 31-42. In these devices, the computer keeps track of the camera position at each frame of the light field capturing process. Such devices are generally expensive, limited to a specific area and range of object sizes it can handle, and require large investments of time and money to produce and use. 
     Another way in which to capture the light fields is to use fiducial points, points in three-dimensional free space R 3  whose exact locations are known with respect to some coordinate frame. Gortler et al., “The Lumigraph”, Computer Graphics Proceedings, SIGGRAPH &#39;96, p. 528, 43-54, discloses one method of using fiducial points to obtain images of a three-dimensional environment. In this method, however, it is necessary to have many fiducial points within the environment and to maintain those fiducial points in the image field while capturing each image. Thus, the range of motion of the camera is limited to the area in which a number of fiducial points are present. 
     Therefore, it would be beneficial and more practical to be able to use a hand-held camera to perform the image capturing of a three-dimensional environment without being constrained to a particular area of movement, i.e. an area containing fiducial points. However, a fundamental difficulty in using a hand-held device is in tracking the position and orientation of the camera at each frame of the image capturing process. 
     Camera tracking is a problem that arises frequently in computer vision. One technique for camera tracking is to measure the optical flow from one image frame to the next. In this technique, each pixel is assumed to correspond to a nearby pixel with the best-matching color. An overall combination of the pixel motions is interpreted as a motion of the camera. This method provides good results when tracking the motion between two successive image frames. However, for multiple frames, such as is needed for capturing of environments, the error in the camera position accumulates rapidly. 
     Another technique for camera tracking is the point correspondence method, in which distinctive looking feature points (such as object corners) are extracted from an image and tracked from one image frame to the next. These points act as fiducial points during camera tracking. This method provides better results than the optical flow method, however, it is difficult to track the same three-dimensional points from image frame to image frame and the method itself is rather slow and cumbersome. 
     Additionally, problems with camera tracking for the capture of images of a three-dimensional environment for later reconstruction of the environment is different from the problem of capturing data from arbitrary video sequences. The camera operator knows that she or he is trying to capture data of the environment, and is willing to move the camera in particular ways in order to do so. Thus, an interactive data collection system could provide feedback to the camera operator, giving the operator indications for keeping the camera tracking robust and to fill in desired data. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and apparatus for tracking the motion of an image recording device. The method and apparatus allow a hand-held image recording device to be used. 
     The invention also provides a simple method and apparatus for tracking image recording device motion using a light field. 
     The invention further provides an interactive system that provides the operator with feedback to capture a sequence of frames that sufficiently cover the light field. 
     The invention additionally provides an interactive system that provides the operator with feedback to provide sufficient data for reconstruction of three-dimensional structures. 
     The method and apparatus of the invention locates the position and the orientation of the image recording device in each frame by checking the radiance along lines in the frame and corresponding lines in previous frames. Thus, a separate device to keep track of the location of the image recording device is not necessary. 
     The method and apparatus of the invention locates the image recording device&#39;s position and orientation in each frame very precisely by checking the radiance seen along lines captured in previous frames. 
     These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein: 
     FIG. 1 shows a light slab representation of a light field; 
     FIG. 2 shows a block diagram of the apparatus of the invention; 
     FIG. 3 is a diagram of one embodiment of the invention in which messages to the operator are displayed and/or announced; 
     FIG. 4 is a diagram of one embodiment of the invention in which an image pickup device path is displayed; 
     FIG. 5 is a diagram of one embodiment of the invention in which movement instructions are displayed to the operator; 
     FIG. 6 is a flowchart outlining one embodiment of the method of this invention; 
     FIG. 7 is a flowchart showing in greater detail the new frame position and orientation determining step of FIG. 6; and 
     FIG. 8 shows two frames along a curve of the movement of an image recording apparatus. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 shows an image recording apparatus  100  including an image pickup device  110 , a processor  120 , a frame storage  130 , a memory  140 , an output controller  150  and an output device  160 . The image pickup device  110  detects an image of the environment in which the image pickup device  110  is operating. This image, once recorded, is a frame. The image pickup device  110  can be a video camera, still photo camera, digital camera, electronic camera, image editing machine, and the like. In one preferred embodiment, the image pickup device  110  is a hand-held camera. Each recorded frame is input to the processor  120 , which stores the frames in the frame storage  130 . The frame storage  130  may include a videotape, photographic film, a CD-ROM, magnetic media, RAM, a hard drive, a floppy disk and disk drive, flash memory or the like. The frame I′ is then sent to the processor  120 . 
