Abstract:
A brightness control system enhances a displayed image. The system selectively alters the brightness of a portion of the image to improve the clarity, noticeability, and intelligibility of information provided on the display. When used with a vehicle navigation system, the brightness adjustment system may emphasize vehicle or destination location display, driving directions, route planning information, or other information provided to the driver.

Description:
PRIORITY CLAIM  
       [0001]     This application claims the benefit of priority from European Patent Application No. 05002975.0, filed Feb. 11, 2005, which is incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The invention relates to a brightness controller for adjusting the brightness of an image on a display. In particular, the invention relates to a brightness control system for modifying the brightness of specific portions of an image.  
         [0004]     2. Related Art  
         [0005]     Both analog displays such as Cathode Ray Tubes (CRTs) and digital displays such as Liquid Crystal Displays (LCDs) create images from individual closely spaced points. Each point in the image is referred to as a picture element or pixel. The image data in the video data stream sent to the display conveys information which controls the brightness of each pixel.  
         [0006]     In the past, dynamic changes to image brightness often required time consuming and computationally expensive brightness recalculations. This was particularly true when the recalculation of image data was done on a pixel-by-pixel basis. With the introduction and widespread use of high resolution displays, processing demands for dynamic brightness changes were often quite significant.  
         [0007]     Displays are common components of vehicle navigation systems, entertainment systems, and game systems. A navigation system, for example, provides core functionality including the display of map location and route information to the driver. In addition to these core functions, the navigation system often provides extensive additional functionality for the driver. Thus, the processing demands on the navigation system (and other systems) are already significant, even without the added burden of dynamic image brightness control.  
         [0008]     Therefore, a need exists for an improved brightness control system.  
       SUMMARY  
       [0009]     A brightness control system adjusts the brightness of a portion of an image, without time consuming or computationally expensive pixel-by-pixel image recalculation. When used with a vehicle navigation system, the brightness control system may emphasize vehicle or destination location display, driving directions, route planning information, or other information provided to the driver. The brightness control system may be used with any other system which displays images, including entertainment systems, game systems, communication systems, or other systems.  
         [0010]     The brightness control system includes a video signal input and a brightness control signal input. A video signal including pixel image data is provided on the video signal input, while a brightness control signal is provided on the brightness control signal input. The brightness control signal is synchronized with the video signal (e.g., with horizontal synchronization information) to specify enhancement for a selected pixel subset of the pixel image data. Modification logic in the brightness control system enhances the brightness of the selected pixel subset. To that end, the modification logic may determine a brightness adjustment responsive to the brightness control signal and apply the brightness adjustment to the selected pixel subset of the pixel image data.  
         [0011]     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views.  
         [0013]      FIG. 1  shows a brightness control system.  
         [0014]      FIG. 2  shows an image in which a selected pixel subset has been brightened.  
         [0015]      FIG. 3  shows an image in which a selected pixel subset has been brightened.  
         [0016]      FIG. 4  shows an image in which a selected pixel subset has been brightened.  
         [0017]      FIG. 5  shows an image in which a selected pixel subset has been brightened.  
         [0018]      FIG. 6  shows an image in which a selected pixel subset has been brightened.  
         [0019]      FIG. 7  shows selective brightness control of specific pixels in a horizontal line of an input video signal.  
         [0020]      FIG. 8  shows a brightness control system.  
         [0021]      FIG. 9  shows the creation of a brightness adjustment from a pulse width modulated brightness control signal.  
         [0022]      FIG. 10  shows pulse width modulated control of the brightness of a portion of an image.  
         [0023]      FIG. 11  shows pulse width modulated control of the brightness of a portion of an image.  
         [0024]      FIG. 12  shows pulse width modulated control of the brightness of a portion of an image.  
         [0025]      FIG. 13  shows pulse width modulation control of a brightness adjustment.  
         [0026]      FIG. 14  shows a vehicle bus interfaced to electronic modules which incorporate brightness control systems.  
         [0027]      FIG. 15  shows a flow diagram of the acts which the brightness control system may take to selectively enhance the brightness of a portion of an image.  
         [0028]      FIG. 16  shows a flow diagram of the acts which a modulator may take to determine a modulated reference voltage.  
         [0029]      FIG. 17  shows a flow diagram of the acts which the brightness control system may take to selectively enhance the brightness of a portion of an image in response to vehicle bus messages.  
