Abstract:
A multi-camera vision system for a vehicle includes first, second and third image capture devices having respective first, second and third fields of view exterior of the vehicle. The first field of view overlaps with the third field of view defining a first overlap zone, and the second field of view overlaps with the third field of view defining a second overlap zone. Responsive to processing of captured image data, a synthesized image is generated without duplication of objects present in the first and second overlap zones and the synthesized image approximates a view as would be seen by a virtual camera at a single location exterior of the vehicle. The synthesized image is displayed by a single display screen that is viewable by a driver of the vehicle and the synthesized image assists the driver in executing a maneuver of the vehicle with a trailer attached thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/800,691, filed Mar. 13, 2013 (Attorney Docket MAG04 P-2059), which is a continuation of U.S. patent application Ser. No. 12/688,146, filed Jan. 15, 2010 (Attorney Docket MAG04 P-1585), which is a continuation of U.S. patent application Ser. No. 12/496,357, filed Jul. 1, 2009 (Attorney Docket MAG04 P-1536), which is a continuation of U.S. patent application Ser. No. 11/122,880, filed May 5, 2005, now U.S. Pat. No. 7,561,181, which is a continuation of U.S. patent application Ser. No. 10/324,679, filed on Dec. 20, 2002, now U.S. Pat. No. 6,891,563, which is a continuation of U.S. patent application Ser. No. 08/952,026, filed under 35 U.S.C. §371, filed Nov. 19, 1997, now U.S. Pat. No. 6,498,620, which is based on International Patent Application No. PCT/US96/07382, filed May 22, 1996. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to vision systems for vehicles and, more particularly, to rearview vision systems which provide the vehicle operator with scenic information in the direction rearward of the vehicle. More particularly, the invention relates to a rearview vision system utilizing image capture devices, such as CMOS imaging arrays and the like. 
         [0003]    A long-felt need in the art of vehicle rearview vision systems is to reduce the amount of time spent gathering information of the condition around the vehicle in order to safely carry out a vehicle maneuver, such as a turn or a lane change. It has been determined that up to about  50  percent of maneuver time is spent gathering information with conventional rearview mirrors. This typically requires viewing one or more mirrors, turning the head and body to check blind spots, and double-checking the mirrors prior to executing the maneuver. Some improvement has been obtained by adjusting mirror optics, for example, to increase the observed field of view rearward of the vehicle. However, this is achieved with an increase in image distortion which makes driver awareness of conditions to the side and rear of the vehicle even more difficult. 
         [0004]    Another long-felt need in the art of vehicle rearview vision systems has been to eliminate exterior rearview mirrors by utilizing image capture devices, such as cameras, in combination with dashboard displays. This would be beneficial because it would reduce wind drag on the vehicle, wind noise and vehicle weight. Furthermore, rearview mirrors protrude a substantial distance from the side of the vehicle, which makes maneuvering in tight spaces more difficult. Image capture devices are capable of positioning in a greater variety of locations on the vehicle, providing more flexibility of vehicle styling. It is further expected that camera systems would greatly reduce the blind spots to the sides and rear of the vehicle common with vehicles equipped with conventional rearview mirror systems. The driver cannot perceive vehicles, objects, or other road users in such blind spots without turning his or her body, which interferes with forward-looking visual activities. 
         [0005]    Camera-based rearview vision systems for vehicles have not obtained commercial acceptance. One difficulty with proposed systems has been that they present a large amount of visual information in a manner which is difficult to comprehend. This difficulty arises from many factors. In order to significantly reduce blind spots, multiple image capture devices are typically positioned at various locations on the vehicle. The image of an object behind the equipped vehicle is usually captured by more than one image capture device at a time and displayed in multiple images. This may confuse the driver as to whether more than one object is present. When multiple image capture devices are positioned at different longitudinal locations on the vehicle, objects behind the vehicle are at different distances from the image capture devices. This results in different image sizes for the same object. This effect is especially noticeable for laterally extending images, such as a bridge, highway crosswalk markings, the earth&#39;s horizon, and the like. Such images are at different vertical angles with respect to the image capture devices. This results in different vertical positions on the display causing the elongated image to appear disjointed. 
         [0006]    A camera system provides a monocular view of the scene, compared to the binocular stereoscopic view obtained when the scene is viewed through a rearview mirror. This makes the ability to judge distances in a camera system a problem. This effect is most noticeable at distances close to the vehicle where stereoscopic imaging is relied upon extensively by the driver in judging relative locations of objects. Therefore, known camera systems fail to provide to the driver important information where that information is most needed at small separation distances from surrounding objects. 
         [0007]    Another difficulty with camera systems is that, in order to provide a sufficient amount of information, the camera system typically presents the driver with a greatly increased field of view. This improves performance by further reducing blind spots at the side and rear of the vehicle. However, an increased field of view is often obtained by utilizing a wide-angle lens which introduces distortion of the scene and further impairs the ability of the driver to judge distances of objects displayed. The problem with such distortion of the scene is that the driver must concentrate more on the display and take a longer time to interpret and extract the necessary information. This further distracts the driver from the primary visual task of maintaining awareness of vehicles and other objects in the vicinity of the driven vehicle. 
         [0008]    Yet an additional difficulty with camera systems is that flat panel displays present the image captured by the rearward-facing image capture device, or devices, at a focal length that approximates the arm length of the vehicle driver. In order to observe the condition of the vehicle utilizing the rearview vision system, the driver must change his or her gaze from the forward field of view of the vehicle to the display. Because the forward field of view of the vehicle is at a focal length that is much greater than the focal length of the displayed image, the eyes of the driver must refocus upon changing gaze. This refocusing further increases the amount of time for the driver to assimilate the information in the displayed image. Furthermore, when the gaze of the driver returns to the forward field of view, the eyes must, again, refocus to the greatly longer distance. 
         [0009]    Yet an additional difficulty with camera systems is that of finding adequate space in the crowded area of the vehicle&#39;s dashboard for the components making up the display. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed towards enhancing the interpretation of visual information in a rearview vision system by presenting information in a manner which does not require significant concentration of the driver or present distractions to the driver. This is accomplished according to an aspect of the invention in a rearview vision system having at least two image capture devices positioned on the vehicle and directed generally rearwardly with respect to the direction of travel of the vehicle. A display is provided for images captured by the image capture devices. The display displays an image synthesized from outputs of the image capture devices which approximates a rearward-facing view from a single location. In order to obtain all of the necessary information of activity, not only behind but also along side of the vehicle, the virtual camera should be positioned forward of the driver. The image synthesized from the multiple image capture devices may have a dead space which corresponds with the area occupied by the vehicle. This dead space is useable by the driver&#39;s sense of perspective in judging the location of vehicles behind and along side of the equipped vehicle. 
         [0011]    The present invention provides techniques for synthesizing images captured by individual, spatially separated, image capture devices into such ideal image, displayed on the display device. This may be accomplished, according to an aspect of the invention, by providing at least three image capture devices. At least two of the image capture devices are side image capture devices mounted on opposite sides of the vehicle. At least one of the image capture devices is a center image capture device mounted laterally between the side image capture devices. A display system displays an image synthesized from outputs of the image capture devices. The displayed image includes an image portion from each of the image capture devices. The image portion from the center image capture device is vertically compressed. 
         [0012]    It has been discovered that such vertical compression substantially eliminates distortion resulting from the spatial separation between the cameras and can be readily accomplished. In an illustrated embodiment, the image compression is carried out by removing selective ones of the scan lines making up the image portion. A greater number of lines are removed further away from the vertical center of the image. 
         [0013]    The compression of the central image portion produces a dead space in the displayed image which may be made to correspond with the area that would be occupied by the vehicle in the view from the single virtual camera. Preferably, perspective lines are included at lateral edges of the dead space which are aligned with the direction of travel of the vehicle and, therefore, appear in parallel with lane markings. This provides visual clues to the driver&#39;s sense of perspective in order to assist in judging distances of objects around the vehicle. 
         [0014]    According to another aspect of the invention, image enhancement means are provided for enhancing the displayed image. Such means may be in the form of graphic overlays superimposed on the displayed image. Such graphic overlap may include indicia of the anticipated path of travel of the vehicle which is useful in assisting the driver in guiding the vehicle in reverse directions. Such graphic overlay may include a distance grid indicating distances behind the vehicle of objects juxtaposed with the grid. 
