Abstract:
Selectively distributing a plurality of data items to a plurality of hardware destinations that share a common bus involves, for each one of the data items, determining which of the hardware destinations the data item should be distributed to, wherein at least one of the data items should be distributed to two or more hardware destinations. The data item is then supplied to the common bus and, for each of the hardware destinations to which the data item should be distributed, a corresponding hardware destination signal is generated that causes the data item to be received in the hardware destination from the common bus, wherein for each data item, the corresponding hardware destination signals are generated substantially simultaneously. This may be applied in a number of scenarios, including the distribution of data items to a plurality of processor boards, and to the distribution of data items to a plurality of channels within a single processor board. A similar approach is provided for forming a sequence of data items by selectively collecting a plurality of data items from a plurality of processor boards in a multiprocessor system.

Description:
BACKGROUND 
     The present invention relates to parallel computing, more particularly to mesh connected computing, and even more particularly to the distribution, processing and reconstruction of images by means of a mesh connected computer having fewer processing elements than the size of the image. 
     In a number of technological fields, such as digital signal processing of video image data, it is necessary to perform substantially identical logical or arithmetic operations on large amounts of data in a short period of time. Parallel processing has proven to be an advantageous way of quickly performing the necessary computations. In parallel processing, an array of processor elements, or cells, is configured so that each cell performs logical or arithmetic operations on its own data at the same time that all other cells are processing their own data. Machines in which the logical or arithmetic operation being performed at any instant in time is identical for all cells in the array are referred to by several names, including Single Instruction-Multiple Data (SIMD) machines. 
     A common arrangement for such a machine is as a rectangular array of cells, with each interior cell being connected to its four nearest neighboring cells (designated north, south, east and west) and each edge cell being connected to a data input/output device. In this way, a mesh of processing elements is formed. Accordingly, the term “Mesh Connected Computer” (MCC) is often applied to this architecture. 
     In a MCC, each cell is connected as well to a master controller which coordinates operations on data throughout the array by providing appropriate instructions to the processing elements. Such an array proves useful, for example, in high resolution image processing. The image pixels comprise a data matrix which can be loaded into the array for quick and efficient processing. 
     Although SIMD machines may all be based upon the same generic concept of an array of cells all performing the same function in unison, parallel processors vary in details of cell design. For example, U.S. Pat. No. 4,215,401 to Holsztynski et al. discloses a cell which includes a random access memory (RAM), a single bit accumulator, and a simple logical gate. The disclosed cell is extremely simple and, hence, inexpensive and easily fabricated. A negative consequence of this simplicity, however, is that some computational algorithms are quite cumbersome so that it may require many instructions to perform a simple and often repeated task. 
     U.S. Pat. No. 4,739,474 to Holsztynski et al., represents a higher level of complexity, in which the logic gate is replaced by a full adder capable of performing both arithmetic and logical functions. This increase in the complexity of the cell&#39;s computational logic allows fewer cells to provide higher performance. 
     U.S. patent application Ser. No. 08/112,540, which was filed on Aug. 27, 1993 now U.S. Pat. No. 6,073,185, in the name of Meeker, and U.S. patent application Ser. No. 09/057,482, which was filed on Apr. 9, 1998 now U.S. Pat. No. 6,173,388, in the name of Andrew P. Abercrombie et al. each describe still further improvements in SIMD architecture computers. 
     As mentioned above, MCCs prove especially useful in applications such as high resolution image processing. Various types of sensors are capable of producing large quantities of data signals (henceforth referred to simply as “data”) that, when taken together, constitute an “image” of the sensed object or terrain. The term “image” is used broadly throughout this specification to refer not only to pictures produced by visible light, but also to any collection of data, from any type of sensor, that can be considered together to convey information about an object that has been sensed. In many applications, the object or terrain is sensed repeatedly, often at high speed, thereby creating many images constituting a voluminous amount of data. Very often, the image data needs to be processed in some way, in order to be useful for a particular application. While it is possible to perform this processing “off-line” (i.e., at a time after all of the data has been collected), the application that mandates the collection of image data may further require that the images be processed in “real-time”, that is, that the processing of the image data keep up with the rate at which it is collected from the sensor. Further complicating the image processing task is the fact that some applications require the sensing and real-time processing of images that are simultaneously collected from two or more sensors. 
     Examples of the need for high-speed image processing capability can be found in both military and civil applications. For example, future military weapon platforms will use diverse suites of high-data-rate infrared, imaging laser, television, and imaging radar sensors that require real-time automatic target detection, recognition, tracking, and automatic target handoff-to-weapons capabilities. Civil applications for form processing and optical character recognition, automatic fingerprint recognition, and geographic information systems are also being pursued by the government. Perhaps the greatest future use of real-time image processing will be in commercial applications like medical image enhancement and analysis, automated industrial inspection and assembly, video data compression, expansion, editing and processing, optical character reading, automated document processing, and many others. 
     Consequently, the need for real-time image processing is becoming a commonplace requirement in commercial and civil government markets as well in the traditional high-performance military applications. The challenge is to develop an affordable processor that can handle the tera-operations-per-second processing requirement needed for complex image processing algorithms and the very high data rates typical of video imagery. 
     One solution that has been applied to image processing applications with some success has been the use of high-performance digital signal processors (DSP), such as the Intel i860 or the Texas Instruments (TI) TMS320C40, which have architectures inspired by high-performance military vector processing algorithms, such as linear filters and the fast Fourier transform. However, traditional DSP architectural characteristics, such as floating point precision and concurrent multiply-accumulate (vector) hardware components, are less appropriate for image processing applications since they process with full precision whether it is needed or not. 
     New hardware architectures created specifically for image processing applications are beginning to emerge from the military aerospace community to satisfy the demanding requirements of civil and commercial image processing applications. Beyond the high input data rates and complex algorithms, the most unique characteristics of image processing applications are the two-dimensional image structures and the relatively low precision required to represent and process video data. Sensor input data precision is usually only 8 to 12 bits per pixel. Shape analysis edge operations can be accomplished with a single bit of computational precision. While it is possible that some other operations may require more than 12 bits, the average precision required is often 8 bits or less. These characteristics can be exploited to create hardware architectures that are very efficient for image processing. 
     Both hard-wired (i.e., algorithm designed-in hardware) and programmable image processing architectures have been tried. Because of the immaturity of image processing-algorithms, programmable image processing architectures (which, by definition, are more flexible than hard-wired approaches) are the most practical. These architectures include Single Instruction Single Data (SISD) uniprocessors, Multiple Data Multiple Instruction (MIMD) vector processors, and Single Instruction Multiple Data (SIMD) two-dimensional array processors. 
     Massively parallel SIMD operating architectures, having two-dimensional arrays of processing elements (PE), each operating on a small number of pixels, have rapidly matured over the last 10 years to become the most efficient architecture for high-performance image processing applications. These architectures exploit image processing&#39;s unique algorithm and data structure characteristics, and are therefore capable of providing the necessary teraoperation-per-second support to image processing algorithms at the lowest possible hardware cost. 
     Where required by the algorithm suite, the SIMD bit serial PE is flexible enough to perform 1 bit or full precision floating point operations. In most cases, the highest possible implementation efficiencies are often achieved because excess hardware in the SIMD architecture is seldom idle, in contrast to those solutions which employ DSP hardware for image processing. Two-dimensional SIMD image processing architectures also mirror the two-dimensional image data structures to achieve maximum interprocessor communication efficiency. These processors typically use direct nearest neighbor (i.e, north, south, east, and west) PE connections to form fine-grained, pixel-to-processor mapping between the computer architecture and the image data structure. The two-dimensional grid of interconnections provides two-dimensional SIMD architectures with inherent scalability. As the processing array is increased in size, the data bandwidth of the inter-PE bus (i.e, two-dimensional processor interconnect) increases naturally and linearly. 
     The fastest image processing time could be achieved by configuring the size of a PE array to exactly match the expected size of the largest image to be processed. In such a configuration, one would need only to load the entire image into the array, control the PE array to perform the image processing algorithm, and then read out the results. However, in order for a parallel processing system to be commercially feasible, the quantity of parallel processing elements in a system must be significantly smaller than the number of pixels in the incoming image. When this is the case, the incoming image must be broken down into smaller sub-images which are then separately processed and then reconstructed for output. For flexibility, the system should also support variable-sized input and output images, preferably by simply reprogramming the sub-image distribution scheme. 