         [0030]      FIG. 18  shows a black-level adjustment to enhance brightness of a portion of an image.  
         [0031]      FIG. 19  shows a contrast adjustment to enhance brightness of a portion of an image.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     In  FIG. 1 , an image brightness control system  100  includes a video signal input  102 , a brightness controller  104 , a brightness control signal input  106 , and a video signal output  108 . A control signal generator  108  provides a brightness control signal on the brightness control signal input  106 . A reference voltage input  110  provides a reference voltage  112  to the brightness controller  104 . In response to the brightness control signal, the brightness controller  104  selectively alters the brightness of the image or portion of the image represented by the output video signal  114 , compared to the original image represented in the input video signal  116 .  
         [0033]     The image may include, for example, navigation information  118  received from a navigation system  120 . The brightness control system  100  may receive and enhance input video signals from many other sources, however, including entertainment systems, video game systems, car phone systems, or other systems. The navigation system  120  may include a receiver for navigation signals, including radio signals from a Global Positioning System (GPS), radio signals from a LOng RAnge Navigation (LORAN) system, or other navigation signals. The navigation information  118  may include map data, such as street, road, intersection, bridge, viaduct, ferry, building, traffic regulation and street closure information; point of interest (POI) information; road and weather conditions; directory and address range information; destination information; route information; or other navigation information The navigation information  118  may also include position and motion data, such as relative position, altitude, bearing, average velocity, instantaneous velocity, acceleration, expected position and/or expected arrival time information. Any of the navigation information  118  may be rendered in the image, and may be selectively enhanced for clarity and intelligibility through brightness adjustment of the pixels which represent the navigation information  118  in the display, or through brightness attenuation of surrounding pixels.  
         [0034]     The input video signal  116  may convey a sequence of individual images to the brightness controller  104 . The input video signal  116  may also include timing and synchronization information such as horizontal and vertical blanking period information, horizontal and vertical synchronization information, luminance and color information, and other video signal information. The input video signal  116  may include or may be encoded as a luminance/chrominance signal (e.g., a YUV signal), red-blue-green (RGB) signal, or as another type of signal. Thus, input video signal  116  may define the brightness and color for each pixel in the image, as well as the horizontal (e.g., line), vertical (e.g., frame), and pixel timing and synchronization information for displaying the image on an analog or digital display.  
         [0035]     The control signal generator  108  may include synchronization logic responsive to the synchronization information included in or accompanying the input video signal  116 . The control signal generator  108  generates a brightness control signal synchronized to the position of pixels to be brightened (or to be attenuated) in one or more frames of an image sequence. The brightness control signal may be synchronized with respect to any combination of horizontal synchronization information, vertical synchronization information, pixel timing information, or other information in or accompanying the input video signal  116 . The synchronized brightness control signal may specify a brightness adjustment or attenuation for any selected pixel subset in the pixel image data represented in the input video signal  116 .  
         [0036]     The control signal generator  108  may also receive the navigation information  118  from the navigation system  120 . The brightness control signal may then be synchronized to the presence and/or location in the image of any desired navigation information  118 , or to the presence and/or location of any other desired information provided by an entertainment system, game system, or other system. For example, the brightness control signal may be asserted for those pixels at which the navigation information  118  (e.g., a vehicle location or destination location) is rendered in the image, for the pixels at which the navigation information  118  is absent, or for image areas (e.g., a bounding box or circle) around those pixels.  
         [0037]     The reference voltage  112  may specify a brightness adjustment for the brightness controller  104 . Alternatively, the brightness controller  104  may derive a brightness adjustment using the reference voltage  112 . For example, when the brightness control signal is asserted, the brightness controller  104  may use pulse width modulation and a resistor-capacitor (RC) filter to derive a brightness adjustment responsive to the reference voltage  112 .  
         [0038]     The brightness controller  104  receives the input video signal  116  and the brightness control signal. In response to the brightness control signal, the brightness controller  104  determines a brightness adjustment and enhances the brightness of the pixel subset to which the brightness control signal is synchronized. The brightness controller  104  avoids time consuming and computationally expensive recalculation of image data on a pixel-by-pixel basis. Instead, the brightness controller responds to the synchronized brightness control signal which specifies the pixel subset for which to enhance brightness.  
