Patent Publication Number: US-2012044327-A1

Title: Device for acquiring stereo image

Description:
TECHNICAL FIELD 
     The present invention relates to a device for acquiring a stereo image, particularly to an on-board device for acquiring a stereo image. 
     BACKGROUND ART 
     In the automotive industry, there is a very active effort going on in terms of how to improve safety, and the trend is moving toward introduction of an increasing number of danger avoidance systems using an image sensor of a camera or radar. One of the well-known systems includes a system that uses an image sensor or radar to get information on the distance of the vehicle from the surrounding object, thereby avoiding danger. 
     In the meantime, in the taxicab industry, the trend is toward introduction of drive recorders. The drive recorder is a device for recording the image before and after an accident, and is effectively used to analyze the cause of the accident. For example, the responsibility for the accident can be identified to some extent by watching the images recorded at the time of collision between vehicles. 
     For example, Patent document 1 discloses the technique wherein long-hour images can be recorded and the required images can be quickly reproduced since the image information is compressed and recorded in a random access recording device. 
     Patent document 2 discloses an operation management device in which the image of a drive recorder and others is used in the training course of the drivers. In this device, on the ocation of reproduction of the image of an accident, when the driving data has reached a risky level, the image reproduction is turned to slow reproduction for the situation of the accident to be easily recognized. 
     In the image pickup operation of the aforementioned devices, it would be very effective if the distance information were obtained by a stereo image. For example, Patent document 3 discloses a method for detecting a moving object within the range of surveillance by extracting a 3D object present within the range of surveillance by using a pair of images captured in a chronological order by a stereo camera, and calculating the three-dimensional motion of the 3D object. However, the object of Patent document 3 is to detect a moving object in front of the vehicle and to avoid collision with the moving object, and no reference is made to such a device for recording an image as the aforementioned drive recorder. 
     In the meantime, Patent document 4 introduces a vehicle black box recorder that records the image obtained by an image pick device for capturing the surroundings of the moving vehicle. In this device, if there are objects in the area of windows provided inside the image, the distance is calculated for each window by a stereoscopic measurement method and the calculated distances are displayed on the screen. The result is stored together with the image information. 
     RELATED ART DOCUMENT 
     Patent Document 
     Patent document 1: Official Gazette of Japanese Patent Laid-open No. 3254946 
     Patent document 2: Unexamined Japanese Patent Application Publication No. 2008-65361 
     Patent document 3: Unexamined Japanese Patent Application Publication No. 2006-134035 
     Patent document 4: Official Gazette of Japanese Patent Laid-open No. 2608996 
     SUMMARY OF THE INVENTION 
     Object of the Invention 
     In the method disclosed in Patent document 4, however, the distance must be calculated from the stereo image on a real-time basis inside the vehicle black box recorder. This requires use of a high-priced electronic circuit exemplified by a high-speed microcomputer and a memory. Further, high-precision calculation of the distance requires high-quality stereo images, and storage of all these images requires a high-priced storage medium having an enormous amount of capacity and high-speed recording capacity. Thus, such a vehicle black box recorder has to be very expensive. 
     In view of the problems described above, it is an object of the present invention to provide a low-priced device for acquiring a stereo image which is capable of recording high-quality stereo images and of obtaining high-precision distance information, without requiring an expensive storage medium or an electronic circuit. 
     Means for Solving the Object 
     The object of the invention is solve by the following configuration. 
     Item 1. A device for acquiring a stereo image which is equipped with a camera section having at least two cameras including a base camera for taking base images of stereo images and a reference camera for taking reference images of the stereo images; a recording section configured to record image data taken by the camera section as record data; a control section configured to control the camera section and the recording section, wherein the device is configured to be mounted on a vehicle to acquire a stereo image of surroundings of the vehicle, the device comprising:
         a frame rate determination section configured to determine a frame rate of the record data to be recorded in the recording section;   a base data generation section to generate base data, from the base images taken by the base camera, on the basis of a first frame rate determined by the frame rate determination section; and   a reference data generation section configured to generate reference data, from the reference images taken by the reference camera, on the basis of a second frame rate which is determined by the frame rate determination section and is equal to or lower than the first frame rate,   wherein the frame rate determination section dynamically determines the second frame rate, depending on conditions and surroundings of the vehicle when the camera section takes images; and the recording section records the base data generated by the base data generation section and the reference data generated by the reference data generation section as the record data.       

     Item 2. The device for acquiring a stereo image of item 1, wherein the frame rate determination section dynamically determines the second frame rate on the basis of any one of or a combination of a plurality of the following conditions:
         (1) a speed of the vehicle;   (2) an operation condition of a steering wheel of the vehicle;   (3) an amount of a change in an optical flow for at least one of the cameras; and   (4) an amount of a temporal change in a parallax between the base camera and the reference camera.       

