Patent Publication Number: US-9426364-B2

Title: Image processing apparatus and image processing method

Description:
TECHNICAL FIELD 
     The present invention relates to an image processing apparatus and an image processing method. 
     BACKGROUND ART 
     Examining small areas in two images captured by a stereoscopic optical system to detect parts in which the same object appears, calculating the offset of the corresponding positions, and calculating the distance to the object by the principle of triangulation is a previously proposed technique. 
     For example, the gradation correction apparatus described in Patent Reference 1 performs stereo matching by calculating a city-block distance between small areas in each of two images to obtain their mutual correlation and identify corresponding small areas. The gradation correction apparatus then carries out distance measurements by obtaining three dimensional image information (a distance image) in which the range information obtained from the pixel offsets (parallaxes) arising from the distance to objects is quantified. 
     The imaging apparatus described in Patent Reference 2 combines images captured by a plurality of cameras, including a super wide angle camera having a field of view of substantially 180°, and displays, on a display means, a multiple-perspective video and a bird&#39;s eye view video (with a single viewpoint) from above a vehicle. This imaging apparatus accordingly enables the driver to recognize the situation around the vehicle. 
     In a position detection apparatus that detects the position of an object to be measured from a pair of images obtained by a stereoscopic image input means, the imaging apparatus described in Patent Reference 3 measures the distance to a point illuminated by light after executing distortion corrections on the left and right image data. 
     PRIOR ART REFERENCES 
     Patent References 
     
         
         Patent Reference 1: Japanese Patent Application Publication No. 11-234701 (paragraphs 0009-0015) 
         Patent Reference 2: Japanese Patent Application Publication No. 2005-236493 (paragraphs 0014-0027) 
         Patent Reference 3: Japanese Patent Application Publication No. 2008-164338 (paragraphs 0014-0026) 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The purpose of the gradation correction apparatus described in Patent Reference 1 is to obtain information about the distance to a vehicle in front of a moving vehicle, so it does not require a wide angle lens capable of imaging the surroundings of the vehicle, and therefore uses a standard angle lens that does not require consideration of lens distortion (distortion aberration). The purpose of the imaging apparatus described in Patent Reference 2 is to assist the driver by displaying an image of the vehicle&#39;s surroundings on the display means, so this imaging apparatus requires a wide angle lens that can capture an image of a wider area. 
     If the gradation correction apparatus described in Patent Reference 1 and the imaging apparatus described in Patent Reference 2 are both mounted on a vehicle to measure distance to a preceding object, which is the purpose of Patent Reference 1, and simultaneously provide assistance in visualizing the vehicle surroundings, which is the purpose of Patent Reference 2, the number of cameras increases and cost problems occur, as well as problems of a lack of installation space. 
     Patent Reference 3 takes the distortion aberration of a wide angle lens into consideration and performs distortion corrections on the right and left image data. Measurement of the distance to the vehicle preceding a moving vehicle, however, demands instant processing, so the time taken for distortion correction is a problem. Furthermore, if a matching process is carried out on images that have been corrected for distortion, then depending on the accuracy of the distortion correction process, the shapes of an object in a stereoscopic image may differ, possibly reducing the accuracy of the matching process. 
     The present invention addresses the problems of the prior art described above, with the object of enabling visual recognition of vehicle surroundings and measurement of distance to objects to be accomplished by a single imaging apparatus using wide angle lenses. 
     Means for Solving the Problem 
     An image processing apparatus according to one aspect of the invention comprises: a plurality of image capturing units with wide angle lenses for capturing at least partly overlapping images; a vehicle speed detecting unit for detecting a speed of a local vehicle; a distance measuring unit for calculating a distance from the local vehicle to an object imaged by the plurality of image capturing units on a basis of a plurality of images captured by the plurality of image capturing units; and a visual recognition image generating unit for generating a visual recognition image for recognition of conditions near the local vehicle from the plurality of images captured by the plurality of image capturing units. The distance measuring unit further comprises a pixel area determining unit for determining, for the plurality of image capturing units, sizes and positions of corresponding pixel areas in images output from each of the plurality of image capturing units, among all pixels usable for imaging in each of the plurality of image capturing units. The pixel area determining unit makes the size of the pixel areas smaller as the speed detected by the vehicle speed detecting unit becomes faster. 
     Effect of the Invention 
     According to one aspect of the invention, visual recognition of vehicle surroundings and measurement of distance to objects can be accomplished by a single imaging apparatus using wide angle lenses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing the configuration of an image processing apparatus according to an embodiment of the invention. 
         FIG. 2  is a block diagram schematically showing the configuration of the image capturing units. 
         FIG. 3  is a diagram showing the two-dimensional positional relationship of the stereo camera and an object. 
         FIG. 4  is a diagram showing the area in which measurement of distance to an object is possible in the distance measurement mode. 
         FIG. 5  is a schematic diagram showing an exemplary visual recognition image furnished to the information conveying unit in the visual recognition mode. 
         FIGS. 6(A) and 6(B)  are schematic diagrams showing captured images captured by the first and second image capturing units. 
         FIG. 7  is a schematic diagram showing the structure of a CMOS imaging device. 
         FIG. 8  is a schematic diagram showing the structure of a CCD imaging device. 
         FIG. 9  is a diagram showing the three-dimensional positional relationship of the stereo camera and an object. 
         FIG. 10  is a flowchart illustrating processing performed by the mode determining unit. 
         FIG. 11  is a flowchart illustrating processing performed in the distance measurement mode. 