         [0039]      FIG. 2  shows an example of brightness control for a subset of image pixel data in a navigation map  200  on a display. In the example shown in  FIG. 2 , the navigation map  200  shows a section of a larger map around the area in which the vehicle currently is located. The current position of the vehicle is indicated by an icon  210  (e.g., an arrow pointing in the direction of vehicle movement) on the map  200 . The brightness controller  104  provides a brightness enhanced region  220  around the icon  210 . While the region  220  is shown as a rectangle, the region  220  may take other shapes, including a circle shape, oval shape, polygonal shape, sector shape, or other shape.  
         [0040]     The driver may select the shape (e.g., square or circular), size (e.g., 100×100 pixels or 50 pixel radius), and position (e.g., centered on the vehicle location, destination, or a point of interest) of the region  220  using any available operator input. The operator input may include a voice recognition system, keyboard including cursor keys, or other input. The brightness controller  104  may enhance the brightness of the region  220  by increasing the overall brightness of a subset of image pixel data corresponding to the region  220 . Alternatively or additionally, the brightness controller  104  may enhance the brightness of the region  220  by attenuating the brightness of the pixels surrounding the region  220 . The selective enhancement is particularly effective at night, when the overall illumination of a vehicle cabin is low. The brightness controller  104  thereby helps to increase the clarity, intelligibility, and speed of recognition of the information provided on the display.  
         [0041]      FIG. 3  illustrates a navigation map  300  including an additional brightness enhanced subset of image pixel data in the form of a directional arrow  302 . The navigation system  120  may generate the directional arrow  302  to indicate the next direction to take on the route to a destination. The brightness control signal may then be asserted for each pixel which makes up the directional arrow  302  and/or the region  220 , or for each pixel which makes up the surrounding regions.  
         [0042]      FIG. 4  illustrates a navigation map  400  including a brightness enhanced subset of image pixel data in the form of a target circle  402 . The target circle  402  highlights a vehicle destination  404 . Alternatively, the target circle  402  may highlight an area of interest, location of traffic congestion, a locus of points of pre-defined distance from a position, or other location of interest. Multiple target circles or other shapes may be generated for multiple locations of interest.  FIG. 5  illustrates another example brightness control for a subset of image pixel data. In  FIG. 5 , a brightness enhanced hazard symbol  502  is rendered on the map  500 . The hazard symbol  502  may represent a serious traffic, road, or weather condition at any given location. Furthermore, the hazard symbol  502  may be displayed to provide a warning about a particular vehicle condition, such as an engine malfunction, low tire pressure, fuel, oil, or other fluid level warning, an electronics error, GPS signal error, or a collision avoidance warning. The brightness control system  106  may flash or otherwise draw attention to the warning symbol  502  (or any other brightness enhanced region on the display) by altering the brightness enhancement between different levels, e.g., on a periodic basis.  
         [0043]      FIG. 6  illustrates another example of brightness enhancement in a navigation map  600 . In  FIG. 6 , the navigation map  600  includes a brightness enhanced road information symbol  602 . The road information symbol  602  may indicate a road or traffic condition, such as a limited access road, construction site, parade route, or any other time or space-restricted road condition. As another example, the road information symbol  602  illustrated in  FIG. 6  may depict a one-way road, with the direction of the arrow pointing in the direction of allowed traffic flow.  
         [0044]      FIG. 7  illustrates voltage-time diagrams which show brightness control selectively activated during a horizontal line of a displayed image.  FIG. 7  shows a portion of the input video signal  116 , a modulated reference voltage  702 , and a brightness enhanced output video signal  114 . Several points in time, t 1 , t 2 , t 3 , and t 4  are labeled on the horizontal axis. The points in time t 1  and t 4  correspond to the beginning and end of a horizontal line of the input video signal  116 . The input video signal  116  includes an active video portion  704  which includes brightness and color information and also includes horizontal blanking intervals  706 . The black-level  708  represents the brightness of the darkest portion of the image.  
         [0045]     The modulated reference voltage  702  assumes a maximum value, Uref,max during the time interval between t 2  and t 3 . In the time periods before t 2  and after t 3 , the modulated reference voltage  702  may be reduced. In the example shown in  FIG. 7 , the modulated reference voltage is reduced by V, resulting in a nominal level Uref,max−V. The brightness control signal may be asserted between t 2  and t 3  (or before t 2  and after t 3 ) to coordinate brightness enhancement with the brightness controller  104 .  