     The processor  120  determines a position of the image pickup device  110  for each frame of the sequence of m frames obtained from the image pickup device  110 . This may be done using any method by which the position of known points in the image frames are used to determine the relative position of the image pickup device  110 . The processor  120  then determines a subset of lines in each frame that pass through the position of the image pickup device  110 . 
     The image pickup device  110  then captures a new frame of image data and sends it to the processor  120 . The position and the orientation of the image pickup device  110  in the new frame I is determined as outlined below. 
     Determining the position and orientation of the image pickup device  110  is performed in real time so that feedback can be provided to the image recording device operator through the output device  160 . For instance, the operator may fail to move the camera slowly or smoothly enough to maintain registration. Registration is the matching up of previous and current images. As shown in FIG. 3, if registration is lost, the image recording apparatus  100  can provide an instruction to the operator to return to the fiducial points. The fiducial points are those points whose exact locations are known a priori. This instruction may be either visual, auditory, or both. For instance, the image recording apparatus  100  may display the message “Return to Fiducial Points” on display  160 , as shown in FIG. 3, and/or may announce a message via the speaker  170 . 
     Likewise, it may be helpful to the operator to obtain a visual feedback of the current path that the operator has traversed. Accordingly, as shown in FIG. 4, in one embodiment of the invention, the processor  120  can output to the display  160  a visual display of dots  162  depicting previously calculated image recording device positions, a curve showing the path traversed, or the like. 
     Additionally, as shown in FIG. 5, the image recording apparatus  100  can provide movement instructions on the display  160  to the operator. These movement instructions instruct the operator to move to positions which provide data for parts of the light field which are sparsely sampled. The image recording apparatus  100  can also provide instructions that direct the operator away from areas for which registration is not yet supported by previous frame data. 
     The elements of the image recording apparatus  100  may be part of a single unit, such that all the elements are housed within a single housing  180 , as shown in FIG. 4, or may be distributed, as shown in FIG.  3 . For instance, the image recording apparatus  100  may be a single, hand-held image recording apparatus  100  in which the image pickup device  110 , the processor  120 , the frame storage  130  and the output devices  160  and  170  are housed. Alternatively, the image pickup device  110  may be a video recording device connected via cables to a device, such as a computer or personal digital assistant, that houses the processor  120 , the memories  130  and  140  and the output devices  160  and  170 . Other combinations may be used to implement the invention, as will be readily apparent to those of ordinary skill in the art. 
     FIG. 6 is a flowchart outlining a preferred method of image recording according to this invention. As shown in FIG. 6, the method starts with step S 100 . 
     Then, in step S 200 , the image recording device captures a sequence of m frames and stores them as a set of pixel lines in the frame storage  130 . Next, in step S 300 , the camera positions, defined as the focal points “p” and “p”′, having orientations “o” and “o”′, respectively, are determined. A simple way in which to produce a sequence of precisely registered frames with which to begin is to use fiducial points, as discussed above. 
     To obtain the fiducial points, many different methods may be employed. In a preferred embodiment, the fiducial points may be obtained by building a physical coordinate frame, such as three distinct arrows attached at right angles. The points of the three arrows, the fiducial points, are easy to find in an image. From the fiducial points, the position and orientation of the camera relative to the coordinate frame is determined. The image recording device can then move slowly about the fiducial points and capture the desired images. In this way, an initial light field is obtained. After the initial light field is obtained, it is no longer necessary to maintain the fiducial points in the field of view of the image pickup device  110 . 
     Then, in step S 400 , for each of these frames, the image is identified with a subset of the set of pixel lines which have been stored in the light field. The sequence of focal points forms a sequence “P” of “m” points along a curve “C” in the three-dimensional free space R 3 . The curve “C” represents the movement of the camera within the environment. 
     Next in step S 500 , the image pickup device  110  obtains a new frame I, i.e., a new subset of lines through some new, unknown focal point p. The goal is to determine p and o by correlating the new frame with previous frames in the light field. 