     For example, consider the case in which an N×M PE array is embodied on a single integrated circuit (IC), with each of the interior PE&#39;s connected to its four nearest neighbors (NORTH, EAST, SOUTH, and WEST). A larger array, for example an 5N×5M array, can be constructed by configuring an array of these ICs (e.g., a 5×5 array of these ICs) on a circuit board (henceforth referred to simply as “board”). Still greater processing power can be arranged by designing a system that includes multiple boards. 
     Within any given IC, each of the PEs is coupled to its nearest neighbors, and is therefore capable of exchanging data with one or more of those neighbors as directed by the master controller. Similarly, the PEs arranged on any one board are often interconnected to enable the PEs along the perimeter of one IC&#39;s PE array to exchange data with a neighboring PE located along the perimeter of a neighboring IC&#39;s PE array. Usually, however, it is impractical to design a system that provides the ability for any PE located on one board to exchange data with any PE located on a different board within the same system. 
     The ability, or lack thereof, of a PE to exchange data with a neighboring PE has ramifications on how an image can best be processed by the array because many of these algorithms require that, in order to process any given pixel, information about one or more of that pixel&#39;s neighboring pixels be available. For example, consider the exemplary image frame  100  depicted in FIG.  1 . The image frame  100  comprises a 3M×2N array of pixels. Assume that a system for processing the image comprises six boards, each having an M×N array of PEs arranged thereon. One might then divide up the image frame  100  into six frame segments  101 ,  103 ,  105 ,  107 ,  109 ,  111 , each consisting of a unique M×N section of the image frame  100 . Each of the frame segments  101 ,  103 ,  105 ,  107 ,  109 ,  111  can then be supplied to a respective one of the six boards for processing. When processing is complete, the processed sections can then be collected from the individual boards and reconstructed to form a complete processed image frame. 
     Less than desirable results are likely to result from the above-described processing strategy. First, if the system is designed in such a way that the PEs on one board are not capable of exchanging data with the PEs located on other boards, then the processing of pixels located along the borders between adjacent frame segments will suffer from “edge effects” due to interaction with “off-array” pixels instead of the actual neighboring pixels. For example, if the rows of the image frame  100  are numbered from 1 to 2N, starting from the top, and if the columns of the image frame  100  are numbered from 1 to 3M, starting from the left, then the processing of the pixel located at row 1, column M (denoted “p(M, 1)”) should take into account the value of the neighboring pixel located at row  1 , column (M+1) (denoted “p(M+1,1)”). However, because these pixels have been distributed to different boards, the processing algorithm applied to each of these pixels will use an erroneous pixel value in place of the actual horizontally neighboring pixel value. Similar edge effects will result at the borders between frame segments  101 ,  103 ,  105 ,  107 ,  109 ,  111  in the vertical direction as well. 
     Furthermore, the edge effect problem can occur in connection with pixels that are located entirely within the PE array of a single board if the size of the frame segment  101 ,  103 ,  105 ,  107 ,  109 ,  111  is larger than the size of a single board&#39;s PE array, thereby requiring that the frame segment  101 ,  103 ,  105 ,  107 ,  109 ,  111  be further subdivided into “subframes” that are sequentially processed by the PE array on the board. For example, suppose that an M×N frame segment  101  is to be processed by a board having only an M/2×N/2 PE array. This can be accomplished by subdividing the M×N. frame segment  101  into four distinct subframes, each sized at M/2×N/2. Because the PE array will have to process each of these in sequence, the PEs that process pixels located along an edge of one subframe will not be able to utilize information about the value of a horizontally or vertically neighboring pixel located along an edge of a neighboring subframe. This will result in edge effect problems. 
     To avoid these edge effect problems, image frames can be divided into overlapping frame segments, whereby some pixels may be assigned to two or more frame segments. For example, consider the image frame  200  shown in FIG.  2 . The exemplary image frame  200  consists of a 720×480 array of pixels. In order to permit the image frame  200  to be processed in a system having six boards, each board having its own PE array that does not exchange data with any other PE array, the image frame  200  can be divided into six frame segments (FSs)  207 , each dimensioned as a 300×300 pixel array. As can be seen in FIG. 2, dimensioning the frame segments  207  in this manner means that there are areas of overlap between adjacent frame segments  207 . In this example, we have the following situation: 
     the pixels located in the rightmost 90 columns of the frame segment  207  assigned to board  1  also make up the leftmost 90 columns of the frame segment  207  assigned to board  2 ; 
     the pixels located in the rightmost 90 columns of the frame segment  207  assigned to board  2  also make up the leftmost 90 columns of the frame segment  207  assigned to board  3 ; 
     the pixels located in the rightmost 90 columns of the frame segment  207  assigned to board  4  also make up the leftmost 90 columns of the frame segment  207  assigned to board  5 ; 
     the pixels located in the rightmost 90 columns of the frame segment  207  assigned to board  5  also make up the leftmost 90 columns of the frame segment  207  assigned to board  6 ; 
     the pixels located in the bottommost 120 columns of the frame segment  207  assigned to board  1  also make up the topmost 120 columns of the frame segment  207  assigned to board  4 ; 
     the pixels located in the bottommost 120 columns of the frame segment  207  assigned to board  2  also make up the topmost 120 columns of the frame segment  207  assigned to board  5 ; and 
     the pixels located in the bottommost 120 columns of the frame segment  207  assigned to board  3  also make up the topmost 120 columns of the frame segment  207  assigned to board  6 . 
     Because there are varying degrees of both horizontal and vertical overlap, pixels may be assigned to one, two or four boards, depending on their location within the frame image  200 . For example, some pixels, such as those located in region  201 , are assigned to four boards. Pixels located on other border regions, such as region  203  and region  205 , are assigned to only two boards. Pixels not located in any overlap region are assigned to just one board. This strategy provides a mechanism for eliminating edge effects, as will be illustrated by the following example. When board  1  processes its frame segment  207 , edge effects will be produced for pixels lying in region  205 , because the PEs on board  1  will not have access to the pixel values lying to the right of region  205 . However, those pixels lying in region  203  do not suffer from this problem because the PEs on board  1  do have access to the pixel values lying to the right in region  205 . 
     Similarly, when board  2  processes its frame segment  207 , edge effects will be produced for pixels lying in region  203 , because the PEs on board  1  will not have access to the pixel values lying to the left of region  203 . However, those pixels lying in region  205  do not suffer from this problem because the PEs on board  2  do have access to the pixel values lying to the left in region  203 . 
     After all of the boards have finished their processing, a complete processed image frame without edge effects is reconstructed by using board  1 &#39;s results for those pixels lying in region  203 , and board  2 &#39;s results for those pixels lying in region  205 . 
     A similar strategy is adopted for processing all other overlapping regions in image frame  200 , both horizontal and vertical. The dotted lines in FIG. 2 illustrate from which board the processed results are taken to reconstruct a complete processed image. 
     This overlapping strategy can similarly be used within a single board, when the frame segment  207  needs to be further divided into subframes that will be sequentially processed by the PE array on that board. 
     It is possible to design and construct dedicated hardware that will perform the necessary input/output (I/O) to move pixels into and out of PE array boards when the size of the image frame, number of boards, and size of the PE array on a board is fixed. However, to make for a more commercially viable, flexible image processing architecture, capable of processing variable sized image frames and further capable of adapting to system configurations having a variable number of boards, it is desirable to provide techniques and apparatuses that simplify the process of inputting frame segments  207  into a plurality of boards, distributing possibly overlapping subframes to PE arrays on a given board, and reconstruct a processed image frame from the processed frame segments generated by the boards. 
     SUMMARY 
     In accordance with one aspect of the present invention, the foregoing and other objects are achieved in methods and apparatuses for selectively distributing a plurality of data items to a plurality of hardware destinations that share a common bus. This involves, for each one of the data items, utilizing a distribution technique that includes determining which of the hardware destinations the data item should be distributed to, wherein at least one of the data items should be distributed to two or more hardware destinations. The data item is then supplied to the common bus; and for each of the hardware destinations to which the data item should be distributed, a corresponding hardware destination signal is generated that causes the data item to be received in the hardware destination from the common bus, wherein for each data item, the corresponding hardware destination signals are generated substantially simultaneously. In this manner, each data item to be distributed need be placed on the common bus only once, even if it is to be distributed to more than one hardware destination. 