         [0015]    According to yet an additional aspect of the invention, a rearview vision system for a vehicle includes at least one image capture device positioned on the vehicle and directed generally rearwardly with respect to the direction of travel of the vehicle. A display system is provided which displays a rear image synthesized from an output of the image capture device. The rear image is substantially contiguous with the forward field of view of the vehicle driver and at a focal length that is forward of the vehicle passenger compartment and preferably within the depth of field of a vehicle driver viewing a distant object. Because the image has a focal length that more closely matches that of the forward field of view observed by the driver, the need for the driver&#39;s eyes to refocus from the forward field of view to a much shorter focus distance each time the gaze of the driver is directed at the display system is minimized. This reduces the amount of time required for the driver to gaze at the displayed image and interpret objects displayed in the image. Furthermore, the reduction in the repeated refocusing of the driver&#39;s eyes reduces driver fatigue. If there are any near field objects in the periphery of the driver&#39;s forward field of view, such as windshield wipers, windshield frame, dashboard, and the like, the display system is preferably positioned in a manner which blocks the view of such near field objects. In this manner, the driver&#39;s gaze may shift between the forward field of view and the long focal length display system without being refocused on the near field objects. This is based upon a recognition that the driver&#39;s eyes will tend to refocus on the near field object momentarily even though the gaze is being redirected between the forward field of view and the display system. 
         [0016]    According to yet an additional aspect of the invention, a rearview vision system for a vehicle includes at least one image capture device positioned on the vehicle and directed generally rearwardly with respect to the direction of travel of the vehicle. A display system is provided for displaying a rear image captured by the image capture device. The displayed image is a unitary image having an aspect ratio that is between approximately  4 : 1  and approximately  2 : 1 . In a most preferred embodiment, the image has an aspect ratio that is approximately  8 : 3 . The aspect ratio, according to this aspect of the invention, is especially useful where the unitary image is synthesized from a plurality of images which are captured by a plurality of image captured devices and are tiled by the display device. 
         [0017]    According to yet an additional aspect of the invention, a rearview vision system for a vehicle includes a plurality of image capture devices positioned on the vehicle and directed generally rearwardly with respect to the direction of travel of the vehicle. A display system which includes at least one image generator and an optical correction system is provided which displays an image synthesized from outputs of the image capture devices as a unitary image. Alternatively, the display system may include a plurality of image generators, each associated with one or more of the image capture devices and an optical correction system which amplifies images generated by the image generators and merges them into a unitary image. The optical correction system additionally increases the focal length, or lengths, of the image, or images, generated by the image generator, or generators. The display system may be an opaque projection display which is positioned approximately at the driver&#39;s arm length in front of the driver. Alternatively, the display system may be a view-through heads-up display which projects the unitary image onto a combiner in order to combine the unitary image with the forward field of view of the driver. 
         [0018]    These and other objects, advantages, and features of this invention will become apparent by review of the following specification in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a top plan view of a vehicle having a rearview vision system according to the invention; 
           [0020]      FIG. 2  is a side elevation of the vehicle in  FIG. 1 ; 
           [0021]      FIG. 3  is a front elevation of a display according to the invention; 
           [0022]      FIG. 4  is the same view as  FIG. 1  illustrating an alternative embodiment of the invention; 
           [0023]      FIG. 5  is a block diagram of an electronic system according to the invention; 
           [0024]      FIG. 6  is the same view as  FIG. 3  illustrating an alternate mode of operation of the system; 
           [0025]      FIG. 7  is the same view as  FIG. 2  illustrating an alternative embodiment of the invention; 
           [0026]      FIG. 8  is the same view as  FIG. 3  illustrating an alternative embodiment of the invention; 
           [0027]      FIG. 9  is the same view as  FIGS. 1 and 4  illustrating an alternative embodiment of the invention; 
           [0028]      FIG. 10  is the same view as  FIGS. 3 and 8  illustrating an alternative embodiment of the invention; 
           [0029]      FIG. 11  is a chart illustrating the horizontal row of pixels (n 1 , n 2 ) on which an object will be imaged from two longitudinally separated image capture devices as that object is spaced at different longitudinal distances from the image capture devices; 
           [0030]      FIG. 12  is a forward elevation of a vehicle passenger compartment as viewed by a vehicle driver; 
           [0031]      FIG. 13  is a sectional view taken along the lines XIII-XIII in  FIG. 12 ; 
           [0032]      FIG. 14  is a sectional view taken along the lines XIV-XIV in  FIG. 12 ; 
           [0033]      FIG. 15  is the same view as  FIG. 14  of an alternative embodiment; 
           [0034]      FIG. 16  is the same view as  FIG. 14  of another alternative embodiment; 
           [0035]      FIG. 17  is an enlarged view of the display system in  FIG. 14  illustrating details thereof; 
           [0036]      FIG. 18  is a block diagram similar to  FIG. 5  of an alternative embodiment of the invention; 
           [0037]      FIG. 19  is a side elevation similar to  FIG. 2  of an alternative embodiment of the invention; 
           [0038]      FIG. 20  is an enlarged side elevation of an image capture device with portions of the housing removed in order to reveal internal structure thereof; 
           [0039]      FIG. 21  is a block diagram similar to  FIG. 5  of another alternative embodiment of the invention; 
           [0040]      FIG. 22  is a block diagram similar to  FIG. 5  of another alternative embodiment of the invention; 
           [0041]      FIG. 23  is a block diagram similar to  FIG. 5  of another alternative embodiment of the invention; 
           [0042]      FIG. 24  is a block diagram similar to  FIG. 5  of another alternative embodiment of the invention; and 
           [0043]      FIG. 25  is a block diagram of a rearview vision system having extended dynamic range capabilities. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0044]    Referring now specifically to the drawings, and the illustrative embodiments depicted therein, a vehicle  10 , which may be an automobile, a light truck, a sport utility vehicle, a van, a bus, a large truck, or the like includes a rearview vision system, generally illustrated at  12 , for providing a driver of the vehicle with a view rearwardly of the vehicle with respect to the direction of travel T of the vehicle ( FIG. 1 ). Vision system  12  includes at least two side image capture devices  14  positioned, respectively, on opposite sides of vehicle  10  and a center image capture device  16  positioned on the lateral centerline of the vehicle. All of the image capture devices are directed generally rearwardly of the vehicle. Rearview vision system  12  additionally includes an image processor  18  for receiving data signals from image capture devices  14 ,  16  and synthesizing, from the data signals, a composite image  42  which is displayed on a display  20 . 
         [0045]    As will be set forth in more detail below, the images captured by image capture devices  14 ,  16  are juxtaposed on display  20  by image processor  18  in a manner which approximates the view from a single virtual image capture device positioned forwardly of the vehicle at a location C and facing rearwardly of the vehicle, with the vehicle being transparent to the view of the virtual image capture device. Vision system  12  provides a substantially seamless panoramic view rearwardly of the vehicle without duplicate or redundant images of objects. Furthermore, elongated, laterally-extending, objects, such as the earth&#39;s horizon, appear uniform and straight across the entire displayed image. The displayed image provides a sense of perspective, which enhances the ability of the driver to judge location and speed of adjacent trailing vehicles. 
         [0046]    Each of side image capture devices  14  has a field of view  22  and is aimed rearwardly with respect to the vehicle about an axis  24  which is at an angle, with respect to the vehicle, that is half of the horizontal field of view of the image capture device. In this manner, each of the image capture devices  14  covers an area bounded by the side of the vehicle and extending outwardly at an angle defined by the horizontal field of view of the respective side image capture device. Center image capture device  16  has a horizontal field of view  26 , which is symmetrical about the longitudinal axis of the vehicle. The field of view of each side image capture device  14  intersect the field of view of center image capture device  16  at a point P which is located a distance Q behind vehicle  10 . 
         [0047]    Rear blind zones  30  are located symmetrically behind vehicle  10  extending from the rear of the vehicle to point P. Side blind zones  25  located laterally on respective sides of the vehicle extend rearwardly of the forward field of view  36  of the driver to the field of view  22  of the respective side image capture device  14 . An object will not be captured by side image capture devices  14  or center image capture device  16  if the object is entirely within one of the blind zones  25 ,  30 . In order for an object, such as another vehicle V or other road user travelling to the side of vehicle  10 , to be observed by an operator of vehicle  10 , the object must be either at least partially within the forward field of view  36  of the driver or be captured by image capture devices  14 ,  16  and displayed on display  20 .  FIG. 4  illustrates vehicle  10  travelling on a three-lane highway having lanes L 1 , L 2 , and L 3  with the vehicle in lane L 2 . Another vehicle V is shown positioned mostly within one of the blind zones  25 , but with the rearmost portion of the vehicle V extending into field of view  22  where the vehicle image will be captured by one of side image capture devices  14 . In the illustrated embodiment, vehicle V is a motorcycle travelling in the center of lanes L 1  or L 3  and represents a worst case for observing a vehicle travelling at least partially within one of the blind zones  25 . In order for a portion of vehicle V to be extending either forwardly or rearwardly of the respective blind zone  25 , where the vehicle V may be observed by either the forward field of view  36  of the driver or by the rearview vision system  12 , the field of view  22  of side image capture devices  14  must be sufficiently wide to capture a portion of vehicle V as illustrated in  FIG. 4 . Preferably, the horizontal field of view  22  of side image capture devices  14  is no greater than that required to provide sufficient coverage which would be in the range of between approximately  55  degrees and approximately  70  degrees. In the illustrated embodiment, the horizontal field of view  22  is  61  degrees. In order for a portion of vehicle V to be within a vertical field of view  40  of one of side image capture devices  14 , the field of view should extend to the pavement at a plane M which intersects vehicle V ( FIG. 2 ). Preferably, vertical field of view  40  is between approximately  60  degrees and approximately  75  degrees. In the illustrated embodiment, vertical field of view  40  is  66  degrees. 