         FIG. 12  is a flowchart illustrating processing performed in the visual recognition mode. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
       FIG. 1  is a block diagram schematically showing the configuration of an image processing apparatus  100  according to an embodiment of the invention. The image processing apparatus  100  includes a first image capturing unit  1   a  and a second image capturing unit  1   b  (these image capturing units will be referred to as image capturing unit  1  when there is no particular need to distinguish between them), a vehicle speed detecting unit  2 , an ECU (Electronic Control Unit)  3 , and an information conveying unit  9 . The ECU  3  includes a mode determining unit  4 , a distance measuring unit  5 , a camera parameter setting unit  6 , a visual recognition image generating unit  7 , and a data bus  8 . 
     The image capturing unit  1  capture a right and left pair of images that become the source of a stereoscopic image. In this embodiment, the right and left pair of images are captured by two cameras: the first image capturing unit  1   a  and second image capturing unit  1   b . The image capturing unit  1  in this embodiment are installed at the front of a vehicle, and image information pertaining to images captured by the image capturing unit  1  (first image information pertaining to a first image captured by the first image capturing unit  1   a  and second image information pertaining to a second image captured by the second image capturing unit  1   b ) is supplied to the mode determining unit  4  through the data bus  8 . 
       FIG. 2  is a block diagram schematically showing the configuration of the image capturing unit  1 . Each image capturing unit  1  includes an optical system  11 , an imaging device  12 , an AD converter  13 , and a sensor driving circuit  14 . 
     The optical system  11  is a wide angle lens, including either a single lens or a plurality of lenses. 
     The imaging device  12  converts light obtained from the optical system  11  to analog electric signals. The imaging device  12  may be an imaging device of, for example, either the CMOS (Complementary Metal Oxide Semiconductor) type or the CCD (Charge Coupled Device) type. 
     The AD converter  13  converts the analog electric signals obtained from the imaging device  12  to digital image information. Incidentally, the AD converter  13  need not necessarily be included in the image capturing unit  1 ; it may be included in the ECU  3 . 
     The sensor driving circuit  14  changes the driving pulses supplied to the imaging device  12  on the basis of a sensor driving pulse control signal obtained from the camera parameter setting unit  6  to control the size of the image captured by the imaging device  12 , as described below. 
     Returning to the description of  FIG. 1 , the vehicle speed detecting unit  2  detects the speed of the vehicle (also referred to below as the local vehicle) in which the image processing apparatus  100  is installed. The vehicle speed detecting unit  2  may be a vehicle speed sensor included in the local vehicle, or it may use GPS (Global Positioning System) equipment in a car navigation system to calculate the speed. The speed detected by the vehicle speed detecting unit  2  is supplied to the mode determining unit  4  and distance measuring unit  5 . 
     The mode determining unit  4  determines the operating mode in the image processing apparatus  100  according to the speed of the vehicle obtained from the vehicle speed detecting unit  2 . For example, when the vehicle speed is equal to or greater than 10 km/h, the mode determining unit  4  selects a distance measurement mode that informs the driver of the distance to a preceding object (such as another vehicle), and when the vehicle speed is less than 10 km/h, the mode determining unit  4  selects a visual recognition mode that informs the driver of conditions near the local vehicle. Incidentally, the threshold vehicle speed for switching between the distance measurement mode and the visual recognition mode is not limited to 10 km/h. An arbitrary threshold speed may be settable by the user, for example. When the distance measurement mode is selected, the mode determining unit  4  sets the distance measuring unit  5  as the output destination of the image information obtained from the image capturing unit  1  through the data bus  8 ; when the visual recognition mode is selected, the mode determining unit  4  sets the visual recognition image generating unit  7  as the output destination of the image information. 
     The distance measuring unit  5  calculates the distance to an object in front of the local vehicle on the basis of the image information supplied from the mode determining unit  4 . The distance measuring unit  5  includes a feature point detecting unit  51 , a pixel area determining unit  52 , an incident angle determining unit  53 , and a distance calculating unit  54 . 
     The feature point detecting unit  51  detects a feature point that forms part of a single object that occurs in both the first image based on the first image information obtained from the first image capturing unit  1   a  and the second image based on the second image information obtained from the second image capturing unit  1   b . The feature point detecting unit  51  determines positions of the feature point (feature point positions) in both images. One exemplary method of detecting a feature point is to extract local feature quantities by relating vectors between pixels in the first and second images and calculate the feature point by evaluating the similarity between the local feature quantities. Alternatively, a block matching method may be used to detect feature points. The block matching method detects a feature point by dividing an image into blocks of a certain size and matching the blocks. The block matching method can detect feature points at high speed by using a sum of absolute values of corresponding pixels (SAD: Sum of Absolute Differences) as an evaluation function to evaluate degree of agreement. The feature point may be any part of the object; the feature point detecting unit  51  has templates of objects and parts thereof (feature points) prestored in the template storage unit  51   a . The feature point detecting unit  51  supplies the pixel area determining unit  52  with the image information obtained from the image capturing unit  1  and feature point position information indicating the detected feature point positions. 
