GB2527091A

GB2527091A - Anti-glare mirror

Info

Publication number: GB2527091A
Application number: GB1410400.4A
Authority: GB
Inventors: Pedro Gabriel Ferreira Caetano Marques
Original assignee: Nissan Motor Manufacturing UK Ltd
Current assignee: Nissan Motor Manufacturing UK Ltd
Priority date: 2014-06-11
Filing date: 2014-06-11
Publication date: 2015-12-16
Also published as: GB201410400D0

Abstract

A dimming system 12 for a vehicle mirror comprises: image capturing means 20 arranged to acquire a captured image that corresponds to a reflected image of a scene appearing in the mirror 14; processing means 22 arranged to analyse the captured image and to determine an image mask (108, figure 6); and masking means arranged to implement the image mask determined by the processing means, wherein the image mask is operable to reduce the brightness of one or more selected zones (112-118) in the reflected image in proportion to the brightness of a respective one or more zones in the captured image. In a further embodiment the dimming system may operate on a vehicle display, such as an LED, LCD or OLED screen which is used to display an image outside of the vehicle. The system is intended to dim only sections of the mirror or display which have areas of glare above a threshold value, thus leaving the remaining areas of the mirror or display undimmed.

Description

Anti-glare mirror

Field of the Invention

This invention relates to an anti-glare mirror system for use in a vehicle.

Background to the Invention

During the course of driving a vehicle, glare arising from the reflection of bright light in vehicle mirrors can temporarily dazzle a driver and create a dangerous situation. Sources of such light vary throughout the day: for example, during the day, the sun can be a source of glare, while at night or in low light conditions, headlamps of other vehicles can become a problem. Glare can be produced in any vehicle mirror, such as a rear-view mirror or in wing' mirrors. is

Anti-glare/anti-dazzle measures for rear view mirrors and wing mirrors are known. In one example, glare-reducing glass is installed in a mirror to reduce the sharpness of the glare.

This has the side-effect of permanently dimming the mirror or significant regions of the mirror, causing a general reduction in rear view visibility. This can be problematic at times when ambient light levels are low, for example at night, during winter, or on an overcast day, when visibility may be reduced to an unacceptable level.

An alternative approach is to automatically switch mirrors between normal and dimmed states to protect the driver from glare. This can be achieved by constructing the mirror using smart glass', for example electrochromic glass which darkens on application of a voltage. In one known arrangement, sensors installed in the mirrors compare the intensity of reflected light with ambient light. The difference between the two is indicative of the level of glare, and the mirror's reflectivity is controlled to switch to the dimmed state when glare exceeds a pre-determined threshold.

An electrochromic layer is typically coated over the full surface of the mirror. Therefore, in the dimmed state the entire mirror is darkened, and so the drawback of reducing visibility also applies to this arrangement. Electrochrornic systems also suffer from a poor response time, taking a second or more to change states.

Use of electrowetting to black out a mirror is also known, for example as described in US 2010/0067093. This arrangement includes an electrowetting element made up of a containment tray sandwiched between a reflective hydrophobic surface and an electrode.

The containment tray defines an array of pockets, each of which holds a pair of liquids: a transparent polar liquid; and an opaque, non-reflective, non-polar liquid, for example a dark oil. In an uncharged state, the polar liquid is held apart from the hydrophobic surface by surface tension forces. The non-polar liquid fills the space between the polar liquid and the hydrophobic surface and forms a non-reflective layer, and so the mirror is dimmed. When charge is applied, the polar liquid is drawn towards the hydrophobic layer and so the non-polar liquid is pushed aside, revealing the reflective hydroscopic surface.

The mirror is divided into two or three horizontal regions, each of which contains a segment of the electrowetting element which can be operated independently the other segments, thereby enabling selective dimming of each region. When glare is detected, only the regions containing the glare are dimmed. In this way, any remaining regions of the mirror continue to reflect image information to the driver.

