Patent Publication Number: US-11024011-B2

Title: Image display apparatus and image display method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority to Japanese Patent Application No. 2018-160618, filed on Aug. 29, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosures herein relate to an image display apparatus and an image display method. 
     2. Description of the Related Art 
     There has been an attempt to replace a vehicle rearview mirror with a camera monitoring system (CMS). The CMS is a technique that displays an image captured by an imaging device on a display. Such a rearview mirror employing the CMS is hereinafter referred to as electronic rearview mirror. The electronic rearview mirror takes the form of a related-art rearview mirror equipped with a built-in display. In the related-art rearview mirror, rearward vision may be obstructed by a vehicle occupant and luggage. Conversely, in the electronic rearview mirror, images of an imaging device provided on the rear of a vehicle are displayed on a display included in the rearview mirror, thus enabling better rear visibility. 
     Generally, it is difficult to install an imaging device on a vehicle without any installation error. Due to installation error of the imaging device, an image captured by the imaging device may be rotated and an image may fail to be captured in a desired direction. Further, in related-art rearview mirrors, it is common to adjust the angle of a rearview mirror to the direction desired by the driver. In electronic rearview mirrors, a function for receiving adjustment of a display area is provided (see Patent Document 1 and Patent Document 2, for example). Patent Document 1 describes a technique that moves a display area displayed by an electronic rearview mirror up, down, left, and right in accordance with the angle of the electronic rearview mirror. Patent Document 2 describes a technique that zooms in or zooms out an image displayed by an electronic rearview mirror by applying the pushing or pulling force to the electronic rearview mirror. 
     However, in the related-art techniques, it may be difficult to adjust a display area to the position intended by a driver. 
     RELATED-ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Laid-Open Patent Publication No. 2012-195634 
         Patent Document 2: Japanese Laid-Open Patent Publication No. 2016-167859 
       
    
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an image display apparatus that readily adjusts a display area to a desired position. 
     According to an embodiment of the present invention, an image display apparatus includes an imaging device configured to generate image data; a receiving unit configured to receive movement of a display area in a vertical direction or in a horizontal direction within the image data, the display area being displayed on a display device, and also rotation of the image data; an image processing unit configured to rotate the image data in accordance with an amount of rotation received by the receiving unit, and, in a case where the movement of the display area in the vertical direction or in the horizontal direction is received by the receiving unit with respect to the rotated image data, move the display area in the vertical direction or in the horizontal direction within the rotated image data; an image cutting unit configured to cut, from the image data rotated by the image processing unit, the display area moved by the image processing unit; and an outputting unit configured to display the display area cut by the image cutting unit on the display device. 
     According to an embodiment of the present invention, an image display method includes generating, by an imaging device configured, image data; receiving, by a receiving unit, movement of a display area in a vertical direction or in a horizontal direction within the image data, the display area being displayed on a display device, and also rotation of the image data; rotating, by an image processing unit, the image data in accordance with an amount of rotation received by the receiving unit, and, in a case where the movement of the display area in the vertical direction or in the horizontal direction is received by the receiving unit with respect to the rotated image data, moving the display area in the vertical direction or in the horizontal direction within the rotated image data; cutting, by an image cutting unit, from the image data rotated by the image processing unit, the display area moved by the image processing unit; and displaying, by an outputting unit, the display area cut by the image cutting unit on the display device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIGS. 1A through 1G  are drawings illustrating positional adjustment of a display area displayed by an electronic rearview mirror; 
         FIGS. 2A through 2D  are drawings illustrating positional adjustment of a CMS area displayed by an electronic rearview mirror; 
         FIGS. 3A and 3B  are drawings illustrating installation examples of the electronic rearview mirror; 
         FIG. 4  is a schematic perspective view of an example of a display unit; 
         FIGS. 5A and 5B  are drawings illustrating an example of a schematic structure of the display unit; 
         FIG. 6  is a functional block diagram illustrating an example of functions of the display unit; 
         FIGS. 7A through 7D  are drawings illustrating an example of a coordinate system of image data and coordinates of a CMS area; 
         FIG. 8  is a flowchart illustrating an example of a process in which the electronic rearview mirror displays CMS area image data; 
         FIGS. 9A through 9F  are drawings illustrating a problem with rotating image data after a CMS area is vertically or horizontally moved; 
         FIGS. 10A through 10G  are drawings illustrating positional adjustment of a CMS area by an electronic rearview mirror; 
         FIGS. 11A through 11D  are drawings illustrating an example of a coordinate system of image data and coordinates of a CMS area; 
         FIG. 12  is a flowchart illustrating an example of a process in which the electronic rearview mirror displays CMS area image data; and 
         FIG. 13  is a flowchart illustrating an example of a process for selecting a method for displaying CMS area image data. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     According to an embodiment of the present invention, it is possible to provide an image display apparatus that readily adjusts a display area to a desired position. 