     From among all pixels available for imaging in each image capturing unit  1 , the pixel area determining unit  52  determines the size of the pixel area corresponding to the image that the image capturing unit  1  will output according to the speed of the vehicle obtained from the vehicle speed detecting unit  2 . For each of the image capturing units  1   a ,  1   b , the pixel area determining unit  52  then positions the pixel area with the determined size so as that the feature point position detected by the feature point detecting unit  51  is included in the pixel area with the determined size. The pixel area determining unit  52  positions the pixel area by, for example, moving the position of the pixel area with the determined size in the right, left, upward, and downward directions so as to include the feature point in the pixel area with the determined size. The pixel area determining unit  52  preferably positions the pixel area so that the feature point is located at the center of the pixel area with the determined size or within a predetermined pixel range of the center. The pixel area in this case is obtained not by clipping unnecessary parts from the captured image but by controlling the driving of the imaging device  12  to make the image capturing unit  1  output only an image with the determined pixel area size. 
     The result output from the vehicle speed detecting unit  2  is used to select the size of the pixel area in the pixel area determining unit  52 . For example, the pixel area is reduced when the vehicle is traveling rapidly, and enlarged when the vehicle is traveling slowly. More specifically, at the threshold speed at which the mode determining unit  4  selects the distance measurement mode, the pixel area determining unit  52  takes the size consisting of all pixels available for imaging in the image capturing unit  1  as the size of the pixel area. As the speed becomes faster than the threshold speed, the pixel area determining unit  52  gradually reduces the size of the pixel area, making it smaller than the size consisting of all the pixels. Although a wide angle lens is advantageous for use in visualizing the vehicle surroundings because of its wide visual recognition area, a wide angle lens is not required for measurement of distance to a preceding object in the distance measurement mode. The frame rate can therefore be increased by controlling the size of the pixel area so as to remove the peripheral part of the image, which is not required for distance measurement, and use only the central part of the image. 
     A low frame rate does not cause problems when the local vehicle is traveling slowly, such as when it is being parked, but when the local vehicle is traveling rapidly, a low frame rate risks collision with a preceding object. For example, when an ordinary driver becomes aware of a preceding object while the local vehicle is traveling at a speed of 50 km/h on an asphalt road on a fine day, and steps on the brake, during the driver&#39;s reaction time of 0.7 seconds and the time of approximately 2.7 seconds it takes to stop, the local vehicle covers a considerable distance: in fact, a distance of 24 m. The time taken to measure the distance to a preceding object therefore has to be as short as possible; the longer it takes to measure the distance, the greater becomes the risk of collision with the object. 
     For example, when images are read at 30 frames per second from the image capturing unit  1 , if charge accumulation for a single image takes 33 ms/frame, readout takes 33 ms/frame, and image processing by the feature point detecting unit  51  takes at least 33 ms/frame, then distance calculation takes 0.1 seconds or more. When only the central part of the image is captured, if the pixel area of the central part that is captured is half the full pixel area, then images can be read at 60 frames per second. The distance calculation time is therefore reduced by half. Accordingly, use of only the central part of the image can be said to be effective in the distance measurement mode. 
     Incidentally, the pixel area determining unit  52  supplies the camera parameter setting unit  6  with the pixel area information indicating the determined size and position of the pixel area. The pixel area determining unit  52  supplies the incident angle determining unit  53  with the feature point position information and image information obtained from the feature point detecting unit  51 . 
     The incident angle determining unit  53  determines the angles (angles of incidence) at which light from the feature point of the object enters the right and left optical systems  11 , on the basis of the pixel coordinates at which the feature point is present in the image information supplied from the pixel area determining unit  52 . Respective angles of incidence are determined for the right and left optical systems  11 . For example, a numeric value storage unit  53   a  in the incident angle determining unit  53  stores numeric information indicating angles of incidence of light from the object that has passed through the centers of the wide angle lenses used as the optical system  11  for different distances from the centers of image planes of the imaging device  12 . The incident angle determining unit  53  calculates distances between the feature point positions and the centers of the images that would be captured with all the pixels, and determines the angles of incidence corresponding to the calculated distances from the numeric information. The incident angle determining unit  53  supplies the distance calculating unit  54  with incident angle information indicating the determined angles of incidence and with the image information supplied from the pixel area determining unit  52 . 
     The distance calculating unit  54  calculates the distance to the object on the basis of the angles of incidence determined by the incident angle determining unit  53 . The distance calculating unit  54  supplies the information conveying unit  9  with the distance information indicating the calculated distance. 
     The camera parameter setting unit  6  generates sensor driving pulse control signals that change the driving pattern of the imaging device  12  in the image capturing unit  1  responsive to the size and position of the pixel area determined by the pixel area determining unit  52 . The size and position of the pixel area determined by the pixel area determining unit  52  are received as, for example, serial data set through serial communication, and the camera parameter setting unit  6  has functions for analyzing the serial data. The camera parameter setting unit  6  supplies the generated sensor driving pulse control signals to the image capturing unit  1  through the data bus  8 . 
     The visual recognition image generating unit  7  generates a visual recognition image by which the driver can see conditions near the local vehicle. The visual recognition image generating unit  7  includes a distortion correcting unit  71  and an image combining unit  72 . 
     The distortion correcting unit  71  corrects distortion in the first image information and second image information supplied from the mode determining unit  4 . The distortion correcting unit  71  supplies the corrected first image information and corrected second image information to the image combining unit  72 . 
     The image combining unit  72  combines the images indicated by the corrected first image information and corrected second image information supplied from the distortion correcting unit  71  to generate visual recognition image information indicating a single visual recognition image. The image combining unit  72  generates the visual recognition image information by, for example, combining the right half of the first image indicated by the first image information and the left half of the second image indicated by the second image information. The image combining unit  72  supplies the generated visual recognition image information to the information conveying unit  9 . 