     In another aspect of the invention, each of the hardware destinations may be a processor board in a multiprocessor system. 
     In yet another aspect of the invention, the hardware destination signal may be generated from one or more control words that are retrieved from respective one or more control memories. 
     In still another aspect of the invention, each bit in the one or more control words may uniquely correspond to one of the processor boards. 
     In yet another aspect of the invention, the hardware destination signal may be generated by logically ANDing two or more control words. For example, one control word may be associated with rows of processor boards, and another control word may be associated with columns of processor boards. If a same bit position in both the row and column control words has an asserted bit (e.g., a binary “1”), then that processor board will be one of the hardware destinations for the data item. 
     In alternative embodiments of the invention, each of the hardware destinations may be one of a plurality of input memory devices that are commonly installed on a processor board. 
     In these embodiments as well, the hardware destination signal may be generated from a control word that is retrieved from a control memory. Furthermore, each bit in the control word may uniquely correspond to one of the input memory devices. 
     In another aspect of the invention, each of the input memory devices may be associated with a corresponding one of a plurality of channels on the processor board, and each of the channels may be associated with a corresponding one of a plurality of processing element arrays. 
     In still another aspect of the invention, the plurality of data items may form a frame segment that is partitioned into a plurality of overlapping subframes; each of the data items that should be distributed to two or more hardware destinations may be associated with an overlap region formed by at least two of the overlapping subframes; each of the input memory devices may be associated with a corresponding one of a plurality of channels on the processor board; and each of the channels may be associated with a corresponding one of a plurality of addressable storage devices. Furthermore, for each of the channels, data items are loaded into the corresponding addressable storage device from the corresponding input memory device. 
     In yet another aspect of the invention, the step of, for each of the channels, loading data items into the corresponding addressable storage device from the corresponding input memory device, may be performed such that, for each of the channels, each data item that is associated with an overlap region associated with vertically overlapping subframes is stored at only one location within the corresponding one of the plurality of addressable storage devices. 
     In still another aspect of the invention, each of the channels is associated with a corresponding one of a plurality of processing element arrays. Furthermore, for each of the channels, data items are loaded into the corresponding one of the processing element arrays from the corresponding addressable storage device. In each of the processing element arrays, a processed subframe is then formed, and the processed subframe is aligned so that at least one edge row of processing elements in the processing element array includes a selected row of processed data items, wherein the selected row of processed data items includes at least one processed data item that will be supplied as an output data item from the processor board. 
     In yet another aspect of the invention, in each of the processing element arrays, a processed subframe may be formed in which each processed data item is marked to indicate whether it is to be retained or discarded. 
     In still another aspect of the invention, for each of the channels, the processed subframe may be loaded from the corresponding processing element array into the corresponding addressable storage device. 
     In yet another aspect of the invention, each of the channels may be associated with a corresponding one of a plurality of output storage devices. Furthermore, for each of the channels, a data item is conditionally loaded from the corresponding addressable storage device into the corresponding output storage device only if the data item is marked for retention. 
     In still another aspect of the invention, the plurality of data items forms an image frame that is partitioned into a plurality of overlapping frame segments; and each of the data items that should be distributed to two or more hardware destinations is associated with an overlap region formed by at least two of the overlapping frame segments. 
     In yet another aspect of the invention, the plurality of data items may form a frame segment that is partitioned into a plurality of overlapping subframes; and each of the data items that should be distributed to two or more hardware destinations is associated with an overlap region formed by at least two of the overlapping subframes. 
     The invention further involves methods and apparatuses for forming a sequence of data items by selectively collecting a plurality of data items from a plurality of processor boards in a multiprocessor system, wherein the processor boards share a common bus. This is done by, for each one of the data items in the sequence to be formed, performing a collection procedure that includes retrieving a board selection word from each of one or more control memories; generating a processor board selection signal from the retrieved one or more board selection words; using the processor board selection signal to selectively cause one of the processor boards to supply the data item to the common bus; and collecting the data item from the common bus, whereby the plurality of data items are collected from the plurality of processor boards in an order that is determined by an order in which the board selection words are retrieved from the one or more control memories. 
     In another aspect of the invention, each bit in the one or more board selection words uniquely corresponds to one of the processor boards. 
     In still another aspect of the invention, the step of retrieving the board selection word from each of one or more control memories includes retrieving a board selection word from each of two or more control memories; and the step of generating the processor board selection signal comprises generating the processor board selection signal by logically ANDing the retrieved two or more board selection words. 
     In yet another aspect of the invention, each of the processor boards comprises a processor array. 
     In other aspects of the invention, methods and apparatuses are provided that process a subframe that comprises a plurality of data items. In accordance with one aspect, this is performed by loading the subframe into a processing element array that comprises a plurality of processing elements arranged in a rectangular array having four processing element array edges, each defined by a respective one of first and second processing element edge rows and first and second processing element edge columns. In the processing element array, a processed subframe is formed that comprises at least one non-retained edge portion and a remaining portion, wherein the non-retained edge portion alternatively comprises one or more contiguous rows, or one or more contiguous columns of processed data items that will not be retained. Then, in the processing element array, the processed subframe is aligned such that at least one of the processing element array edges stores an edge row or column of the remaining portion of the processed subframe. 
     In yet another aspect, the step of, in the processing element array, aligning the processed subframe includes shifting the processed subframe within the processing element array until a first processing element array edge stores the edge row or column of the remaining portion of the processed subframe. As a result, a first rectangular group of the processing elements is formed that has an edge that is opposite the first processing element array edge, and that stores data items that will not be retained, wherein the data items stored in the first rectangular group of the processing elements constitute a first rectangular group of non-retained data items. 
     In still another aspect, the shifted processed subframe is then moved from the processing element array to an addressable memory device, wherein the edge row or column of the remaining portion of the processed subframe overwrites a second rectangular group of non-retained data items that was previously moved from the processing element array to the addressable memory device. This is useful for assembling a larger processed image in the addressable memory device. 
     In other aspects, subframe processing is performed by loading the subframe into a processing element array, and forming a processed subframe in which each processed data item is marked to indicate whether the processed data item is to be retained or discarded. 
     In yet another aspect of the invention, one of the processed data items is then conditionally loaded into an output storage device only if the processed data item is marked for retention. 
     In still another aspect of the invention, the processed subframe is first loaded from the processing element array into an addressable memory. In these embodiments, one of the processed data items may be conditionally loaded from the addressable memory into the output storage device only if the processed data item is marked for retention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and advantages of the invention will be understood by reading the following detailed description in conjunction with the drawings in which: 
     FIG. 1 depicts an exemplary image frame; 
     FIG. 2 depicts an exemplary image frame comprising a number of overlapping frame segments; 
     FIG. 3 is a block diagram of an exemplary image processor system in accordance with an aspect of the invention; 
     FIG. 4 is a block diagram of an exemplary embodiment of an I/O board in accordance with an aspect of the invention; 
     FIG. 5 a  illustrates an input column control memory and an input row control memory, in accordance with an aspect of the invention; 
     FIG. 5 b  illustrates how values for the input column board select words and the input row board select words may be readily determined; 
     FIG. 6 a  illustrates an output column control memory and an output row control memory, in accordance with an aspect of the invention; 
     FIG. 6 b  illustrates how values for the output column board select words and the output row board select words may be readily determined; 
     FIG. 7 is a block diagram of an exemplary processor board in accordance with an aspect of the invention; 
     FIG. 8 is a block diagram of those components of the processor board that are most relevant to receiving and supplying data in connection with the board&#39;s input and output functions, in accordance with an aspect of the invention; 
     FIG. 9 is a diagram of a frame segment that has been divided up into one or more array-sized overlapping “subframes”, in accordance with an aspect of the invention; 
     FIG. 10 illustrates a 300×416 pixel frame segment that has been partitioned into six overlapping subframes, each dimensioned at 160×160 pixels; 
     FIG. 11 is a block diagram of an exemplary embodiment of the frame buffer, in accordance with an aspect of the invention; 
     FIG. 12 illustrates how the pixels of the exemplary subframes of FIG. 10 would be distributed among the five channels of the input FIFO in accordance with an aspect of the invention; 
     FIG. 13 is a block diagram of an input buffering stage, in accordance with an aspect of the invention; 
     FIG. 14 depicts an input control memory and exemplary contents, in accordance with an aspect of the invention; 
     FIG. 15 illustrates an arrangement of horizontally subframed data in the frame buffer in accordance with an aspect of the invention; 
     FIG. 16 illustrates how a border about a processed subframe  1601  may be marked for non-retention, the creation of a shifted processed subframe, and the movement of a shifted processed subframe to the frame buffer, in accordance with various aspects of the invention; 
     FIG. 17 illustrates how the pixels of an exemplary processed frame segment would be distributed after being moved from the frame buffer to the five channels of the output FIFO, in accordance with an aspect of the invention; 
     FIG. 18 is a block diagram of an exemplary output buffering stage, in accordance with an aspect of the invention; and 
     FIG. 19 is a diagram of an exemplary output control memory and exemplary contents, in accordance with an aspect of the invention. 
