Abstract:
An object identifier is provided with a data-forming unit and a data-retrieving unit. The data-forming unit forms an integral image having an image region of the same size as an original image and an image to be integrated. The data-retrieving unit obtains compressed data derived by extracting, from a plurality of pixels constituting the integral image, at least every other pixel in at least one direction.

Description:
TECHNICAL FIELD 
     The present invention relates to an object identifying apparatus for generating integral images from an input image, and identifying whether a particular object exists in the image using the integral images. 
     BACKGROUND ART 
     Recently, various image processing technologies have been proposed for reducing the amount of processing operations using integral images, when an image processing sequence is carried out to identify whether or not a particular object exists within an input image. Such integral images are equivalent to interim data for calculating a total of the pixel values residing within a region of interest that is extracted from an image area. 
     Japanese Patent No. 4582079 proposes an apparatus for and a method of dividing a single image area into a plurality of integration areas, and then generating respective integral data corresponding to the integration areas. The publication states that the method is capable of cutting back the maximum value of the pixel values contained in the integral data, thereby reducing the amount of data per pixel. 
     SUMMARY OF INVENTION 
     However, the method, etc., disclosed in Japanese Patent No. 4582079 is only capable of reducing the amount of data by a number of bits on the order of a logarithm of the number of the integration areas. More specifically, according to the description (see paragraph [0066]) of this patent document, a reduction of only about 7% is achieved by removing 1 bit from 15 bits. 
     The present invention has been made with the aim of solving the aforementioned problems. An object of the present invention is to provide an object identifying apparatus, which is capable of maintaining the accuracy with which an object can be identified while greatly reducing the amount of data that makes up the integral images. 
     According to the present invention, there is provided an object identifying apparatus including a data generator for successively integrating pixel values of integration object images representing feature quantities at respective pixels that make up an original image, from a starting point at either one of angular positions of the integration object images, thereby generating integral images, each including an image area having a size in common with the original image and the integration object images, a data acquirer for acquiring compressed data by extracting at least every other pixel along at least one direction from a plurality of pixels that make up the integral images generated by the data generator, an identification object region determiner for determining an identification object region from within an original image area represented by the original image, and an object identifier for calculating a total of the pixel values of the integration object images within the identification object region that was determined by the identification object region determiner, as an image feature quantity using the compressed data acquired by the data acquirer, and on the basis of the image feature quantity, identifying whether or not an object exists within the identification object region in the original image. 
     Since the object identifying apparatus includes the data generator, which generates integral images, each including an image area having a size in common with the original image and the integration object images, and the data acquirer, which acquires compressed data by extracting at least every other pixel along at least one direction from a plurality of pixels that make up the integral images, the object identifying apparatus is capable of obtaining the compressed data that makes up the integral images, in which original information of the integration object images at the positions of the extracted pixels is retained without change. By appropriately using the retained image information, the accuracy with which an object is identified can be maintained, while the amount of data that makes up the integral images is greatly reduced. 
     The identification object region determiner preferably determines the identification object region, which includes at least four pixels at vertices thereof, selected respectively from pixel groups extracted as the compressed data. Therefore, although compressed data are used, a total of the pixel values of the integration object images can accurately be calculated. 
     The data acquirer preferably acquires the compressed data by reducing the amount of data of each of the pixels of the integral images by at least one bit. Thus, the amount of data that makes up the integral images can further be reduced. 
     If the identification object region determiner determines the identification object region, and the object identifier identifies the object while a position of the identification object region is changed sequentially along a scanning direction at a scanning interval, thereby identifying whether or not the object exists within the original image, the identification object region determiner preferably determines the identification object region at a different scanning interval depending on a position in the original image area. Thus, the identification object region determiner is capable of maintaining the accuracy with which an object can be identified within the entire image area of the original image, while also reducing the amount of processing operations. 
     The object identifying apparatus preferably further includes an image capturing unit that is mounted on a vehicle for acquiring an image in a vicinity of the vehicle by capturing the image while the vehicle is traveling, and the scanning interval in a lower position in the original image area, which is represented by the captured image acquired as the original image by the image capturing unit, preferably is greater than the scanning interval in an upper position in the original image area. An object that is far away from the vehicle tends to be displayed in a smaller size in an upper zone of the image area, whereas an object that is near to the vehicle tends to be displayed in a greater size in a lower zone of the image area. Consequently, the amount of processing operations can further be reduced. 