     The information conveying unit  9  outputs the distance indicated by the distance information supplied from the distance measuring unit  5  and the visual recognition image indicated by the visual recognition image information supplied from the visual recognition image generating unit  7  to enable the driver recognize them. For example, the information conveying unit  9  includes some type of display unit (not shown) that displays the distance indicated by the distance information or the visual recognition image indicated by the visual recognition image information. 
       FIG. 3  is a diagram showing the two-dimensional positional relationship of the stereo camera and an object. In  FIG. 3 , point P denotes the feature point position in the object,  11   a  denotes the optical system of the first image capturing unit  1   a , and  11   b  denotes the optical system of the second image capturing unit  1   b . In  FIG. 3 , θ 1  denotes the angle of incidence at which light from point P enters optical system  11   a, θ   2  denotes the angle of incidence at which light from point P enters optical system  11   b , B denotes the distance (stereo base length) between the optical systems  11   a  and  11   b , x denotes an x-axis, and z denotes a z-axis. Z denotes the object distance measured by perpendicular descent from point P to the x-axis, W 1  denotes the distance (true distance) between point P and optical system  11   a , and W 2  denotes the distance (true distance) between point P and optical system  11   b . Although the optical systems  11   a  and  11   b  have identical configurations, in distance measurement, one of the optical systems must be selected as a reference. In this embodiment, optical system  11   a  is taken as the reference system and W 1  is taken to be the distance to the preceding object in the distance measurement mode. 
       FIG. 4  is a diagram showing the area in which measurement of distance to an object is possible in the distance measurement mode. The first image capturing unit  1   a  and second image capturing unit  1   b  include wide angle lenses with the same field angle, and are installed at the front of a vehicle  110 . Reference characters  111   a  and  111   b  denote optical axes that pass through the centers of the lenses in the first image capturing unit  1   a  and second image capturing unit  1   b , respectively. The two axes  111   a ,  111   b  are mutually parallel. Reference characters  112   a  and  112   b  denote the field angles of the lenses used in the first image capturing unit  1   a  and second image capturing unit  1   b , respectively. The field angle  113  formed by overlap of field angles  112   a  and  112   b  encloses the area in which an object is seen in images captured by both the first image capturing unit  1   a  and second image capturing unit  1   b , in other words, the area in which measurement of distance to the object is possible. Reference characters  114  denote the minimum distance at which measurement of distance to the object is possible. The minimum distance  114  lengthens in proportion to the stereo base length. The minimum distance  114  also lengthens as the field angles of the first image capturing unit  1   a  and second image capturing unit  1   b  become narrower. 
       FIG. 5  is a schematic diagram showing an exemplary visual recognition image  120  furnished to the information conveying unit  9  in the visual recognition mode. In  FIG. 5  the local vehicle is about to turn into a road from an intersecting side road; the visual recognition image  120  shows vehicle surroundings that would be blind spots for the driver in this situation. 
     The right half  121   a  of the visual recognition image  120  is the right half part of the full pixel image captured by the first image capturing unit  1   a . The left half  121   b  of the visual recognition image  120  is the left half part of the full pixel image captured by the second image capturing unit  1   b . Parts  121   a  and  121   b  are identical in size. Reference characters  122  and  123  denote vehicles approaching from the right and left, respectively, travelling in opposite lanes. Reference characters  124   a  and  124   b  denote the white center line on the road, and reference characters  125   a  and  125   b  denote wall surfaces. Distortion in both parts  121   a  and  121   b  of the image has been corrected by the distortion correcting unit  71 . When operating in the visual recognition mode as described above, using the full pixel images captured by the first image capturing unit  1   a  and the second image capturing unit  1   b  through their wide angle lenses, the visual recognition image generating unit  7  can forestall collisions and other accidents by enabling the driver to see areas in the images that would otherwise be blind spots. 
       FIGS. 6(A) and 6(B)  are schematic diagrams showing images  130   a  and  130   b  captured by the first and second image capturing units  1   a ,  1   b  in the distance measurement mode. The captured image  130   a  shown in  FIG. 6(A)  is the full pixel image captured by the first image capturing unit  1   a ; captured image  130   b  is the full pixel image captured by the second image capturing unit  1   b . Distortion corrections are not executed on captured images  130   a  and  130   b . After processing has been executed in the feature point detecting unit  51  and the pixel area determining unit  52  and the camera parameter setting unit  6  has changed the size of the images captured by the first and second image capturing units  1   a ,  1   b , however, the image size changes, for example, from captured images  130   a  and  130   b  to captured images  140   a  and  140   b . Reference characters  131  denote a moving vehicle preceding the local vehicle, reference characters  132  denote a wall surface, reference characters  133  denote the white line, reference characters  134   a  and  134   b  denote the center (pixel) of captured images  130   a  and  130   b , respectively, and reference characters  135   a  and  135   b  denote the feature point detected by the feature point detecting unit  51  in captured images  130   a  and  130   b , respectively. The feature point detected by the feature point detecting unit  51  may be any part of the object (vehicle); the exemplary feature point shown in  FIGS. 6(A) and 6(B)  is the lower right corner of the preceding vehicle&#39;s rear license plate, but this is not a limitation. 
     In the description below, m will denote a number of pixels in the horizontal direction, and n will denote a number of pixels in the vertical direction. M will denote the number of pixels in the horizontal direction in the full pixel image captured with all the pixels available in the image capturing unit  1 ; N will denote the number of pixels in the vertical direction in the full pixel image. 
