Abstract:
A system mountable in a motor vehicle. The system includes a camera and a processor configured to receive image data from the camera. The camera includes a rolling shutter configured to capture the image data during a frame period and to scan and to read the image data into multiple image frames. A near infra-red illuminator may be configured to provide a near infra-red illumination cone in the field of view of the camera. The near infra-red illumination oscillates with an illumination period. A synchronization mechanism may be configured to synchronize the illumination period to the frame period of the rolling shutter. The frame period may be selected so that the synchronization mechanism provides a spatial profile of the near infra-red illumination cone which may be substantially aligned vertically to a specific region, e.g. near the center of the image frame.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/283,701, filed Oct. 28, 2011, now U.S. Pat. No. 9,179,072, issued Nov. 2, 2015, which claims the priority benefit of U.S. Provisional Application No. 61/421,424, filed Dec. 9, 2010, and the priority benefit of U.S. Provisional Application No. 61/408,617, filed Oct. 31, 2010, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to driver assistance systems in motor vehicles, and in particular to providing a system and a method for night vision using a near infra-red (NIR) Illumination and a rolling shutter for driver assistance systems (DAS) applications. 
     2. Description of Related Art 
     During the last few years camera based driver assistance systems (DAS) have been entering the market; including lane departure warning (LDW), automatic high-beam control (AHC), traffic sign recognition (TSR) and forward collision warning (FCW). 
     Lane departure warning (LDW) systems may be designed to give a warning in the case of unintentional lane departure. The warning is given when the vehicle crosses or is about to cross the lane marker. Driver intention is determined based on use of turn signals, change in steering wheel angle, vehicle speed and brake activation. There may be various LDW systems available. One algorithm for lane departure warning (LDW) used by the Applicant/assigne (Mobileye Technologies Ltd., Nicosia, Cyprus, hereinafter “Mobileye”) of the present application is predictive in that it computes time-to-lane crossing (TLC) based on change in wheel-to-lane distance and warns when the time-to-lane crossing (TLC) is below a certain threshold. Other algorithms give a warning if the wheel is inside a certain zone around the lane marker. In either case, essential to the lane departure warning system is the lane marker detection algorithm. Typically, the lane markers may be detected in the camera image and then, given the known camera geometry and camera location relative to the vehicle, the position of the vehicle relative to the lane is computed. The lane markers detected in the camera image may be then collected over time, for instance using a Kalman filter. Wheel-to-lane marker distance may be given with an accuracy of better than 5 centimeters. With a forward looking camera, wheel-to-lane marker distance is not observed directly but is extrapolated from the forward view of the camera. The closer road markings may be observed, less extrapolation is required for determining wheel-to-lane marker distance and more accurate estimates of wheel-to-lane marker distance may be achieved especially on curves of the road. Due to the car hood and the location of the camera, the road is seldom visible closer than six meters in front of the wheels of the car. In some cars with longer hoods, minimal distance to visible road in front of the car is even greater. Typically the lane departure warning system of Mobileye works on sharp curves (with radius down to 125 m). With a horizontal field of view (FOV) of 39 degrees of the camera, the inner lane markers may be still visible on curves with a radius down to 125 meters. In order to correctly perform lane assignment on curves, lane markings may be detected at 50 meters and beyond. With a horizontal field of view (FOV) of 39 degrees for the camera, a lane mark of width 0.1 meters at 50 m distance corresponds in the image plane to just under two pixels wide and can be detected accurately. The expectation from the lane departure warning systems is greater than 99% availability when lane markings may be visible. Expectation with 99% availability is particularly challenging to achieve in low light conditions when the lane markings may be not freshly painted (have low contrast with the road) and the only light source is the car halogen headlights. In low light conditions, the lane markings may be only visible using the higher sensitivity of the clear pixels (i.e. using a monochrome sensor or a red/clear sensor). With the more powerful xenon high intensity discharge (HID) headlights it is possible to use a standard red green blue (RGB) sensor in most low light conditions. 