         [0046]     Specifically, the brightness control signal may be asserted between t 2  and t 3  to prevent the brightness controller  104  from applying an attenuation to the active video portion  704  of the input video signal. The assertion of the brightness control signal may cause the modulated reference voltage to rise (e.g., to the value Uref,max) to a value which prevents attenuation of the input video signal between t 2  and t 3 . The assertion of the brightness control signal may also cause the modulated voltage to rise to an intermediate level which causes the brightness controller  104  to apply less attenuation between t 2  and t 3  than before t 2  or after t 3 . Alternatively, the brightness control signal may be asserted before t 2  and after t 3  to instruct the brightness controller  104  to apply an attenuation to the active video portion  704  of the input video signal outside of the range t 2  to t 3 . The brightness control signal may specify the amount of attenuation (e.g., the brightness adjustment), the brightness controller  104  may determine the amount of attenuation using pulse width modulation techniques, or the brightness controller may determine the brightness adjustment in other manners.  
         [0047]     The brightness controller  104  modifies the input video signal  116  to enhance the brightness of the pixel subset selected through synchronization of the brightness control signal with the input video signal. The resulting output video signal  114  conveys a brightness enhanced pixel subset to the display. In particular,  FIG. 7  shows the original level  710  of the input video signal  116  superimposed on the enhanced level  712  of the output video signal  114 .  
         [0048]     The brightness controller  104  responds to the brightness control signal and the modulated reference voltage  702  to hold the voltage level of the input video signal unchanged between t 2  and t 3 . In addition, the brightness controller  104  attenuates the voltage level of the input video signal before t 2  and after t 3 . Thus, the image area around the selected pixel subset is lowered in brightness, thereby enhancing the perceived brightness of the selected pixel subset. Brightness enhanced pixel data  714  results between t 2  and t 3 . Depending on the sampling rate of the active portion of the output video signal  114 , there many be one or more pixels defined in the pixel data  714 . The brightness control signal and modulated reference voltage  702  may change line-by-line and pixel-by-pixel to specify the shape or shapes to be brightness enhanced.  
         [0049]     The attenuation applied to the input video signal  116  may be set as a configurable gain factor (e.g., an operator settable gain factor) for brightness enhancement. Alternatively, the brightness control signal may specify the gain factor. The gain factor may also be based on, and vary according to, the value of V. In other implementations, the brightness controller  104  may enhance the selected pixel subset by applying a configurable amplification factor to the input video signal  116  between t 2  and t 3 , while holding constant, attenuating, or applying a smaller gain to the input video signal before t 2  and after t 3 .  
         [0050]      FIG. 8  shows a more detailed implementation of the brightness control system  100 . The control signal generator may include a control signal source  802  and synchronization logic  804 . The control signal source  802  supplies a control signal to the synchronization logic  804 . The control signal source  802  may include a DC voltage source, a pulse generator, waveform generator, look-up-table, mathematical function generator, square-wave generator, an external source of control signal input, a pulse width modulated signal source, or other source.  
         [0051]     The synchronization logic  804  determines when to assert the brightness control signal to the brightness controller  104 . The brightness control signal is synchronized to horizontal, vertical, and/or pixel synchronization information in or accompanying the input video signal  116  and/or to other sources of information, such as the navigation information  118 . The control signal generator  108  may specify in the brightness control signal (or with separate signals) the brightness adjustment associated with selected subsets of image pixel data. The synchronization logic  804  may be implemented with counters, timers, and/or other timing logic using integrated or discrete circuits, including a microprocessor, microcontroller, digital signal processor (DSP), ASIC, FPGA, custom IC, or other circuitry. Alternatively or additionally, the synchronization logic  804  may include programs or firmware encoded or stored in a memory to count lines, frames, pixel clock transitions or any other synchronization data to locate any specific pixel or pixels in an image, and responsively assert the brightness control signal.  
         [0052]     In the brightness controller  104 , a modulator  806  may establish the brightness adjustment for the selected pixel subset (e.g., by varying the modulated reference voltage  702  or the voltage V). To that end, the modulator may switch a reference voltage ‘on’ and ‘off’ at a specified duty ratio. The reference voltage  112  may provide the DC voltage, while the control signal generator  108  may specify the duty ratio (i.e., the fraction of a period during which the voltage is switched ‘on’). However, the duty ratio and reference voltage may be set according to an operator preference setting for brightness enhancement, may be a pre-configured parameter, or the duty ratio may be established in other manners.  