     Prior to describing an embodiment of the present invention, the above-described related art will be described with reference to  FIGS. 1A  through  1 G.  FIGS. 1A through 1G  are drawings illustrating an example of positional adjustment of a display area displayed by an electronic rearview mirror.  FIG. 1A  illustrates image data captured by an imaging device  11 . The imaging device  11 , installed on the rear of a vehicle, is rotated clockwise (rotated counterclockwise when viewed in the optical axis direction) due to installation error.  FIG. 1B  illustrates an image of a display area (hereinafter referred to as a CMS area  8 ) that is cut out from the image data and is displayed by an electronic rearview mirror  10 . Because the imaging device  11  is rotated, the image data is tilted, and the image displayed by the electronic rearview mirror  10  is also tilted. Accordingly, a driver desires to cause the image to be horizontal. 
     The driver can adjust the image displayed by the electronic rearview mirror, by using a setting screen provided and available to the driver. In this case, the driver rotates the image clockwise.  FIG. 1C  illustrates an example of the image data rotated clockwise by the driver. As illustrated in  FIG. 1D , a CMS area  8  is cut out from the image data after rotation. Therefore, an image displayed by the electronic rearview mirror  10  is not tilted. 
     In the state of  FIG. 1C , the driver may desire to move the CMS area  8  up, down, left, or right. However, even if the driver moves the CMS area  8  up, down, left, or right in the state of  FIG. 10 , an internal process of the electronic rearview mirror  10  causes the CMS area  8  to be moved within the image data before rotation.  FIG. 1E  illustrates a state in which the CMS area  8  is moved to the right by the driver within the image data before rotation. 
     Because the driver has operated to rotate the image of the CMS area  8 , the electronic rearview mirror  10  rotates the image data after the CMS area  8  is moved to the right by the driver. 
       FIG. 1F  illustrates the image data rotated clockwise after the movement of the CMS area  8 . As compared to  FIG. 1E , the CMS area  8  in  FIG. 1F  is moved not only to the right but also downward. Namely, although the driver desires to move the CMS area  8  to the right only, the CMS area  8  is moved obliquely downward to the right. 
       FIG. 1G  illustrates an image of the CMS area  8  displayed by the electronic rearview mirror  10  after the CMS area  8  is moved to the right, and further the image data is rotated clockwise. As compared to  FIG. 1D , it appears to the driver that the CMS area  8  has moved to the lower right. In order to move the CMS area  8  to the right only, the driver is required to perform an extra operation for moving the CMS area  8  upward. 
     As described above, even if the driver moves the position of the CMS area  8  up, down, left, or right after rotating the image data so as to allow the CMS area  8  to become horizontal, the electronic rearview mirror  10  moves the CMS area  8  up, down, left, or right within the image data before rotation. Thus, it is difficult to adjust the CMS area  8  to the position intended by the driver. 
     In the following, an electronic rearview mirror and an image display method performed by the electronic rearview mirror according to an embodiment of the present invention will be described with reference to the accompanying drawings. 
     First Embodiment 
     Overview of Electronic Rearview Mirror According to First Embodiment 
     In a case where a driver corrects an image captured by an imaging device  11  having installation error, an electronic rearview mirror according to a first embodiment performs a “process for moving a CMS area in a vertical direction or in a horizontal direction within the image that has been rotated”. The CMS area is a display area within image data and is displayed on a display. 
       FIGS. 2A through 2D  are drawings illustrating positional adjustment of a CMS area  8  displayed by the electronic rearview mirror of the present embodiment. Similar to  FIG. 1C ,  FIG. 2A  illustrates image data that has been rotated. Similar to  FIG. 1C ,  FIG. 2B  illustrates a CMS area  8  after the rotation of the image data. In this state, if the driver moves the CMS area  8  up, down, left, or right, the electronic rearview mirror  10  moves the CMS area  8  in the vertical direction or in the horizontal direction within the image data that has been rotated. 
       FIG. 2C  illustrates the CMS area  8  that has moved to the right within the rotated image data. The above-described process moves the CMS area  8  to the right only, and does not move the CMS area  8  in the vertical direction.  FIG. 2D  illustrates the CMS area  8  cut out from the image data of  FIG. 2C . As compared to  FIG. 2B , it can be seen that the CMS area  8  of  FIG. 2D  is moved to the right only. 
     As described above, the electronic rearview mirror  10  according to the present embodiment moves the CMS area  8  in the vertical direction or in the horizontal direction within the rotated image data. Thus, it becomes possible to move the CMS area  8  up, down, left, or right only as intended by the driver, without obliquely moving the CMS area  8 . 