     Item 3. The device for acquiring a stereo image of item 1 or 2, wherein
         the reference data generation section generates the reference data, in uncompressed form or after performing compression with a low compression rate, on the basis of the second frame rate, and the reference data generation section generates second reference data compressed with a compression rate higher than the compression rate for the reference data, by using the reference image which is of the reference image not used to generate the reference data and is synchronized in the first frame rate; and   the recording section records the base data, the reference data, and the second reference data as the record data.       

     Advantage of the Invention 
     According to the present invention, the base data is produced from the image taken by the base camera on the basis of the first frame rate and is recorded; the reference data is produced from the image taken by the reference camera on the basis of the second frame rate which is equal to or lower than the first frame rate and is dynamically determined based on the vehicle and the surroundings of the vehicle at the time of image taking, whereby high quality stereo images are recorded, a high-precision distance information is obtained, and an expensive recording medium and electronic circuit are not required, thereby providing an inexpensive device for acquiring a stereo image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the structure of a first embodiment of a device for acquiring a stereo image; 
         FIG. 2  is a block diagram showing the structure of a frame rate determination section; 
         FIGS. 3   a  and  3   b  are block diagrams showing the structure of a data generation section; 
         FIG. 4  is a schematic diagram showing the process of generating base data and reference data under the normal states; 
         FIG. 5  is a schematic diagram showing the process of generating the base data, reference data and second reference data under the normal states; 
         FIG. 6  is a schematic diagram showing the process of generating the base data and reference data under the conditions that recording is needed; 
         FIG. 7  is a schematic diagram showing the process of generating the base data and reference data in the case that the condition changes from the normal state to the record-demanding condition and back to the normal state; 
         FIG. 8  is a block diagram showing the structure of a second embodiment of a device for acquiring a stereo image; and 
         FIG. 9  is a schematic diagram showing the process of generating the base data and reference data in a third embodiment of the device for acquiring a stereo image. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes the present invention with reference to embodiments, without the present invention being restricted thereto. The same or equivalent portions in the drawings will be assigned the same reference numbers, and duplicated explanations will be omitted. 
     Referring to  FIG. 1 , the following describes the structure of the first embodiment of the device for acquiring a stereo image in the present invention.  FIG. 1  is a block diagram showing the structure of the first embodiment of a device for acquiring a stereo image in the present invention. 
     In  FIG. 1 , the device for acquiring a stereo image  1  includes a camera section  11 , a recording section  13 , a control section  15 , a sensor section  17 , and a data generation section  19 . 
     The camera section  11  includes: at least two cameras, a base camera  111  and a reference camera  112 . The base camera  111  and the reference camera  112  are arranged apart from each other by a prescribed base line length D. In synchronism with a camera control signal CCS from a camera control section  151  (to be described later), base images Ib are outputted from the base camera  111  at a prescribed frame rate FRO, and a reference images Ir are outputted from the reference camera  112 . 
     The recording section  13  includes a hard disk or a semiconductor memory. Base data Db and reference data Dr are recorded on the basis of a recording control signal RCS from a recording control section  152 . A second reference data Dr 2  is also recorded if required. 
     The control section  15  includes the camera control section  151 , the recording control section  152  and a frame rate determination section  153 . The components of the recording section  13  may be made of hardware, or the functions of the components may be implemented by using a microcomputer and software. 
     The camera control section  151  outputs the camera control signal CCS for synchronizing the image capturing operations of the base camera  111  and reference camera  112 . 
     The recording control section  152  outputs a recording control signal RCS at the frame rate determined by the frame rate determination section  153  (to be described later), and controls the recording operation of the recording section  13 . 
     The frame rate determination section  153  determines a first frame rate FR 1  and outputs the first frame rate FR 1  to the data generation section  19 , and the base images Ib, which are taken by the base camera  111  at the prescribed frame rate FR 0  in synchronism with the camera control signal CCS from the camera control section  151 , are thinned out with respect to the first frame rate FR 1  to generate and record the base data Db. 
     In a similar manner, the frame rate determination section  153  determines a second frame rate FR 2  which is equal to or lower than the first frame rate FR 1 , and outputs the second frame rate FR 2  to the data generation section  19 , and the reference images Ir, which are taken by the reference camera  112  at the prescribed frame rate FR 0  in synchronism with the camera control signal CCS from the camera control section  151 , is thinned out with respect to the second frame rate FR 2  to generate and record the reference data Dr. How to determine the first frame rate FR 1  and the second frame rate FR 2  is described in detail with reference to  FIG. 2 . 