     Traffic sign recognition (TSR) modules may be designed typically to detect speed limit signs and end-of-speed limit signs on highways, country roads and urban settings. Partially occluded, slightly twisted and rotated traffic signs may be preferably detected. Systems implementing traffic sign recognition (TSR) may or should ignore the following signs: signs on truck/buses, exit road numbers, minimum speed signs, and embedded signs. A traffic sign recognition (TSR) module which focuses on speed limit signs does not have a specific detection range requirement because speed limit signs only need to be detected before they leave the image. An example of a difficult traffic sign to detect is a 0.8 meter diameter traffic sign on the side of the road when the vehicle is driving in the center lane of a three lane highway. Further details of a TSR system is disclosed by the present assignee in patent application publication US20080137908. 
     A typical automatic headlight or high/low beam control (AHC) system detects the following conditions and switches from high beams to low beams: headlights of oncoming vehicles, taillights of preceding vehicles, street lights or ambient light indicating that high beams may be not required and a low vehicle speed. The host vehicle lights may be switched back to high beams when none of these conditions exist (often after a specified grace period). One approach for detecting taillights is to compare images from two sensors: one with a red filter and the second with a cyan filter. The cyan filter responds to non-red light sources and will give zero response to red light. By comparing corresponding pixels from two imaging sensors one can detect the color of the light source. The number of pixels of each color above a certain intensity is counted and if the count is above a threshold the systems switches to low beams. The use of color filters with imaging sensors may preclude the simultaneous use of the same image frames for other driver assistance applications. 
     A second approach for automatic high-beam control (AHC) uses an RGB sensor to give better color differentiation. Typical light sources can be located in the full CIE color space as defined by the International Commission on Illumination. This approach distinguishes between green, yellow and red lights. A powerful green traffic light is not confused with an oncoming vehicle. Since a single sensor with a color mosaic filter i.e. Bayer pattern mosaic is used, the lens is defocused so as to spread a light source over multiple pixels. The use of the color mosaic filter reduces both the effective image sensor resolution (by 50%) and the intensity response (to less than one third). The color mosaic filter may preclude the use of the same sensor for traffic sign recognition (TSR) or lane departure warning (LDW) because of the intensity response penalty. 
     Ego-motion estimation is disclosed in U.S. Pat. No. 6,704,621 by Stein. Image information is received from images recorded as the vehicle moves along a roadway. The image information is processed to generate an ego-motion estimate of the vehicle, including the translation of the vehicle in the forward direction and the rotation. 
     Some driver assistance systems may rely on changing camera exposure parameters (e.g. aperture, exposure, magnification). The use of a color camera equipped for instance with an RGB (red/green/blue) filter and an infra-red filter achieves good spectral separation for detecting taillights or brake lights but reduces imaging sensitivity by a factor of six or more. A reduction in sensitivity by such a factor has an adverse impact on other vehicle control application such as LDW performance in dark scenes. The presence of an infra-red filter also negates the use of the camera as a near infra-red sensor for applications, such as pedestrian detection. Thus, a brake light detection system which uses color or spectral analysis in the captured images may be less compatible with other driver assistance systems without sacrificing performance. 
     The core technology behind forward collision warning (FCW) systems and headway distance monitoring is vehicle detection. Assume that reliable detection of vehicles in a single image a typical forward collision warning (FCW) system requires that a vehicle image be 13 pixels wide, then for a car of width 1.6 m, a typical camera (640×480 resolution and 40 deg FOV) gives initial detection at 115 m and multi-frame approval at 100 m. A narrower horizontal field of view (FOV) for the camera gives a greater detection range however; the narrower horizontal field of view (FOV) will reduce the ability to detect passing and cutting-in vehicles. A horizontal field of view (FOV) of around 40 degrees was found by Mobileye to be almost optimal (in road tests conducted with a camera) given the image sensor resolution and dimensions. A key component of a typical forward collision warning (FCW) algorithm is the estimation of distance from a single camera and the estimation of scale change from the time-to-contact/collision (TTC) as disclosed for example in U.S. Pat. No. 7,113,867. 
     BRIEF SUMMARY 