     The image feature quantity preferably is represented by a histogram, and the integration object images preferably are represented by binary voting data, which indicate whether each pixel of the original image belongs to any one of ranges of the histogram. If the image feature quantity is represented by a histogram, the number of integral images to be handled tends to be large. Consequently, in particular, a reduction in the amount of data based on the compression process is rendered significantly effective. 
     The object identifying apparatus preferably further includes an image capturing unit that is mounted on a vehicle for acquiring an image as the original image by capturing the image while the vehicle is traveling. The image capturing unit, which has high image resolution, typically is mounted on the vehicle in order to detect objects at a long distance and over a wide angle of view. The images captured in this manner are of high resolution, thus tending to increase the number of pixels of the integral images, as well as increasing the number of significant bits of data strings. Thus, in particular, a reduction in the amount of data based on the compression process also is rendered significantly effective. 
     The object identifying apparatus according to the present invention includes the data generator, which generates integral images, each including an image area having a size in common with the captured image and the integration object images, and the data acquirer, which acquires compressed data by extracting at least every other pixel along at least one direction from a plurality of pixels that make up the integral images. Consequently, the object identifying apparatus is capable of obtaining the compressed data that makes up the integral images, in which original information of the integration object images at the positions of the extracted pixels is retained without change. By appropriately using the retained image information, the accuracy with which an object is identified can be maintained, while the amount of data that makes up the integral images is greatly reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of an object identifying apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic perspective view of a vehicle in which the object identifying apparatus shown in  FIG. 1  is incorporated; 
         FIG. 3  is a flowchart of an operation sequence of an ECU shown in  FIG. 1 ; 
         FIGS. 4A and 4B  are views showing images captured by a camera; 
         FIGS. 5A through 5C  are diagrams illustrating a process for calculating a feature quantity represented by histograms of oriented gradient (HOG); 
         FIG. 6  is a diagram showing integration object images by way of example; 
         FIG. 7  is a diagram illustrating a process, using integral images, for calculating an integral pixel value within an optional region of integration object images; 
         FIG. 8A  is a diagram of an integral image before a decimating process is carried out; 
         FIG. 8B  is a diagram of an integral image after a decimating process has been carried out; 
         FIGS. 9A through 9C  are diagrams schematically showing captured images that are input to an integration object image generator shown in  FIG. 1 ; 
         FIG. 10  is a diagram illustrating a bit eliminating process; and 
         FIG. 11  is a diagram illustrating a process for determining an identification object region. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An object identifying apparatus according to an embodiment of the present invention will be described below with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing a configuration of an object identifying apparatus  10  according to an embodiment of the present invention.  FIG. 2  is a schematic perspective view of a vehicle  12  in which the object identifying apparatus  10  shown in  FIG. 1  is incorporated. 
     As shown in  FIGS. 1 and 2 , the object identifying apparatus  10  includes a color camera  14  (hereinafter simply referred to as a “camera  14 ”) for capturing a color image, hereinafter referred to as a “captured image Im” (original image), in a plurality of color channels, a vehicle speed sensor  16  for detecting a vehicle speed Vs of the vehicle  12 , a yaw rate sensor  18  for detecting a yaw rate Yr of the vehicle  12 , a brake sensor  20  for detecting a depressed angle Br of a brake pedal operated by the driver of the vehicle  12 , an electronic control unit  22  (hereinafter referred to as an “ECU  22 ”) for controlling the object identifying apparatus  10 , a speaker  24  for outputting a sound warning or the like, and a display unit  26  for displaying the captured image that is output from the camera  14 , etc. 
     The camera  14  is a camera using light having wavelengths mainly in the visible range, and functions as an image capturing means for capturing an image around the periphery of the vehicle  12 . The camera  14  has characteristics such that, the greater the amount of light reflected from the surface of a subject is, the higher the level of an output signal from the camera becomes, thereby resulting in an increase in brightness, e.g., RGB values, of the image. As shown in  FIG. 2 , the camera  14  is fixedly disposed (mounted) in a substantially central region of a front bumper of the vehicle  12 . 
     The image capturing means for capturing an image around the periphery of the vehicle  12  is not limited to the above structure (a so-called monocular camera), but may be a compound-eye camera (stereo camera). The image capturing means may be one or both of a monochromatic camera instead of a color camera, or an infrared camera. If the image capturing means is a monocular camera, then the monocular camera may be combined with a range finding means (radar apparatus). 
     Referring back to  FIG. 1 , the speaker  24  produces a warning sound or the like on the basis of a command from the ECU  22 . The speaker  24  is mounted on a non-illustrated dashboard of the vehicle  12 . Alternatively, a speech output function, which belongs to another apparatus (e.g., an audio system or a navigation system), may alternatively be used as the speaker  24 . 