     The sensor section  17  is constituted by a vehicle speed sensor  171  for detecting the speed of a vehicle (hereinafter referred to as “own vehicle”) which is provided with a device  1  for acquiring a stereo image, and a steering angle sensor  172  for detecting the operating conditions of the steering wheel of the own vehicle. An own vehicle speed signal SS, which is the output from the own vehicle speed sensor  171 , and a steering angle signal HS, which is the output from the steering angle sensor  172 , are inputted into the frame rate determination section  153 , and are used for the determination of the second frame rate FR 2 . To detect the operating conditions of the steering wheel of the own vehicle, instead of the steering angle sensor  172 , an acceleration sensor can be used to detect the acceleration perpendicular to the traveling direction of the own vehicle. 
     The data generation section  19  includes a base data generation section  191  and a reference data generation section  192 . The components of the data generation section  19  may be made of hardware, or the functions of the components may be implemented by using a microcomputer and software. 
     The base data generation section  191  thins out the base images lb of the base camera  111  at the first frame rate FR 1 , and generates the base data Db with the image not compressed or compressed at a low compression rate. The base data Db is outputted to the recording section  13 . The compression method applied at a low compression rate is preferably a lossless compression method. The base images Ib subjected to the thinning out at the first frame rate FR 1  are discarded. 
     Similarly, the reference data generation section  192  thins out the reference images Ir of the reference camera  112  at the second frame rate FR 2  and generates the reference data Dr with the image not compressed or compressed at a low compression rate. The reference data Dr is outputted to the recording section  13 . The compression method applied at a low compression rate is preferably a lossless compression method. The reference image sIr subjected to the thinning out at the second frame rate FR 2  are discarded or are subjected to the following processing if required. 
     When required, on the reference images Ir thinned out in synchronism with the first frame rate FR 1  of the reference images Ir having been subjected to the thinning at the second frame rate FR 2 , the reference data generation section  192  performs the process of compression at a high compression rate and generates the second reference data Dr 2 . The second reference data Dr 2  is then outputted to the recording section  13 . The compression at a high compression rate can be lossy compression. The reference images Ir which have not been used to generate the reference data Dr or the second reference data Dr 2  will be discarded. 
     Referring to  FIG. 2 , the following describes the method of the first embodiment for determining the first frame rate FR 1  and the second frame rate FR 2  in the aforementioned frame rate determination section  153 .  FIG. 2  is a block diagram showing the structure of the frame rate determination section  153 . 
     In  FIG. 2 , the frame rate determination section  153  includes a first frame rate determination section  1531 , a second frame rate determination section  1532 , a parallax change calculating section  1533  and an optical flow change calculating section  1534 . The components of the frame rate determination section  153  may be made of hardware or the functions of the components may be implemented by using a microcomputer and software. 
     The frame rate determination section  153  determines the first frame rate FR 1 . The first frame rate FR 1  is set at a prescribed value independent of the conditions of the own vehicle and the surroundings, and is not changed even if there is a change in the conditions of the own vehicle and the surroundings. For example, when the base camera  111  captures images at a prescribed frame rate FR 0 =30 frames/sec. (hereinafter referred to as “fps”), the first frame rate FR 1  is set at half that value, i.e., 15 fps. In this manner, one out of two frames of the base images lb captured by the base camera  111  is used to generate the base data Db. The other frames are discarded. 
     The second frame rate determination section  1532  determines the second frame rate FR 2 . The second frame rate FR 2  is equal to or lower than the first frame rate FR 1 , and is determined depending on the conditions of the own vehicle and its surroundings. The second frame rate FR 2  is dynamically changed if there is a change in the conditions of the own vehicle and the surroundings. 
     In  FIG. 2 , the own vehicle speed signal SS, which is an output from the own vehicle speed sensor  171  and indicate the current conditions of the own vehicle, and the steering angle signal HS, which is an output from the steering angle sensor  172 , are inputted into the second frame rate determination section  1532 . 
     The base images Ib and the reference images Ir are inputted into the parallax change calculating section  1533  and the change in parallax is calculated in the parallax change calculating section  1533 . The parallax change signal Pr is inputted into the second frame rate determination section  1532 . 
     Similarly, the base images Ib and the reference images Ir are inputted into the optical flow change calculating section  1534  and the shift in optical flow is calculated in the optical flow change calculating section  1534 . The optical flow change signal Of is inputted into the second frame rate determination section  1532 . The parallax change signal Pr and the optical flow change signal Of indicate the surroundings around the own vehicle. 
     The second frame rate determination section  1532  determines and dynamically changes the second frame rate FR 2  based on any one or a combination of more than one of the aforementioned vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr, and the optical flow change signal Of. 