     Captured images  140   a  and  140   b  are obtained by controlling the readout control pulses of the imaging device  12  on the basis of the pixel area determined by the pixel area determining unit  52 ; reference characters  140   a  and  140   b  denote the images captured by the first image capturing unit  1   a  and the second image capturing unit  1   b , respectively. Captured images  140   a  and  140   b  are identical in size. 
     Captured image  140   b  has an m×n size. The size can be changed in the vertical and horizontal directions. The pixel area determining unit  52  can change the values of m and n, for example, in proportion to the vehicle speed detected by the vehicle speed detecting unit  2 . 
     When the vehicle speed detecting unit  2  detects that the local vehicle is traveling at a fast speed, the pixel area determining unit  52  can increase the frame rate at which images are read from the image capturing unit  1  by reducing the image size m×n. This enables the distance measuring unit  5  to reduce the time for calculating the distance to a preceding object, increasing the possibility that the user can avoid a collision with the preceding object. When the vehicle speed detecting unit  2  detects that the local vehicle is traveling at a slow speed, the pixel area determining unit  52  enlarges the image size m×n. This enables the distance measuring unit  5  to recognize objects present in a wider area. 
     In regard to the captured image size for a given local vehicle speed, if the full pixel image with size M×N has area S, the pixel area determining unit  52  controls the values of m and n so that, for example, the area of the captured image is S when the speed is equal to or greater than 10 km/h and less than 20 km/h, S/2 when the speed is equal to or greater than 20 km/h and less than 40 km/h, S/3 when the speed is equal to or greater than 40 km/h and less than 60 km/h, S/4 when the speed is equal to or greater than 60 km/h and less than 80 km/h, and S/5 when the speed is equal to or greater than 80 km/h. The pixel area determining unit  52  preferably makes the m×n captured image similar to the M×N full pixel image. 
     Although the combinations of values of m and n are not fixed, as the value of m approaches the value M of the full pixel image, the distance measuring unit  5  can recognize objects present in a wider area in the horizontal direction. This capability is effective when the road has a plurality of lanes and the distance measuring unit  5  recognizes objects present in each lane, and when the distance measuring unit  5  recognizes objects present on the shoulders of the road. The initial setting is therefore m=M. 
     Even when the local vehicle is traveling at a slow speed, if the distance to the preceding object is reduced, that alone increases the risk of collision. The pixel area determining unit  52  can therefore also change the image size m×n according to the vertical position of a preceding object seen in the full pixel images  130   a  and  130   b . For example, as the position of an imaged object approaches the bottoms of captured images  130   a  and  130   b  (in the direction of the ‘down’ arrow in captured image  130   b ), the pixel area determining unit  52  assumes that the distance between the local vehicle and the object is decreasing and reduces the pixel area. As the position of the object approaches the tops of the images (in the direction of the ‘up’ arrow in captured image  130   b ), the pixel area determining unit  52  assumes that the distance between the local vehicle and the object is increasing and enlarges the pixel area. The position of an object seen in captured images  130   a  and  130   b  may be recognized from the relations between the centers  134   a  and  134   b  of captured images  130   a  and  130   b  and the feature points  135   a ,  135   b . For example, as the feature points  135   a ,  135   b  lie further below the centers  134   a ,  134   b  of captured images  130   a  and  130   b , the pixel area determining unit  52  decides that the position of the imaged object is approaching the bottoms of captured images  130   a  and  130   b . As the feature points  135   a ,  135   b  lie further above the centers  134   a ,  134   b  of captured images  130   a  and  130   b , the pixel area determining unit  52  decides that the position of the imaged object is approaching the tops of captured images  130   a  and  130   b . Because, as described above, the distance measuring unit  5  can reduce the time for calculating the distance to the preceding object as the distance to a preceding object decreases, the user&#39;s chances of avoiding a collision with the preceding object increase. 
     Control by vertical position as described above may be performed in place of control by local vehicle speed, or may be performed together with control by local vehicle speed. For example, the pixel area determining unit  52  may first specify a size for the pixel area according to the local vehicle speed, and then modify the specified size on the basis of the vertical position of the feature point to determine the size of the pixel area. 
     The distance information pertaining to the distance to the preceding object calculated by the distance calculating unit  54  may be fed back to the pixel area determining unit  52 , and the pixel area determining unit  52  may change the m×n image size on the basis of the actually measured value of the distance. For example, as the distance to the preceding object becomes increasingly shorter than a predetermined distance, the pixel area determining unit  52  can make the m×n image size increasingly smaller than the image size for the predetermined distance. Because the distance measuring unit  5  can then reduce the time for calculating the distance to the preceding object as the distance to the preceding object becomes increasingly shorter than the predetermined distance, the user&#39;s chances of avoiding a collision with the preceding object improve. 
     The positions of the centers  134   a ,  134   b  of the captured images and the feature points  135   a ,  135   b  are preferably represented by coordinates in the full pixel captured images  130   a ,  130   b . For example, if among the four corners of captured image  130   a  (captured image  130   b ) the lower left corner and upper right corner have coordinates (0, 0) and (M, N), the centers  134   a ,  134   b  can be represented by coordinates (M/2, N/2). The feature points  135   a ,  135   b  can be represented by coordinates (α,β) (0≦α≦M, 0≦β≦N). When captured images  140   a ,  140   b  are generated, the centers  134   a ,  134   b  and the feature points  135   a ,  135   b  are represented by coordinates (M/2, N/2) and (α,β). 