         [0053]      FIG. 9  illustrates a filtering operation which the modulator  806  may employ to convert a pulse width modulated control signal into a DC brightness adjustment level. The left hand portion of  FIG. 9  depicts a sequence of voltage pulses  1405  having a period T, a duty ratio d and a maximum voltage Vm. The duty cycle, d, may be determined by dividing the period during which the pulse has a high value, here dT, by the total period of the waveform, here T. This yields dT/T=d for the duty cycle. The right hand part of  FIG. 8  indicates the DC brightness adjustment  1410  resulting from filtering the pulse width modulated control signal using an RC integrator. The brightness adjustment  1410  may correspond to the duty ratio d. Thus, by adjusting the duty ratio d, every voltage value between 0 and Vm may be generated as a brightness adjustment. The brightness adjustment may be determined in other manners, however, including using a digital to analog converter (DAC), or other circuitry.  
         [0054]     The brightness control signal may cause the modulator  806  to generate or change the brightness adjustment. For example, the brightness control signal may itself be a pulse width modulated signal, converted to the brightness adjustment. Alternatively, the brightness control signal may trigger the application, generation, or modification to another source of a pulse width modulated signal (e.g., by gating ‘on’ and ‘off’ the reference voltage input  112 ) to obtain the brightness adjustment. Using an RC filter, the duty ratio, d, over the cycle period T, may set the brightness adjustment according to the following equation:  
         1   T     ⁢       ∫   T     T   +   d       ⁢       U   ⁡     (   t   )       ⁢     ⅆ   t             
 
         [0055]     where the voltage function U(t) is given by  
                   U   ⁡     (   t   )       ⁢     {           U   0         for         kT   ≤   t   ≤       (     k   +   d     )     ⁢   T               0       for             (     k   +   d     )     ⁢   T     ≤       (     k   +   1     )     ⁢   T             }     ⁢   k     =   0     ,   1   ,   2   ,   …           (   2   )             
 
         [0056]     The result is  
                 1   T     ⁢       ∫   T     T   +   d       ⁢       U   ⁡     (   t   )       ⁢     ⅆ   t           =     U   0             (   3   )             
 
         [0057]     Thus, by varying the duty ratio, d, between 0 and 100%, voltage values in the range between 0 and U 0  may be obtained.  
         [0058]      FIGS. 10, 11 , and  12  illustrate a sequence of brightness enhanced images controlled by pulse width modulation. During the sequence, the brightness controller  104  changes the degree of enhancement of a selected pixel subset corresponding to a turn arrow  1002 . The turn arrow  1002  becomes darker during the sequence to slowly fade the turn arrow  1002  from the image.  
         [0059]     In  FIG. 10 , a pulse width modulated control signal  1004  determines the brightness adjustment applied to the turn arrow  1002 . With the duty ratio set at 90%, a relatively large brightness enhancement is selected. Thus, the pixels corresponding to the turn arrow  1002  may be amplified in brightness, or the surrounding pixels may be attenuated in brightness according to the relatively large brightness enhancement. In  FIG. 11 , the pulse width modulated control signal  1102  has a duty ratio of 50%. The turn arrow  1002  is therefore less enhanced than the turn arrow  1002  shown in  FIG. 10 . In  FIG. 12 , the pulse width modulated control signal  1202  has a duty ratio of 10%. Accordingly, the turn arrow  1002  is less enhanced than that shown in  FIG. 11 . As a result, the brightness controller  104  initially enhances the turn arrow  1002  then fades out the enhancement.  
         [0060]     In  FIG. 8 , the enhancement logic  808  modifies the input video signal  116  to apply the desired brightness adjustment to any selected pixel subset of the image. For example, the enhancement logic  808  may include an analog multiplier, attenuator, or level shifter which applies a gain or attenuation based on the brightness adjustment to a selected part of the active portion  704  of the input video signal  116 . Alternatively, the enhancement logic  808  may include an analog to digital converter, a digital adder or multiplier, and a digital to analog converter to apply the brightness adjustment and output the enhanced video signal. As another example, the enhancement logic  808  may include a digital adder or multiplier which modifies digital RGB, YCbCr, or other digital values, according to the brightness adjustment, in a digital video signal for a digital display to change the perceived brightness of the image. The brightness adjustment may be expressed in terms of a ratio (e.g., 50% brighter or dimmer than the original signal), and absolute measure (e.g., 2V signal level or a specific brightness determined by R, G, and B values), or in other manners.  