     The following describes the parallax change signal Pr and the optical flow change signal Of The parallax change signal Pr will be described first. The parallax is defined as a difference between the positions, of the same object, in the base image Ib and the reference image Ir. The parallax is proportional to the reciprocal umber of the distance from the subject. The greater the parallax is, the smaller the distance from the subject is. The smaller the parallax is, the greater the distance from the subject is. 
     Distance to the subject can be calculated from the base line lengths D of the base camera  111  and the reference camera  112 , the focal distances of the pickup lenses of the base camel a  111  and the reference camera  112 , and the value of the parallax. 
     The amount of the change in parallax can be defined as the amount of temporal change in the parallax. When the change in parallax is 0 (zero) or small, there is no change or a small change in the change in the distance from the subject. When the parallax is getting larger, the object is getting closer, and when the parallax is getting smaller, the object is getting farther. 
     Therefore, when the change in the parallax is 0 (zero) or is getting smaller, the object, i.e., another vehicle, a human body or an obstacle ahead is at the same distance or is getting away, which situation means that there is little possibility of collision with the object. In contrast, when the change in the parallax is increasing, the object is getting closer, which situation means that there is a risk of collision. In this manner, by using the parallax change signal Pr, the change in the distance between the own vehicle and the object is detected without calculating the distance to the object. 
     The following describes the optical flow change signal Of. The optical flow can be defined as the vector indicating the temporal change in the position of an object in the captured image. A 3D optical flow can be obtained by calculating the optical flow of the object ahead such as another vehicle from a stereo image, and if the extension of the 3D optical flow crosses the moving direction of the own vehicle, there is a risk of collision. 
     Thus, by using a 3D optical flow there can be detected not only a situation change, such as the change in the distance to the object indicated by the parallax change, in the traveling direction of the own vehicle, but also a situation change in the surroundings of the own vehicle such as a situation change like a cutting-in in the direction perpendicular to the traveling direction of the own vehicle, whereby the situation change in the surroundings of the own vehicle is more effectively detected. 
     Getting back to the second frame rate determination section  1532 , the second frame rate determination section  1532  determines and dynamically changes the second frame rate FR 2  based on any one or a combination of more than one of the aforementioned four signals; the vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr, and the optical flow change signal Of. 
     Here the situation of the own vehicle and the surroundings are classified into two states; a normal state CS 1  and a record-demanding state CS 2 . The second frame rate FR 2  will be determined for each of the states. 
     (Normal state CS 1 ) 
     Vehicle speed signal SS: Moving at a constant speed or at an accelerated or decelerated speed within a prescribed range. 
     Steering angle signal HS: Straight moving or the steering angle is within a prescribed range. 
     Parallax change signal Pr: 0 (zero) or small, or the parallax is reducing. 
     Optical flow change signal Of: There is no risk of collision. 
     When the aforementioned four conditions are met, it is judged that there is little risk of collision, and the second frame rate FR 2  is set low. For example, when images are captured by the base camera  111  and reference camera  112  at FRO=30 fps, and the first frame rate FR 1  is 15 fps, the second frame rate FR 2  is set at half that value, i.e., 7.5 fps. Thus, one out of four frames of the reference images Ir captured by the reference camera  112  is used to generate the reference data Dr. 
     (Record-demanding state CS 2 ) 
     Vehicle speed signal SS: Accelerated or decelerated speed exceeding a prescribed range. 
     Steering angle signal HS: Steering angle out of a prescribed range. 
     Parallax change signal Pr: Parallax is increasing. 
     Optical flow change signal Of: There is a risk of collision. 
     If any one of the aforementioned conditions is met, it is judged that there is a risk of collision, and the second frame rate FR 2  is set higher. For example, if images are captured by the base camera  111  and reference camera  112  at FRO=30 fps, and the first frame rate FR 1  is 15 fps, the second frame rate FR 2  is set at 15 fps, which is the same as the first frame rate FR 1 . Thus, one out of two frames of the reference images Ir captured by the reference camera  112 , similarly to the case of base data Db, is used to generate the reference data Dr. 
     To determine the conditions of the own vehicle and the surroundings, it is preferred to use all of the four signals, the vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr and the optical flow change signal Of. However, it is also possible to use one of these four signals or a combination of a plurality of these signals. For example, the own vehicle speed signal SS alone may be used, and when the own vehicle speed signal SS indicates that the vehicle is moving at a constant speed or at an accelerated or decelerated speed within a prescribed range, the state is determined to be the normal state CS 1 . Instead, when the own vehicle speed signal SS indicates that the vehicle is moving at an accelerated or decelerated speed beyond the prescribed range, the state is determined as the record-demanding state CS 2 . 