    
    
     DETAILED DESCRIPTION 
     The various features of the invention will now be described with respect to the figures, in which like parts are identified with the same reference characters. 
     The invention relates to methods and apparatuses for distributing, processing, and reconstructing variable-sized images using multiple processor arrays. Techniques and hardware are provided for quickly and easily distributing pixels to one or more boards in accordance with their mapping in overlapping frame segments  207 . The same approach may also be applied within a single board, to quickly and easily distribute pixels to one or more portions of that board&#39;s PE array in accordance with their mapping in overlapping subframes. To accomplish this, one or more control memories plus support logic are provided for controlling a pixel&#39;s distribution as soon as it is made available. The same approach may further be applied to selectively retrieve pixels from boards to reconstruct a processed image frame that is free from edge effects. 
     The invention will now be described with reference to an exemplary one of many possible embodiments. To facilitate an understanding of the invention, particular hardware configurations are shown. However, the practice of the invention is not limited to the exemplary embodiments presented herein. Rather, the design principles presented herein may be generally applied to many other embodiments of image processors. 
     An exemplary image processor system  300  embodying the invention is shown in FIG.  3 . For purposes of illustration, the image to be processed in this example is video data, supplied by a video sensor. However, this is not an essential feature of the invention, which is equally applicable to all types of image processors, regardless of the source of the images. The image processor system  300  includes an input/output (I/O) board  301 , one or more processor boards  303 , a system controller  305 , and a user interface  307 . The I/O board  301  and processor boards  303  are coupled to the system controller  305  by means of a bus  309 , which may be, for example, a Compact PCI bus. The I/O board  301  is also coupled to the processor boards  303  by control and data lines  311 . The user interface  307  permits a user to get controllable access to the system controller  305 . 
     The image processor system  300  receives a video signal in a given video format, processes the video data, and transmits the processed video signal in the same or other video format. The I/O board  301  provides minimal buffering of the video data for the purpose of synchronizing the video transmission and reception rates to internal data rates. Input and output data flows are continuous and independent of each other. Storage of video data for processing is performed by the processor boards  303 . The I/O board  301  controls data flow to and from the processor boards and provides video frame synchronization. 
     An exemplary embodiment of the I/O board  301  is shown in greater detail in FIG.  4 . The I/O board  301  provides independent input and output control and data paths. The input video signal is decoded by a decoder  401 , and supplied to an input distributer  403 , which synchronizes the decoded video data with internal data rates and processes. The input distributer  403  then immediately sends the synchronized input data (via Data_In  405 ) to the processor boards  303  for processing. Each received pixel is sent to one or more processor boards as selected by the Board_Select_In signal  407 . An Input_Sync signal  409  is also generated for indicating an input video frame boundary to the processor boards  303  so that they may synchronize data input with the I/O board. 
     After processing, the processed video data is read as needed from the processor boards  303  (via Data_Out  411 ). The processed video data is received from the boards under the control of an output collector  413 , which synchronizes the processed video data with the video output device (not shown) and supplies it to an encoder  415  for suitable encoding prior to transmission. Each pixel is read from the appropriate processor board  303  as selected by a Board_Select_Out signal  417 , which is generated by the output collector  413 . An Output_Sync signal  419  is also generated by the output collector  413  for indicating an output video frame boundary to the processor boards  303  so that they may synchronize data output with the I/O board  301 . 
     Each received video frame is segmented into overlapping tiles known as “frame segments” (FSs)  207  for processing by the processor boards  303  as shown and described earlier with respect to FIG.  2 . As previously explained, the overlap of frame segments  207  is necessitated by the fact that the image processing algorithms employed by the processor boards  303  produces “edge effects” (pixels on the periphery of a frame segment  207  that are corrupted during processing due to interaction with “off-array” pixels). These edge effects must be eliminated whenever possible to avoid visible artifacts in the processed video image. The use of an overlap region allows the frame segment  207  boundary region which contains the edge effects to be discarded prior to reconstruction of the processed video frame. As shown in FIG. 2, the outer half of the overlap region for each processed frame segment is discarded, allowing the video frame to be reconstructed, free of edge effects, by abutting the resulting frame segments. 
     The distribution of the video data to processor boards  303  is performed for each pixel as it is received by the I/O board  301 . In the following exemplary embodiments, pixels are received in a raster order, for example, from left to right and top to bottom of the video frame. The invention is equally applicable to other raster orders as well, including but not limited to any combination of right to left, bottom to top, and interlaced or non-interlaced. As shown in FIG. 2, each pixel is sent to one or more boards  303  depending upon its position in the image frame  200 . In this example, the first 210 (=300−90) pixels of the first row are sent to board  1  only, the next 90 pixels are sent to both board  1  and board  2 , the next 120 pixels are sent to board  2  only, the next 90 pixels are sent to both board  2  and board  3 , and the final 210 pixels are sent to board  3  only. In the 181st (=300−120+1) row, the first 210 pixels are sent to boards  1  and  4 ; the next 90 pixels are sent to boards  1 ,  2 ,  4  and  5 ; the next 120 pixels are sent to boards  2  and  5 ; the next 90 pixels are sent to boards  2 ,  3 ,  5  and  6 ; and the final 210 pixels are sent to boards  3  and  6 . 
     In accordance with one aspect of the invention, distribution of pixels can be configured for different input and output video frame sizes, different frame segment sizes and overlaps, and different numbers of processor boards  303 . The example of FIG. 2 is provided to illustrate one of many possible processor board input (solid lines) and output (dotted lines) configurations that can easily be effected by means of the invention. An important aspect to the configurability of the invention is in the strategy adopted for controlling data distribution and collecting. 
     In order to provide programmable control of pixel distribution from the I/O board  301  to the processor boards  303 , the input distributer  403  on the I/O board  301  includes an input column control memory  501  and an input row control memory  503  as shown in FIG. 5 a.  The input column and row control memories  501 ,  503  are each configured to operate as a rotating buffer, and may be implemented by many different types of memory components, including but not limited to first-in-first-out (FIFO) memory components. The input column control memory  501  stores, for each pixel column (as measured in the image frame  200 ), a board column select word. Similarly, the input row control memory  503  stores, for each pixel row (as measured in the image frame  200 ), a board row select word. Thus, for the exemplary image frame  200 , the input column control memory  501  would store  720  board column select words, and the input row control memory  503  would store  480  input row select words. For each pixel that the input distributer  403  is to supply to the processor board interface  421 , a board column select word is retrieved from the input column control memory  501  as a function of that pixel&#39;s column address, and a board row select word is retrieved from the input row control memory  503  as a function of that pixel&#39;s row address. 
     The width of each of the board row and board column select words corresponds to the number of processor boards  303  in the system  300 . Each bit in the board column select word uniquely corresponds to one of the processor boards  303 . Similarly, each bit in the board row select word uniquely corresponds to one of the processor boards  303 . In the exemplary embodiment, in which 6 processor boards  303  are employed, each of the board column and board row select words is 6 bits in width. For a pixel to be loaded into any given one of the processor boards  303 , that board&#39;s corresponding bits must be set to a value indicating “enable” (e.g., a logical “1”, with a value of logical “0” denoting“non-enable”) in both the board column and board row select words that correspond to the pixel&#39;s column and row addresses. The Board_Select_In signal  407  (which in this example is 6-bits wide) is generated by ANDing the input row board select word with the input column board select word. In the exemplary embodiment, an AND gate  505  is provided for this purpose. However, it will be readily apparent to those of ordinary skill in the art that the logical ANDing function could alternatively be provided by any number of alternative logic configurations. The individual bits of the Board_Select_In signal  407  may be routed to their corresponding processor boards  303  to alternatively enable or disable the loading of the present pixel (supplied on the Data_In line  405 ) onto that board  303  as a function of the value of the Board_Select_In bit. 