     It is important to observe that a line “I” through p and any other point p′ in the set “P” is likely to have been captured already, by a previous frame I′ corresponding to p′ and stored in the light field. Specifically,  1  is captured in both frames I and I′ if the image recording device is oriented at p′ such that the projection of p onto the image plane of the previous frame I′ falls within the previous frame I′ and conversely, the image recording device is oriented at p′ so that the projection of p′ onto the image plane of the new frame I falls within the new frame I. Thus, knowing the correct p and o for the image recording device, the radiance assigned to l in I should be identical to the radiance assigned to l in I′. The values for p and o correspond to a prediction of the positions of the lines L={l 1  , . . . ,l m } in I and of the radiance at those positions. When m is large, this gives a very precise test for the correct values of p and o. 
     Then, in step S 600 , the image recording device position p and orientation o is determined for the new frame. In step S 700 , the control routine determines whether operation should be repeated for a new image frame. This may be done by either determining if the image pickup device  110  is still recording, determining if the image pickup device  110  has moved, and the like. If the operation is to be repeated, control returns to step S 500 . Otherwise, control continues to step S 800  where the control routine stops. 
     FIG. 7 shows the position and orientation determination step S 600  in greater detail. In particular, starting from step S 600 , control continues to step S 610 . In step S 610 , a rough estimate “e” of the position p and the orientation o are determined by identifying a small volume “v” of a space “S” which is likely to contain the image pickup device  110 . The space S is a six-dimensional space of possible positions for the image pickup device  110 . 
     In step S 610 , to get the rough estimate e of the image recording device&#39;s position and orientation, some assumptions about the motion of the image pickup device  110  are made. If the image pickup device  110  does not move too fast, the position p cannot be too different from the previous position p&#39;. If the image pickup device  110  does not change its translational or rotational trajectory suddenly, the new position p should roughly extrapolate some number of previous positions p&#39;. These assumptions are likely to be good since the operator knows that he/she is trying to capture an image for later reconstruction of the environment and is likely to operate the image pickup device  110  accordingly. 
     Traditional methods may also be used to obtain a rough estimate of the position p in the new frame I. For instance, the optical flow (interpreting the difference between two images as a motion of a set of points of constant radiance) between the previous frame I′ and the current frame I is not expensive to compute and can give an estimate of the motion of the image pickup device  110 . Likewise, the point correspondence method may be used to obtain a rough estimate of the image pickup device&#39;s position. 
     Then, in step S 620 , the volume v is densely sampled to find a close match to the image. In step S 620 , given the rough estimate e in the space S, a small surrounding volume of S is sampled to find a point or pixel which best matches one or more lines in I to corresponding lines stored in the light field. Next, in step S 630 , the error of the match is determined as a function “F” on points of the space S. The value of F at a point “s” in the space S is the difference between the predicted radiance of the portion of L overlapping I, when I is assumed to lie at s, and the observed values of the corresponding points I. The differences are to allow for the existence of outliers, to account for random image noise, but to require fairly precise matching of non-outliers, which should suffer only from quantization error. 
     Lastly, in step S 640 , the result is optimized to provide a close fit. In step S 640 , the match is optimized to minimize the error in order to provide the closest correspondence. In step S 650 , control returns to step S 700 . 
     Thus, in the present invention, the position of an image recording device can be accurately tracked by correlating radiance lines in a newly captured frame with radiance lines in a previously captured light field. 
     These correlated frames may then be added to the existing light field in order to enlarge the light field. The addition of these frames to the light field may be performed dynamically as frames are being captured or may be performed off-line. In this way, the light field may be preprocessed off-line before use in further camera tracking and image recording. 
     Furthermore, this process of adding frames to the initial light field may be repeated in order to obtain larger and large light fields. In this way, entire environments may be captured for later reconstruction. 
     Using the tracking of the present invention, it is not necessary to maintain fiducial points in the image frame once the initial light field has been obtained. After the initial light field is obtained, tracking is accomplished by correlating radiance lines in a new frame with those in the light field. This allows the image recording apparatus to be used in a much larger environment. 
     As shown in FIG. 4, the image recording apparatus  100  is preferably implemented using a programmed general purpose computer. However, the image recording apparatus  100  can also be implemented using a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an ASIC or other integrated circuit, a hardware electronic or logic circuit such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, or the like. In general, any device on which a finite state machine capable of implementing the flowchart shown in FIGS. 6 and 7 can be used to implement the image recording of this invention. 
     While the invention has been described as using fiducial points to obtain an initial light field, it is not limited to such an arrangement. The initial light field may be obtained using a gantry type device, the point correspondence method, or any other method readily apparent to one of skill in the art. 
     While this invention has been described with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.