     Referring to  FIG. 3 , the following describes the method for generating the base data Db in the base data generation section  191  of the data generation section  19 , and the method for generating the reference data Dr and second reference data Dr 2  in the reference data generation section  192 .  FIGS. 3   a  and  3   b  are block diagrams showing the structure of the data generation section  19 .  FIG. 3   a  shows the structure of the base data generation section  191 , and  FIG. 3   b  shows the structure of the reference data generation section  192 . 
     In  FIG. 3   a , the base data generation section  191  is made up of a basic thin-out section  1911  and a low compression rate compressing section  1912 . The components of the base data generation section  191  may be made of hardware, or the functions of the components may be implemented by using a microcomputer and software. 
     The base images Ib captured at a prescribed frame rate FR 0  (e.g., 30 fps) by the base camera  111  are inputted into the basic thin-out section  1911 . The basic thin-out section  1911  thins out the base images Ib according to the first frame rate FR 1  (e.g., 15 fps) determined by the frame rate determination section  153 , and generates and outputs the basic thinned-out image Ib 1 . The image of the frame not used to generate the basic thinned-out image Ib 1  is discarded. 
     The basic thinned-out image Ib 1  is subjected to compression of a low compression rate by the low compression rate compressing section  1912 , and is outputted as the base data Db from the base data generation section  191 . However, the basic thinned-out image Ib 1  may be outputted as the base data Db without being compressed. 
     In  FIG. 3   b , the reference data generation section  192  is made up of a reference thin-out section  1921 , a low compression rate compressing section  1922 , a high compression rate compressing section  1923  and others. The components of the reference data generation section  192  may be made of hardware, or the functions of the components may be implemented by using a microcomputer and software. 
     The reference images Ir captured at a prescribed frame rate FR 0  (e.g., 30 fps) by the reference camera  112  is inputted into the reference thin-out section  1921 . The reference thin-out section  1921  thins out the reference images Ir according to the second frame rate FR 2  (e.g., 7.5 fps) determined by the frame rate determination section  153 , and generates and outputs the reference thinned-out images Ir 1 . 
     The reference thinned-out images Ir 1  are subjected to compression of a low compression rate by the low compression rate compressing section  1922 , and is outputted as the reference data Dr from the reference data generation section  192 . However, the reference thinned-out images Ir 1  may be outputted as the reference data Dr without being compressed. 
     Of the images of the frame not used to generate the reference thinned-out images Ir 1 , the images of the frame synchronized with the first frame rate FR 1  (e.g., 15 fps) determined by the frame rate determination section  153  are inputted as the second reference thinned-out images Ir 2  into the high compression rate compressing section  1923  and are subjected to compression with a high compression rate. These images are then outputted as the second reference data Dr 2 . The images of the frame not used to generate the reference thinned-out images Ir 1  or the second reference thinned-out images Ir 2  are discarded. 
     The step of generating the second reference data Dr 2  is not essential, and can be omitted. Compression of the high compression rate in the high compression rate compressing section  1923  can be lossy compression. 
     As described above, by using the images of the frame synchronized with the first frame rate FR 1  of the images of the frame not used to generate the reference thinned-out images Ir 1 , the second reference data Dr 2  is generated. With this arrangement, the distance is calculated from a stereo image method in conformity to the stereo image, although the precision is not good due to a high compression rate. Thus, the precision analysis of the cause for an accident can be conducted. There is only a small increase in the amount of recording data in the second reference data Dr 2  because a high compression rate is used for its compression. 
     Referring to  FIGS. 4 through 7  showing the process of forming an image file, the following describes the operation of the first embodiment.  FIG. 4  is a schematic diagram showing the process of forming the base data Db and reference data Dr in the normal state CS 1 . 
     In  FIGS. 4 through 7  and  FIG. 9  (to be described later), it is assumed that images are captured in chronological order along the time axis “t” from the left to the right. In  FIGS. 4 through 7  and  FIG. 9 , the first frame rate FR 1  is set at half the prescribed frame rate FR 0 , and the second frame rate FR 2  is set at half the first frame rate FR 1  in the normal state CS 1 , and the value equal to the first frame rate FR 1  in the record-demanding state CS 2 . Further, in  FIGS. 4 through 7  and  FIG. 9 , the images drawn by the broken line have been discarded through thinning. 
     In  FIG. 4 , the base images Ib are outputted from the base camera  111  at a prescribed frame rate FR 0 . The base images Ib are subjected to thinning at the first frame rate FR 1 , and the thinned-out data is recorded in the recording section  13  as the base data Db without being compressed or after being compressed at a low compression rate. 
     In the meantime, the reference images Ir are outputted from the reference camera  112  at a prescribed frame rate FR 0 . The reference images Ir are subjected to thinning at the second frame rate FR 2 , and the thinned-out data is recorded in the recording section  13  as the reference data Dr without being compressed or after being compressed at a low compression rate. 