A disadvantage with this arrangement is that the system dims at least one third of the mirror surface in response to glare, which represents a significant reduction in visibility. If the glare is widespread, or if there are multiple sources of glare, the entire mirror surface may be dimmed. Furthermore, the electrowetting arrangement operates digitally, in that each region of the mirror is either in a dimmed state or in a normal state. Thus, complete dimming of a region is applied irrespective of the intensity of the glare. The effect of this is that a region may be fully dimmed unnecessarily in response to mild glare, when partial dimming would be sufficient, resulting in loss of image information.

It is one object of the present invention to provide an improved glare-reducing mirror for use in a vehicle, which overcomes or alleviates the aforementioned disadvantages known in the

prior art.

Summary of the Invention

According to a first aspect of the invention, there is provided a dimming system for a vehicle mirror. The system comprises image capturing means arranged to acquire a captured image that corresponds to a reflected image of a scene appearing in the mirror. The system further comprises processing means arranged to analyse the captured image and to determine an image mask, and masking means arranged to implement the image mask determined by the processing means. The image mask is operable to reduce the brightness of one or more selected zones in the reflected image in proportion to the brightness of a respective one or more zones in the captured image.

An alternative implementation of the invention is expressed in a second aspect in which a dimming system for a vehicle display system is provided. The display system comprises an electronic display, image capturing means arranged to acquire a captured image of a scene, and processing means arranged to analyse the captured image to produce a presented image that is displayed to a user on the display. The dimming system comprises masking means arranged to analyse the captured image to determine an image mask, and to implement the image mask. The image mask is operable to reduce the brightness of one or more selected zones in the presented image in proportion to the brightness of a respective one or more zones in the captured image.

The first and second aspects of the invention are applicable to alternative known methods of relaying visual information pertaining to a scene external to a vehicle to a driver. The first of these is a mirror such as a rear-view mirror or, alternatively, a wing mirror, and the second is an electronic display, for example a display configured to emulate a rear-view mirror or wing mirror. Each of these methods may be improved by the respective aspect of the invention in that glare, which may otherwise dazzle the driver, is reduced.

A particular benefit that is provided in each aspect of the invention is that the brightness of an image viewed by the driver is reduced in a proportional manner, such that brighter areas are dimmed most, and less dimming is applied to duller areas. In this way, a maximum level of information is preserved in the image. This sits in contrast with prior art arrangements in which a significant loss of valuable visual information is caused by blanket dimming.

The term brightness' is intended to cover a range of terms of the art that may be used to measure brightness within an image, including luma', luminance', luminosity', and lightness'.

In the second aspect, the display of the vehicle display system may be of a type selected from the group comprising: OLED screen; LED screen; LCD screen. The dimming system and the display system may be integrated, in which case the processing means may comprise the image masking means.

In either of the above aspects of the invention, the masking means may comprise transparent display technology. As an alternative, the masking means may comprise reflective display technology. In either of these embodiments the image mask may comprise an image arranged to be superimposed over the reflected image or the presented image to reduce the brightness of the selected zones. These alternatives provide convenient practical implementations based on known technologies.

The processing means may be arranged to select the respective one or more zones in the captured image if the brightness in those zones is above a threshold. By applying a threshold, this embodiment ensures that no visual information is lost from areas of the image where the brightness is below the threshold, as no dimming is applied to these areas. The threshold may be pre-determined, or alternatively the processing means may be arranged to adjust the threshold dynamically in response to environmental conditions. In either case, conveniently the threshold may be user adjustable.

In an embodiment, the image capturing means is a vehicle-mounted camera. This provides a convenient practical implementation, since suitable cameras are readily available and are routinely used in vehicles for other purposes. For example, the camera may be associated with a parking assist system.

The inventive concept is also expressed in a third aspect as a method of dimming glare in a vehicle mirror. The method comprises captuling an image that corresponds to a reflected image of a scene appearing in the mirror, analysing the captured image to determine an image mask, and implementing the image mask. The image mask is operable to reduce the brightness of one or more selected zones in the reflected image in proportion to the brightness of a respective one or more zones in the captured image.

A fourth aspect of the invention provides a method of dimming glare in a vehicle display system, the display system comprising an electronic display, image capturing means arranged to acquire a captured image of a scene, and processing means arranged to analyse the captured image to produce a presented image that is displayed to a user on the display. The method comprises, analysing the captured image to determine an image mask, and implementing the image mask. The image mask is operable to reduce the brightness of one or more selected zones in the presented image in proportion to the brightness of a respective one or more zones in the captured image.