     According to the present invention there is provided a system mountable in a motor vehicle. The system includes a camera and a processor configured to receive image data from the camera. The camera includes a rolling shutter configured to capture the image data during a frame period and to read the image data into multiple image frames, for instance alternately line by line. A near infra-red illuminator may be configured to provide a near infra-red illumination cone in the field of view of the camera. The near infra-red illumination oscillates with an illumination period. A synchronization mechanism may be configured to synchronize the illumination period to the frame period of the rolling shutter. The image frames include a long exposure frame and a short exposure frame. The synchronization mechanism is configured to turn on the near infra-red illumination during exposure of at least one of the middle rows of the long exposure frame. The frame period may be selected so that the synchronization mechanism provides a spatial profile of the near infra-red illumination cone which may be substantially aligned vertically to a specific region of the image frame. 
     The processor may include multiple driver assistance modules configured to perform respective multiple driver assistance functions. A first driver assistance module may be configured to use the near infra-red illumination and a second driver assistance module may be configured to use parts of an image that do not have the near infra-red illumination. The first driver assistance module may be configured to perform pedestrian detection responsive to the near infra-red illumination in the first multiple image frames. The second driver assistance module may use color information from the second image frames to detect headlights of an oncoming vehicle in the field of view of the camera, to detect taillights of a leading vehicle in the field of view of the camera, to detect taillights of a leading vehicle in the field of view of the camera and/or to detect a traffic sign in the field of view of the camera. The camera may be configured to provide to the processor the image frames. The image frames may be partitioned into first image frames for use by the first driver assistance module and into second image frames for use by a second driver assistance module. A parameter of the camera may be configurable to alternate respective values between the first and second image frames. The parameter may be the frame period of the rolling shutter. 
     The synchronization module may be configured to time the near infra-red illumination to be in the first image frames and to avoid the near infra-red illumination in the second image frames. 
     According to the present invention there is provided a method operable in a system mountable in a motor vehicle. The system include a processor, a camera having a rolling shutter configured to capture image data during a frame period and to scan line by line to read the image data of the image frame and to read the image data into a plurality of image frames. The method receives image data from the camera. Near infra-red (NIR) illumination may be provided in the field of view of the camera. The near infra-red illumination oscillates at an illumination frequency with an illumination period with an illumination duty cycle. The illumination period may be synchronized and to the rolling shutter. The image frames include a long exposure frame and a short exposure frame. The synchronization turns on the near infra-red illumination during exposure of at least one of the middle rows of said long exposure frame. 
     The camera may be configured to provide to the processor multiple image frames. The image frames may be partitioned into first image frames for use by a first driver assistance module and into second image frames for use by a second driver assistance module. The synchronization may provide the near infra-red illumination in the first image frames and avoids the near infra-red illumination in the second image frames. The values of a camera parameter may be alternated respectively in alternating first and second image frames. The camera parameter may be the frame period of the rolling shutter. 
     These, additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIGS. 1 a  and 1 b    illustrate a system including a near infra-red (NIR) illuminator and a camera mounted in a vehicle, according to an aspect of the present invention. 
         FIG. 2  shows a schematic description of exposure settings with a rolling shutter, according to a feature of the present invention. 
         FIG. 3  shows schematically graphs of NIR intensity in percent for NIR an illuminator that is on for 15 ms and off for 30 ms, according to a feature of the present invention. 
         FIG. 4 a    shows a NIR illuminator that is on for 22 ms and off for 30 ms, where the on time (indicated by horizontal arrow) aligns with the middle row of a long exposure frame, according to a feature of the present invention. 
         FIG. 4 b    shows an NIR spatial profile overlaid on an image with respective percentage (%) NIR intensities for two frames, according to a feature of the present invention. 
         FIG. 5  shows schematically a typical spatial distribution in image space coordinates of an NIR illumination cone, according to a feature of the present invention. 
         FIG. 6  shows the NIR spatial profile overlaid on an image, according to an feature of the present invention. 
         FIG. 7  shows a method, according to an aspect of the present invention. 
     