     Next, control of the driving of the imaging device  12  for changing the image size of the captured image will be described with reference to  FIGS. 7 and 8 .  FIG. 7  is a schematic diagram showing the structure of a CMOS imaging device  12 A. Reference characters  150  denote a horizontal scanning circuit, reference characters  151  denote a vertical scanning circuit, and reference characters  1521  to  1525  denote column selectors. The area indicated by dashed line  153  is the area occupied by a single pixel. The area indicated by dashed line  153  includes a MOS transistor, a photodiode amplifier, and a pixel selector. Reference characters  1541  to  1545  denote vertical signal lines, and reference characters  1551  to  1554  denote horizontal signal lines. The charge accumulated for each pixel is both converted to a voltage and then amplified by the amplifier. The amplified voltages are supplied to the vertical signal lines  1541  to  1545  row by row by controlling the horizontal signal lines  1551  to  1554  to turn the pixel selectors on and off. The voltages are temporarily held in a CDS circuit disposed in each of the vertical signal lines  1541  to  1545  and then output by turning the column selectors  1521  to  1525  on and off. An image of a partial area  156  can be read as the captured image by controlling some of the horizontal signal lines  1552 ,  1553  and some of the column selection switches  1522 ,  1523  from the sensor driving circuit  14 . 
       FIG. 8  is a schematic diagram showing the structure of a CCD imaging device  12 B. Reference characters  160  denote a horizontal transfer CCD, reference characters  161  denote an output amplifier, reference characters  162  denote a horizontal synchronization signal, and reference characters  163  denote a vertical synchronization signal. Reference characters  164  denote the direction in which charge is transferred within vertical transfer CCDs (not shown). In  FIG. 8 , the hatched area  165  is the pixel area; areas  166  and  167  are undesired pixel areas. In a CCD imaging device  12 B, desired pixels cannot be specified and read as they can in a CMOS imaging device. Therefore, all the pixel charges are read out through the vertical transfer CCDs (not shown), but the frame rate can be increased by reading the desired pixels in the pixel area  165  at a normal vertical charge transfer rate, and reading the undesired pixels in areas  166  and  167  at a faster charge transfer rate than the normal vertical charge transfer rate and discarding them. The sensor driving circuit  14  controls the vertical charge transfer rate. 
       FIG. 9  is a diagram showing the three-dimensional positional relationship of the stereo camera and an object. In  FIG. 9 , x denotes the x-axis, y denotes the y-axis, and z denotes the z-axis. Point P denotes the feature point position in the object,  11   a  denotes the optical system of the first image capturing unit  1   a , and  11   b  denotes the optical system of the second image capturing unit  1   b. θ   1  denotes the angle of incidence at which light from point P enters optical system  11   a, θ   3  denotes the angle formed by the z-axis and the light path in which deviation has occurred because of distortion aberration when the light passes through optical system  11   a, θ   2  denotes the angle of incidence at which light from point P enters optical system  11   b , and θ 4  denotes the angle formed by an axis parallel to the z-axis and the light path in which deviation has occurred because of distortion aberration when the light passes through optical system  11   b . Reference characters  170   a ,  170   b  denote the surfaces of the imaging devices in the first image capturing unit  1   a  and the second image capturing unit  1   b , and B denotes the distance between the optical systems  11   a  and  11   b  (the stereo base length). O 1  denotes the point of intersection of the z-axis and a plane parallel to the image planes  170   a ,  170   b  and including point P, and O 2  denotes the point of intersection of a z 1 -axis parallel to the z-axis and the plane parallel to the image planes  170   a ,  170   b  and including point P. Z denotes the object distance measured by perpendicular descent from point O 1  to the x and y axes, W 1  denotes the distance (true distance) between point P and optical system  11   a , and W 2  denotes the distance (true distance) between point P and optical system  11   b . Reference characters  134   a  and  134   b  denote the centers of the image planes  170   a  and  170   b , respectively, f denotes focal length, and reference characters  135   a  and  135   b  denote the feature point on the image planes  170   a  and  170   b , respectively. r 1  denotes the distance from feature point  135   a  to image center point  134   a , r 2  denotes the distance from feature point  135   b  to image center point  134   b , h 1  denotes the length of a segment descending perpendicularly from the feature point  135   a  to an x 1 -axis parallel to the x-axis on the image planes  170   a ,  170   b , and h 2  denotes the length of a segment descending perpendicularly from the feature point  135   b  to the x 1 -axis parallel to the x-axis. θ 5  denotes the angle formed by segment r 1  and the x 1 -axis, and θ 6  denotes the angle formed by segment r 2  and the x 1 -axis. 
     A method of calculating the distance W 1  to an object will be described on the basis of  FIG. 9 . From  FIG. 9 , the following equations (1) to (4) hold. The distance W 1  to the object, which is the distance to be calculated, is given by equation (4). 
     
       
         
           
             
               
                 
                   B 
                   = 
                   
                     
                       
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                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
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     Values r 1 , r 2 , h 1 , and h 2  in equations (1) to (4) are determined after the processing by the feature point detecting unit  51  is executed, and are measured in pixels. θ 1  and θ 2  can be determined by referring to the numeric information referred to by the incident angle determining unit  53 . This numeric information is, for example, a reference table from which the angles of incidence can be read according to the distance between the feature point and the center of the image, with the distortion aberration characteristic of the optical system  11  taken into consideration in the example in  FIG. 9 . The reason why the input value for obtaining the output value (angle of incidence) is r 1  or r 2 , which is the distance between the feature point and the center of the image, is based on the assumption that all points on concentric circles referenced to the center of the image have identical distortion factors. Once the angles at which light rays from the object enter the optical systems  11   a ,  11   b  have been found, the distance to the object can be calculated from equations (1) to (4). 