It is noted that the third and fourth aspects share in common with the first and second aspects the feature of an image mask that is operable to dim a presented or reflected image in proportion to a reference captured image. Accordingly, these aspects of the invention offer the same advantage of optimising the level of visual information that is retained in the image after dimming is applied.

It will be appreciated that preferred and/or optional features of the first, second, third or fourth aspects of the invention may be incorporated alone or in appropriate combination in any other aspect of the invention also.

Brief Description of the Drawings

In order that the invention may be more readily understood, preferred non-limiting embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an illustration of a vehicle including a selective dimming mirror system according to an embodiment of the invention; Figure 2 is a schematic illustration of an electrical sub-system of the system of Figure 1; Figure 3 is a flow diagram showing in overview a method for operating the system of Figure 1; Figure 4 is a flow diagram showing a more specific embodiment of the method of Figure 3; Figure 5 is a graphical representation of ranges of possible luminance values relevant to the system; and Figure 6 is an illustration of a portion of the mirror of the system of Figure 1 in use.

Detailed Description of Embodiments of the Invention Figure 1 shows in schematic form a vehicle 10 including a mirror dimming system 12 according to an embodiment of the invention. The mirror dimming system 12 includes a display element 14 in the form of a mirror including a mirror mask display 16 which is adapted to selectively obscure portions, regions or zones of the mirror in response to a

S

control input, as will be described in more detail below. The mirror 14 and the mirror mask display 16 together form a mask display unit 18.

In this embodiment, the mask display unit 18 acts as a rear view mirror, although it will be appreciated that the mirror dimming system 12 can be adapted for use with a twing' mirror, or indeed any other conceivable vehicle mirror.

The mirror dimming system 12 further comprises an image acquisition system 20, for example a rear-facing camera associated with a park assist and blind spot warning system, and an image processing means in the form of a controller unit 22.

In overview, the camera acquires an image of a rear view behind the vehicle 10, which is then analysed by the controller unit 22 to identify zones of the image containing glare. Each zone may relate to a single pixel of an image produced by the camera, or to a collection of pixels. The controller unit 22 then determines an appropriate mask to apply to the mask display unit 18 so as to dim points of glare in a corresponding reflected image in the mirror 14.

As depicted in the Figure 1, the image captured by the image acquisition system 20 is taken at a wide angle, and so includes significantly more of the rear view than would be shown in the mirror 14 as a reflected image. This image is passed to the controller unit 22, which analyses the image in order to determine which part of the image corresponds to the reflected image in the mirror 14. In order to do this, the controller unit 22 takes into account various factors, including: the height difference between the mask display unit 18 and the image acquisition system 20; the location of the mask display unit 18; radial distortion arising from a lens of the image acquisition system; and the angle at which the mask display unit 18 is positioned, which provides information relating to the driver's point of view, since the driver typically angles a rear-view mirror so as to provide a view of and through a rear window of the vehicle 10.

Figure 2 shows further components of the miiror dimming system 12. As described above, the mirror dimming system 12 comprises three units: the controller unit 22; the mask display unit 18; and the image acquisition system or camera 20. The three units are linked using a dedicated communication channel 24, although in other embodiments other suitable communication channels could be used to link the units, for example a vehicle CAN bus.

As shown in Figure 2, each of the units of the mirror dimming system 12 is composed of various sub-components, as described below.

The image acquisition system 20 includes an image capturing element 26 and an image processing subsystem 28. The image capturing element 26 is a device that is configured to react to incoming light to create a raw image, for example a CCD (charge coupled device) image sensor such as those commonly found in digital cameras. The raw image is then passed to the image processing subsystem 28, which processes the image into a digital format which is compatible with other devices. This image file is then output to the controller unit 22.

In this embodiment the image is processed into an RGB signal, although in other embodiments other types of colour space' may be used, for example YUV', 110', YCBR' or YpbPr' type colour spaces. Indeed, in practical implementations an NTSC'/'PAL' type camera configured to transmit in YIQ'/'YUV4200' would be likely. However, the signal format should enable the extraction of a relative luminance array, as will be described later.