    
    
     The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to aspects of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The aspects are described below to explain the present invention by referring to the figures. 
     Before explaining features of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other features or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     By way of introduction, features of the present invention may be directed to a system that performs multiple Driver Assistance (DAS) applications concurrently on the same hardware platform wherein for instance at least one of the applications requires near infra-red (NIR) illumination and at least one other application requires for instance color information. For example, one application might be pedestrian detection at night requiring near infra-red (NIR) illumination and another application might be Automatic Highbeam Control (AHC) which requires color information. 
     Rolling shutter is a method of image acquisition in which each frame is recorded not from a snapshot of a single point in time, but rather by scanning across the frame, for instance row by row. With a rolling shutter, not all parts of the image are recorded at exactly the same time, even though the whole frame is displayed at the same time during playback. The rolling shutter is in contrast with a global shutter where the entire frame is exposed for the same time window. 
     Since near infra-red (NIR) illumination is almost monochrome, all color information from reacted objects may be lost. With a global shutter image sensor, it may be possible to alternate between illuminated and non illuminated NIR frames. That is, the near infra-red (NIR) illumination is on for some image frames (those used for pedestrian detection for example) and off for image frames (those used for color based detection for example). A more complex situation is when a rolling shutter sensor is used. The rolling shutter may be more complex because rows in one image are actually exposed at the same time as other rows for the following frame and there may be no period when it is possible to switch on the illuminator to uniformly expose all the rows of one image and none of the rows of the next image. According to different features of the present invention, when the rolling shutter sensor is used, an illumination frequency and duty cycle may be used which are carefully tuned to the camera exposure, combined with precise definition of the illumination cone. 
     Pedestrians may be detected up to a distance of 20 m using a standard automotive CMOS camera such as the Aptina MT9V024 (Aptina Imaging Corporation, San Jose, Calif., USA) under low beams. The detection distance increases significantly when using high beams but of course the use of high beams is limited when there is traffic so as not to blind other drivers. One possible way to increase detection distance is to use NIR illumination. NIR illuminators may be standard halogen highbeams with a long pass filter or dedicated IR illuminators such 850 nm light emitting diodes or LEDs. However with NIR illumination all color information from reflective surfaces may be lost, particularly for the case of narrow band 850 nm LEDs but even if a wide NIR spectrum is used, the NIR response of typical red, green and blue pixels of a typical color sensor is the same for all three channels. 
     Reference is now made to  FIGS. 1 a  and 1 b    which illustrate a system  16  including a near infra-red (NIR) illuminator  4  and a camera  12  mounted in a vehicle  18 , according to an aspect of the present invention. Illuminator  4  may be mounted in the headlights of vehicle  18  or in proximity of camera  12  to project NIR illumination  4   a  forward of vehicle  18 . Camera  12  includes a sensor equipped with a rolling shutter  6 . Camera  12  images a field of view in the forward direction which captures image frames  204 / 206  in real time and transfers image frames  204 / 206  to processor  14 . Image processor  14  may be used to process image frames  204 / 206  simultaneously and/or in parallel and to provide multiple driver assistance functions. By way of example in  FIG. 1 b   , image frames  204 / 206  may be used to serve forward collision warning (FCW) system  22 , lane departure warning (LDW)  21 , traffic sign recognition (TSR)  20 , automatic headlight control  24  and pedestrian detection warning (PDW)  23 . Synchronization mechanism  2  provides synchronization between the rolling shutter  6  of camera  12  and NIR illuminator  4 . 
     Image processor  14  is used to process image frames  204 / 206  to detect a pedestrian in an image in the forward field of view of camera  12  as required for pedestrian detection warning (PDW)  23 . Pedestrian detection warning (PDW)  23  works in parallel with the other driver assistance functions. 
     Table 1 shows typical high dynamic range (HDR) exposure and gain settings for the Aptina MT9M024 1.3 Megapixel camera  12 , according to a feature of the present invention. Sensor  6  is set to produce images at 45 frames per second (FPS) or 22.2 ms per frame. Image frames  204 / 206  may be partitioned so that the exposures may alternate between between a longer exposure setting used primarily for detecting road markings and pedestrian targets, and a shorter exposure which may give better color information and less motion blur to support AHC  24  and TSR  20  driver assistance systems. 
     