     Terminology 
     An image display apparatus is an apparatus that displays peripheral image data captured by the imaging device  11  on a display device. The imaging device  11  is not limited to a device mounted on a vehicle. In the present embodiment, the term “electronic rearview mirror” is used; however, “electronic back-view mirror” or “electronic mirror” may also be used. 
     Further, the imaging device may be mounted on a moving object other than a vehicle. For example, the imaging device may be mounted on a motorized bicycle, a non-motorized vehicle such as a bicycle, a wheelchair, or a robot. 
     Further, the image display apparatus may be applied to a surveillance camera. The surveillance camera refers to an imaging device for performing monitoring for various purposes. The main uses of the surveillance camera include crime prevention, disaster prevention, and recording. 
     Further, although an “image” might refer to an image displayed on a display and “image data” might refer to intangible information, in the present embodiment, an image and image data are not strictly distinguished. 
     Configuration Example 
       FIGS. 3A and 3B  are drawings illustrating installation examples of the electronic rearview mirror  10 .  FIG. 3A  illustrates an example in which a display unit  12  is installed within a vehicle.  FIG. 3B  illustrates an example in which the imaging device  11  and the display unit  12  are installed. 
     As illustrated in  FIG. 3A , similar to a related-art rearview mirror, the display unit  12  of the electronic rearview mirror  10  is located between a driver&#39;s seat and a passenger&#39;s seat, and is fixed to the upper portion of the windshield. In some countries and regions, the position at which to install the display unit  12  is not clearly defined by laws and regulations. Thus, the illustrated installation position is merely an example. In countries and regions where the position at which to install the display unit  12  is clearly defined by laws and regulations, the display unit  12  is installed at the position defined by the laws and regulations. Further, the number of display units  12  is not limited to one, and a plurality of display units  12  may be installed. 
     The imaging device  11  is provided on the rear of a vehicle  101 . It is preferable for the imaging device  11  to capture an image in an optical axis direction close to the direction of light incident on a related-art rearview mirror. The related-art rearview mirror is a mirror that is installed at the upper portion of the windshield and that reflects the rear view of the vehicle. Therefore, it is often preferable to install the imaging device  11  at an upper portion of the vehicle. For example, the imaging device  11  may be installed on the roof or on the roof spoiler. The imaging device  11  is preferably installed at the center or in the vicinity of the center of the vehicle in the horizontal direction. However, even if the imaging device  11  is installed on the right end or the left end of the vehicle, it is possible to capture images. Therefore, image processing may be performed such that an image, captured by the imaging device  11  installed on the right end or the left end of the vehicle, appears to be captured in the vicinity of the center of the vehicle. 
     The position of the imaging device  11  may be different not only on a per-vehicle-model basis, but also on a per-vehicle basis. For example, the imaging device  11  may be installed in the vicinity of a license plate, in the vicinity of a vehicle manufacturer&#39;s emblem, or at an appropriate position of a rear bumper. 
     Further, the vehicle  101  may include a rearview camera. The rearview camera is a camera that projects the rear view of the vehicle when the vehicle is moved backward (when the driver moves the shift lever to the reverse position) for parking in a garage, for example. In most cases, captured images are displayed on a display such as an automotive navigation system. In the present embodiment, images captured by the rearview camera may be displayed on the display unit  12 . Alternatively, as images captured by the rearview camera, images captured by the imaging device  11  of the electronic rearview mirror  10  may be displayed on a display such as an automotive navigation system. 
     Further, a relatively wide-angle camera such as a fisheye camera may be used together with the electronic rearview mirror  10  and the rearview camera. Namely, a part of an image captured by the wide-angle camera may be displayed on the display unit  12 , and another part of the image (which may overlap with the part displayed on the display unit  12 ) may be displayed on a display such as an automotive navigation system. 
     Further, a wire connecting between the imaging device  11  and the display unit  12  may be laid on the ceiling or near the floor, which may be determined by taking into account the length or the layout of the wire. Further, the imaging device  11  may transmit images to the display unit  12  via wireless communication. 
     Although not illustrated in  FIG. 3 , a power supply line, a reverse signal line, turn signal lines, and video input of other imaging devices (such as a side-view camera and the rearview camera) may be connected to the display unit  12 . For example, in response to a signal from the reverse signal line, the display unit  12  may display an image of the rearview camera, or in response to a signal from a turn signal line, the display unit  12  may display an image of the side-view camera. 
       FIG. 4  is a schematic perspective view of an example of the display unit  12 . The display unit  12  includes a display  15 , four operation buttons  1  through  4 , and a lever  17 . Note that a wire interface is not depicted. Similar to a drive recorder, the display unit  12  may have a memory card slot in addition to the illustrated configuration. The shape of the display unit  12 , and the number and positions of the buttons are merely examples. 