     The generation of proper address sequences for controlling the input column control memory  501  and the input row control memory  503  may be performed by means of counters and the like. However, in accordance with another aspect of the invention, proper address sequencing is advantageously generated without the use of extra counters by storing an extra address control bit along with each input column board select word and each input row board select word. More particularly, the input column control memory  501  stores an extra bit, referred to herein as the final column indicator bit  511 , with each input column board select word  507 . The input row control memory  503  similarly stores an extra bit, referred to herein as the final row indicator bit  513 , with each input row board select word  509 . Only the last final column indicator bit  511 , stored with the last input column board select word  507 , is initialized to a value indicating assertion (e.g., a binary “1”); all of the remaining final column indicator bits  511  are initialized to a value indicating non-assertion (e.g., a binary “0”). Similarly, only the last final row indicator bit  513 , stored with the last input row board select word  509 , is initialized to a value indicating assertion (e.g., a binary “1”); all of the remaining final row indicator bits  513  are initialized to a value indicating non-assertion (e.g., a binary “0”). 
     During operation, addresses for the input column control memory  501  and input row control memory  503  are each initialized to point to the start of their respective control memories. The Board_Select_In signals  407  for distributing the first row of pixels are then generated by reading the first input row board select word  509  from the input row control memory  503 , while simultaneously selecting and reading in turn the input column board select words  507  from the input column control memory  501 , one for each pixel in the row. When the final input column board select word  507  has been read, its associated final column indicator bit  511  is output from the input column control memory  501  as well, thereby asserting the final column indicator signal  515 . Assertion of the final column indicator signal  515  causes the address for the input column control memory  501  to be reset (so that it will start again from the first stored input column board select word  507 ), and also causes the address for the input row control memory  503  to be incremented, so that the next input row board select word  509  will be supplied by the input row control memory  503 . Operation continues in this manner until the final input row board select word  509  and its associated final row indicator bit  513  are emitted from the input row control memory  503 . Because of its initial setting, the final row indicator bit  513  causes the final row indicator signal  517  to be asserted. Addresses for the input column control memory  501  continue to increment until the last address is reached, at which point the final column indicator signal  515  is asserted. Assertion of both the final column indicator signal  515  and the final row indicator signal  517  causes addresses for both the input column control memory  501  and the input row control memory  503  to be reset to point to the first entries contained therein. The input distributer  403  is then ready to generate Board_Select_In signals  407  for the next image frame  200 . 
     In the above example, pixels that make up the image frame  200  were distributed to the processor boards  303  one row at a time. This is not essential to the invention, however. Those skilled in the art will readily understand how to adapt the principles underlying this aspect of the invention to develop a logic control structure whereby pixels that make up an image frame  200  are distributed to the processor boards  303  one column at a time instead of one row at a time. 
     As is illustrated in FIG. 5 b,  values for the input column board select words  507  and the input row board select words  509  are readily determined by observing, for each row or column of the image frame  200 , which boards are to receive the pixels in that row or column. For example, with columns numbered  1  through  720  (left-to-right) and rows numbered  1  through  480  (top-to-bottom), it can be seen that column  250  is an overlap region with some pixels being sent to only boards  1  and  2 , some pixels sent to boards  1 ,  2 ,  4  and  5 , and some pixels sent to only boards  4  and  5 . The input column board select word  507  is generated by taking the union of these sets, so the input column board select word  507  for column  250  is therefore “011011” (where the 6 bits in the input column board select word  507  board control bits respectively represent boards  6 - 5 - 4 - 3 - 2 - 1 , in that order). In this example, it can be seen that all of the pixels located in columns  211  through  300  are similarly situated, and will therefore also have an input column board select word  507  having a value of “ 011011 ”. 
     Taking an example from the rows, it can be seen that row  350  includes some pixels that are to be sent only to board  4 , some pixels that are to be sent to both boards  4  and  5 , some pixels that are to be sent only to board  5 , some pixels that are to be sent to both boards  5  and  6 , and some pixels that are to be sent only to board  6 . The input row board select word  509  is generated by taking the union of these sets, so the input row board select word  509  for the pixels in row  350  is “111000”, again with the bits in the input row board select word  509  respectively representing boards in the order  6 - 5 - 4 - 3 - 2 - 1 . 
     Completing the example, the Board_Select_In signal  407  for the pixel located at row  350 , column  250  is determined by ANDing 011011 with 111000 to give 011000. That pixel will therefore be sent only to boards  4  and  5 . 
     The strategy for generating the Board_Select_Out signals  417  is the same as that used for generating the Board_Select-In signals  407 . As shown in FIG. 6 a,  the output collector  413  includes an output column control memory  601  and an output row control memory  603 . The output column control memory  601  stores output column board select words  607 , and in some embodiments also final column indicator bits  611 . In these embodiments, the output column control memory  601  also supplies a final column indicator signal  615  at an output. (As indicated earlier, the use of final column indicator bits  611  and a final column indicator signal output  615  are not essential to practicing the invention.) The output row column control memory  603  similarly stores output row board select words  609 , and in some embodiments also final row indicator bits  613 . In these embodiments, the output row control memory  603  also supplies a final row indicator signal  617  as an output. (As indicated earlier, the use of final row indicator bits  613  and final row indicator signal  617  output are not essential to practicing the invention.) An AND gate  605  may be provided for generating the board select out signal  417  from the output column board select words  607  and output row board select words  609  supplied by the output column control memory  601  and output row control memory  603 , respectively. Use of the AND gate  605  is not essential to practicing the invention; alternative circuit arrangements could be used to generate the board select out signal  417  from the output column board select words  607  and output row board select words  609 . 
     The elements depicted in FIG. 6 a  operate analogously to those described above with respect to FIG. 5 a,  and so a detailed discussion of their operation will not be repeated. Rather, only the differences will be discussed. As can be seen from the processed image frame  650  depicted in FIG. 6 b,  the primary difference between the input and output strategies is that no overlap is employed during output. Output column board select words  607  combine with output row board select words  609  to form the Board_Select_Out signals  417  with one and only one select bit set for each pixel. 
     In some embodiments, there is the potential for two or more processor boards  303  to attempt to simultaneously drive data onto the same bus. This can occur during brief moments when a transition is being made from selection of one board  303  to selection of another. To prevent such occurrences, one or both of the output column and row control memories  601 ,  603  can be further programmed to include, at appropriate locations, output column and/or board select words  607 ,  609  in which none of the select bits are set. In the exemplary embodiment illustrated in FIG. 6 a,  one of these “all zero” output column board select words  607  is inserted between those output column board select words  607  which, if applied in succession, cause a transition between one or more boards  303  to occur, that is, after the first  255  output column board select words  607 , again after the next  210  output column board select words  607 , and again after the next  255  output column board select words  607 . 
     The use of the input and output row and column control memories  501 ,  503 ,  601 ,  603  is a simple means for providing programmable control of the distribution and collecting processes. The contents of these control memories can easily be changed to accommodate different sized image frames  200 , different numbers of processor boards  303 , different degrees of overlap between frame segments  207 , and different raster orders including multiple fields associated with interlaced images. Each processor board  303  receives a frame segment  207  from the I/O board  301 , processes the frame segment  207 , and provides an output frame segment (processed frame segment  651 ) to the I/O board  301 . While some areas of the frame segment  207  may represent overlap with frame segments  207  sent to other processor boards  303 , the output frame segment is “trimmed” prior to output so that the I/O board  301  can easily reconstruct the processed image frame  650  by abutment of output (processed) frame segments  651 . 