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Aptina Grabbing Scheme 
               
               
                 Sensor running on 45 FPS, 12 bit compacted image. 
               
               
                 Algorithm running on two high dynamic range (HDR) images up to 22.5 FPS. 
               
               
                 Texture Exposure - gain up to 2.75 × 8, integration periods up to {14, 0.875, 0.054} ms, knee 
               
               
                 factors 16, 16. 
               
               
                 Color Exposure - gain 1, integration periods {11, 0.68, 0.042} ms, knee factors 16, 16. 
               
               
                 Frame capture at constant rate of 45 images per second. 
               
               
                 Processing 
               
               
                 Produce 8 bit Tone Mapped image from each of Texture and Color exposures for texture 
               
               
                 analysis. 
               
               
                 Work directly on 12 bit images for color processing mainly on Color exposure. 
               
               
                 Night Vision Display 
               
               
                 the tone mapped image of Texture exposure should be used 
               
               
                 pedestrian enhancement to be performed on tone mapped image. 
               
               
                 running as separate process parallel and asynchronous to the main processing. 
               
               
                   
               
             
          
         
       
     
     Reference is now made to  FIG. 7  which shows a method  701  according to an aspect of the present invention. Rolling shutter  6  scans line by line over an image frames  204 / 206  to read the image data alternately line by line into image frames  204  and  206 . In step  703 , the image data of multiple image frames  204 / 206  from camera  12  may be received by processor  14 . In step  705 , near infra-red (NIR) illumination may be provided from illuminator  4  to give NIR illumination  4   a  forward of vehicle  18 . Illumination  4   a  oscillates with an oscillation frequency and an illumination duty cycle synchronized line by line with the exposure of frames  204 / 206  of rolling shutter  6 . 
     The term “oscillation” as used herein in the context of near infra-red illumination refers to a repetitive variation of illumination intensity. The term “illumination frequency” as used herein is the frequency of the illumination intensity oscillation and the term “illumination period” is the inverse of the illumination frequency of the oscillation. The term “duty cycle” as used herein duty cycle refers to a substantially square wave infra-red oscillation between a state of higher illumination and a state of lower illumination. The term “duty cycle” is substantially equal to the time spent in the higher state divided by total time. 
     The term “exposure” as used herein refers to a time period during which a line of a frame of the rolling shutter is sensitive to light. 
     Synchronization mechanism  2  and synchronization step  707  may be performed if camera  12  has a general purpose input output (GPIO) pin which can be switched between 1 and 0 with a certain delay after the vertical synch of sensor  6 . The signal from the general purpose input output (GPIO) pin may be then sent to the controller of NIR illuminator  4  to switch the near infra-red illumination on and off synchronously with rolling shutter  6 . In some cases having the general purpose input output (GPIO) pin may not be available for synchronization  707 . In such cases, NIR illumination controller may be set to a nominal frequency and duty cycle and rolling shutter  6  detects and locks into the exact frequency and phase. For example if the frequency is 22.5 Hz and the duty cycle is 50%, rolling shutter  6  frequency may be controlled very precisely (to an accuracy of 12 ns) by adjusting the horizontal and vertical blanking time of sensor  6 . 
     Alternatively, synchronization (step  707 ) may be achieved for step  707  by maximizing the brightness of a central horizontal strip of an image frame  204 / 206 . 
     Reference is now made to  FIG. 2  which shows a schematic description of the exposure settings with rolling shutter  6 , according to a feature of the present invention. Two types of image frames  206  and  204  may be shown. In the example, each image row starts and ends exposure approximately 22 μs after the previous row shown as interval  202 . In the example, the long exposure in frame  204  is 15 milliseconds and the shorter exposure in frame  206  is 11 milliseconds. One important point may be that the exposure of the last rows in one image frame, e.g.  206  partially overlaps with the first rows of the next image frame, e.g.  204 . 
     In both image frames  204  and  206  the frame interval  202  is 22.2 ms. Therefore the first row of the long exposure frame  204  starts 7.2 ms after the start of last row of the short exposure frame  206 . The short exposure frame is 11 ms and the first row starts 11.2 ms after start of the last row of the long exposure frame. 
     Reference is now also made to  FIG. 3  which shows schematically graphs of NIR intensity in percent for NIR illuminator  4  that is on for 15 ms and off for 30 ms. The on time aligns with the middle row (shown as a horizontal arrow in  FIG. 2 ) of the long exposure frame  204 . This timing would give very little NIR illumination during the short frame  206  (only at the tail end) which may be desirable. However the NIR illumination during long frame  204  will also be reduced. There will only be a number of rows or perhaps even just one row (shown as a horizontal arrow in  FIG. 2 ) which has NIR illumination  4   a  for its full length. The issue becomes more complex when high dynamic range (HDR) sensor may be used. 
       FIG. 3  also shows pedestrians at 15 m. The pedestrians may be located in the central horizontal third of the image. 
     Reference is now made to  FIG. 4 a    which shows a NIR illuminator  4  that is on for 22 ms and off for 30 ms, where the on time (indicated by horizontal arrow) aligns with the middle row of the long exposure frame. The center rows in the image have NIR illumination for their full length with the period of illumination dropping at the top and bottom. However there may be significant NIR illumination during the exposure time of the top and bottom row of short exposure frame  206 . 
     Below is Table 2 which shows typical different DAS applications and the illumination requirements: 
     