         [0061]      FIG. 13  provides an additional example of pulse width modulated brightness control. The enhancement logic  808  receives a digital input video signal  1302 , which includes RGB image data. The enhancement logic  808  enhances selected pixels of the input video signal by modifying the levels of the RGB data in response to a synchronized brightness control signal and according to the brightness adjustment to generate the output video signal  1304 .  
         [0062]     The brightness adjustment is represented in  FIG. 13  by the Uref signal  1312 . The brightness adjustment signal  1312  may vary in accordance with a pulse width modulated control signal  1306  output by the pulse width modulator  1308 . An integrating stage  1310  (e.g., an RC filter) converts the pulse width modulated control signal  1306  to the brightness adjustment signal  1312 . As the duty cycle of the modulated control signal  1306  varies, the brightness adjustment signal  1312  changes to specify a new level of enhancement for the selected pixel subset.  
         [0063]     The brightness control system  100  may be incorporated into stand alone devices (e.g., portable game systems or personal data assistants) as well as any vehicle component.  FIG. 14  shows a vehicle component system  1400  in which the components interface and communicate on a vehicle bus  1402 . The vehicle component system  1400  includes electronic modules which display information for the driver and passengers. Examples of the electronic modules include vehicle entertainment systems  1404  (e.g., a DVD player, CD player, portable music player, or other entertainment system), video game systems  1406 , and a navigation system  1408 .  
         [0064]     The electronic modules  1404 - 1408  may accept input from the operator inputs  1410 , including a voice recognition system, keyboard, function switches, or other inputs. The operator inputs  1410  may communicate operator selections over the vehicle bus  1402  to any of the electronic modules  1404 - 1408 . Alternatively, one or more of the electronic module  1404 - 1408  may include local operator inputs. Sensors  1412  may measure and communicate levels of ambient light, noise, vehicle performance, or other data to a bus interface (e.g., the bus interface  1416  in the navigation system  1408 ) of any of the electronic modules  1404 - 1408 .  
         [0065]     Each electronic module  1404 - 1408  may include a brightness control system  100  and a display. Thus, for example, the navigation system  1408  may enhance the clarity, intelligibility, or visibility of any portion of navigation information provided on the navigation display  1414 . In the navigation system  1408 , the processor  1418  executes, from a memory  1420 , a navigation program  1422 . The navigation program  1422  may provide route guidance data  1424 , location information  1426 , or any other navigation data to the driver on the navigation display  1414 . A navigation database  1428  may provide a source of map data, route data, points of interest data, or any other navigation data for the navigation program  1422 . In addition, the memory  1420  may store shape data. The shape data may define the shapes for image enhancement regions. For example, the vehicle position region shape data  1432  may define the shape and/or size of the enhancement region  220 , while the point-of-interest region shape data  1434  may define the shape and/or size of the enhancement region  402 .  
         [0066]     In response to the data provided by the sensors  1412 , any of the electronic modules  1404 - 1408  may initiate brightness control over displayed images. As one example, a light level sensor may detect and report ambient light levels. The navigation system  1408 , for instance, may respond to low ambient light levels (e.g., below a low-light threshold) to enhance the brightness of navigation information on the navigation display  1414 . Such light levels may be present at night, when the vehicle is underground, or when the vehicle travels through a tunnel. Similarly, when the ambient light levels are significant (e.g., above a high-light threshold), the navigation system  1408  may also enhance brightness to make selected navigation information more visible on the navigation display  1414 .  
         [0067]      FIG. 15  illustrates acts which the brightness control system  100  may take to enhance brightness of an image or selected pixels in the image, particularly in the context of a vehicle navigation system. The navigation system  120  supplies the navigation information to the brightness control system  100 . In addition, the brightness controller  104  receives an input video signal  116  (Act  1504 ). The input video signal  116  may be an analog or digital video signal which defines a sequence of images.  