         [0048]    A left overlap zone  32  and a right overlap zone  34  extend rearward from respective points P where the horizontal fields of view of the side image capture devices intersect the field of view of center image capture device  16 . Overlap zones  32 ,  34  define areas within which an object will be captured both by center image capture device  16  and one of the side image capture devices  14 . An object in an overlap zone  32 ,  34  will appear on display  20  in multiple image portions in a redundant or duplicative fashion. In order to avoid the presentation of redundant information to the driver, and thereby avoid confusion and simplify the task of extracting information from the multiple images or combined images on display  20 , the object should avoid overlapping zones  32 ,  34 . In practice, this may be accomplished to a satisfactory extent by moving points P away from the vehicle and thereby increasing distance Q. It is desirable to increase distance Q to a length that will exclude vehicles travelling at a typical separation distance behind vehicle  10  from overlapping zones  32 ,  34 . This separation distance is usually a function of the speed at which the vehicles on the highway are travelling. Therefore, optionally, distance Q may be made variable, not fixed. In such embodiment, the faster the vehicles are travelling, the further Q should be moved behind vehicle  10  to keep overlap zones  32  and  34  outside of the recommended vehicle spacing. If, however, the vehicles are travelling at a slower speed, then the generally accepted recommendation for vehicle spacing decreases and it is more likely that a vehicle will be within overlap zone  32 ,  34 . Therefore, the distance Q may be selected to accommodate expected vehicle spacing for an average driving speed of vehicle  10 . 
         [0049]    Distance Q is a function of the effective horizontal field of view  26  of center image capture device  16 . As field of view  26  decreases, points P move further rearward of the vehicle from a distance Q 1 , to a distance Q 2 , as best illustrated in  FIG. 4 . In order to increase distance Q to eliminate redundant and duplicative information displayed on display  20  for most driving conditions of vehicle  10 , field of view  26  is preferably less than  12  degrees. In the illustrated embodiment, field of view  26  is between 6 and 8 degrees. Alternatively, distance Q may be dynamically adjusted according to some parameter, such as the speed of vehicle  10 . This would allow Q to be greater when the vehicle is travelling at a faster speed, where vehicle separation tends to be larger, and vice versa. Field of view  26  may be adjusted by utilizing a selective presentation of pixels of the captured image in the displayed image. 
         [0050]    Referring to  FIG. 3 , image display device  20  displays a composite image  42  made up of a left image portion  44 , a right image portion  46 , and a center image portion  48 . Each image portion  44 - 48  is reversed from the image as captured by the respective image capture device  14 ,  16  utilizing conventional techniques. These techniques include reading the image in reverse with the image capture device, writing the image in reverse to display device  20 , or reversing the image in image processor  18 . Left image portion  44  is joined with central image portion  48  at a boundary  50 . Central image portion  48  is joined with right image portion  46  at a boundary  52 . As may best be seen in  FIG. 3 , the image portions at boundaries  50  and  52  are continuous whereby composite image  42  is a seamless panoramic view rearwardly of the vehicle. As also is apparent from  FIG. 3 , central image portion  48  is narrower than either left image portion  44  or right image portion  46 . This is a result of reducing the horizontal field of view  26  of center image capture device  16  sufficiently to move points P, and thus overlap zones  32  and  34 , a sufficient distance behind vehicle  10  to reduce redundant and duplicative images between image portions  44 - 48 . Composite image  42  provides a clear image, which avoids confusion and simplifies the task of extracting information from the multiple image portions  44 - 48 . As also may be seen by reference to  FIG. 3 , display  20  may additionally include indicia such as the readout of a compass  54 , vehicle speed  56 , turn signals  58 , and the like as well as other graphical or video displays, such as a navigation display, a map display, and a forward-facing vision system. In this manner, rearview vision system  12  may be a compass vision system or an information vision system. 
         [0051]    In the embodiment of rearview vision system  12  having a dynamically adjusted value of distance Q, the spacing between boundaries  50  and  52  will dynamically adjust in sequence with the adjustment of distance Q. Thus, as overlap zones  32 ,  34  move further away from the vehicle; for example, in response to an increase in speed of the vehicle, boundary lines  50  and  52  will move closer together and vice versa. In this manner, composite image  42  is dynamic, having image portions of dynamically adaptive sizes. 
         [0052]    Display  20  is of a size to be as natural as possible to the driver. This is a function of the size of the displayed image and the distance between the display and the driver. Preferably, the displayed image simulates an image reflected by a rearview mirror. As such, the size of the displayed image is approximately the combined areas of the three rearview mirrors (one interior mirror and two exterior mirrors) conventionally used with vehicles. As best seen by reference to  FIGS. 2 and 12 , display  20  is preferably positioned within the driver&#39;s physiological field of view forward of the vehicle, generally illustrated at  70 , through the windshield  72  without significantly obstructing the forward field of view. It is known that the driver&#39;s field of view, with the head and eyes fixed forward, extends further in a downward direction than in an upward direction. Display  20  could be located above the field of view  70  wherein the display may be observed at the upward portion of the driver&#39;s field of view. However, the position for the display illustrated in  FIGS. 2 and 12  is preferred wherein the display is within the lower portion of the driver&#39;s field of view. 
         [0053]    Display  20  may be a direct view flat panel display, such as a back-lit or reflective liquid crystal display, a plasma display, a field emission display, a cathode ray tube electroluminescent, light-emitting diode or deformable mirror display. The display may be mounted/attached to the dashboard, facia or header, or to the windshield at a position conventionally occupied by an interior rearview mirror. However, the synthesized image could be displayed using other display techniques such as to provide a projected or virtual image. Alternatively, a virtual image may be displayed on an opaque display adjacent the forward field of view. Alternatively, a virtual image may be displayed on a view-through heads-up display in which the image is superimposed on the forward field of view. 
         [0054]    In the embodiment illustrated in  FIGS. 12-17 , display  20  displays an image at a focal length that is forward of the passenger compartment of vehicle  10 . Preferably, the image displayed by display  20  is at a focal length that is within the driver&#39;s normal depth of field when viewing a distant object. Display  20  includes an image generator  74 , which produces an image captured by one or more image capture devices  14 ,  16 , and an optical correction system  76 , which increases the focal distance of the image generated by image generator  74 . In the illustrated embodiment, optic correction system  76  increases the focal distance by collimating the rays, generally indicated at X, from diverging rays to generally parallel rays projected from the display. Optical correction system  76  additionally magnifies the image. In the illustrated embodiment, the magnification is a factor of two. In this manner, optical corrective system  76  has the advantage of extending the focal distance of the image generated by image generator  74  and enlarging the image by the factor of magnification thereof. This advantageously allows each image generator  74  to project all or a portion of an image captured by one of image capture devices  14 ,  16 , or a combination of portions of images from one or more image capture devices, by tiling the images or image portions. This is accomplished because the images projected from optical correction system  76  may abut even though the corresponding image generators  74  do not. This provides a convenient technique for joining the images synthesized from the image capture devices into a unitary image which represents a panoramic view rearward of the vehicle. 
         [0055]    In the embodiment illustrated in  FIG. 14 , display  20  is an opaque projection display which projects the image directly toward the driver. In the embodiment illustrated in  FIG. 15 , a display device  120  is a view-through heads-up display in which the rays X are projected generally upwardly by image generator  74  and optical correction system  76  which are generally vertically aligned, or aligned forward of vertical. The rays X are reflected off a first surface of windshield  72  in the direction of the driver. Windshield  72  acts as a combiner which combines the image displayed by display  120  with a portion of the forward field of view  70  observed by the driver. In the embodiment illustrated in  FIG. 15 , a combiner other than the windshield may be used. Examples may be holographic or diffractive optical film elements or beam splitters of metal or dielectric thin films. Furthermore, image processor  18  may generate a line in the shape of a polygon, such as a rectangle, around rear image  42 . This provides a border around the image to differentiate the rear view from the view forward of the vehicle. 