       
         
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Vehicle Detection: Vehicle detection at night at long distances by detecting headlights and 
               
               
                 taillights NIR illumination may be detrimental to assessing taillight color. Close vehicle 
               
               
                 detection may benefit from NIR illumination but NIR illumination may be not required. 
               
               
                 Pedestrian (and animals) Detection 23: up to 20 m pedestrian can typically be detected well 
               
               
                 with the host vehicles low beams. Beyond that range NIR illumination may be 
               
               
                 recommended. 
               
               
                 Traffic sign recognition 20: Regular reflective traffic signs may be typically located at the 
               
               
                 side of the road and infrequently overhead. Electronic signs may be most often located 
               
               
                 overhead. For correct color recognition and to avoid over-saturation it may be important that 
               
               
                 the reflective signs may be not illuminated by the NIR 4 during the recognition stage. Thus it 
               
               
                 may be important the central strip of the image not be illuminated by NIR during the short 
               
               
                 exposure frame 206 and ideally neither the top. Dark, distant signs may benefit from NIR 
               
               
                 illumination during the long exposure frame. 
               
               
                 Automatic headlight control 24: Oncoming headlights and preceding taillights may be 
               
               
                 detected without any host vehicle illumination. NIR illumination may be detrimental to 
               
               
                 distinguishing, using color, between reflectors at the side of the road and distant taillights. It 
               
               
                 may be important the central strip of the image not be illuminated by NIR during at least one 
               
               
                 frame. 
               
               
                 Lane departure warning 21: LDW can benefit from NIR illumination for distance road 
               
               
                 markings. In order to determine lane mark color it would be best to not have NIR 
               
               
                 illumination in the lower region of the image which may be well illuminated by the low 
               
               
                 beams. 
               
               
                   
               
             
          
         
       
     