     The display unit  26  (see  FIGS. 1 and 2 ) is a head-up display (HUD), which is positioned so that a display screen thereof is displayed on the front windshield of the vehicle  12 , at a position where the display screen does not obstruct the forward vision of the driver. The display unit  26  is not limited to a HUD, but may be a display unit for displaying a map, etc., of a navigation system incorporated in the vehicle  12 , or a display unit (multi-information display unit: MID) disposed in a meter unit for displaying mileage information, etc. 
     The ECU  22  basically includes an input/output unit  28 , a processor  30 , a display controller  32 , and a memory  34 . 
     Signals from the camera  14 , the vehicle speed sensor  16 , the yaw rate sensor  18 , and the brake sensor  20  are input to the ECU  22  through the input/output unit  28 . Signals from the ECU  22  are output through the input/output unit  28  to the speaker  24  and the display unit  26 . The input/output unit  28  includes a non-illustrated A/D converting circuit for converting input analog signals into digital signals. 
     The processor  30  performs processing sequences on the signals from the camera  14 , the vehicle speed sensor  16 , the yaw rate sensor  18 , and the brake sensor  20 , and on the basis of results of the processing operations, generates signals that are supplied to the speaker  24  and the display unit  26 . The processor  30  functions as an integration object image generator  40 , a data generator  42  (data generating means), a data compression processor  44 , a data acquirer  46  (data acquiring means), an identification object region determiner  48  (identification object region determining means), an object identifier  50  (object identifying means), and an object detector  52 . 
     The various functions of the components of the processor  30  are realized by reading and executing programs stored in the memory  34 . Alternatively, the programs may be supplied from an external source via a non-illustrated wireless communications unit (mobile phone, smartphone, or the like). 
     The display controller  32  is a control circuit for energizing and controlling the display unit  26 . By supplying a display control signal to the display unit  26  through the input/output unit  28 , the display controller  32  energizes the display unit  26  in order to display various images (the captured image Im, markings, etc.). 
     The memory  34  is composed of a random access memory (RAM) for storing captured image signals that have been converted into digital signals, temporary data used in various processing operations, etc., and a read only memory (ROM) for storing executable programs, tables and maps, etc. 
     The object identifying apparatus  10  according to the present embodiment is basically configured as described above. An outline of operations of the object identifying apparatus  10  will be described below. 
     The ECU  22  converts an analog video signal output from the camera  14  into a digital signal, and temporarily stores the digital signal in the memory  34 , in each prescribed frame clock interval or period, e.g., in each of thirty frames per second. The ECU  22  then performs various processing operations on the captured image Im, i.e., an image in front of the vehicle  12 , which is read from the memory  34 . 
     The ECU  22 , and in particular the processor  30 , comprehensively evaluates the results of the processing sequences carried out on the captured image Im. Further, if necessary, the ECU  22  comprehensively evaluates signals representing the vehicle speed Vs, the yaw rate Yr, and the depressed angle Br, which indicate the traveling state of the vehicle  12 , and detects a pedestrian, an animal, or the like, which exists in front of the vehicle  12 , as an object to be monitored (hereinafter referred to as a “monitoring object” or simply an “object”). 
     If the processor  30  assesses that there is a high possibility for the vehicle  12  to collide with the monitoring object, the ECU  22  controls the output units of the object identifying apparatus  10  in order to seek the attention of the driver. For example, the ECU  22  controls the speaker  24  to output a warning sound, e.g., a succession of blips, and highlights the monitoring object in the captured image Im that is displayed visually on the display unit  26 . 
     The object identifying apparatus  10  according to the present embodiment is mounted on the vehicle  12 , which serves as a mobile body. However, the object identifying apparatus  10  is not limited to this configuration and application. Alternatively, the object identifying apparatus  10  may be configured to acquire a sequence of captured images from the camera  14 , which may be fixed to any of various stationary bodies including a building. 
     A detailed sequence of operations of the object identifying apparatus  10  will be described below with reference to the flowchart shown in  FIG. 3 . The sequence of operations is carried out for each of respective captured frames while the vehicle  12  is traveling. 
     In step S 1 , for each frame, the ECU  22  acquires and inputs a captured image Im of an area that lies within a given angle of view in front of the vehicle  12 , which is represented by an output signal from the camera  14 . 