         [0056]    In the embodiment illustrated in  FIG. 16 , display  20 A is oriented at an angle with respect to the forward field of view of driver D. Image rays X are reflected by a mirror  140  toward the driver. Display  20 A is an opaque display, with mirror  140  blocking the driver&#39;s view of near field objects, such as wipers  98  and the like. Display  20 A has the advantage of being capable of location within a forward portion of the dashboard. Additionally, the only portion of the display visible to the driver is mirror  140 . This allows near field portions of the display to be significantly reduced. 
         [0057]    Because display  20  has a relatively long focus distance, display  20  defines an observation cone, generally designated  78 , within which the displayed image can be observed. Therefore, the head of the driver must be properly oriented with respect to observation cone  78  in order to allow the driver to observe the displayed image. However, drivers come in various sizes. Therefore, a driver may be too tall or too short to have his or her head properly positioned within observation cone  78 . In order to provide for various size drivers along with various adjustments in seating positions and the like, an accommodation means, generally illustrated at  80 , may be provided in order to accommodate variations in the relationship between a driver&#39;s head and observation cone  78 . In the illustrated embodiment, accommodation means  80  includes adjustment means  82  for adjusting the position of observation cone  78 . The adjustment means may adjust the position of the observation cone either vertically, horizontally, or both vertically and horizontally. A vertical adjustment means  82  is illustrated in  FIG. 13  in which the adjustment means includes an electric actuator  84  which is joined by linkage  86  with a portion of a housing  88  of display  20 . Actuator  84  is electrically interconnected through a reversing switch  90  with a driver-operated actuator  92  which may be positioned on dashboard  94  or other convenient position accessible to the driver. Housing  88  may be adjustably mounted, such as by a pivot  96 , in order to allow housing  88  to be adjustably repositioned with respect to dashboard  94 . In this manner, by operation of actuator  92 , housing  88  may be pivoted upwardly or downwardly with respect to pivot  96  in order to adjust the direction of observation cone  78 . In this manner, the location of observation cone  78  may be adjusted in order to coincide with the location of the driver&#39;s head. In a similar fashion, the position of observation cone  78  may be adjusted laterally, if desired. If a view-through heads-up display of the type illustrated in  FIG. 15  is utilized, the position of the observation cone may be adjusted vertically and laterally, in a similar manner, by mechanical or optical adjustments of display  20 . 
         [0058]    Accommodation means  80  may include extending the rearward field of view displayed by display  20  laterally outwardly with respect to the bezel  89  beyond that normally observed by a driver. In this manner, a driver&#39;s head located generally centrally within observation cone  78  will observe a view generally rearwardly of the vehicle. As the driver&#39;s head is moved laterally within observation cone  78 , the driver will observe images more laterally to the side of the vehicle as would occur if the driver&#39;s head were to be moved with respect to a conventional optical rearview mirror system. 
         [0059]    Vehicle  10  may include one or more near field view objects adjacent forward field of view  70 . One such object is a windshield wiper  98  of the vehicle. Other such objects may include the top of dashboard  94 , the frame around windshield  72 , the hoodline, and the like. The housing of display  20  in  FIG. 14  and mirror  140  in  FIG. 15  are positioned with respect to forward field of view  70  such that housing  88  or mirror  140  covers any near field of view objects in the portion of the forward field of view adjacent display  20 ,  20 A. In this manner, the gaze of the driver can switch between forward field of view  70  and the image displayed on display  20 , without the eyes of the driver focusing on any significant near field objects. This is based upon a discovery that, even though the eyes of the driver are switching between the long focal distance of the forward field of view and the long focal distance of the image displayed by display  20 , the eyes of the operator will unconsciously momentarily focus on any near field object positioned between the long focal distance views. Therefore, by blocking the driver&#39;s gaze of any near field objects, the eyes of the driver will be less stimulated to refocus during the transition from field of view  70  to display  20  and back again. 
         [0060]    Image processor  18 , which supplies a video signal  100  to image generator  74 , may have a second input  102  which modulates the intensity level of the image generated by image generator  74  and displayed by display  20  ( FIG. 14 ). The illumination level of the display is set in response to an ambient light input  104  which is an indication of the ambient light level around vehicle  10 . Image processor  18  responds to the value of ambient light input  104  by producing a luminance intensity signal  102  which increases the intensity of the display in response to increases in ambient light level and decreases the intensity of the display in response to decreases in ambient light level. However, the level of display luminance may be limited to vary between upper and lower limits such that, once ambient light reaches a particular upper level, further increases in ambient light level will not result in a further increase in display intensity. Likewise, once the ambient light level decreases below a particular value, further reductions in ambient light level will not result in further reduction in display intensity. Ambient light input  104  may be produced by a separate ambient light sensor of the type which produces a continuously variable output in response to variations in ambient light levels, in which case, the intensity of display  20  may be proportionately adjusted. Alternatively, ambient light input  104  may be produced by a vehicle headlight control system (not shown) which switches the vehicle headlights on, or to a nighttime condition, in response to decreases in ambient light levels and switches the vehicle headlights off, or to a daytime running light condition, in response to increasing ambient light levels. Such system is disclosed in commonly assigned U.S. patent application Ser. No. 08/277,674 filed on Jul. 19, 1994, by Kenneth L. Schierbeek and Niall R. Lynam for an AUTOMATIC REARVIEW MIRROR SYSTEM WITH AUTOMATIC HEADLIGHT ACTIVATION, the disclosure of which is hereby incorporated herein by reference. If the ambient light signal supplied to ambient light input  104  is a binary signal representative of a daytime ambient light level and a nighttime ambient light level, image processor  18  would typically provide a signal on luminance intensity line  102  that would switch the intensity level of display  20  between two intensity levels. Alternatively, ambient light input  104  may be supplied with a signal developed by one or more image capture devices  14 ,  16 . The ambient light signal would be based upon an average intensity value sensed by all, or a group of, pixels in the image capture device or devices. This embodiment eliminates the necessity for a separate ambient light sensor. Alternatively, ambient light input  104  may be responsive to manual actuation of the vehicle&#39;s headlights by the driver. Additionally, a comfort level setting may be provided to allow the driver to adjust to a preferred brightness at one ambient light condition. Thereafter, the system automatically adjusts display brightness according to ambient light changes. 
         [0061]    In the illustrated embodiment, display  20  incorporates a combined image generator and optical correction system  106  which provides for both image magnification and light ray collimation. In this manner, the image projected from display  20  is larger than the image generated by image generator  74  and has a focal length that is greater than the separation distance between the image generator and the driver and, preferably, is generally at infinity ( FIG. 17 ). Combined image generator and an optical correction system  106  is disclosed in detail in U.S. Pat. Nos. 5,050,966 for an OPTICAL COMBINER COLLIMATING APPARATUS; 4,859,031 for an OPTICAL COLLIMATING APPARATUS; 4,900,133 for a HEADS-UP DISPLAY COMBINER UTILIZING A CHOLESTERIC LIQUID CRYSTAL ELEMENT; 4,987,410 for a MULTIPLE IMAGE FORMING APPARATUS; and 5,408,346 for an OPTICAL COLLIMATING DEVICE EMPLOYING CHOLESTERIC LIQUID CRYSTAL AND NON-TRANSMISSIVE REFLECTOR, the disclosures of which are hereby incorporated herein by reference and will not be repeated. Suffice it to say, combined image generator and optical correction system  106  includes a light source  108  which generates broad band white light which is gathered and reflected by a parabolic reflector  110 . In the illustrative embodiment, light source  108  is a tungsten halogen incandescent lamp. The light rays then pass through a dielectric green filter  112  which passes light in a specific region of the green portion of the spectrum and through a hot mirror  114  which removes the infrared content of the spectrum. Light then passes through a holographic diffuser  116  which homogenizes and shapes the light pattern. The light rays then pass through a monochrome liquid crystal display with opposing linear polarizers  118  which is supplied with a video signal by image processor  18 . Items  108 - 118  make up image generator  74 , which, in the illustrative embodiment, is a transmissive backlit liquid crystal display. However, image generator  74  could additionally be an emissive display or a reflective display, all of which are well known in the art. 
         [0062]    Light rays of the image generated by image generator  74  next pass through an anti-reflective coated cover glass  120  which is joined with a left-hand circular polarizer  122  which is bonded to this cover glass. The opposite surface of circular polarizer  122  is bonded to a lens  124  having a 50/50 dielectric coating. Such dielectric coating allows light rays to be both transmitted through the lens and reflected by the lens. The left-hand polarized light X′ transmitted through lens  124  contacts a cholesteric liquid crystal layer (CLC)  126  which is left-hand polarized, which is what gives efficient reflection of left-hand polarized light X′, as illustrated at X. Fifty percent (50%) of light rays X″ get efficiently reflected by the 50/50 beam splitter on lens 124 as right-hand circular polarized light X′″. Right-hand polarized light X′″ is transmitted by CLC layer  126  and passes through a right-hand circular polarizer  128  and an anti-reflective coated cover glass  130 . 