     Next, the operation of the image processing apparatus  100  according to the embodiment of the invention will be described with reference to  FIGS. 10 to 12 . 
       FIG. 10  is a flowchart illustrating processing performed by the mode determining unit  4 . 
     When the information indicating the local vehicle speed is received from the vehicle speed detecting unit  2 , the mode determining unit  4  decides whether or not the local vehicle speed is equal to or greater than a predetermined threshold speed S (km/h) (S 10 ). If the local vehicle speed is equal to or greater than the predetermined threshold speed S (km/h) (step S 10 : Yes), the mode determining unit  4  proceeds to the processing in step S 11 ; if the local vehicle speed is less than the predetermined threshold speed S (km/h) (step S 10 : No), the mode determining unit  4  proceeds to the processing in step S 12 . 
     In step S 11 , operating in the distance measurement mode, the mode determining unit  4  supplies the distance measuring unit  5  with the image information obtained from the image capturing unit  1 . 
     In step S 12 , operating in the visual recognition mode, the mode determining unit  4  supplies the visual recognition image generating unit  7  with the image information obtained from the image capturing unit  1 . 
       FIG. 11  is a flowchart illustrating the processing performed in the distance measurement mode. 
     First, the feature point detecting unit  51  decides whether or not image information has been obtained from the mode determining unit  4  (S 20 ). If the image information has been obtained (step S 20 : Yes), the processing proceeds to step S 21 . 
     In step S 21 , the feature point detecting unit  51  detects a predetermined feature point of a predetermined object on the basis of the image information obtained from the mode determining unit  4 . The feature point is detected in both of the first image information supplied from the first image capturing unit  1   a  and the second image information supplied from the second image capturing unit  1   b.    
     Next, the pixel area determining unit  52  determines the size of a pixel area corresponding to the local vehicle speed obtained from the vehicle speed detecting unit  2 , and positions the pixel area so that the feature point detected by the feature point detecting unit  51  is included in an image with that pixel area size (S 22 ). The pixel area is positioned in both of the first image information supplied from the first image capturing unit  1   a  and the second image information supplied from the second image capturing unit  1   b.    
     The pixel area determining unit  52  supplies the camera parameter setting unit  6  with the pixel area information indicating the pixel area determined in step S 22 . 
     The camera parameter setting unit  6  generates driving pulse control signals that change the driving pattern of the imaging device  12  in each image capturing unit  1  responsive to the pixel area determined by the pixel area determining unit  52 , and supplies the generated driving pulse control signals to the image capturing unit  1  through the data bus  8  (S 24 ). 
     In parallel with the processing in steps S 22  to S 24 , the incident angle determining unit  53  obtains the image information output from the mode determining unit  4  through the pixel area determining unit  52  and the feature point position information indicating the position of the feature point detected by the feature point detecting unit  51 , and determines the angles of incidence at which light enters the lenses in the first image capturing unit  1   a  and second image capturing unit  1   b  on the basis of the information described above (S 25 ). 
     Next, the distance calculating unit  54  calculates the distance to the object on the basis of the angles of incidence determined by the incident angle determining unit  53  and the feature point position detected by the feature point detecting unit  51  (S 26 ). 
     The distance calculating unit  54  supplies the information conveying unit  9  with the distance information indicating the distance calculated in step S 26  (S 27 ). A display unit or the like, for example, in the information conveying unit  9  that obtains the distance information described above displays the distance indicated by the obtained distance information. 
     Next, the feature point detecting unit  51  decides whether or not image information has been obtained from the mode determining unit  4  (S 28 ). If image information has been obtained (step S 28 : Yes), the processing proceeds to step S 29 . 
     In step S 29 , the feature point detecting unit  51  decides whether or not the predetermined feature point of the predetermined object can be detected on the basis of the image information obtained from the mode determining unit  4 . If the feature point can be detected (step S 29 : Yes), the feature point detecting unit  51  detects the feature point, and proceeds to the processing in step S 29 ; if the feature point cannot be detected (step S 29 : No), the processing proceeds to step S 34 . 
     In step S 30 , the pixel area determining unit  52  decides whether or not there is a need to change the size and position of the pixel area. For example, if the local vehicle speed obtained from the vehicle speed detecting unit  2  has changed by a predetermined amount, the pixel area determining unit  52  decides that there is need to change the size of the pixel area; if the feature point position has moved away from the center of the pixel area by a predetermined amount, the pixel area determining unit  52  decides that there is need to change the position of the pixel area. When the pixel area determining unit  52  decides that there is a need to change at least one of the size and position of the pixel area (step S 30 : Yes), the processing returns to steps S 22  and S 25 ; when the pixel area determining unit  52  decides that there is no need to change the size and position of the pixel area (step S 30 : No), the processing proceeds to step S 31 . 
     In step S 31 , the incident angle determining unit  53  obtains the image information output from the mode determining unit  4  through the pixel area determining unit  52  and the feature point position information indicating the position of the feature point detected by the feature point detecting unit  51 , and determines the angles of incidence at which light is incident on the lenses in the first image capturing unit  1   a  and second image capturing unit  1   b  on the basis of the information described above. 
     Next, the distance calculating unit  54  calculates the distance to the object on the basis of the angles of incidence determined by the incident angle determining unit  53  and the feature point position detected by the feature point detecting unit  51  (S 32 ). 