     Up to this point, it has been assumed that each processor board  303  is capable of performing the required processing of the input frame segment  207  supplied without generating its own edge effect-related artifacts. However, as mentioned earlier, if the size of the PE array on a processor board  303  is smaller than the input frame segment  207 , some strategy, such as overlapping subframes must be applied. In accordance with another aspect of the invention, the same strategy as described above, employing one or more control memories to provide pixel input and output controls, can be used to facilitate the distribution of pixels to and from particular PEs located on a single processor board  303 . This aspect of the invention will now be described with reference to an exemplary embodiment. 
     Referring now to FIG. 7, an exemplary processor board  303  is shown. The processor board  303  comprises an input buffering stage  701 , an output buffering stage  703 , a frame buffer  705 , and a processor array  707 . Instructions for controlling the processor array  707  are supplied by a microcode memory  709 , which receives its addresses from a sequencer  711 . The sequencer in turn is controlled by commands supplied by a command buffer  713 . The sequencer  711  further interacts with an on-board I/O controller  715 . The sequencer  711  provides commands to the I/O controller  715 , and receives state (“handshake”) signals from the I/O controller  715 . 
     The processor array  707  is coupled to the input buffering stage  701 , output buffering stage  703  and frame buffer  705  by means of a common image bus  717 , which in the exemplary embodiment is 5×32=160 bits wide and operates at 60 MHz. The I/O controller  715  is responsible for controlling the movement of data between the input buffering stage  701  and the frame buffer  705 ; between the frame buffer  705  and the processor array  707  (both directions); and between the frame buffer  705  and the output buffering stage  703 . 
     In the exemplary embodiment, the processor array  707  is a 160×160 array of PEs, each having a 1-bit wide architecture (i.e., the processing of multi-bit operands by any one of the PEs requires the performance of multiple instructions). The PE array is arranged as a SIMD architecture, so that the PEs operate in lock-step as each new microcode instruction is supplied by the microcode memory  709 . The processor array  707  is preferably implemented as a 5×5 array of processor array ICs, each IC itself comprising a 32×32 array of 1-bit wide PEs. The processor array IC may, for example, be that which is described in U.S. patent application No. 08/112,540, filed on Aug. 27, 1993 in the name of Woodrow L. MEEKER (“Parallel Data Processor”), which is hereby incorporated herein by reference in its entirety. However, use of this particular IC is not essential to practicing the invention. Any similar type of processor array IC, such as, but not limited to, that which is described in U.S. patent application Ser. No. 09/057,482, filed on Apr. 9, 1998 in the name of Abercrombie et al. (“Mesh Connected Computer”) can be used instead. The disclosure of U.S. patent application No. 09/057,482 is hereby incorporated herein by reference in its entirety. The processor array ICs are interconnected with one another so that, during processing, they behave like a 160×160 PE array, with each PE being able to exchange information with its neighbors which may be found to its NORTH, EAST, SOUTH AND WEST. 
     FIG. 8 illustrates those components of the processor board  303  that are most relevant to receiving and supplying data in connection with the board&#39;s input and output functions. In particular, FIG. 8 shows a 5×5 array of 32×32 PE array ICs  801  that make up the processor array  707 . Also shown is the frame buffer  705  as well as an input FIFO  803  that is part of the input buffering stage  701 . The input FIFO  803  receives data from the Data_In line  405 , and makes data available on the common image bus  717 . The outputting of data from the processor board  303  is supported by an output FIFO  805  in conjunction with a master FIFO  807 , as will be explained in greater detail below. 
     Because of the high data throughput required within the processor board  303 , the input FIFO  803 , output FIFO  805 , frame buffer  705  and processor array  707  are arranged as multiple column channels. This permits data to flow on each of the five channels simultaneously. As shown in FIG. 8, the exemplary embodiment of the processor board employs five column channels. Specifically, each column of PE array Cs  801  provides a 32 bit path for the input and output of image data. Each channel of the input FIFO  803 , output FIFO  805  and frame buffer  705  also provides a 32 bit path for data movement. The 160-bit common image bus (CIB)  717  interconnects the five 32-bit channels of the input FIFO  803 , output FIFO  805 , frame buffer  705  and processor array  707  channels. At any given time one of the following tasks may be performed: 1) transfer of a frame segment  207  from the input FIFO  803  to the frame buffer  705 , 2) input of a subframe (described below in detail) from the frame buffer  705  to the processor array  707 , 3) output of a processed subframe from the processor array  707  to the frame buffer  705  and 4) transfer of a processed frame segment from the frame buffer  705  to the output FIFO  805 . In practice these tasks may be time multiplexed so that effective sharing of the common image bus  717  is provided. 
     It is emphasized that the partitioning of the processor board  303  into channels as described above prevents the cross-communication of data between channels during the above mentioned data movement tasks. However, the processor array  707  is not partitioned in this manner with respect to its ability to process subframe data. That is, pixel data may freely move horizontally (east-west) as well as vertically (north-south) within the processor array  707  during processing. 
     Continuing the examples described above with reference to FIGS. 2,  5   b  and  6   b,  there is an apparent sizing discrepancy: The size of each frame segment  207  that is sent to a processor board  303  for processing is 300×300, but the size of the processor array  707  located on that board is only 160×160. In accordance with another aspect of the invention and referring to FIG. 9, this discrepancy is resolved by having the processor board  303  divide each frame segment  207  into one or more array-sized overlapping “subframes”  901  for processing by the array processor  707 . Using the numbers of the present example, the 300×300 frame segment  207  may be segmented into 4 overlapping subframes of 160×160 pixels for processing by the 160×160 processor array  707 . The processor board  303  processes each subframe  901  in turn and provides processed subframes for constructing a processed frame segment by discarding the appropriate overlap region of each processed subframe, thereby allowing edge effect regions to be discarded in the same way as was fully described with respect to overlapping frame segments  207 . 
     In accordance with an aspect of the invention, distributing and reconstructing subframes  901  into processed frame segments  651  is the same in principle as distributing and reconstructing processed frame segments  651  into processed image frames  650 . While frame segments  207  are distributed to separate processor boards  303  within a system  300 , subframes  207  are distributed for processing in separate time slots within a single processor array  707 . Because subframes  207  are processed separately, vertical and horizontal overlap must be employed in order to eliminate edge effects, just as overlap is employed at the frame segment level of processing. 
     The 300×300 subframes  901  depicted in FIG. 9 have only one vertical and one horizontal overlap region. To aid in the understanding of how the inventive board level input/output techniques facilitate the use of overlapping subframes  901 , and also to illustrate the versatility that the invention affords the designer, the next examples will assume that a frame segment  207  to be processed by a processor board  303  is even wider. In particular, it will be assumed that a frame segment  207  to be processed is dimensioned at 300×416. This permits the use of an example in which there are two vertical overlap regions between subframes  901 , as shown in FIG.  10 . More particularly, FIG. 10 illustrates a 300×416 pixel frame segment  207  that has been partitioned into six overlapping subframes  901 , each dimensioned at 160×160 pixels. To illustrate the versatility of the invention, the amount of horizontal overlap is not evenly distributed between subframes  901 . Rather, the amount of horizontal overlap between the first and second subframes  901 , as well as that between the fourth and fifth subframes  901 , is 20 pixels. By contrast, the amount of horizontal overlap between the second and third subframe  901 , as well as that between the fifth and sixth subframe  901  is 44 pixels. 
     As each frame segment  207  is received by the processor board  303 , it is moved from the input buffering stage  701  to the frame buffer  705  for storage. A subframe  901  is processed by moving it from the frame buffer  705  to the processor array  703 , processing the subframe  901  in the array, and moving the processed subframe back to the frame buffer  705 . The arrangement of data in the frame buffer  705  must therefore accommodate the movement of subframe data to and from the processor array. 