     Thus, in the case that both the base data Db and reference data Dr are uncompressed, the amount of the base data Db is half that of the base images Ib, and the amount of the reference data Dr is a quarter of that of the reference images Ir, and the recording capacity can be saved by that amount. It should be noted that the distance can be calculated from a stereo image between the corresponding base data Db and reference data Dr, which are indicated by two-way arrows. 
       FIG. 5  is a schematic diagram showing the process of forming the base data Db and reference data Dr in the normal state CS 1 . The difference of  FIG. 5  from  FIG. 4  is that, of the images fro which images were thinned out at the second frame rate FR 2 , the second reference thinned-out images Ir 2  captured synchronously with the first frame rate FR 1  are subjected to compression of a high compression rate and is recorded in the recording section  13  as a second reference data Dr 2 . Otherwise,  FIG. 5  is the same as  FIG. 4 . 
     Since the second reference data Dr 2  is compressed at a high compression rate, there is only a smaller increase in the amount of data, when compared with example of  FIG. 4 . Further, calculation of the distance from a stereo image can be performed between the corresponding base data Db and second reference data Dr 2 , which are indicated by two-way arrows, although the precision is not good because the second reference data Dr 2  is compressed at a high compression rate. 
       FIG. 6  is a schematic diagram showing the process of forming the base data Db and the reference data Dr in the aforementioned record-demanding state CS 2 . The difference of  FIG. 6  from  FIG. 4  is that the second frame rate FR 2  used to record the reference images Ir of the reference camera  112  in the recording section  13  is the same as the first frame rate FR 1 , and the reference data Dr is recorded at the same density as the base data Db. Otherwise,  FIG. 6  is the same as  FIG. 4 . 
     Thus, in the case that the base data Db and the reference data Dr are uncompressed, the amount of the base data Db is half that of the base images Ib, and the amount of the reference data Dr is also a half that of the reference images It As a result, the recording capacity is increased by a quarter of the amount of the reference images Ir, when compared to  FIG. 4 . Despite that, the capacity is reduced to half the amount of the original image. Further, the distance can be calculated from a stereo image between the corresponding base data Db and reference data Dr, which are indicated by the two-way arrows in the diagram. 
       FIG. 7  is a schematic diagram showing the process of forming the base data Db and the reference data Dr in the case that the state changes from the normal state CS 1  to the record-demanding state CS 2 , and changes again to the normal state CS 1 . 
     In  FIG. 7 , until time t 1 , the base data Db and reference data Dr have been recorded in the normal state CS 1  of  FIG. 4 . At time t 1 , one of the four signals of the vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr, and the optical flow change signal Of has met the judging criterion for the record-demanding state CS 2 , and the normal state CS 1  of  FIG. 4  has been changed to the record-demanding state CS 2  of  FIG. 6 , with the result that the reference data Dr is recorded at the same high density as the base data Db. 
     This state remains unchanged until time t 2 . During this time, the reference data Dr continues to be recorded at the same high density as the base data Db. Thus, if there is an traffic accident, the distance is calculated from the stereo image, based on the base data Db and the reference data Dr recorded at a high density, and the higher-precision analysis of the accident can be conducted. 
     AT time t 2 , all the aforementioned four signals have returned to the normal state CS 1 ; accordingly, the record-demanding state CS 2  of  FIG. 6  has changed back to the normal state CS 1  of  FIG. 4 , and the base data Db and the reference data Dr in the normal state CS 1  start to be recorded. 
     As described above, the second frame rate FR 2  for recording the reference data Dr is determined dynamically based on the four signals, which are the surroundings, consisting of the vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr, and the optical flow change signal Of and indicate the state of the own vehicle and its surrounding, and if a collision is likely to occur, the stereo images are recorded at a high density, whereby the higher-precision analysis of the accident will be conducted. 
     As described above, according to the first embodiment, the image captured by the base camera is subjected to thinning at the first frame rate, and the base data without being compressed or after being compressed at a low compression rate is generated and recorded. The image captured by the reference camera is subjected to thinning at the second frame rate which is the same as, or lower than, the first frame rate, and which is dynamically determined depending on the conditions of the own vehicle and its surroundings. Then the reference data without being compressed or after being compressed at a low compression rate is generated and recorded. This arrangement provides a less expensive device for acquiring a stereo image capable of recording a high-quality stereo image and obtaining high-precision distance information, without using an expensive storage medium or electronic circuit. 
     The second frame rate for recording the aforementioned reference data is determined dynamically based on the four signals, which are the vehicle speed signal, the steering angle signal, the parallax change signal, and the optical flow change signal and indicate the state of the own vehicle and its surroundings; thus a collision is likely to occur, the stereo image is recorded at a higher density, whereby a higher-precision analysis of the accident will be conducted. 