     The above requirements in Table 2, lead to an illuminator  4  design which spreads the energy vertically over +/−5 deg and horizontally +/−20 degrees. 
       FIG. 4 b    shows an NIR spatial profile  60  overlaid on an image with respective percentage (%) NIR intensities for frames  204 / 206 . 
     Reference is now made to  FIG. 5  which shows schematically a typical spatial distribution  60  in image space coordinates of NIR illumination cone, according to a feature of the present invention. The profile may be not optimal as its horizontal spread should be greater. It may be most interesting to look at how such an illumination profile matches the sensor timing. 
     Reference is now made to  FIG. 6  which shows the NIR spatial profile  60  overlaid on an image, according to an feature of the present invention. By carefully aligning the row timing and the NIR profile  60  maximum illumination on the important regions of the image frames during the long exposure frame  204  and virtually no illumination during the top and bottom rows of the short exposure image frame  206 . Thus the short exposure image frame  206  may be almost clean of NIR artifacts. As an added bonus, the flickering of a reflector between short and long exposures  206  and  204  respectively, can further aid in distinguishing between reflectors at the side of the road and distant taillights. 
     Effects of High Dynamic Range (HDR) Imagers 
     Consider the high dynamic range (HDR) integration settings for the long exposure frame shown in Table 1. The first integration period of 14 ms is followed by integration period of 0.845 ms and then 0.054 ms. Typically a very bright patch in the image will saturate the long and medium integration periods and the pixel value will be determined by the short integration period. Less bright patches will perhaps saturate during the long integration period and the pixel value will be determined by the medium integration period. This high dynamic range (HDR) method is not designed for flickering light sources and has the following behavior: for patches that are saturated at the long integration period but then have no response during the medium and short integration periods, the pixel has a maximum value of about 256 (in 12 bit image with a maximum of 4095). For patches that are saturated at the medium integration period but then have no response during the short integration period, the pixel has a maximum value of about 1024. Referring again to  FIGS. 4 a  and 4 b   , the bottom rows in the image the NIR illumination will not continue till the end of the frame. This means that for a few rows there will be only partial illumination during the short integration. Then for about 37 rows there will be no illumination during the short integration period and for only part of the medium integration period. Then for about 292 rows there will not be illumination during the short or medium integration periods. For these lines the maximum pixel value on reflectors will be limited to a maximum of 256. 
     Pedestrian Detection 
     Consider now a pedestrian illuminated by the NIR beams. Clothing, even black clothing, may be often very reflective to NIR and can appear saturated in the image. However due to the effect described above, the bottom part of the pedestrian might not reach above a value of 256. Then there will be a band with a value of 1024 and only the top part will reach the true brightness levels above 1024. This makes pedestrian detection difficult as the pedestrian will look different depending on the exact vertical location in the image. It will also look bad in the display. These two issues may be addressed separately. 
     Pedestrian detection can be divided into four steps: (1) quickly finding initial candidates (2) more expensive pattern recognition often using an example based classifier (3) tracking over time and accumulating classification score (4) final approval or rejection. Each may be affected differently by the NIR and high dynamic range (HDR) timing:
     1. Finding initial candidates: This first step often uses vertical and horizontal edges. The vertical edges, based on horizontal derivatives, are significantly not affected be the timing issue. The horizontal derivatives of the two lines, above and below the transition should be computed without vertical smoothing. The horizontal edges can be computed as usual but suppressing fake edges at the transition rows if the lower row value may be at the relevant saturation level (256 or 1024). The high dynamic range (HDR) effect can be used as a rejection cue since any pixel below the 1024 or 256 cutoff lines and with values above 1024 or 256 respectively must be a light source and not a reaction.   2. Classifier: The classifier cannot work directly on the images where the lower part is clipped at 256 and 1024 since the cutoff line will hit each pedestrian at a different vertical point on the body. One may simply clip the whole patch to be classified at 256 but then useful information might be lost. An alternative may be to leave the top half of the patch unchanged and the lower half (which includes both the 1024 and the 256 cutoff lines) is then clipped to 256. This gives uniform images but allows for full detail at least in the upper part of the body.   3. Tracking: Tracking may be performed on the patch and image after is clipped to 256.   4. Final approval: This stage combines information from previous stages and does not address the image directly.   