     To understand how data is arranged in the frame buffer  705 , it is important to recognize that there are two possible types of overlapping regions within frame segment  207 : horizontal overlap, and vertical overlap. The exemplary frame segment  207  depicted in FIG. 10 has two (first and second) horizontal overlap regions  1001 ,  1003  and one vertical overlap region  1005 . To accommodate the storage of vertically overlapping subframes  901 , these may be treated as super-subframes that each comprise the combined data from such overlapping subframes  901 . Considering for example the frame segment  207  depicted in FIG. 10, the first and fourth subframes  901  form a first super-subframe comprising  160  columns and  300  rows. Similarly, the second and fifth subframes  901  form a second super-subframe, and the third and sixth subframes  901  form a third super-subframe, with each of the second and third super-subframes comprising  160  columns and  300  rows. It is then necessary only to store each of the first, second and third super-subframes into the frame buffer  705  one row at a time (i.e., in which sequential frame buffer addresses access horizontally adjacent pixels within a given row). Nothing else need be done for the purpose of accommodating vertical overlap regions, such as the vertical overlap region  1005 . When, for example the higher of two vertically overlapping subframes  901  (e.g., the first subframe  901 ) is to be moved into the processor array  707 , the frame buffer&#39;s start address may simply be set to point to the first pixel in the first row, and then incremented to point to the next pixels in the same row and then to the next row until the final pixel in the final row of the subframe  901  has been accessed. When it is next desired to move the lower of the two vertically overlapping subframes  901  (e.g., the fourth subframe  901 ) into the processor array  707 , the same process is followed, but with the initial address set to point to the first row and column of the lower subframe  901  (e.g., the fourth subframe  901 ). Thus, for purposes of storing vertically overlapping subframes  901  within the frame buffer  705 , the fact that there are distinct subframes  901  can be ignored, with all of the pixels lying anywhere within the vertically overlapping subframes  901  (i.e., the super-subframe) being stored in sequence. 
     The same is not true with respect to horizontally overlapping subframes  901 , however. This is because, as mentioned earlier, the frame buffer  705  and processor array columns are partitioned into channels. FIG. 11 is a block diagram of an exemplary embodiment of the frame buffer  705 , in which it can be seen that five 32-bit wide memory components  1101 , such as SSRAM components, are arranged in parallel to form a 160-bit wide memory resource. Each of the memory components  1101  receives and supplies 32 bits of data for movement on a corresponding one of the five channels. There is no possibility for a pixel stored in the memory component  1101  associated with one of the channels to be loaded into or retrieved from a PE associated with a different one of the channels. 
     Since, for purposes of data movement on the processor board  303 , there is no cross-communication between channels, each frame buffer channel must contain all information necessary for processing subframes  901  within the corresponding channel of the processor array  707 . The distribution of horizontally overlapping subframes  901  must therefore provide for data located in horizontal overlap regions (e.g., the first and second horizontal overlap regions  1001 ,  1003 ) to be stored within more than one frame buffer channel. 
     FIG. 12 illustrates how the pixels of the exemplary subframes  901  of FIG. 10 would be distributed among the five channels of the input FIFO  803 . Pixels are received from the I/O board  301  one frame segment row at a time. Thus, for each row in the frame segment  207 , pixels associated with columns  0  through  31  would be stored into channel  1  of the input FIFO  803 ; pixels associated with columns  32  through  63  would be stored into channel  2  of the input FIFO  803 , and so on. Pixels located in horizontally overlapping regions are stored into more than one channel of the input FIFO  803 . For example, pixels associated with columns  140  through  159  are distributed to both channel  1  as well as channel  5  of the input FIFO  803 . Pixels associated with columns  160  through  171  are then stored only into channel  1  of the input FIFO  803 , since these are not in a horizontally overlapping region. The process of distributing pixels associated with the remaining columns of the frame segment  207  continues in a like manner. When the pixel associated with the final column (i.e., column  415  in this example) is received and stored into the input FIFO  803 , the process continues with the pixels associated with the columns of the next row in the frame segment  207 . Because pixels are received and stored one frame segment row at a time, the rows of any one super-subframe are not stored contiguously within the input FIFO  803 , but rather are interleaved with the rows of the other super-subframes that make up the frame segment  207 . 
     In order to provide for distribution of pixels as just described, an input buffering stage  701  as depicted in greater detail in FIG. 13 is provided. The input buffering stage  701  provides the appropriate distribution of frame segment data to the channels of the input FIFO  803  as the data is received from the I/O board  301 . Control for the distribution of the data is similar to that used by the I/O board  301 . Since only horizontal overlap of subframes  901  is handled by the distribution logic, only a single control memory (the input control memory  1301 ), analogous to the input column control memory  501  of the I/O board  301 , is required. This input control memory  1301  provides a FIFO select word for each pixel that is received. The input control memory  1301  is configured to operate as a rotating buffer, and may be implemented by many different types of memory components, including but not limited to FIFO memory components. 
     The input control memory  1301  and exemplary contents are shown in greater detail in FIG.  14 . The input control memory  1301  stores input FIFO select words  1401 , one for each column in the frame segment  207 . For each input FIFO select word  1401 , the input control memory  1301  also stores a corresponding final column indicator bit  1403 , which indicates whether the input FIFO select word  1401  corresponds to the final column in the frame segment  207 . Each bit of the input FIFO select word  1401  provides a write enable signal for a corresponding channel of the input FIFO  803 . At the completion of receipt of each frame segment row, the final column indicator signal  1405 , which is supplied at the output of the input control memory  1301 , is asserted. The final column indicator signal  1405  is supplied to input FIFO control logic  1303 , which responds by causing the input control memory  1301  to again begin retrieving values from the initial starting point, so that the input FIFO  803  can receive and store pixels associated with a next row in the frame segment  207 . Preferably at about the same time, the I/O controller  715  moves the data from the head of each channel of the input FIFO  803  to corresponding channels of the frame buffer  705 , in order to prevent the input FIFO  803  from filling up. In this manner, frame segment data is moved from the input FIFOs  803  to the frame buffer  705 . 
     The arrangement of horizontally subframed data in the frame buffer  705  is shown in FIG.  15 . It may be observed that, within each frame buffer channel, the pixels associated with a particular row of the entire frame segment  207  are stored together in sequence. This arrangement reflects the order in which data is received and stored in the input FIFOs  803 . This is also the order in which data is stored in the output FIFOs  805  for output to the I/O board  301 . 
     Once the data has been loaded into the frame buffer  701 , the next step is typically for it to be processed by the processor array  707 . The movement of subframe data from the frame buffer  701  to the processor array  707  requires that the data for each row of the subframe be read in sequence. As shown in FIG. 15, the first 160 pixels (row  0 ) of subframe  0  are retrieved from addresses  0  through  31  of each of the five frame buffer channels. The next row, row  1 , is reached by skipping the next 64 pixels, which belong to subframes  1  and  2  respectively. This interleaved arrangement of subframe data in the frame buffer  705  reflects the order in which input data is received from the input buffering stage  701  as well as the order in which output data is sent to the output buffering stage  703 . For purposes of moving data between the processor array  707  and the frame buffer  705 , however, it is necessary to provide an offset value reflecting the number of horizontally adjacent subframes to be skipped and the number of pixels per subframe per frame buffer channel (e.g., (3−1)*32=64). This offset allows the sequence of pixel accesses to skip from the end of one row to the beginning of the next for a given subframe  901  in the frame buffer  705 . 
     After each subframe  901  is processed by the processor array  707 , it is prepared for vertical and horizontal reconstruction. The preparation for horizontal reconstruction comprises tagging each pixel for retention or non-retention. This tagging is performed by the processor array  707 . As shown in FIG. 16, a border  1603  about the processed subframe  1601  is marked for non-retention. The remaining pixels  1605  are marked for retention. The marking may be accomplished by setting one bit of each pixel (e.g., the 32nd bit of each pixel) to 1 to indicate retention or to 0 to indicate non-retention. The preparation of the processed subframe  1601  for vertical subframing is accomplished by further using the processor array  707  to shift the image north until the non-overlap region aligns with the north edge of the array. The number of pixels to shift is preferably predetermined, and coded into the microsequence that controls the processor array  707 . Also, the values of the pixels shifted in from the south of the processor array  707  are unimportant, and may simply be the same pixels that are shifted out of the north end of the processor array  707 . The shifted processed subframe  1601 ′ is depicted in FIG.  16 . As further shown in FIG. 16, the shifted processed subframe  1601 ′ is then moved to the frame buffer  705 , under the control of the I/O controller  715 . As each subsequent processed subframe is marked, shifted and written to the frame buffer  705 , the north edge of the subsequent shifted processed subframe  1601 ′ overwrites the overlap region  1603  on the southern boundary of the previous processed shifted subframe  1601 ′. In this manner, vertical reconstruction of frame segments  207  is accomplished by abutment of processed shifted subframes  1601 ′ within the frame buffer  705 . In order to simplify the depiction of the shifted processed subframes  1601 ′ within the frame buffer  705 , the interleaving of data associated with horizontally overlapping subframes has been omitted. It should be apparent from the earlier discussion that the consecutive rows of the first and second vertical subframes depicted in FIG. 16 are stored at addresses that are sufficiently offset with respect to one another to permit the storage of rows associated with horizontally adjacent subframes. 