     As shown in  FIG. 4A , it is assumed that the ECU  22  acquires a captured image Im in one frame at a given time from the camera  14 . The captured image Im represents an original image area  60  (hereinafter referred to as an “image area  60 ”) having a horizontally elongate rectangular shape made up of horizontal rows of 1200 pixels and vertical columns of 600 pixels, for example. The image area  60  includes a road region  62  (hereinafter referred to simply as a “road  62 ”) along which the vehicle  12  travels, a plurality of utility pole regions  64  (hereinafter referred to simply as “utility poles  64 ”), which are installed at substantially regular intervals along the road  62 , and a pedestrian region  66  (hereinafter referred to simply as a “pedestrian  66 ”) on the road  62 . 
     As shown in  FIG. 4B , it is assumed that a position in the captured image Im, where an existing road surface (road  62 ) and a human body (pedestrian  66 ) are in contact with each other, is referred to as a reference position  68 . A rectangular identification object region  70 , i.e., a region within a rectangular frame indicated by the broken lines, is established according to predetermined rules using the reference position  68  as a landmark. The identification object region  70  corresponds to a region of interest for identifying whether or not an object including the pedestrian  66  exists, as well as for identifying the type of such an object. 
     In step S 2 , the integration object image generator  40  generates integration object images  81  through  88  (see  FIG. 6 ) from the captured image Im that was input in step S 1 . In order to clarify the purpose of generating the integration object images  81  through  88 , a feature quantity represented by histograms of oriented gradient (HOG), which indicate the brightness intensities and features of the gradient in a local region of an image, will first be described below. 
     As shown in  FIG. 5A , the identification object region  70  is divided into a grid pattern, thereby defining a plurality of blocks  72 . In  FIG. 5A , the rectangular identification object region  70  is divided into a two-dimensional pattern of equal blocks made up of eight rows and six columns of blocks. It is assumed that one of the blocks  72  is selected from the identification object region  70  as a unit for producing a histogram. It also is assumed that the selected block  72  is made up of a total of thirty-six pixels  74 , which may be arranged in six vertical columns of pixels and six horizontal rows of pixels, for example. 
     As shown in  FIG. 5B , a two-dimensional gradient (Ix, Iy) of brightness is calculated for each of the pixels  74  that make up the block  72 . A brightness gradient intensity I and a brightness gradient angle θ (0≦θ&lt;2π) are calculated according to the following equations (1) and (2).
 
 I =( Ix   2   +Iy   2 ) 1/2   (1)
 
θ=tan −1 ( Iy/Ix )  (2)
 
     Arrows marked on the respective cells of the first row illustrate in plan the directions of the brightness gradients. Although brightness gradient intensities I and brightness gradient angles θ actually are calculated for all of the pixels  74 , the arrows in the respective pixels of the second and subsequent rows are omitted from illustration. 
     As shown in  FIG. 5C , a histogram of brightness gradient angles θ is generated for each block  72 . The horizontal axis of the histogram represents the brightness gradient angle θ, whereas the vertical axis represents the number of pixels. In  FIG. 5C , the brightness gradient angles θ are grouped into a first range (0≦θ&lt;π/4), a second range (π/4≦θ&lt;π/2), . . . , and an eighth range (7π/4≦θ&lt;2π). 
     The histograms (brightness gradient intensities I and brightness gradient angles θ) for the respective blocks  72  are joined together according to a predetermined order, for example, in an ascending order, in order to obtain a HOG feature quantity of the identification object region  70 . Each time that the position of the identification object region  70  (see  FIG. 4B ) changes within the image area  60 , the values of the HOG feature quantity are changed. In particular, when a HOG feature quantity, which is represented by a histogram or the like, is calculated, the amount of processing operations that are required is vast, due to the fact that addition and conditional branching are involved for each pixel. In order to avoid repetitive processing operations, it is convenient to generate auxiliary interim data (an integration object image as a feature quantity of each pixel  74 ) in advance, and to use the auxiliary interim data in the process of calculating an image feature quantity. 
     As shown in  FIG. 6 , respective image areas  80 , which are represented by the integration object images  81  through  88 , are identical in shape and have a common size with the image area  60  (see  FIGS. 4A and 4B ) represented by the captured image Im. Pixels in the integration object images  81  through  88 , which correspond respectively to the positions of pixels  90  in the image areas  80 , will hereinafter be referred to as corresponding pixels  91  through  98 . 