     The display  15  is a flat panel display, such as a liquid crystal display or an organic electro-luminescence (EL) display. The display  15  may be a rear-projection-type display. A half-silvered mirror is attached to the front (interior side) of the display  15  so as to be superimposed on the display  15 , as will be described below. 
     The four operation buttons  1  through  4  are input devices for displaying a setting screen of the electronic rearview mirror  10  and receiving operations from a user. An on-screen display (OSD) is known as a function of displaying a simple setting screen; however, a method for displaying a user interface may vary depending on resources of the display unit. For example, if an operating system (OS) is installed on a board, which will be described below, a screen display function supported by the OS may be used. 
     The functions of the operation buttons are as follows. 
     Operation button  1 : Displaying a menu (including luminance, contrast, color temperature, language, and CMS area settings) 
     Operation button  2 : Selecting one direction (including moving to the right, moving up, rotating to the right, and scale-up) 
     Operation button  3 : Selecting another direction (including moving to the left, moving down, rotating to the left, and scale-down) 
     Operation Button  4 : OK 
     The driver can make settings for luminance, contrast, color temperature, and language, rotate image data, and move a CMS area  8 . 
     The lever  17  is an input device for switching between turning on and off the power of the electronic rearview mirror  10 . When the power is turned on, the display unit  12  outputs an image captured by the imaging device  11 . When the power is turned off, the display unit  12  does not display an image captured by the imaging device  11 . In the latter case, light is reflected by the half-silvered mirror, and the display unit  12  functions as a mirror that reflects light inside of the vehicle in the same manner as an ordinary mirror, which will be described with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  are drawings illustrating an example of a schematic structure of the display unit  12 .  FIG. 5A  illustrates a state of a half-silvered mirror  16  when the power is turned on, and  FIG. 5B  illustrates a state of the half-silvered mirror  16  when the power is turned off. For example, if the transmittance of the half-silvered mirror  16  is T % (for example, 50% to 90%), T % of light from the display  15  passes through the half-silvered mirror  16  when the power is turned on. Even if only T % of light passes through the half-silvered mirror  16 , the driver can visually recognize an image displayed on the display  15  because the display  15  is brighter than the inside of the vehicle. 
     When the power is turned off, the inner side of the half-silvered mirror  16  becomes dark because the display  15  does not emit light. In this case, the inside of the vehicle is brighter than the inner side of the half-silvered mirror  16 , and thus, the intensity of light reflected by the half-silvered mirror  16  becomes large. As a result, light transmitted through the half-silvered mirror  16  is obscured by bright reflected light inside the vehicle. Thus, the display unit  12  looks like a mirror when viewed from the inside of the vehicle. Accordingly, by pushing the lever backward and forward, the driver can readily switch between the display  15  displaying images and the ordinary mirror reflecting light inside of the vehicle. For example, if the visibility of the display  15  is decreased because of strong light of the westering sun incident from the rear of the vehicle, the driver may switch to the ordinary mirror. 
     As illustrated in  FIGS. 5A and 5B , the display unit  12  includes a board  14 , and uses the board  14  to perform image processing, such as controlling the imaging device  11  and cutting out a CMS area  8 . The board  14  includes general computer functions, and also a CPU, a RAM, a ROM, a flash memory, and an input and output interface (I/F). 
     &lt;Functions of Display Unit&gt; 
     Referring to  FIG. 6 , functions of the display unit  12  will be described.  FIG. 6  is a functional block diagram illustrating an example of functions of the display unit  12 . The display unit  12  includes an image obtaining unit  21 , an operation receiving unit  22 , an image rotating and moving unit  23 , an image cutting unit  24 , and an image outputting unit  25 . The functions of the display unit  12  are functions or means implemented by causing the CPU included in the board  14  of the display unit  12  to execute a program. 
     The board  14  performs general control of the imaging device  11 . For example, the board  14  performs exposure control that controls one or both of the shutter speed and sensitivity (gain level), such that the brightness of an image is at an appropriate level. In addition, the board  14  performs a process called an auto-white balance control (AWB control), in which proportions of R, G, and B are set to be approximately the same so as to avoid changes in a white color and other colors of an object under different color temperatures of light sources. In addition, the board  14  generates a drive signal so that an imaging element can perform an imaging operation at an appropriate timing, and transmits a clock signal to an A/D converter. The board  14  may perform other processes such as removing noise, and adjusting the sharpness of an image by extracting and emphasizing edge components only. 
     The image obtaining unit  21  sequentially obtains image data from the imaging device  11  at a predetermined frame rate. Accordingly, an image is assumed to be a moving image, but may be a still image. 
     The operation receiving unit  22  receives various operations for the electronic rearview mirror, in response to the driver operating the operation buttons  1  through  4 . In the present embodiment, the operation receiving unit  22  receives rotation of an image and movement of a CMS area  8 . It is assumed that the relationship between the operation of the operation buttons  1  through  4  versus the amount of rotation and the amount of movement is preset as follows, for example. 