         [0063]    As can be seen by reference to  FIG. 17 , the optical configuration of lens  124  in combination with the left-hand and right-hand circular polarizers  122 ,  128  and cholesteric liquid crystal layer (CLC)  126 , provide image magnification as well as collimate the image light in order to produce a very long focal distance image. Advantageously, this structure allows image portions from multiple image capture devices to be tiled into a unitary image.  FIG. 17  illustrates an approach using a single image generator. Merging of multiple image portions would require additional combined image generator and optical correction systems. Although image generators  74  for each of the image portions are laterally spaced apart from each other, the amplification produced by combined image generator and optical correction system  106  causes the image portions to merge at their periphery.  FIG. 17  illustrates an approach using a single image generator. Merging of multiple image portions would require additional combined image generators and optical correction systems. Other optical elements such as prisms, or other lenses, may be necessary to merge images to form a unitary image. Although the invention is illustrated with a combined image generator and optical correction system using cholesteric liquid crystal optical processing, other optical correction systems, as are known in the art, may be used. What is required is that the optical system generally collimates the light generated by the image generator and, preferably, provides amplification to the generated image. 
         [0064]    In the illustrated embodiment, rear image  42 , synthesized from the output of image capture devices  14 ,  16 , has a lateral width verses vertical height aspect ratio that is between approximately 4:1 and 2:1. Most preferably, the aspect ratio of image  42  is 8:3. This allows a panoramic view rearwardly of the vehicle with an optimum information content while reducing display of irrelevant information. The aspect ratio of display  20  may be different from that of the displayed synthesized image  42 . The remaining portion of the display, either above or below image  42 , may be utilized to display images other than synthesized image  42 . For example, the remaining portion of the display can be used to display auxiliary information such as one or more vehicle-operating parameters, such as vehicle speed indicia  56 , heading indicia  54 , or turn signal indicia  58 . Alternatively, the remaining portion of the display can be a reconfigurable high-information content display area to selectively display various types of information. Such information may include incoming facsimile or pager information, phone numbers, and navigational aids including pull-up maps, route guidance information, global positioning system (GPS) data, intelligent vehicle highway system (IVHS) information, as well as radio and environmental system control settings, and the like. Display  20  is especially useful for displaying such alternative data. Because display  20  has a very long focal length, the driver may consult the alternative data by switching the gaze of the driver between forward field of view  70  and to display  20  which does not require extensive refocusing of the driver&#39;s eyes. This allows the driver to consult the alternative data quickly with reduced fatigue and distraction. The content of the auxiliary information displayed may be user-selectable by a keypad, trackball, or other input device on the dashboard, steering column, or other position readily accessible to the driver. 
         [0065]    Although various camera devices may be utilized for image capture devices  14 ,  16 , an electro-optic, pixilated imaging array, located in the focal plane of an optical system, is preferred. Such imaging array allows the number of pixels to be selected to meet the requirements of rearview vision system  12 . The pixel requirements are related to the imaging aspect ratio of the respective image capture devices, which, in turn, are a function of the ratio of the vertical-to-horizontal field of view of the devices, as is well known in the art. In the illustrated embodiment, the imaging aspect ratio of side image capture devices  14  is 2:1 and the image aspect ratio of central image capture device  16  is variable down to 0.1:1. Such aspect ratio will produce images which will not typically match that of commercially available displays. A commercially available display may be used, however, by leaving a horizontal band of the display for displaying alpha-numeric data, such as portions of an instrument cluster, compass display, or the like, as illustrated in  FIG. 3 . 
         [0066]    In the illustrated embodiment, image capture devices  14 ,  16  are CMOS imaging arrays of the type manufactured by VLSI Vision Ltd. of Edinburgh, Scotland, which are described in more detail in U.S. patent application Ser. No. 08/023,918 filed Feb. 26, 1993, now U.S. Pat. No. 5,550,677, the disclosure of which is hereby incorporated herein by reference. However, other pixilated focal plane image-array devices, which are sensitive to visible or invisible electromagnetic radiation, could be used. The devices could be sensitive to either color or monochromatic visible radiation or near or far infrared radiation of the type used in night-vision systems. Each image capture device could be a combination of different types of devices, such as one sensitive to visible radiation combined with one sensitive to infrared radiation. Examples of other devices known in the art include charge couple devices and the like. 
         [0067]    Preferably, image capture devices  14  and  16  are all mounted at the same vertical height on vehicle  10 , although compromise may be required in order to accommodate styling features of the vehicle. The horizontal aim of image capture devices  14  and  16  is preferably horizontal. However, the portion of the image displayed is preferably biased toward the downward portion of the captured image because significantly less useful information is obtained above the horizontal position of the image capture devices. 
         [0068]    Each image-capturing device  14 ,  16  is controlled by appropriate supporting electronics (not shown) located in the vicinity of the imaging array such that, when operating power is supplied, either an analog or a digital data stream is generated on an output signal line supplied to image processor  18 . The support electronics may be provided partially on the image chip and partially on associated electronic devices. For each exposure period, a value indicative of the quantity of light incident on each pixel of the imaging array during the exposure period is sequentially outputted in a predetermined sequence, typically row-by-row. The sequence may conform to video signal standards which support a direct view such that, when a scene is viewed by an image-capturing device, the image presented on a display represents directly the scene viewed by the image-capturing devices. However, when looking forward and observing a displayed image of a rearward scene, the driver will interpret the image as if it were a reflection of the scene as viewed through a mirror. Objects to the left and rearward of the vehicle, as viewed by the rearward-looking camera, are presented on the left-hand side of the display and vice versa. If this reversal is effected in image processor  18 , it may be by the use of a data storage device, or buffer, capable of storing all of the pixel values from one exposure period. The data is read out of the data storage device in a reversed row sequence. Alternatively, the imaging array electronics could be constructed to provide the above-described reversal at the image-capturing device or at the display. 
         [0069]    Data transmission between image capture devices  14 ,  16  and image processor  18  and/or between image processor  18  and display  20  may be by electrically conductive leads. The leads may comprise either a serial or parallel bus. Alternatively, the data transmission may be via plastic or glass fiber-optic cable or an RF link. It is possible, for particular applications, to eliminate image processor  18  and direct drive display  20  from image capture devices  14 ,  16  at the pixel level. This may be accomplished by providing an interface between the output of image capture device  14 ,  16  and display  20  which synchronously maps image pixels captured by the image capture device, or devices, to the display. This synchronous mapping may be accomplished by providing a one-to-one mapping in which each pixel measurement is communicated to the display. Alternatively, the interface may only transmit pixel data which represents changes in the captured image. This allows for a reduction in the communication bandwidth necessary to transmit data between the image capture device, or devices, and the display. This may be accomplished by encoding the pixel data which represents changes in the captured image with additional data which designates the position of the pixel or other relevant information. Communication between the image capture device, or devices, may be multiplexed. 
         [0070]    The data streams from image-capturing devices  14 ,  16  are combined in image processor  18  and directly mapped to the pixel array of display  20 . This process is repeated preferably at a rate of at least  30  times per second in order to present an essentially real time video image. The image captured by side image capture device  14  on the right side of the vehicle is presented in right image portion  46  and the image from side image capture device  14  on the left side of the vehicle is displayed on left image portion  44 . The image from center image capture device  16  is displayed on central image portion  48 . The three image portions  44 - 48  are presented in horizontal alignment and adjacent to each other. However, the composite image may be positioned at any desired vertical position in the display  20 . It is also possible to display image portions  44 - 48  on separate image devices which are adjacent each other. 
         [0071]    In vision system  12 , side image capture devices  14  are positioned preferably at a forward longitudinal position on vehicle  10  and center image capture device  16  is positioned at a rearward longitudinal position on the vehicle. As best seen by reference to  FIG. 7 , this positioning creates a difference in the vertical angle between each side image capture device  14  and center image capture device  16  with respect to a fixed location P 1  that is a distance D 1  behind the vehicle. This difference in sensing angle will cause each side image capture device  14  to image an object located at P 1  on a horizontal row of pixels that is different from the horizontal row of pixels that center image capture device  16  will image the same object. If the image is below the horizontal centerline of the image capture device, it will be imaged on a lower row of pixels by center image capture device  16  than the row of pixels it will be imaged by the side image capture devices  14 , as illustrated in  FIG. 7 . This mismatch between horizontal pixel rows of the captured image is furthermore a function of the distance of the captured image from the rear of the vehicle. This can be understood by reference to  FIG. 11  which presents a chart  90  having a first column  92  of pixel lines n 1 , measured from the array centerline, at which an object will be imaged by side image capture device  14  and a second column  94  of pixel lines n 2 , measured from the array vertical centerline, at which the same object will be imaged by center image capture device  16 . The result is that an object, which is captured by both side and center image capture devices  14 ,  16 , will be vertically disjointed at the boundary of the displayed image, if the object is captured by more than one image capture device. The amount of disjointment will be greater closer to the vehicle and less at further distances. If the object is elongated in the horizontal direction, such as earth&#39;s horizon, bridges, or cross-markings on highways, then the object will appear to be either broken or crooked. 