     According to the present embodiment, an attribute of each pixel  90  is determined depending on the brightness gradient angle θ as the image feature quantity. For example, if the brightness gradient angle θ of a pixel  90  is θ=2π/3, the pixel  90  belongs to the third range (π/2≦θ&lt;3π/4). In this case, the integration object image generator  40  assigns a pixel value “1” to a corresponding pixel  93  of the integration object image  83  depending on the third range. The integration object image generator  40  assigns a pixel value “0” to respective corresponding pixels  91 ,  92 ,  94  through  98  of the remaining integration object images  81 ,  82 ,  84  through  88 . 
     In this manner, the integration object image generator  40  successively determines pixel values of the corresponding pixels  91  through  98  by alternatively assigning the pixel value “1” with respect to all of the pixels  90  in the image areas  80 . Stated otherwise, the integration object images  81  through  88  serve as binary voting data for indicating whether or not each pixel  90  belongs to any one of the first through eighth ranges. In addition to the value of the brightness gradient angle θ, the value of the brightness gradient intensity I may also be taken into account as a criterion for assessing whether or not each pixel  90  belongs to any one of the first through eighth ranges. For example, the integration object image generator  40  may assign a pixel value “1” to a corresponding pixel only if the brightness gradient intensity I is of a value equal to or greater than a prescribed threshold value, e.g., an integral value of 4, thereby reducing adverse effects such as camera noise or the like. The integration object images  81  through  88  are not necessarily limited to the above voting data, but may employ various definitions insofar as such definitions represent an image feature quantity of each of the pixels  74 , such as the value of the brightness gradient intensity I or the captured image Im (original image) itself. 
     In step S 3 , the data generator  42  generates integral images  101  through  108 , respectively, from the integration object images  81  through  88  that were generated in step S 2 . The term “integral image” refers to an image that is produced by successively integrating pixel values of the integration object images  81  through  88  (image areas  80 ) from a starting point at any one of the angular positions of the integration object images  81  through  88 . A pixel value integrated according to the above rule will hereinafter be referred to as an “integrated pixel value”. 
     As shown in  FIG. 7 , respective image areas  100 , which are represented by the integral images  101  through  108 , are identical in shape and have a common size with the image areas  80 , which are represented respectively by the integration object images  81  through  88  (see  FIGS. 6A and 6B ). Each of the integral images  101  through  108  is generated, starting from a starting point  110  and ending at an ending point  112 . The integrated pixel value at a position  114 , i.e., a position within the image areas  100 , for example, is represented by the total of the pixel values that lie within a rectangular area, the diagonal line of which is given as a line segment interconnecting the starting point  110  and the position  114 . 
     By way of example, the total of the pixel values will be calculated within a rectangular calculation range  116 , the diagonal line of which is given as a line segment interconnecting the position  114  and a position  115 . The total of the pixel values in the calculation range  116  is calculated by subtracting the sum of integrated pixel values at positions  117  and  118  (two vertices) from the sum of integrated pixel values at the positions  114  and  115  (other two vertices). Such a calculation process makes it possible to significantly reduce the number of times that the memory (memory  34 ) is accessed, compared with a process of successively reading pixel values of all of the pixels in the image areas  80  and adding the pixel values. As a result, the processing operation for adding the pixel values can be performed at a higher speed. 
     In step S 4 , the data compression processor  44  performs a decimating process for reducing the number of image pixels for each of the integral images  101  through  108  that were generated in step S 3 . The decimating process refers to a process of extracting at least every other pixel along at least one direction from the image area  100  (see  FIG. 7 ). According to the present embodiment, the data compression processor  44  performs the decimating process two-dimensionally, or more specifically, along an X direction and a Y direction perpendicular to the X direction. However, the data compression processor  44  may perform a decimating process along either one of the directions, i.e., along the X direction or along the Y direction. 
     As shown in  FIG. 8A , each of the integral images  101  through  108  is made up of a plurality of pixels  120  arranged in a two-dimensional matrix, including rows of 1200 pixels along the X direction and columns of 600 pixels along the Y direction. For example, the data compression processor  44  extracts every third pixel along the X direction and every third pixel along the Y direction. In this manner, the data compression processor  44  extracts pixels  122 , i.e., the rectangular cells shown in hatching, which make up 1/9 of all of the pixels  120 , and excludes decimated pixels  124 , i.e., the blank rectangular cells, which make up 8/9 of all of the pixels  120 . 
     As a result, as shown in  FIG. 8B , the data compression processor  44  produces integral images, hereinafter referred to as “first compressed images  131  through  138 ” (compressed data), each of which is made up of extracted pixels  122  arranged in a two-dimensional matrix, including rows of 400 pixels along the X direction and columns of 200 pixels along the Y direction. The first compressed images  131  through  138  are stored in a memory area of the memory  34 , which may be the same as or different from the memory area in which the integration object images  81  through  88  are stored. 