     By pressing an operation button once in the case of the rotation of an image being received via the setting screen, the image is rotated by θ degrees. By pressing an operation button once in the case of the movement of a CMS area  8  being received via the setting screen, the CMS area  8  is moved by N pixels. 
     The image rotating and moving unit  23  obtains one or both of the amount of rotation and the amount of movement received by the operation receiving unit  22 , rotates image data, and moves a CMS area  8  vertically or horizontally. In the present embodiment, in a case where the driver desires to move a CMS area  8  up, down, left, or right after rotating image data, the image rotating and moving unit  23  moves the CMS area  8  in the vertical direction or in the horizontal direction within the image data that has been rotated. 
     The image cutting unit  24  cuts the CMS area  8  from the image data, and generates CMS area image data. The CMS area image data is image data having approximately the same size as the display  15 . Note that the CMS area  8  may be scaled up or scaled down in accordance with an operation performed by the driver. A cutting process may be referred to as trimming. The image outputting unit  25  outputs the CMS area image data to the display  15 , and causes the display  15  to display the CMS area image data. 
     &lt;Process for Rotating Image and Moving CMS Area&gt; 
     Next, a process for rotating an image and moving a CMS area  8  will be described with reference to  FIGS. 7A through 7D .  FIGS. 7A through 7D  are drawings illustrating an example of a coordinate system of image data and coordinates of a CMS area  8 . 
     First, as illustrated in  FIG. 7A , a coordinate system (an orthogonal coordinate system) having the X-axis and the Y-axis is provided, and image data  201  is placed in the coordinate system. The coordinate system is fixed, and does not rotate when image data is rotated. The image rotating and moving unit  23  places image data captured by the imaging device  11  having installation error (namely, an object in the image data is tilted) in the coordinate system, in such a manner that the horizontal direction of the image data becomes parallel to the X-axis. As an example, in  FIG. 7A , the origin (0, 0) is located at the lower left corner of the image data  201 . Because the width W and the height H of the image data  201  are fixed, coordinates P1 to P4 of the corners of the image data  201  can be readily obtained. 
     The CMS area  8  is set at a predetermined position of the image data  201 . The position of the CMS area  8  in  FIG. 7A  may be a predetermined initial position, or may be a position adjusted by a driver. The width w and the height h of the CMS area  8  are fixed. Thus, when any one of coordinates of the center and coordinates Q1 through Q4 of the corners is determined, the rest of the coordinates can be readily obtained.
 
 Q 1( X 1, Y 1), Q 2( X 2, Y 2), Q 3( X 3, Y 3), Q 4( X 4, Y 4)
 
       FIG. 7B  illustrates the image data  201  rotated by θ degrees by the driver. The image data  201  is rotated by θ degrees around the center O. If the CMS area  8  were rotated together, the position of the CMS area  8  would not be corrected. Thus, no change is made to the coordinates Q1 through Q4 (namely, the CMS area  8  is not rotated). Accordingly, the tilt of CMS area image data to be displayed on the display  15  is corrected. 
     The rotation of the image data  201  by the image rotating and moving unit  23  is expressed as follows. A formula (1) expresses that, after rotating with the center of image data moved to the origin (0, 0), the image data is translated to the original position. Coordinates of any pixel of the image data  201  after rotation are calculated by the formula (1). P1′ through P4′ denote coordinates of the corners of the image data after rotation. 
     
       
         
           
             
               
                 
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     Next,  FIG. 7C  illustrates the CMS area  8  that has moved to the right, and  FIG. 7D  illustrates the CMS area  8  that has moved upward. For example, assuming that the CMS area  8  is moved to the right by 100 pixels, coordinates of the moved CMS area  8  are represented by Q1′ (X1′, Y1′) through Q4′ (X4′, Y4′).
 
 X 1′= X 1+100, X 2′= X 2+100, X 3′= X 3+100, X 4 ′X 4+100
 
     No change is made to Y1 through Y4. The image cutting unit  24  cuts a rectangular area identified by the coordinates Q1′ (X1′, Y1′) through Q4′ (X4′, Y4′). If the CMS area  8  is moved upward, coordinates Y′ are calculated in accordance with the amount of movement. 
     Accordingly, the electronic rearview mirror  10  can move a CMS area  8  in the vertical direction or in the horizontal direction within image data after rotation. 
     &lt;Operation Procedure&gt; 
       FIG. 8  is a flowchart illustrating an example of a process in which the electronic rearview mirror displays CMS area image data. The process illustrated in  FIG. 8  is repeatedly performed while the electronic rearview mirror is turned on. 
     First, the image obtaining unit  21  obtains image data from the imaging device  11  (S 10 ). Image data is sequentially transmitted from the imaging device  11 . 