     It should also be noted that the border  1603 , indicating those pixels of the processed subframe  1601  that are marked for non-retention, is merely illustrative of one possibility, and in general will vary in dependence on how much horizontal and vertical overlapping the processed subframe  1601  has with each of its north, south, east and west neighboring subframes  901 . It is further noted that whether a pixel is to be retained is determined not only by its status with respect to overlapping with other subframes  901 , but also with respect to overlapping with other frame segements  207 . It is the goal of this marking to indicate, for each processed pixel stored in the frame buffer  705 , whether that processed pixel will ultimately make up a part of the processed image frame  650  (i.e., whether that pixel is part of a processed frame segment  651 ). 
     The horizontal construction of a processed frame segment  651  is accomplished during output of the processed frame segment  651  to the I/O board  301 . FIG. 17 illustrates how the pixels of an exemplary processed frame segment  1701 , comprising the processed shifted subframes  1601 ′ of FIG. 16, would be distributed after being moved from the frame buffer  705  to the five channels of the output FIFO  805 . The data of the processed subframe  1701  is stored in the channels of the output FIFO  805  in the same format as in the frame buffer, except that all pixels marked for non-retention are discarded, that is, not written to the corresponding channels of the output FIFO  805 . 
     This is accomplished by the I/O controller  715 , which causes each of the channels of the frame buffer  705  to supply a sequentially next pixel at its output port. The I/O controller  715  tests the tag associated with each pixel, and only if that tag indicates that the pixel is to be retained does the I/O controller  715  cause it to be written into the corresponding channel of the output FIFO  805 . Otherwise, the pixel is simply discarded. 
     The example of FIG. 17 is intended to illustrate the case in which a processed frame segment  651  is trimmed at both its left and right sides, presumably because these were horizontally overlapping regions with other frame segments  207  on these sides. Thus, none of the pixels located in columns  0  through  31  or in columns  396  through  415  are moved into any of the channels of the output FIFO  805 . Additional trimming has taken place to account for overlapping regions associated with neighboring subframes  901 . Thus, in this example, the pixels associated with columns  140  through  149  are moved only into channel  5  of the output FIFO  805 , while the pixels associated with columns  150  through  159  are moved only into channel  1  of the output FIFO  805 . (Formerly, pixels in this region had been associated with both of these channels.) Similarly, the pixels associated with columns  256  through  275  are moved only into one of channels  4  and  5  of the output FIFO  805 , while the pixels associated with columns  276  through  299  are moved only into one of channels  1  and  2  of the output FIFO  805 . Vertical trimming is also evident: the output FIFO  805  stores only 240 rows of pixels instead of the  300  that were part of the originally supplied frame segment  207 . 
     The frame segment data is then “collected” from the separate channels of the output FIFO  805  to form a contiguous frame segment as it is written to the master FIFO  807  via an output FIFO data bus  1805 . For example, referring to FIG. 17, each frame segment row of processed pixels would be moved from the output FIFO  805  to the master FIFO  807  as follows: 
     12 pixels of the row (columns  20  through  31 ) would be moved from channel  1  of the output FIFO  807  to the master FIFO  807 ; 
     the next 32 pixels of the row (columns  32 - 63 ) would be moved from channel  2  of the output FIFO  807 ; the next 32 pixels of the row (columns  64 - 95 ) would be moved from channel  3  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  96  through  127 ) would be moved from channel  4  of the output FIFO  807 ; 
     the next 22 pixels of the row (columns  128  through  149 ) would be moved from channel  5  of the output FIFO  807 ; 
     the next 22 pixels of the row (columns  150  through.  171 ) would be moved from channel  1  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  172  through  203 ) would be moved from channel  2  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  204  through  235 ) would be moved from channel  3  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  236  through  267 ) would be moved from channel  4  of the output FIFO  807 ; 
     the next 8 pixels of the row (columns  268  through  275 ) would be moved from channel  5  of the output FIFO  807 ; 
     the next 12 pixels of the row (columns  276  through  287 ) would be moved from channel  1  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  288  through  319 ) would be moved from channel  2  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  320  through  351 ) would be moved from channel  3  of the output FIFO  807 ; 
     the next 32 pixels of the row (columns  352  through  383 ) would be moved from channel  4  of the output FIFO  807 ; and 
     the next 12 pixels of the row (columns  384  through  395 ) would be moved from channel  5  of the output FIFO  807 . 
     In order to control this data movement, the output buffering stage  703  includes the elements depicted in FIG.  18 . The output buffering stage  703  provides the appropriate distribution of processed frame segment data from the channels of the output FIFO  803  to the master FIFO  807 . An output control memory  1801  is provided for controlling the distribution of the data in a manner similar to that performed by the input control memory  1301 . This output control memory  1801  provides a FIFO select word for each pixel that is to be moved. The output control memory  1801  is configured to operate as a rotating buffer, and may be implemented by many different types of memory components, including but not limited to FIFO memory components. 
     The output control memory  1801  and exemplary contents are shown in greater detail in FIG.  19 . The output control memory  1801  stores output FIFO select words  1901 , one for each column in the processed frame segment  1701 . For each output FIFO select word  1901 , the output control memory  1801  also stores a corresponding final column indicator bit  1903 , which indicates whether the output FIFO select word  1901  corresponds to the final column in the processed frame segment  1701 . Each bit of the output FIFO select word  1901  provides a read enable signal for a corresponding channel of the output FIFO  805 . Because only one channel of the output FIFO  805  at a time can drive the output FIFO data bus  1805 , each of the output FIFO select words  1901  enables no more than 1 channel of the output FIFO. In some embodiments, it is preferable to insert an output FIFO select word  1901  having a value of all zeros just prior to an output FIFO select word  1901  that will change which of the channels is enabled. This is to provide a “bus turnaround” time period during which none of the channels of the output FIFO  805  are enabled. This permits enough time for one channel of the output FIFO  805  to completely stop driving the output FIFO data bus  1805  before the next channel begins, thereby preventing any possibility of the two channels attempting to drive the output FIFO data bus  1805  at the same time. Upon completing the transference of each frame segment row from the output FIFO  805  to the master FIFO  807 , the final column indicator signal  1905 , which is supplied at the output of the output control memory  1801 , is asserted. The final column indicator signal  1905  is supplied to output FIFO control logic  1803 , which responds by causing the output control memory  1801  to again begin retrieving values from the initial starting point, so that the output FIFO  805  can supply pixels associated with a next row in the processed frame segment  1701 . Data is read from the master FIFO  807  upon assertion of the board select out signal  417  supplied by the I/O board  301 . In this manner, processed frame segment data is moved from the processor board  303  to the I/O board  301 . 
     It is reiterated that the frame segment data in the master FIFO  807  represents the processed frame segment  1701  that has been reconstructed from the processed subframes  1601 . This processed frame segment  1701  in turn is ready for reconstruction with other processor board frame segments into a processed image frame  650 . As explained earlier, the output collector  413  on the I/O board  301  controls this function by selectively enabling the data output from particular ones of the processor boards  303  in an order that builds a processed image frame  650  one row at a time. 
     The invention has been described with reference to a particular embodiment. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the preferred embodiment described above. This may be done without departing from the spirit of the invention. 
     For example, the descriptions of the exemplary embodiments presented herein make numerous references to the distribution and collection of pixels and other image-related (e.g., sensor-derived) data items. However, this is not an essential feature of the invention. Rather, the various techniques described above can easily, and without any need for modification, be applied to the distribution and collection of any type of data item. Thus, the term “data item” is used herein to refer to all types of data, including but not limited to pixels and other imagerelated data. 
     Thus, the preferred embodiment is merely illustrative and should not be considered restrictive in any way. The scope of the invention is given by the appended claims, rather than the preceding description, and all variations and equivalents which fall within the range of the claims are intended to be embraced therein.