         [0072]    In order to provide uniform display of laterally elongated images, a rearview vision system  12 ′ is provided having a central image portion  48 ′ which is processed differently from the image display portions  44 ′ and  46 ′ produced by the side image capture devices ( FIG. 8 ). Central image portion  48 ′ is reduced vertically, or compressed, by removing specified scan lines, or pixel rows, from the image captured by center image capture device  16  in a graduated fashion. The difference in the pixel line at which an object will be imaged by each of the side and center image capture devices is a function of the distance D of the object from the rear of the vehicle, with a greater variation occurring at shorter distances and the variation reducing to zero for infinite distances. Therefore, the compression of central image portion  48 ′ is non-linear, with substantially no compression at the vertical center of the image and greater compression at greater distances above and below the vertical center point of the image. This is accomplished by removing specific lines from the center display in a graduated fashion with a greater number of lines removed further from the vertical center of the image. The removed lines may be merely discarded in order to vertically reduce the image. Alternatively, the data contained in the removed lines may be utilized to modify the value of adjacent pixels above and below the removed line in order to enhance the quality of the compressed image. Averaging, median filtering, or other such known techniques may also be used. 
         [0073]    Each of right image portion  46 ′ and left image portion  44 ′ includes an upper portion  64  which extends above the compressed upper portion of the central image portion  48 ′. In the illustrated embodiment, upper portions  64  are deleted in order to present a uniform upper horizontal boundary for display  20 ′. In the illustrated embodiment, the mismatch between the lower horizontal boundary of central image portion  48 ′ and each of the left and right image portions provides a dead space  66  which provides a visual prompt to the user of the approximate location of the rearward corners S of vehicle  10 . This dead space  66  in the image displayed on display  20 ′ approximates the footprint occupied by vehicle  10  when viewed from point C. This is particularly useful because it provides a visual indication to the driver that a vehicle passing vehicle  10 , as viewed in either left image portion  44 ′ or right image portion  46 ′, is at least partially adjacent vehicle  10  if the image of the approaching vehicle is partially adjacent to dead space  66 . 
         [0074]    In an alternative embodiment, the vertical compression technique may be applied to only a lower vertical portion of central image portion  48 ′. In most driving situations, objects imaged by rearward-facing image capture devices above the horizon are at a long distance from the vehicle while those below the horizon get progressively closer to the vehicle in relation to the distance below the horizon in the displayed image. Therefore, compression of the upper vertical portion of the central image portion may be eliminated without significant reduction in performance. 
         [0075]    Compression of the central image portion may also advantageously be provided horizontally, as well as vertically. Spatial separation of center image capture device  16  from side image capture devices  14  causes similar distortion, as that described above, in the horizontal direction. This effect is spherical in nature and would require a more complex corrective action, such as compressing the image based upon the removal of pixels from an approximation to concentric circles centered on the center of the imaging array, or other techniques which would be apparent to those skilled in the art. 
         [0076]    A rearview vision system  12 ″ includes an image display  20 ″ having a compressed central image portion  48 ″ and left and right image portions  44 ″ and  46 ″, respectively ( FIG. 10 ). A border  50 ′ between left side image  44 ″ and central image  48 ″ includes a vertical central border portion  50   a ′, an upper border portion  50   b ′, and a lower border portion  50   c ′. Upper border portion  50   b ′ and lower border portion  50   c ′ diverge laterally outwardly, vertically away from central portion  50   a ′. A border  52 ′ between central image portion  48 ″ and right image portion  46 ″ includes a central boundary portion  52   a ′, an upper boundary portion  52   b ′, and a lower boundary portion  52   c ′. Upper boundary portion  52   b ′ and lower boundary portion  52   c ′ diverge laterally outwardly vertically away from central portion  52   a ′. This creates an upper portion of central image portion  48 ″ and a lower portion of central image portion  48 ″ which extend beyond the center portion thereof. This configuration is based upon the realization that the surface of the road immediately behind the vehicle is captured by central image capture device  16 . Likewise, the horizontal plane above the vehicle, which is symmetrical with the road surface, is captured by the center image capture device. This may be seen by referring to point P in  FIG. 10 , which illustrate the points where the effective radius  68  of the virtual image capture device intersects dead zones  30  and by referring to point S in  FIG. 10  which illustrates the corners or the rear of the vehicle (S). 
         [0077]    The image displayed on display  20 ″ includes a dead space  66 ′ having diverging lateral sides  68   a,    68   b.  Diverging sides  68   a  and  68   b  are configured in order to extend in the direction of travel of vehicle  10  which is parallel to lane markings of a highway on which vehicle  10  is travelling. This further enhances the visual perception of the driver by providing a visual clue of the location of images appearing on display  20 ″ with respect to the vehicle  10 . Side portions  68   a,    68   b,  in the illustrated embodiment, are natural extensions of lower boundary portions  50   c ′ and  52   c ′ and extend from point S on each respective side of the vehicle to point R, which represents the intersection of the lower extent of the vertical field of view  40  of each side image capture device  14  with the pavement ( FIG. 7 ). 
         [0078]    Rearview vision systems  12 ′ and  12 ″ utilize a displayed synthesized image which takes into account the use of perspective in enhancing the driver&#39;s understanding of what is occurring in the area surrounding the vehicle. The images produced on displays  20 ′ and  20 ″ effectively remove the vehicle bodywork and replace the bodywork with a vehicle footprint as would be viewed by virtual camera C. The image displayed on display  20 ″ further includes perspective lines which further enhance the roll of perspective in the driver&#39;s understanding of what is occurring. 
         [0079]    In order to further enhance the driver&#39;s understanding of what is occurring in the area surrounding the vehicle, a rearview vision system  12 ′″ includes a display  20 ′″ having image enhancements ( FIG. 6 ). In the illustrative embodiment, such image enhancements include graphic overlays  70   a,    70   b  which are hash marks intended to illustrate to the driver the anticipated path of movement of vehicle  10 . In the illustrated embodiment, the anticipated vehicle motion is a function of the vehicle direction of travel as well as the rate of turn of the vehicle. The forward or rearward direction of vehicle travel is determined in response to the operator placing the gear selection device (not shown) in the reverse gear position. The degree of turn of the vehicle may be determined by monitoring the movement of the vehicle steering system, monitoring the output of an electronic compass, or monitoring the vehicle differential drive system. In the embodiment illustrated in  FIG. 6 , the configuration of graphic overlays  70   a,    70   b  indicates that the vehicle is in reverse gear and that the wheels are turned in a manner that will cause the vehicle to travel toward the driver&#39;s side of the vehicle. If the wheels were turned in the opposite direction, graphic overlays  70   a,    70   b  would curve clockwise toward the right as viewed in  FIG. 6 . If the vehicle&#39;s wheels were straight, graphic overlays  70   a,    70   b  would be substantially straight converging lines. If the vehicle is not in reverse gear position, graphic overlays  70   a,    70   b  are not presented. Other types of graphic overlays of the displayed image are comprehended by the invention. 
         [0080]    Horizontal grid markings on the display may be provided to indicate distances behind the vehicle at particular markings. Such grid would allow the driver to judge the relative position of vehicles behind the equipped vehicle. In one embodiment, short horizontal lines are superimposed on the displayed image at regular rearward intervals in horizontal positions which correspond to the boundaries of the lane in which the vehicle is travelling. In order to avoid confusion when the vehicle is travelling in a curved path, from a lack of correspondence between the graphic overlay and the road, a signal indicative of the vehicle&#39;s rate of turn may be taken into account when generating the graphic overlay. In this manner, the distance indications may be moved laterally, with reduced horizontal separation, to correspond to the positions of the curved lane boundaries and vertically on the image to compensate for the difference between distances along a straight and curved path. 
         [0081]    Another image enhancement is to alter the appearance of an object in a particular zone surrounding the vehicle in order to provide an indication, such as a warning, to the driver. As an example, a vehicle that is too close to the equipped vehicle for safe-lane change, may be displayed in a particular color, such as red, may flash, or otherwise be distinguishable from other images on the display. Preferably, the speed of the equipped vehicle  10 , which may be obtained from known speed transducers, may be provided as an input to the rearview vision system in order to cause such warning to be a function of the vehicle speed which, in turn, affects the safe separation distance of vehicles. The operation of the turn signal may also be used to activate such highlighting of other road users or to modify the scope of the image displayed. In order to determine the distance of objects behind vehicle  10 , a separate distance-measuring system may be used. Such separate system may include radar, ultrasonic sensing, infrared detection, and other known distance-measuring systems. Alternatively, stereoscopic distance-sensing capabilities of side image capture devices  14  may be utilized to determine the separation distance from trailing objects utilizing known techniques. 