     The operation receiving unit  22  receives rotation of the image data in response to the driver&#39;s operation of the operation buttons  1  through  4  (S 20 ). Although the image data in real time switches to next image data, the amount of rotation received by the operation receiving unit  22  may be commonly applied to all images. 
     The image rotating and moving unit  23  rotates the image data in accordance with the amount of rotation received by the operation receiving unit (S 30 ). 
     Further, the operation receiving unit  22  receives movement of a CMS area  8  in response to the driver&#39;s operation of the operation buttons  1  through  4  (S 40 ). 
     The image rotating and moving unit  23  moves the CMS area  8  in the vertical direction or in the horizontal direction within the rotated image data (S 50 ). 
     The image cutting unit  24  cuts, from the image data, the CMS area  8  located at a position determined by the amount of rotation and the amount of movement received by the operation receiving unit (S 60 ). 
     The image outputting unit  25  displays CMS area image data on the display  15  (S 70 ). 
     SUMMARY 
     As described above, the electronic rearview mirror  10  according to the present embodiment moves the CMS area  8  in the vertical direction or in the horizontal direction within the rotated image data. Thus, it becomes possible to move the CMS area  8  up, down, left, or right only as intended by the driver, without obliquely moving the CMS area  8 . 
     Second Embodiment 
     In the first embodiment, an example in which a CMS area  8  is moved vertically or horizontally after the rotation of image data has been described. In a second embodiment, an example in which image data is rotated after the vertical or horizontal movement of a CMS area  8  will be described. 
     &lt;Problem with Rotating Image Data after CMS Area is Vertically or Horizontally Moved&gt; 
       FIGS. 9A through 9F  are drawings illustrating a problem with rotating image data after a CMS area is vertically or horizontally moved. As illustrated in  FIG. 9A  and  FIG. 9B , because the imaging device  11  installed on the rear of the vehicle is rotated clockwise due to installation error, CMS area image data displayed by an electronic rearview mirror  10  is tilted. 
     Before rotating the image data, the CMS area  8  has moved vertically or horizontally by a driver.  FIG. 9C  illustrates an example of image data in which the CMS area  8  has moved to the left. In this state, the CMS area  8  displayed on the display  15  of the electronic rearview mirror  10  is cut out. As illustrated in  FIG. 9D , CMS area image data displayed by the electronic rearview mirror  10  remains tilted. 
     Next, the driver rotates the image data. Namely, the image data is rotated clockwise.  FIG. 9E  illustrates the image data after rotation. By rotating the image data, the tilt of the image data due to the installation error of the imaging device  11  is corrected, thus allowing the object to become horizontal. 
     However, the CMS area  8  is not rotated by the image rotating and moving unit  23 . Thus, only the image data is rotated while the CMS area  8  remains fixed.  FIG. 9F  illustrates the CMS area image data cut from the image data of  FIG. 9E . As compared to  FIG. 9D , the CMS area data has moved downward. Thus, it appears to the driver that the CMS area image data has moved downward. 
     As described above, if the driver only desires to correct the tilt of the image data by rotation without moving the CMS area  8  in the vertical direction, operability would not be provided as desired by the driver. 
     The problem described with reference to  FIGS. 9A through 9F  may arise when the center of the image data does not coincide with the center of the CMS area  8 . In  FIGS. 9A through 9F , because the CMS area  8  is located at the left end of the image data, the CMS area image data appears to be moved downward after the rotation of the image data. If the CMS area  8  is located at the right end of the image data, the CMS area image data would appear to be moved upward after the rotation of the image data. Similarly, if the CMS area  8  is located at the upper side of the image data, the CMS area image data would appear to be moved to the left after the rotation of the image data, and if the CMS area  8  is located at the lower side of the image data, the CMS area image data would appear to be moved to the right after the rotation of the image data (the image data is assumed to be rotated clockwise in all the cases). 
     &lt;Overview of Electronic Rearview Mirror According to Second Embodiment&gt; 
     In a case where a CMS area of an image captured by the imaging device  11  having installation error is moved first in the vertical direction or in the horizontal direction, and the image is thereafter rotated for correction by a driver, an electronic rearview mirror according to the second embodiment performs a “process for moving the CMS area in the vertical direction or in the horizontal direction in accordance with the amount of rotation”. 
       FIGS. 10A through 10G  are drawings illustrating positional adjustment of a CMS area  8  by the electronic rearview mirror according to the present embodiment.  FIGS. 10A through 10E  are the same as  FIGS. 9A through 9E . 
     As a solution to the above-described problem in that it appears to the driver that CMS area image data has moved downward, the image rotating and moving unit  23  of the present embodiment moves a CMS area  8  upward.  FIG. 10F  illustrates image data in which a CMS area  8  is moved upward.  FIG. 10G  illustrates CMS area image data cut from the image data of  FIG. 10F . By moving the CMS area  8  upward after the rotation of the image data, the CMS area image data is not moved upward as is seen from  FIG. 10G . 