     The distance calculating unit  54  supplies the information conveying unit  9  with the distance information indicating the distance calculated in step S 32  (S 33 ). 
     In step S 34 , because the feature point is not included in the captured image, the pixel area determining unit  52  assumes that the feature point has moved out-of-frame from the pixel area, and moves the position of the pixel area in the direction in which the feature point has moved out-of-frame. For example, the pixel area determining unit  52  stores feature point position information in time series order in the feature point position storage unit  52   a , and determines the direction in which the feature point has moved out-of-frame by the path of the feature point positions indicated by the feature point position information. For example, the pixel area determining unit  52  may decide the direction in which the feature point has moved out-of-frame by the direction in which the path of feature point positions has moved away from the center of the captured image, or may decide by a vector in which the feature point position at a time t 1  is taken as a starting point, and the feature point position at a time t 2  after a predetermined time has elapsed since time t 1  is taken as an endpoint. 
     The pixel area determining unit  52  supplies the camera parameter setting unit  6  with pixel area information indicating the size and position of the pixel area as altered in step S 34  (S 35 ). 
     The camera parameter setting unit  6  generates sensor driving pulse control signals that change the driving pattern of the imaging device  12  in each image capturing unit  1  responsive to the pixel area indicated by the pixel area information supplied in step S 35 , and supplies the generated sensor driving pulse control signals to the image capturing unit  1  through the data bus  8  (S 36 ). 
       FIG. 12  is a flowchart illustrating the processing performed in the visual recognition mode. 
     When a switchover to the visual recognition mode is decided upon, the mode determining unit  4  instructs the camera parameter setting unit  6  to initialize the pixel area (S 40 ). On receiving this instruction, the camera parameter setting unit  6  generates sensor driving pulse control signals that change the driving pattern of the imaging device  12  in each image capturing unit  1  so that the pixel area is the full pixel area (maximum image size), and supplies the generated sensor driving pulse control signals to the image capturing unit  1  through the data bus  8 . 
     Next, the distortion correcting unit  71  checks whether or not image information has been obtained from the mode determining unit  4  (S 41 ). If image information has been obtained (step S 41 : Yes), the distortion correcting unit  71  proceeds to the processing in step S 42 . 
     In step S 42 , the distortion correcting unit  71  corrects distortion in the image information obtained from the mode determining unit  4 . Distortion in both the first image obtained from the first image capturing unit  1   a  and the second image obtained from the second image capturing unit  1   b  is corrected. 
     The image combining unit  72  generates the combined image by combining at least part of the first image and at least part of the second image as corrected by the distortion correcting unit  71  (S 43 ). The image combining unit  72  supplies the information conveying unit  9  with the combined image information indicating the combined image. When it obtains the combined image information, the information conveying unit  9  causes a display unit or the like to display the combined image indicated by the combined image information. 
     As described above, in this embodiment, because the operating mode can be switched according to the vehicle speed between a visual recognition mode that displays images of the surroundings of the local vehicle and a distance measurement mode that outputs the distance to a preceding vehicle, appropriate information can be communicated to the driver of the vehicle according to the vehicle speed. Because processing is appropriately performed in the visual recognition mode and the distance measurement mode on the basis of the image information from the image capturing unit  1 , which are wide angle cameras, there is no need to install cameras with field angles suitable to the respective modes. Moreover, since the size of the images captured by the image capturing unit  1  is changed according to the vehicle speed in the distance measurement mode, the distance measuring unit  5  can obtain image information from the image capturing unit  1  at a frame rate responsive to the vehicle speed. 
     In the embodiment described above, the image capturing unit  1  is installed at the front of the local vehicle, but installation at the front of the vehicle is not a limitation; the image capturing unit  1  may be installed at the rear or on the sides of the local vehicle. 
     Because the charge exposure time of the imaging device  12  can be adjusted by driving pulse control of the imaging device  12 , the camera parameter setting unit  6  can control the charge exposure time as well as the pixel area of the image. 
     Although it is the position of the pixel area that is changed in step S 34  in  FIG. 11  in the embodiment described above, the pixel area may be enlarged, for example, to include the feature point. 
     Although the distance calculating unit  54  supplies the information conveying unit  9  with the distance information indicating the calculated distance in the embodiment described above, a mark that changes in color or shape according to the calculated distance, for example, or a warning tone that changes according to the calculated distance may be supplied to the information conveying unit  9 . If the information transmitted to the information conveying unit  9  is a distance or a mark, the information conveying unit  9  operates as a display in the interior of the vehicle and the information is displayed on a screen. If the information transmitted to the information conveying unit  9  is a sound, the information conveying unit  9  operates as a speaker, and makes the driver aware of the distance to the preceding object by sound. 
     REFERENCE CHARACTERS 
       100 : image processing apparatus,  1   a : first image capturing unit,  1   b : second image capturing unit,  11 : optical system,  12 : imaging device,  13 : AD converter,  14 : sensor driving circuit,  2 : vehicle speed detecting unit,  3 : ECU,  4 : mode determining unit,  5 : distance measuring unit,  51 : feature point detecting unit,  51   a : template storage unit,  52 : pixel area determining unit,  52   a : feature point position storage unit,  53 : incident angle determining unit,  53   a : numeric value storage unit;  54 : distance calculating unit,  6 : camera parameter setting unit,  7 : visual recognition image generating unit,  71 : distortion correcting unit,  72 : image combining unit,  8 : data bus,  9 : information conveying unit.