     Further, of the images of the frame not used to generate the first reference thinned-out frame, the images of the frame synchronized with the first frame rate are used to generate the second reference data; thus, the distance is calculated from the stereo image, and the high precision analysis of the accident can be conducted in return for a mall increase in the data amount although the precision is not good due to a higher compression rate. 
     In the first embodiment, if as the base camera  111  and reference camera  112 , a camera capable of capturing an image not at a prescribed frame rate FR 0  but at the first frame rate FR 1  is employed, and the first frame rate FR 1  is equal to a prescribed frame rate FR 0 , it is possible to omit at least the basic thin-out section  1911  of the base data generation section  191 . 
     Referring to  FIG. 8 , the following describes the second embodiment of the device for acquiring a stereo image according to the present invention.  FIG. 8  is a block diagram showing the structure of the second embodiment of a device for acquiring a stereo image. 
     In reference to  FIG. 8 , the data generation section  19  of the first embodiment is omitted in the second embodiment, and the function of the base data generation section  191  of the data generation section  19  is provided in the base camera  111 , and the function of the reference data generation section  192  is provided in the reference camera  112 . 
     The base camera  111  and the reference camera  112  of the camera section  11  are synchronized with the camera control signal CCS from the camera control section  151 , and the base images Ib are outputted from the base camera  111 , and the reference images Ir are outputted from the reference camera  112  at a prescribed frame rate FR 0 . The base images Ib and the reference images Ir are inputted into the frame rate determination section  153 , and the first frame rate FR 1  and the second frame rate FR 2  are determined as shown in  FIG. 2 . 
     The determined first frame rate FR 1  is inputted into the base camera  111  and the reference camera  112 , and the second frame rate FR 2  is inputted into the reference camera  112 . 
     The base camera  111  performs thinning process on the base images Ib at the first frame rate FR 1 , and outputs the base images Ib as the base data Db to the recording section  13  without being compressed or after being compressed at a low compression rate. 
     The reference camera  112  performs thinning process on the reference images Ir at the second frame rate FR 2 , and outputs the reference images Ir as the reference data Dr to the recording section  13  without being compressed or after being compressed at a low compression rate. Further, of the images of the frame not used to generate the reference data Dr, the images of the frames synchronized with the first frame rate are compressed at a high compression rate by the reference camera  112  and are outputted as the second reference data Dr 2  to the recording section  13 . Other operations are the same as those of the first embodiment and will not be described to avoid duplication. 
     In the second embodiment in particular, the base data Db and the reference data Dr are uncompressed and the second reference data Dr 2  is not generated. With this structure, it is not required to perform compression in the camera, and the data generation section  19  of the first embodiment can be omitted, thereby putting much load on the base camera  111  and the reference camera  112 . This arrangement ensure a higher speed in the processing of the device for acquiring a stereo image  1 , a simplified structure, and reduction of the manufacturing cost. The process of generating the base data Db and the reference data Dr in this arrangement is the same as that of  FIG. 7 . 
     Instead, the second embodiment can employ as the base camera  111  a camera capable of capturing an image not at a prescribed frame rate FR 0  but at the first frame rate FR 1 , and as the reference camera  112  a variable-frame-rate camera capable of capturing not at a prescribed frame rate FR 0  but at an image at the second frame rate FR 2 . 
     The following describes a third embodiment of the device for acquiring a stereo image according to the present invention. In the third embodiment, when the state of the own vehicle and its surroundings falls in the normal state CS 1  of the first and second embodiments, the base data Db and the reference data Dr are not recorded in the recording section  13 , and only when the state of the own vehicle and its surroundings falls in the record-demanding state CS 2 , the base data Db and reference data Dr are recorded in the recording section  13 . 
       FIG. 9  shows the process of generating the base data Db and the reference data Dr.  FIG. 9  is a schematic diagram showing the process of generating the base data Db and the reference data Dr in the third embodiment of the device for acquiring a stereo image of the present invention, and the schematic view shows the process of generating the base data Db and the reference data Dr in the case that the state changes from the normal state CS 1  to the record-demanding state CS 2  and changes again to the normal state CS 1 . 
     In  FIG. 9 , until time T 1 , the state is the aforementioned normal state CS 1 , and the base camera  111  captures the base images Ib at a prescribed frame rate FR 0 , but the base data Db is not recoded. Similarly, the reference camera  112  captures the reference images Ir at a prescribed frame rate FR 0 , but the reference data Dr is not recorded. 