     Accordingly, the electronic rearview mirror of the present embodiment moves a CMS area  8  up, down, left, or right in accordance with the rotation of image data. Thus, image data can be rotated only as intended by the driver. 
     In the present embodiment, the functional block diagram of  FIG. 6  described in the first embodiment can be incorporated. 
     &lt;Process for Moving CMS Area and Rotating Image&gt; 
     A process for moving a CMS area  8  and rotating an image will be described with reference to  FIGS. 11A through 11D .  FIGS. 11A through 11D  are drawings illustrating an example of a coordinate system of image data and coordinates of a CMS area  8 . 
     As illustrated in  FIG. 11A , in the illustrated coordinate system, it is assumed that a driver has moved a CMS area  8  to the left within image data  201  before rotation. O denotes the center of the image data  201 , and Q 0  denotes the center of the CMS area  8 . 
       FIG. 11B  illustrates the CMS area  8  after the image data  201  is rotated by the driver, and illustrates a virtual CMS area  81  rotated together with the image data  201 . In reality, the CMS area  8  is not rotated together with the image data  201 , and the virtual CMS area  81  is illustrated for convenience of description. Coordinates of the center Q 0  of the CMS area  8  are assumed to be (X 0 , Y 0 ), and coordinates of the center Q 0 ′ of the virtual CMS area  81  are assumed to be (X 0 ′, Y 0 ′). 
     By moving the center Q 0  of the CMS area  8  to the center Q 0 ′, CMS area image data can be properly displayed even after the rotation of image data. The image rotating and moving unit  23  moves the CMS area  8  based on differences between Q 0  and Q 0 ′. 
       FIG. 11C  illustrates an example of differences between Q 0  and Q 0 ′. The coordinates (X 0 , Y 0 ) of Q 0  are values input by the driver by using the operation buttons  1  through  4 . The coordinates (X 0 ′, Y 0 ′) of Q 0 ′ can be calculated by the formula (1) of the first embodiment. Accordingly, differences (ΔX, ΔY) can be calculated as follows.
 
Δ X=X   0   ′−X   0  
 
Δ Y=Y   0   ′−Y   0  
 
       FIG. 11D  illustrates an example of the CMS area  8  whose position is corrected in the rotational direction of the image data based on the differences (ΔX, ΔY). The image rotating and moving unit  23  calculates coordinates Q1′ through Q4′ of the CMS area  8  that have moved in a direction conforming to the rotational direction of the image data, by adding the differences (ΔX, ΔY) to the coordinates Q1 through Q4 of  FIG. 11B . 
     &lt;Operational Procedure&gt; 
       FIG. 12  is a flowchart illustrating an example of a process in which the electronic rearview mirror  10  displays CMS area image data. The process illustrated in  FIG. 12  is repeatedly performed while the electronic rearview mirror is turned on. 
     First, the image obtaining unit  21  obtains image data from the imaging device  11  (S 110 ). Image data is sequentially transmitted from the imaging device  11 . 
     The operation receiving unit  22  receives movement of a CMS area  8 , in response to the driver&#39;s operation of the operation buttons  1  through  4  (S 120 ). 
     The image rotating and moving unit  23  moves the CMS area  8  in accordance with the amount of movement received by the operation receiving unit (S 130 ). 
     Next, the operation receiving unit  22  receives rotation of the image data in response to the driver&#39;s operation of the operation buttons  1  through  4  (S 140 ). Although the image data in real time switches to next image data, the amount of rotation received by the operation receiving unit  22  may be commonly applied to all images. 
     The image rotating and moving unit  23  calculates coordinates of a virtual CMS area  81  in accordance with the amount of rotation (S 150 ). Because the width w and the height h of the CMS area  8  are known, the image rotating and moving unit  23  may calculate coordinates of the center of the virtual CMS area  81  or coordinates of one of the four corners of the virtual CMS area  81 . 
     Next, the image rotating and moving unit  23  calculates differences between coordinates of the virtual CMS area  81  and coordinates of the CMS area  8  of step S 120 , and corrects the position of the CMS area  8  (S 160 ). 
     The image cutting unit  24  cuts, from the image data, the CMS area  8  located at the position corrected by the image rotating and moving unit  23  (S 170 ). 
     The image outputting unit  25  displays CMS area image data on the display  15  (S 180 ). 
     SUMMARY 
     As described above, the electronic rearview mirror according to the present embodiment can avoid the movement of a CMS area  8  in the direction not intended by a driver, even when image data is rotated for correction, after the CMS area  8  is moved in the vertical direction or in the horizontal direction. 