     Advantages that accrue from the decimating process (step S 4 ) will be described below with reference to  FIGS. 9A through 9C . Each of  FIGS. 9A through 9C  shows schematically a captured image Im that is input to the integration object image generator  40  shown in  FIG. 1 . 
       FIG. 9A  shows schematically a captured image Im, the resolution of which has not been changed. Although details are rendered appropriately in the captured image Im shown in  FIG. 9A , a problem occurs in that a vast amount of data must be handled in a subsequent identification process. 
       FIG. 9B  shows schematically a captured image Im, the resolution of which has been changed to ⅓, i.e., the pixel size of which has been increased by three times both vertically and horizontally. The captured image Im shown in  FIG. 9B  allows the amount of data of the integral images to be greatly reduced, by about 90%, but suffers from a new problem in that the amount of information of the image itself becomes reduced due to the reduced resolution. More specifically, on account of the reduced resolution of the original image, detailed features, i.e., brightness gradient intensities I and brightness gradient angles θ of the object, are lost. 
       FIG. 9C  shows schematically a captured image Im, the resolution of which has not been changed. As can be understood from  FIG. 9C , the captured image Im renders details appropriately, in the same manner as with the captured image Im shown in  FIG. 9A . The decimating process, which is performed after the integral images  101  through  108  are generated, is effective to significantly reduce the amount of data by about 90%. Nevertheless, detailed information remains for each of the integrated pixel values, which is about the same as the detailed information shown in  FIG. 9A . This is because an integral image retains an integrated pixel value for each pixel, and the integrated pixel value is not adversely affected by the subsequent decimating process. 
     If the image feature quantities are represented by histograms, and the integration object images  81  through  88  are represented by binary voting data, which indicates whether or not each pixel  74  of the captured image Im belongs to any one of the first through eighth ranges, then the number of integral images to be handled tends to be large. Consequently, based on the compression process, in particular, a reduction in the amount of data is rendered significantly effective. 
     Furthermore, the camera  14 , which has high image resolution, typically is mounted on the vehicle  12  in order to detect objects at a long distance and over a wide angle of view. Captured images Im, which are generated by the camera  14 , are of high resolution, thus tending to increase the number of pixels of the integral images, as well as increasing the number of significant bits of data strings (pixel values). Thus, in particular, a reduction in the amount of data based on the compression process also is rendered significantly effective. 
     In step S 5 , the data compression processor  44  carries out a bit number reducing process, in order to reduce the amount of data of each pixel by at least one bit, on each of the first compressed images  131  through  138  on which the decimating process of step S 4  has been performed. 
     As shown in  FIG. 10 , a data string  140  is defined by a binary data string, which represents, in binary notation, an integrated pixel value (3 bytes) of an extracted pixel  122  (see  FIG. 8A ) depending on the ending point  112  (see  FIG. 7 ). Within the data string  140  and the data strings  142 ,  144 , to be described later, the least significant bit (LSB) is referred to as a first bit, and the most significant bit (MSB) is referred to as a 24th bit. 
     According to the present embodiment, since each of the pixel values of the integration object images  81  through  88  (see  FIG. 6 ) is a non-negative value (0 or 1), the value represented by the data string  140  corresponds to a maximum value of the extracted pixel  122 . Stated otherwise, the number of effective bits of the data string  140  is 18, and the values of the 19th through 24th bits are 0. The image area  100  shown in  FIG. 8A  can theoretically assume a maximum value of 19.5 bits. However, if the distribution of the brightness gradient angles θ is essentially uniform, then the maximum value is about 17 bits. 
     As shown in  FIG. 10 , the data compression processor  44  carries out a bit shifting operation on the data string  140 , thereby producing a data string  142  having an 18th bit that coincides with the 24th bit (MSB) of the data string  140 . Thereafter, as shown in  FIG. 10 , the data compression processor  44  deletes the lower-order 8 bits (one byte) of the data string  142  while retaining the higher-order 16 bits (2 bytes), thereby producing a 2-byte data string  144 , which is of a substantially effective value. Eliminating the lower-order bits also is effective to digitally remove electric noise, which may be added when the image is captured by the camera  14 . 
     Alternatively, the data compression processor  44  may determine the number of bits to be eliminated depending on the data transfer protocol between the processor  30  and the memory  34 . For example, the data compression processor  44  is capable of reducing the total amount of data to be transferred by eliminating bits, such that the compressed data string  144  becomes an integral multiple of a unit of transferred data. 