         [0068]     The brightness control system  100  may determine whether to buffer the input video signal  116  (Act  1506 ). The video buffer may store image data for later processing (including brightness enhancement), frame synchronization, error checking, or other processing. When the brightness control system  100  determines to buffer the input video signal  116 , the brightness control system  100  stores the image data in a memory (Act  1508 ).  
         [0069]     The control signal generator  108  synchronizes to horizontal, vertical, and/or pixel synchronization information in or accompanying the input video signal  116  (Act  1510 ). The control signal generator outputs a brightness control signal which specifies a selected pixel subset in the image pixel data for brightness enhancement (Act  1512 ). The brightness control signal may directly specify a brightness adjustment, may specify the duty ratio of am modulated control signal which determines a brightness adjustment, or the brightness control system  100  may determine the brightness adjustment in other manners (e.g., by retrieving a configurable operator preference setting from a memory).  
         [0070]     In one implementation, the brightness control system  100  modulates a reference voltage to determine the brightness adjustment. To that end, the brightness control system  100  receives the reference voltage input  112  (Act  1514 ). The modulator  806  may apply a pulse width modulated control signal to switch the reference voltage ‘on’ and ‘off’ through an RC filter. A DC brightness adjustment results, at a level which is set according to the duty ratio of the modulated control signal.  
         [0071]     The brightness control system  100  may enhance image brightness in many different ways. Accordingly, the brightness control system  100  determines which enhancement technique to apply (Act  1518 ) using preset preferences or settings, by accepting operator input, or in other manners. As examples, the brightness control system  100  may select between black level adjustment, contrast adjustment, direct manipulation of an active portion of the input video signal  116 , direct manipulation of RGB or other pixel data, or other techniques. The brightness control system  100  may also determine whether to amplify brightness of a selected pixel subset, attenuate brightness of surrounding pixels, or perform the enhancement in another manner. The brightness controller  104  applies the brightness adjustment to the selected pixel subset (Act  1520 ). The brightness control system may then provide the output video signal  114  to the display (Act  1522 ).  
         [0072]      FIG. 16  illustrates the acts which the modulator  806  may take to establish a modulated reference voltage for use as a brightness adjustment. The modulator  806  obtains an input reference voltage  112  (Act  1602 ). The reference voltage may establish a voltage level which the modulator will pulse width modulate to obtain the modulated reference voltage.  
         [0073]     To that end, the modulator  806  determines a duty ratio, d, for the pulse width modulation (Act  1604 ). The reference voltage is switched ‘on’ and ‘off’ at the duty ratio. (Act  1606 ). The modulator  806  integrates the switched reference voltage (e.g., using an RC filter) to obtain the modulated reference voltage (Act  1608 ). The modulated reference voltage may take any value between 0 and the value of the reference voltage, and may be set as the brightness adjustment applied to image data through black level adjustment, contrast adjustment, direct manipulation of the active portion of the input video signal  116 , or through other techniques (Act  1610 ).  
         [0074]      FIG. 17  illustrates the acts which the brightness control system  100  may take to enhance brightness in response to bus messages on the vehicle bus  1402 . A bus interface  1416  receives bus messages transmitted on the vehicle bus  1402  (Act  1702 ). Any of the electronic modules  1404 - 1408  may prepare and send the bus messages. In addition, the bus messages may be sent by any of the sensors  1412  or operator inputs  1410 .  
         [0075]     For example, the operator inputs  1410  may send bus messages which convey operator commands to move, resize, reshape, or change the brightness adjustment to brightness enhanced regions on the display, such as those shown in  FIGS. 2-6 . As another example, the sensors  1412  may transmit bus messages conveying the ambient light level, vehicle status and warnings, noise levels, speed, proximity to other objects, or any other information. The brightness control system  100  extracts the data from the bus message (Act  1706 ). Based on the data, the brightness control system  100  may determine whether to enhance the brightness of a selected pixel subset of an image (Act  1706 ).  
         [0076]     For example, when the brightness control system  100  receives a bus message that ambient light is relatively low, the brightness control system  100  may enhance the brightness of selected navigational information in the image. The navigational information is thereby made more noticeable and intelligible for the driver. As another example, when the brightness control system  100  receives warning information about speed, proximity to other objects, engine (or other vehicle system) operation, the brightness control system  100  may respond by enhancing the brightness of a portion (e.g., a warning message) of an image.  