         [0082]    Thus, it is seen that the image displayed on display  20 - 20 ′″ may be different under different circumstances. Such different circumstances may relate to the vehicle&#39;s direction of travel, speed, rate of turn, separation from adjacent objects, and the like. 
         [0083]    Various other forms of image processing may be utilized with rearview vision system  12 - 12 ′″. Luminant and chrominant blending may be applied to the images captured by image capture devices  14 ,  16  in order to produce equality of the image data whereby the image portions appear as if they were produced by one image capture device. The dynamic range of the image capture devices may be extended in order to provide high quality images under all lighting conditions. Furthermore, individual pixel groups may be controlled in order to selectively compensate for bright or dark spots. For example, anti-blooming techniques may be applied for bright spots. Multiple exposure techniques may be applied to highlight dark areas. Image morphing and warping compensation techniques may additionally be applied. Resolution of the image capture devices and display may be selected in order to provide sufficient image quality for the particular application. 
         [0084]    A heater may be applied to each image capture device in order to remove dew and frost that may collect on the optics of the device. Although, in the illustrative embodiment, the optical centerline of the camera coincides with the field of view, particular applications may result in the centerline of the camera pointing in a direction other than the centerline of the field of view. Although, in the illustrative embodiment, the image capture devices are fixed, it may be desirable to provide selective adjustability to the image capture devices or optical paths in particular applications. This is particularly desirable when the system is used on articulated vehicles where automated and coordinated camera aim may be utilized to maintain completeness of the synthesized image. 
         [0085]    When operating the vehicle in the reverse direction, it may be desirable to provide additional data concerning the area surrounding the immediate rear of the vehicle. This may be accomplished by utilizing non-symmetrical optics for the center image capture device in order to provide a wide angle view at a lower portion of the field of view. Alternatively, a wide angle optical system could be utilized with the electronic system selectively correcting distortion of the captured image. Such system would provide a distortion-free image while obtaining more data, particularly in the area surrounding the back of the vehicle. 
         [0086]    The invention additionally comprehends the use of more than three image capture devices. In addition to side image capture devices positioned at the front sides of the vehicle and a center image capture device positioned at the center rear of the vehicle, additional image capture devices may be useful at the rear corners of the vehicle in order to further eliminate blind spots. It may additionally be desirable to provide an additional center image capture device at a higher elevation in order to obtain data immediately behind the vehicle and thereby fill in the road surface detail immediately behind the vehicle. Such additional detail is particularly useful when operating the vehicle in the reverse direction. Of course, each of the image capture devices could be a combination of two or more image capture devices. 
         [0087]    Although the present invention is illustrated as used in a rearview vision system, it may find utility in other applications. For example, the invention may be useful for providing security surveillance in an area where a building or other object obstructs the view of the area under surveillance. Additionally, the invention may find application in night-vision systems and the like. For example, the invention may be applied to forward-facing night-vision systems, or other vision enhancement systems such as may be used in adverse weather or atmospheric conditions such as fog, applied to provide an enhanced display of a synthesized image, which approximates a forward-facing view from a single virtual camera located rearwardly of the driver, taking advantage of the perspective features of the image. 
         [0088]    A rearview vision system  150  is provided which, in addition to displaying a rear image on display  20  which is synthesized by image processor  18  from the output of image capture devices  14 ,  16 , also supplies drive signals to an electrically operated optical device such as electro-optic mirror  152 , an electro-optic window  154 , or both. Even though a panoramic view rearward of the vehicle is displayed on display  20 , it may be desired to provide the driver with a rearview mirror of the type which has conventionally been provided on vehicles. One such mirror is an electro-optic mirror, such as an electrochromic mirror, a liquid crystal mirror, or a solenoid-operated prismatic mirror and the like. Additionally, vehicles may be provided with electro-optic windows, such as sunroofs, rear windows, side windows, and the like, which change transmissivity in response to a drive signal to a partial light transmittance level. In U.S. patent application Ser. No. 08/023,918 filed Feb. 26, 1993, now U.S. Pat. No. 5,550,677, the disclosure of which is hereby incorporated herein by reference, a technique is disclosed for producing a drive signal for an electrically operated optical device, such as an electro-optic mirror or window, from the image captured by a rearward-facing array. Utilizing the techniques disclosed therein, image processor  18  produces a drive signal on line  156  in order to control the partial reflectance level of electro-optic mirror  152  and a drive signal on line  158  in order to control the partial light transmittance level of electro-optic window  154 . 
         [0089]    A rearview vision system  160  is provided which includes a near infrared illumination device  162  in order to enhance an image captured by image capture devices  14 ,  16  ( FIG. 19 ). In the illustrated embodiment, infrared illumination device  162  illuminates an area immediately behind the vehicle. Preferably, the output of illumination device  162  has a greater near infrared light output than visible light output. This allows an enhanced image to be captured by the image capture device without increasing the visible light perceived by drivers surrounding the vehicle. Infrared illumination device  162  may be actuated in response to the vehicle being placed in reverse gear. This provides backup illumination which is greatly enhanced without having an increased effect on other drivers. Alternatively, infrared illumination devices may be positioned, for example, at other locations on the side or even the front of a vehicle in order to enhance the image captured by the image capture device or devices. This is especially useful in order to utilize rearview vision system  160  with a large truck, such as a trailer truck. This infrared illumination device may flood the area around the trailer with infrared light in order to enhance the image captured by the image capture device, or devices, without distracting other drivers. 
         [0090]    Image capture device  14 ,  16  may include a housing  164  in which an antenna  166  is positioned. This provides a convenient and functional location for a receiving antenna, such as the type used with a global positioning system, cellular telephone, garage door opener, radar distance sensing device, and the like, as disclosed in patent application Ser. No. 08/569,851 filed by O&#39;Farrell et al., now U.S. Pat. No. 5,971,552, the disclosure of which is hereby incorporated herein by reference. A heater  168  may be associated with the image capture device in order to stabilize the temperature of the device in low ambient temperature conditions. A similar heater may be supplied in display  20  in order to improve its performance in low ambient temperature conditions. A heater control  170  is provided in order to control the energization of heater  168  and, if utilized, the heater in the display. Heater control  170 , preferably, energizes heater  168  prior to the vehicle being started. This allows the temperature of the image capture device to be elevated to a more desirable temperature prior to the driver operating the vehicle. This may be accomplished by heater control  170  being a proximity detector which detects a device carried by the driver as the driver approaches the vehicle. Alternatively, heater control  170  may be responsive to a signal produced by a remote keyless entry device concurrently with the doors being activated. Alternatively, heater control  170  may be responsive to the vehicle device being opened. 
         [0091]    A rearview vision system  172  is provided which provides an output, generally referred to at  174 , from image processor  18  to display  20 . Output  174  provides an indication when an object bears a predetermined relationship to the vehicle. Such object may be of interest because the object is in a blind spot of the vehicle, may be tailgating the vehicle, or may be an object in front of the vehicle which the vehicle is too close to. Display  20  may respond to output  174  by highlighting the displayed vehicle, such as by displaying the vehicle in an artificial color, such as red, by flashing the image of the vehicle, or, otherwise, drawing the attention of the driver to the vehicle. Output  174  may be developed by image processor  18  from the outputs of image capture devices  14 ,  16 . This may be accomplished by, for example, utilizing redundant image portions captured by the image capture devices, even though not displayed by display  20 , in order to calculate relative position of the object with respect to the vehicle. Alternatively, an object sensor  176  may be provided in order to supply an output  178  indicative of a predetermined positional relationship of an object sensed by the object sensor with respect to the vehicle. In the illustrated embodiment, object sensor  176  may be a passive infrared sensor which senses the presence of an object in the vehicle&#39;s blind spot. Alternatively, object sensor  176  may be a distance-measuring device, such as an active infrared sensor, an ultrasonic sensor, a radar sensor, or the like. Such object sensor is especially useful in determining the separation distance between the vehicle and objects in front of the vehicle. Preferably, object sensor  176  has a sensing field of view that is substantially coincident with the field of view of one or more of the image capture devices  14 ,  16 . 
         [0092]    A rearview vision system  178  is provided which has the capability of displaying stereoscopic images rearward of the vehicle. Rearview vision system  178  includes at least one pair of image capture devices  14   a,  which are closely positioned on the vehicle and have overlapping fields of view. Because the image capture device pairs are closely positioned, they capture substantially the same image but from a slightly different angle. This allows image processor  18  to produce a video signal  100 ′ with stereoscopic information. This signal is utilized by a stereoscopic display  320  in order to produce a stereoscopic image rearward of the vehicle. Such stereoscopic displays are known in the art. Although one pair of image capture devices are illustrated in  FIG. 22 , rearview vision system  178  may include multiple pairs of image capture devices. This allows a rear image to be synthesized from the multiple pairs of image capture devices in order produce a panoramic view rearward of the vehicle and stereoscopic image. Preferably, utilizing other aspects of the invention, the stereoscopic image is a rearward-facing view from a single location. 