     Synchronously with time t 1 , any one of the four signals consisting of vehicle speed signal SS, steering angle signal HS, parallax change signal Pr and optical flow change signal Of has met the decision condition under the record-demanding state CS 2 . Accordingly, the normal state CS 1  changes over to the record-demanding state CS 2 . In this state, the base images Ib are thinned out at the first frame rate FR 1  and the basic thinned-out image Ib 1  is generated. This is recorded as base data Db. Similarly, the reference images Ir are thinned out at the second frame rate FR 2  and reference thinned-out images Ir 1  are generated. This is recorded as reference data Dr. 
     In  FIG. 9 , similarly to the case of  FIG. 6 , the first frame rate FR 1  is equal to the second frame rate FR 2 , and the reference data Dr is recorded at the same high density as the base data Db. 
     The record-demanding state CS 2  continues until the time t 2 . During that period, the reference data Dr is kept to be recorded at the same high density as the base data Db. Thus, if an accident happens, since the distance is calculated from a stereo image based on the base data Db and reference data Dr recorded at high density, whereby the higher-precision analysis of an accident can be conducted. 
     At time t 2 , all the aforementioned four signals return to the normal state CS 1 . Accordingly, the record-demanding state CS 2  changes back to the normal state CS 1 , and images are taken by the base camera  111  and reference camera  112 , but neither the base data Db nor reference data Dr is recorded. 
     As described above, the second frame rate FR 2  for recording the reference data Dr is determined dynamically based on the four signals, which are the vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr, and optical flow change signal Of and indicates state of the own vehicle and its surroundings. Only when a collision is likely to occur, the stereo images are recorded at a high density, whereby a higher-precision analysis of an accident can be conducted. At the same time, if there is no danger, data is not recorded, with the result that the recording time gets longer and the capacity of such a recording medium such as a hard disk and semiconductor memory can be reduced, whereby the apparatuses are downsized and the manufacturing cost is reduced. 
     As described above, according to the third embodiment, only when a collision is likely to occur, the stereo images are recorded at a high density, based on the four signals, which are the vehicle speed signal SS, the steering angle signal HS, the parallax change signal Pr, and the optical flow change signal Of, and indicates the state of the own vehicle and the surroundings. This arrangement provides higher-precision analysis of an accident. If there is no danger, data is not recorded. This prolongs the recording time and reduces the capacity of the recording medium such as a hard disk and semiconductor memory, with the result that a downsized apparatus and reduced manufacturing cost are ensured. 
     In the third embodiment, if the cameras capable of capturing an image at the first frame rate FR 1 , not at a prescribed frame rate FRO are used as a base camera  111  and reference camera  112 , or if the first frame rate FR 1  is equal to a prescribed frame rate FR 0 , it is possible to omit the base data generation section  191 . 
     As described above, according to the present invention, the base data is generated at the first frame rate from the image captured by a base camera, and is recorded. The reference data is generated from the image captured by the reference camera and is recorded at the second frame rate which is the same as or lower than the first frame rate and which is dynamically determined depending on the conditions of the own vehicle and its surroundings. This arrangement provides a less expensive device for acquiring stereo images which is capable of recording high-quality stereo images and of obtaining high-precision distance information, without the need of using an expensive storage medium or electronic circuit. 
     The details of the structures constituting the device for acquiring a stereo image of the present invention can be modified without departing from the spirit of the present invention. 
     DESCRIPTION OF THE NUMERALS 
       1  Device for acquiring a stereo image 
       11  Camera section 
       111  Base camera 
       112  Reference camera 
       13  Recording section 
       15  Control section 
       151  Camera control section 
       152  Recording control section 
       153  Frame rate determination section 
       1531  First frame rate determination section 
       1532  Second frame rate determination section 
       1533  Parallax change calculating section 
       1534  Optical flow change calculating section 
       17  Sensor section 
       171  Vehicle speed sensor 
       172  Steering angle sensor 
       19  Data generation section 
       191  Base data generation section 
       1911  Basic thin-out section 
       1912  Low compression rate compressing section 
       192  Reference data generation section 
       1921  Reference thin-out section 
       1922  Low compression rate compressing section 
       1923  High compression rate compressing section 
     CCS Camera control signal 
     CS 1  Normal state 
     CS 2  Record-demanding state 
     D: Base line length 
     Db: Base data 
     Dr: Reference data 
     Dr 2 : Second reference data 
     FR 0 : Prescribed frame rate 
     FR 1 : First frame rate 
     FR 2 : Second frame rate 
     HS: Steering angle signal 
     Ib: Base image 
     Ib 1 : Base thinned-out image 
     Ir: Reference image 
     Ir 1 : First reference thinned-out image 
     Ir 2 : Second reference thinned-out image 
     Of: Optical flow change signal 
     Pr: Parallax change signal 
     SS: Vehicle speed signal