     Night Vision Display 
     According to an aspect of the present invention the near infra-red illumination may be switched on and off in alternating frames  204 / 206 . Multiple NIR illuminated frames  204  may be provided to processor  14  which may be used by pedestrian detection  23  for example. Multiple non-NIR illuminated frames  206  may be provided to processor  14  which may be used by AHC  24  and TSR  20  driver assistance systems for example. Partitioning of frames  204 / 206  into either NIR illuminated  204  or non NIR illuminated frames  206  may also alternate between a longer or a shorter sensor  6  exposure time. The shorter sensor  6  time may be useful to give better color information and less motion blur when compared with the longer exposure time of sensor  6 . 
     A consequence of switching the NIR illumination on and off in alternate frames may be that for a night vision display, only half the frames may be usable and the effective frame rate may be halved. If 20 Hz is the minimum acceptable frame rate for night vision display then the acceptable frame rate must be 40 Hz or higher. If instead of switching off the NIR illuminator  4 , the power is reduced to 50% every frame  206 , the low reduced NIR power frame  206  can still be used for night vision display. The power values cannot simply be multiplied by a factor of two since not all the image brightness may be due to the NIR illumination. Some of the image brightness may be from actual light sources and the lower part of image frames  204 / 206  may also be illuminated by low beams of host vehicle  18 . 
     The first step may be to calibrate the low beams versus the NIR illuminator  4  to determine a smooth image function that describes by what value may be needed to multiply that particular pixel when the NIR illumination may be halved. This calibration may be done automatically when vehicle  18  may be stationary. During driving the 50% frame  206  may be multiplied by a factor α. Pixels which are bright and suspected of being a light source may be tracked between consecutive frames  204 / 206  by first assuming a scaling of a and secondly using a scaling of 1 for a light source. The match that gives the best tracking score. 
     The above discussion assumed that the exposures are linear and remain constant between consecutive frames. If a high dynamic range (HDR) sensor may be used then the image values can be mapped to a linear image before scaling. If alternating exposure values are used then scaling values may be adapted accordingly. 
     The near infra-red (NIR) and high dynamic range (HDR) timing produce two horizontal cutoff lines in the image. In order to produce a smooth transition, pedestrians and other regions may be treated separately. For pedestrians, if the upper part of the pedestrian, just above the cutoff line is above the 1024 cutoff value, first compute an average value for all pixels of 1024 and up. Then all pixels are colored of the detected pedestrian that are below the cutoff line, that are of the cutoff values of 256 or 1024 according to this average pixel value. A similar technique may be used to blend the pixels below the lower cutoff line if need be. 
     For non pedestrian regions a similar idea may be used, however:
     (1) We only extend it to 100 rows below the upper cutoff line with a linear transition between the average value and the cutoff value of 256.   (2) We only affect pixels that are connected to pixels above the cutoff line which are above the cutoff value. This may be done using a recursive pixel fill (or paint) operation.   

     Switching the Near Infra-Red (NIR) Source on Each Odd Row 
     If LED illuminators  4  are used then can be switched on and off at very high frequencies. Consider what would happen if during the 22 ms when the near infra-red (NIR) is typically on, the near infra-red (NIR) source was switched off during the short integration period of the odd rows. If the short integration period matched the shift between start times of consecutive rows then reflectors would not reach values above 1024 on odd rows. Thus it would be simple to tell if a bright patch was a light source or a reflector: it would have even rows reaching levels above 1024 and odd rows clipped at 1024. 
     Close and mid-range pedestrians will often reach values above 1024. For near infra-red (NIR) display, if there is a pixel on an odd row with a value of 1024 and higher values above and below, the 1024 may be replaced with the average value. It may be possible to do the same for pedestrian detection: the edge detection for initial candidates will still work well and so will a classifier if the classifier is trained to examples that were processed in a similar manner. 
     The indefinite articles “a”, “an” is used herein, such as “an illuminator”, “a driver assist system” have the meaning of “one or more” that is “one or more illuminators” or “one or more driver assist systems”. 
     Examples of various features/aspects/components/operations have been provided to facilitate understanding of the disclosed features of the present invention. In addition, various preferences have been discussed to facilitate understanding of the disclosed features of the present invention. It is to be understood that all examples and preferences disclosed herein are intended to be non-limiting. 
     Although selected features of the present invention have been shown and described individually, it is to be understood that at least aspects of the described features may be combined. Also although selected features of the present invention have been shown and described, it is to be understood the present invention is not limited to the described features. Instead, it is to be appreciated that changes may be made to these features without departing from the principles of the invention, the scope of which is defined by the claims and the equivalents thereof.