         [0077]     The enhancement logic  808  may enhance image brightness in other manners than that described above. As additional examples, the enhancement logic  808  may enhance image brightness through black level adjustment or contrast adjustment using level shifters, analog or digital adders or multipliers, or other circuitry.  FIG. 18  illustrates a black level adjustment for a selected pixel subset of an image. The enhancement logic  808  may apply a modulated reference voltage as an additive component to the selected pixel subset or as a subtractive component to pixels around the selected pixel subset to enhanced the brightness of the selected pixel subset in the output video signal  114 .  
         [0078]     In  FIG. 18 , a segment of an active portion  1802  of an input video signal conveys pixel image data for a portion of a line in the image. The active portion  1802  varies in voltage between the black level  1804  and the maximum brightness voltage  1806 . The brightness controller  104  level shifts the active portion  1802  to enhance brightness of a portion of the image.  
         [0079]     In particular, the brightness controller  104  level shifts the active portion  1802  of the input video signal by the reference voltage  1808 . The reference voltage  1808  may be provided by the reference voltage input  112 , by modulating a reference voltage (e.g., using an RC filter), by the control signal generator  108 , or by other sources of a brightness adjustment. After level shifting, the segment of the active portion  1802  is lowered in voltage.  
         [0080]      FIG. 18  shows the corresponding segment of the active portion  1810  of the output signal. The black level  1812  has been reduced by the reference voltage  1808 . In addition, there is a corresponding decrease in the maximum brightness voltage  1814  in the output video signal. A decrease in brightness for the segment of the active portion  1810  results, thereby enhancing the brightness of surrounding pixels in the image.  
         [0081]     The brightness controller  104  may also enhance brightness through a contrast adjustment. The contrast adjustment may be implemented as the application of a gain to a selected pixel subset of an image.  FIG. 19  provides an example of a gain adjustment for a portion of an image. The brightness controller  104  applies an attenuation factor ‘g’ to a segment of an active portion  1902  of an input video signal which defines one or more pixels of the image. The attenuation factor may be defined as the ratio of a voltage value U′ after attenuation (or amplification) to the corresponding voltage value U before attenuation (or amplification):  
             g   =       U   ′     U             (   4   )             
 
         [0082]     The attenuation factor ‘g’ may be constant or may vary over the segment of the active portion  1902 . Holding the attenuation factor constant may mitigate signal distortion.  
         [0083]     In  FIG. 19  the segment of the active portion  1902  of the input video signal varies between the black level  1904  and the maximum brightness voltage  1906 . The segment of the active portion  1906  of the output video signal results from the application of the attenuation factor by the brightness controller  104 . After attenuation, the segment of the active portion  1906  has a decreased modulation depth. In other words, the difference between the upper limit of the maximum brightness voltage  1908  and the black level  1904  has been decreased compared with the corresponding values of the maximum brightness voltage  1906  and the black level  1904  prior to attenuation.  
         [0084]     Both the black level adjustment illustrated in  FIG. 18  and the gain adjustment illustrated in  FIG. 19  may be performed on a selected subset of image pixel data. The brightness control signal may be synchronized vertically, horizontally, and by individual pixel to select a pixel subset for enhancement on a line-by-line basis. In response to the brightness control signal, the brightness controller  104  adjusts the contrast and/or black level of the input video signal  116  to generate an output video signal  114  with brightness adjustments made to the selected subset of pixel image data. The brightness control system  100  thereby avoids individual pixel-by-pixel recalculation of image data.  
         [0085]     Although selected aspects, features, or components of the implementations are depicted as being stored in hardware memories, all or part of systems and methods consistent with the brightness control system  100  may be stored on, distributed across, or read from other machine-readable media, for example, secondary storage devices such as hard disks, floppy disks, and CD-ROMs; other forms of ROM or RAM; transmitted signals; or other machine readable media. Furthermore, the brightness control system  100  may include additional or different components than described above. For example, a processor may be implemented as a microprocessor, microcontroller, application specific integrated circuit (ASIC), discrete logic, or a combination of other types of circuits or logic. Level shifting, multiplication, modulation, and other logic may be implemented in hardware, software, or a combination of hardware and software.  
         [0086]     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. For example, the brightness control system may additionally or alternatively modify red, green, and/or blue color information to enhance an image by color, rather than overall brightness. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.