     As a result, the data compression processor  44  produces integral images, hereinafter referred to as “second compressed images  151  through  158 ” (compressed data), each of which is made up of extracted pixels  122  arranged in a two-dimensional matrix of 2-byte integrated pixel values. The second compressed images  151  through  158  are stored in a memory area of the memory  34 , which may be the same as or different from the memory area in which the first compressed images  131  through  138  are stored. 
     In step S 6 , the processor  30  initiates a raster scanning process on the captured image Im. The raster scanning process refers to a process for sequentially identifying whether or not an object exists in the captured image Im, while the reference position  68  (a pixel in the captured image Im; see  FIG. 4B ) is moved a prescribed distance (scanning interval) along the scanning direction. Thereafter, the identification object region determiner  48  sequentially determines the presently scanned reference position  68  and the position of the identification object region  70  (see  FIG. 4B ) that is identified from the reference position  68 . At first, the identification object region determiner  48  initializes the reference position  68  and the identification object region  70 . 
     Each time that the raster scanning process is performed, the data acquirer  46  reads and acquires from the memory  34  the second compressed images  151  through  158  on a timely basis. According to the present embodiment, the data acquirer  46  is only required to directly read the integrated pixel values at designated addresses of the second compressed images  151  through  158  stored in the memory  34 , and there is no need for the data acquirer  46  to perform other processing sequences. When reading the integration object images  81  through  88 , the data acquirer  46  may perform processes equivalent to the decimating process (see step S 4 ) and/or the bit number reducing process (see step S 5 ) carried out by the data compression processor  44 . 
     In step S 7 , the object identifier  50  identifies whether at least one type of object exists within the determined identification object region  70 . Prior to the identifying process, the data acquirer  46  reads and acquires from the memory  34  various data, e.g., the second compressed images  151  through  158 , required for the identifying process. 
     If the data compression processor  44  has carried out the bit number reducing process (refer to step S 5 ), the data acquirer  46  changes the data definition inherent in the second compressed images  151  through  158  to an appropriate data definition, which is suitable for input to the object identifier  50 . For example, the data acquirer  46  may restore the original data definition using the reduced number of bits and the amount of bit shift (number and direction of bits), or may acquire the data in an unchanged form. 
     More specifically, the object identifier  50  successively acquires the integrated pixel values of the second compressed images  151  through  158  corresponding to the blocks  72 , and joins respective values obtained by calculating the number of pixels that belong to the first through eighth ranges, thereby generating histograms corresponding to the blocks  72 . Then, the object identifier  50  successively joins the histograms of the respective blocks  72 , thereby producing a HOG feature quantity, which is a type of image feature quantity, in the identification object region  70 . The image feature quantity may be of any type or may be calculated by any of various processes. For example, the image feature quantity may be an edge feature quantity, which is typified by a HOG feature quantity, or a brightness feature quantity, which is typified by a Haar-like feature quantity. 
     The object identifier  50  is an identifying device, which is generated using a machine learning process, with an image feature quantity, e.g., a HOG feature quantity, at the identification object region  70  being input thereto, and with information as to whether or not an object exists that is output therefrom. The machine learning process may be based on any of various algorithms, including a supervised learning algorithm, an unsupervised learning algorithm, and a reinforcement learning process. Examples of learning architecture include a boosting process including AdaBoost, a support vector machine (SVM), a neural network, and an expectation maximization (EM) algorithm. 
     Examples of an object that can be identified include human bodies (a pedestrian  66  as shown in  FIG. 4A ), various animals (specifically, mammals such as deer, horses, sheep, dogs, cats, etc., birds, etc.) and artificial structures (specifically, vehicles, markings, utility poles, guardrails, walls, etc.). 
     In step S 8 , the identification object region determiner  48  assesses whether or not the scanning process has been completed on the designated pixel. If the identification object region determiner  48  assesses that the scanning process has not been completed (step S 8 : NO), then control proceeds to the next step S 9 . 
     In step S 9 , the identification object region determiner  48  changes the position or size of the identification object region  70 . More specifically, the identification object region determiner  48  moves the designated pixel to be scanned, e.g., the reference position  68 , by a prescribed distance, e.g., a distance corresponding to three pixels, along a prescribed direction, e.g., a rightward direction. 
     As shown in  FIG. 11 , in the same manner as with the integration object image  81 , the integral image  101 , etc., the captured image Im (image area  60 ) is made up of a plurality of pixels  74  arranged in a two-dimensional matrix, including rows of 1200 pixels along the X direction and columns of 600 pixels along the Y direction. 