     Third Embodiment 
     In a third embodiment, an electronic rearview mirror  10  that can be employed in both the image processing of the first embodiment and the image processing of the second embodiment will be described. 
     A process may be separated in accordance with whether a CMS area  8  is moved after the rotation of image data (first embodiment) or image data is rotated in a state where the center of the image data does not coincide with the center of the CMS area  8  (second embodiment). 
       FIG. 13  is a flowchart illustrating an example of a process for selecting a method for displaying CMS area image data. 
     First, the operation receiving unit  22  determines whether a rotation operation of image data is received (S 210 ). If it is determined that the rotation operation is not received (no in step S 210 ), the process proceeds to step S 250 . 
     If it is determined that the rotation operation is received (yes in step S 210 ), the image rotating and moving unit  23  rotates the image data (S 220 ). Namely, as in the first embodiment and the second embodiment, the rotation of the image data can be applied. 
     Next, the image rotating and moving unit  23  determines whether the center of the image data coincides with the center of a CMS area  8  (S 230 ). If it is determined that the center of the image data coincides with the center of the CMS area  8  (yes in step S 230 ), the process proceeds to step S 250 . 
     If it is determined that the center of the image data does not coincide with the center of the CMS area  8  (no in step S 230 ), the image rotating and moving unit  23  moves the CMS area  8  by using the method of the second embodiment (S 240 ). Specifically, because CMS area image data appears to have moved in the vertical direction or in the horizontal direction due to the rotation of the image data, the image rotating and moving unit  23  corrects the position of the CMS area  8 . 
     In step S 250 , the operation receiving unit determines whether a movement operation of the CMS area  8  is received (S 250 ). If it is determined that a movement operation of the CMS area  8  is not received (no in step S 250 ), it means that the operation is not performed by the driver. Thus, the process of  FIG. 13  ends. 
     If it is determined that the movement operation of the CMS area  8  is received (yes in step S 250 ), the image rotating and moving unit  23  determines whether the image data has already been rotated (S 260 ). The reason is because if the CMS area  8  were to be moved after the rotation of the image data without correction, it would appear to the driver that CMS area image data has obliquely moved. If it is determined that the image data has not been rotated (no in step S 260 ), the process of  FIG. 13  ends. 
     If it is determined that the image data has already been rotated (yes in step S 260 ), the image rotating and moving unit  23  moves the CMS area  8  by using the method of the first embodiment (S 270 ). 
     SUMMARY 
     Accordingly, the electronic rearview mirror according to the present embodiment integrates the imaging processing according to the first embodiment with the imaging processing according to the second embodiment, and readily adjusts the position of a CMS area  8  to an intended position regardless of whether the driver rotates image data first or moves the CMS area  8  first. 
     Other Application Examples 
     Although the embodiments have been specifically described above, the present invention is not limited to these embodiments. Various variations and modifications may be made to the described subject matter without departing from the scope of the present invention. 
     For example, in the above-described embodiments, the driver corrects image data captured by the imaging device  11  having installation error by rotating the image data. However, installation error of the imaging device  11  may be detected by the imaging device  11  or the display unit. For example, when the imaging device  11  includes an acceleration sensor, the acceleration sensor may detect the orientation (installation error) of the imaging device  11 , and the image rotating and moving unit  23  may automatically cancel the amount of rotation calculated based on the orientation (installation error) of the imaging device  11 . Alternatively, imaging processing may be used to detect the amount of rotation of an image captured by the imaging device  11 . For example, machine learning may learn the relationship between images and the orientation of the imaging device  11  beforehand, and may detect the orientation of the imaging device  11  mounted on a vehicle. The image rotating and moving unit  23  may rotate image data so as to cancel the rotation of the image data based on the detected orientation. 
     Further, in the above-described embodiments, an example in which a rearview mirror including a CMS has been described; however, a display device including the CMS is not limited to the rearview mirror. For example, if a side-view mirror includes the CMS, the CMS can be preferably used to correct the tilt and movement of image data at the rear lateral side of a vehicle. Further, the CMS can be preferably used to correct the tilt and movement of image data displayed on a display of a navigation system or on a back-seat display. Further, the CMS can be preferably used to correct the tilt and movement of image data displayed on a display disposed within a vehicle. 
     The example configuration of  FIG. 6  illustrates main functions in order to facilitate understanding of the processes performed by the electronic rearview mirror  10 . The present invention is not limited by the configuration of the divided processing units or the names of the processing units. A process performed by the electronic rearview mirror  10  may be divided into more processing units in accordance with details of the process. Further, the process performed by the electronic rearview mirror  10  may be divided such that one processing unit include more steps. 
     Further, the operation receiving unit  22  is an example of a receiving unit, the image rotating and moving unit  23  is an example of an image processing unit, the image cutting unit  24  is an example of an image cutting unit, and the image outputting unit  25  is an example of an outputting unit.