         [0093]    A rearview vision system  180  is provided which produces an indication  182  of road line markings. Indication  182  may also indicate road edges. Image processor  18  detects the road line markings and the road edges from the images captured by image capture devices  14 ,  16 . This feature may be further enhanced by combining it with an infrared illumination device  162  in order to further illuminate areas behind and around the vehicle in order to enhance the image of the road line markings and the road edges. Indication  182  may be utilized by display  20  in order to provide an indication of the vehicle with respect to the road line markings and road edges. The indication may further be utilized by an indicator which indicates the relative position of the vehicle in its lane. Additionally, the indication may be utilized to determine erotic vehicle operation, such as may occur when the driver begins to fall asleep, in order to provide a suitable alarm, or the like. 
         [0094]    A rearview vision system  184  is provided with capabilities for infrared communication with other vehicles and stationary beacons. Rearview vision system  184  produces a communication data output  186  which includes communication data decoded from infrared signals detected by image capture device or devices  14 ,  16 . For example, suitable standards may be developed wherein vehicles are equipped with a pair of spaced apart infrared transmitters on a forward portion thereof. Image processor  18  may respond to the temporal and spatial patterns of infrared signals detected by image capture devices  14 ,  16  in order to determine the speed and distance and, thereby, the separation of the vehicles as well as the rate of change of separation of the vehicles. Such information may be communicated to the trailing vehicle by an infrared transmitter (not shown) in order to control the speed of the trailing vehicle. This feature provides adaptive cruise control in which the speed of the trailing vehicle is controlled according to separation distance with leading vehicles. This allows high-speed convoying between vehicles. The communication system may additionally provide for the identification of emergency vehicles, and the like, which may transmit a unique temporal and/or spatial pattern of an infrared signal. The IR communication signal may additionally be utilized to receive signals from stationary devices, such as location beacons and Intelligent Vehicle Highway System (IVHS) data. Because rearview vision system  184  has a field of view which extends generally rearwardly of the vehicle, the system provides the capability for sensing information after the vehicle has passed the beacon. This provides an adjunct to infrared communication systems having a field of view generally forward of, or to the side of, the vehicle. 
         [0095]    A rearview vision system  188  is provided having extended dynamic range ( FIG. 25 ). Rearview vision system  188  includes a pair of image capture devices  14  and/or  16 , each of which has an image-sensing array  190 . Each image capture device is capable of operating in either a color mode, in which a color image is displayed on display  20 , or a monochrome mode, in which a monochrome image is displayed on display  20 . System  188  includes an image luminance sensor  192  which senses the luminance level of images captured by image capture devices  14 ,  16 . Image luminance sensor  192  may switch the image capture devices between the color mode and the monochrome mode such that, when the image luminance is sufficiently high, the image capture device, or devices, operate in a color mode. During low image luminance conditions, the image capture device, or devices, are operated in a monochromatic mode which does not require as much image luminance. This extends the dynamic range of the system. Rearview vision system  188  may additionally include an exposure control  194  which determines the exposure period for capturing each frame by arrays  190 . In order to extend the dynamic range of system  188 , exposure control  194  may produce exposure intervals for arrays  190  which vary in length from interval-to-interval. Thus, a series of normal exposure intervals may be occasionally supplanted by a longer exposure interval during which greater detail of the image may be captured. This enhanced image may then be combined with the image captured during the shorter intervals into a merged image of enhanced detail. 
         [0096]    Rearview vision system  188  may additionally include a plurality of infrared shutters  196  which are in the optical paths  198  of arrays  190 . Each infrared shutter  196  has at least one state in which infrared energy is generally not attenuated to array  190 . In another state, the infrared shutter generally blocks infrared radiation from the array. The state of infrared shutters  196  is controlled by image luminance sensor  192 . During periods of high image luminance, sensor  192  may switch the infrared shutters  196  to a state blocking near infrared radiation from arrays  190 . However, during low image luminance conditions, sensor  198  may switch the infrared shutters  196  to a state in which the near infrared energy is transmitted to arrays  190 . The addition of the near infrared radiation at low luminance levels enhances the image luminance sensed by arrays  190 . In the illustrated embodiment, infrared shutters  196  are either electrochromic shutters or liquid crystal shutters, both of which are known in the art. Rearview vision system  188  additionally includes means for coordinating the image intensity received from multiple image capture devices and displayed on display  20 . This allows a balance composite image to be displayed on the display. This may be provided by a display intensity control  200  which regulates the intensity of the output of both arrays  190  in order to produce color and luminance balancing between the multiple image capture devices. Instead of a separate display intensity control, a direct communication channel may be developed between the image capture devices in order to provide color and luminance balancing. 
         [0097]    Each image pixel captured by image capture devices  14 ,  16  and displayed on display  20  has a resolution which affects the amount of image detail displayed. While it is desirable to have a high degree of detail of displayed image, the increase in image resolution produces a commensurate increase in system cost. While it is desirable to reduce system cost, this should not be achieved by sacrificing necessary detail in the displayed image. According to the invention, it has been determined that sufficient image detail may be obtained at a suitable system cost by having a pixel resolution in the range of between approximately 2 arc minutes and approximately 8 arc minutes. Preferably, system resolution is approximately 6 arc minutes. 
         [0098]    It is known in the art to provide imaging array capture devices having mosaic filters which mask image radiation in order to produce pixels which respond respectively to red, green, and blue light. Because such known pixel filter masks do not adequately absorb near infrared radiation, it is known to supply infrared filters in order to block infrared radiation from the pixels so that the pixels respond to only the designed radiation band. However, such additional filters have undesirable characteristics including costs. It has been discovered that a pixel filter mask may be made responsive to red, green, or blue light while filtering out near infrared by adding appropriate dyes to the dyes making up the filter mask. 
         [0099]    The heater supplied with each image capture device may include a transparent conductive coating applied to a window covering the device lens. Alternative heater constructions include ITO or a series of fine wire mesh. This provides protection of the lens of the image capture device from physical harm while allowing moisture and frost to be removed from the window. 
         [0100]    Various manipulation techniques may be applied to image capture devices  14 ,  16 . For example, when the invention is applied to semi-tractor/trailer combinations, the image capture devices may be motorized and responsive to signals representing the relative position of the trailer and the cab in order to produce full panoramic view during various maneuvers of the vehicle. For example, as the vehicle turns, which may cause the trailer to otherwise block the image captured by a side image capture device, the image capture device may pivot to a different panoramic view which is not blocked by the trailer. Additionally, the panoramic view sensed by the image capture device may be different when the trailer is attached than when the trailer is not attached. In a similar fashion, rearward-mounted image capture device  16  may be motorized to move vertically upwardly and downwardly between a first position when the vehicle is moving in a forward direction and a second position when the vehicle is in reverse gear. In the second position, the motorized camera is directed more downwardly in order to capture images closer to the rear of the vehicle which may be contacted by the vehicle. Image capture devices  14 ,  16  may be supplied with electrical opto-mechanical zoom devices as well as vibration isolation and compensation stabilizing devices. 
         [0101]    Image processor  18  may provide an input to a control for the intensity of the brake lights, turn signals, and the like for the vehicle. In this manner, the image processor may control the intensity of such lights based upon ambient lighting conditions. This allows the intensity of the rearward directed lights to be less distracting for following vehicles. 
         [0102]    Additionally, the present invention may be utilized for providing guidance for a tractor/trailer or like vehicle backing to a loading dock. Additionally, the invention may provide guidance for proper alignment of the trailer and tractor which are being joined by backing of the tractor. The present invention may additionally provide accident monitoring by capturing a predetermined continuous stream of images, such as, for example,  15  seconds. This running store of image may be frozen upon impact of the vehicle, in order to record events leading up to the impact, or may be stopped by a manual input. Furthermore, the invention may be utilized to alert the driver to an impending rear-end collision. The vehicle may respond to such indication by deploying an appropriate device, such as a smart headrest or the like. 
         [0103]    Thus, it is seen that the present invention enhances the relationship between the driver&#39;s primary view and the image presented on the rearview vision system. This is accomplished in a manner which provides ease of interpretation while avoiding confusion so that the driver does not have to concentrate or look closely at the image. In this manner, information presented on the display is naturally assimilated. This is accomplished while reducing blind spots so that other vehicles or objects of interest to the driver will likely be displayed to the driver. Additionally, the use of perspective allows distances to be more accurately determined. 
         [0104]    Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention, which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.