     In view of the relationship with the integral images  101  through  108  (see  FIG. 8A ), the pixels  74  generally are classified into referable pixels  76  (the rectangular cells shown in hatching) for which integrated pixel values can be referred to, and non-referable pixels  78  (the blank rectangular cells) for which integrated pixel values cannot be referred to. The identification object region determiner  48  determines identification object regions  70 , which may be of a rectangular or cuboidal shape, for example, having at least four referable pixels  76  located at vertices thereof, the identification object regions  70  being selected respectively from pixel groups extracted as the second compressed images  151  through  158 . Thus, the total of the pixel values of the integration object images  81  through  88  can be calculated accurately, even from the second compressed images  151  through  158  in which the number of pixels  120  (see  FIG. 8A ) thereof has been reduced. 
     An object that exists remotely, i.e., far away, from the vehicle  12  tends to be displayed in a smaller size and in an upper zone of the image area  60 , whereas an object that exists in close proximity, i.e., near, to the vehicle  12  tends to be displayed in a greater size and in a lower zone of the image area  60 . In view of the above tendency, in order to further reduce the amount of processing operations, the identification object region determiner  48  changes the scanning interval, i.e., the number of times that the scanning cycle is carried out, depending on the position within the image area  60 . As shown in  FIG. 11 , the identification object region determiner  48  may set a relatively small scanning interval (every third pixel in both vertical and horizontal directions) in the upper zone of the image area  60 , and may set a relatively large scanning interval (every sixth pixel in both vertical and horizontal directions) in the lower zone of the image area  60 . 
     When the identification object region determiner  48  determines an identification object region  70  and the object identifier  50  identifies a given object, the identification object region determiner  48  changes the scanning interval depending on the position, i.e., features of the captured image Im, within the image area  60 . Consequently, the accuracy with which an object is identified within the entire image area  60  can maintained, while also reducing the amount of processing operations. 
     Thereafter, the control returns to step S 7 , and the processor  30  repeats steps S 7  through S 9  until the scanning process on all of the designated pixels has been completed. If it is judged that the scanning process is completed (step S 8 : YES), then in step S 10 , the processor  30  terminates the raster scanning process on the captured image Im. 
     In step S 11 , the object detector  52  detects an object that exists within the captured image Im. The object detector  52  may use an identification result in a single frame, or may take into account identification results in a plurality of frames, so that a motion vector of one object can be calculated. 
     In step S 12 , the ECU  22  stores in the memory  34  data that are needed for the next cycle of processing operations. For example, the data may include an attribute of the object (pedestrian  66  in  FIG. 4A ), which was obtained in step S 7 , the reference position  68 , or the like. 
     By successively carrying out the above operation sequence, the object identifying apparatus  10  is capable of monitoring an object that exists in front of the vehicle  12  at prescribed time intervals. 
     As described above, the object identifying apparatus  10  includes the data generator  42  for generating integral images  101  through  108 , each including an image area  100  having a size in common with a captured image Im, and integration object images  81  through  88 , the data acquirer  46  for acquiring compressed data (first compressed data  131  through  138 , second compressed data  151  through  158 ) by extracting at least every other pixel (extracted pixels  122 ) along at least one direction from a plurality of pixels  120  that make up the integral images  101  through  108 , the identification object region determiner  48  for determining an identification object region  70  from within an image area  60 , and the object identifier  50  for calculating the total of the pixel values of the integration object images  81  through  88  in the identification object region  70  as an image feature quantity using the compressed data, and on the basis of the image feature quantity, identifying whether or not an object (e.g., pedestrian  66 ) exists within the identification object region  70  in the captured image Im. 
     The object identifying apparatus  10 , which is configured as described above, is capable of obtaining the first compressed images  131  through  138 , in which original information of the integration object images  81  through  88  at the positions of the extracted pixels  122  is retained without change. By appropriately using the retained image information, the accuracy with which an object is identified can be maintained, while the amount of data of the integral images  101  through  108  is greatly reduced. 
     The present invention is not limited to the embodiment described above. The embodiment may be changed or modified without departing from the scope of the present invention. 
     For example, in the present embodiment, the aforementioned image processing sequence is carried out on the captured image Im, which is produced by the monocular camera (camera  14 ). However, the same advantages as those described above can be obtained by performing the image processing sequence on a captured image that is produced by a compound-eye camera (stereo camera). The image area represented by the original image is not limited to a planar area (two-dimensional area), but may be a cubic area (three-dimensional area), or an image area in which a temporal concept is incorporated.