The image capturing element 26 is configured to capture images at a resolution that is sufficient to provide a mask with an acceptable resolution to ensure that areas of glare within the image are covered/dimmed while minimising or avoiding dimming portions of the image that do not contain glare. Images are also captured at a frame rate that enables a fast response to an instance of glare. For example, it has been found that capturing images at a resolution of 640x480 pixels at a rate of 60 frames per second satisfies these requirements, although a wide range of alternative values for resolution and capture rate could be used.

The controller unit 22 includes a micro-controller 30, a working memory 32, such as a RAM module, and a vehicle network transceiver 34. The micro-controller 30 receives the digital image file as an input, and then uses the memory 32 to process the image file in order to identify a portion of the image corresponding to an image presented in the mask display unit 18, and any points of glare within that portion.

In order to do this accurately, the micro-controller 30 takes into account the environmental factors outlined above, such as the angle at which the mask display unit 18 is positioned. To this end, signals indicative of these variables are received as inputs through the vehicle network transceiver 34 from a vehicle network bus 36. As indicated by the dashed lines in Figure 1, there may also be additional sensor inputs 38 for additional image processing to take into account further variables. These may include differences between the image capturing element 26 and the mask display unit 18, or the position offset of the driver relative to the mask display unit 18.

Once the points of glare have been identified, the micro-controller 30 then determines an appropriate mask to apply to the mask display unit 18, and outputs a corresponding control signal to the mirror mask display 16.

As noted above the mask display unit 18 comprises a display element 14 such as a mirror, and a mirror mask display 16 which acts as an interface subsystem. An image of the rear view is displayed to the driver on the display element 14, which is typically a mirror, although it may alternatively be a display screen such as an LCD screen, an OLED screen, or an LED screen for example. The mirror mask display 16 applies a mask to the display element 14 which dims areas of glare and high contrast, under the influence of the controller unit 22.

The method by which masking is achieved depends upon the type of display element 14 used. For a mirror, the masking involves reducing the reflectivity of the mirror in localised areas, using any one of several suitable techniques. For example, transparent display surface technology can be used in front of a reflective optical surface (i.e. if the display element 14 is in the form of a mirror). An alternative is to use reflective display technology, whereby the display element 14 acts as both a mirror and a display. In either case, an image of a mask is displayed and superimposed over the reflected image in the mirror to cover zones of glare thereby to attenuate the intensity of the reflected light in those zones.

For a display screen, such as an LCD or OLED screen, masking entails adjusting the displayed image to reduce the contrast range, again under the influence of the controller unit 22.

For either masking approach, dimming is applied selectively and locally, such that only points of glare are dimmed. The mask display unit 18 does not dim entire regions of the display, as this causes a significant reduction in visibility. Instead, the areas of glare are identified and masked in isolation. Furthermore, the intensity of the glare is determined throughout the displayed image, and masking is applied in a manner that is proportionate to the level of glare. Therefore, areas of glare are not simply darkened: dimming is tuned to reduce glare to an acceptable level, so as to provide an optimised image with minimal information loss.

Turning now to Figures 3 and 4, a method for operating the mirror dimming system 12 is shown. Figure 3 provides an overview of the method, while Figure 4 shows the method in more detail.

As shown in Figure 3, a top-level process 50 for operating the mirror dimming system 12 initiates with the image acquisition system 20 acquiring at Step 52 an image of the rear view behind the vehicle 10. The image acquisition system 20 then converts at Step 54 this image into a form that is suitable for processing. The image is then analysed at Step 56 by the controller unit 22, which then creates at Step 58 a mask according to points of glare identified within the image. The mask is then applied at Step 60 by the mask display unit 18 in order to attenuate zones of glare in an image viewed by the driver, so as to avoid dazzling.

Referring now to Figure 4, an embodiment of a complete dimming process 70 initiates on activation of a vehicle ignition. An analogue signal is captured at Step 72 by the image capturing element 26, which is then processed at Step 74 by the image processing subsystem 28. Processing entails converting the analogue signal into a digital signal, and then encoding at Step 76 the converted digital signal into an RGB colour model format, i.e. a two-dimensional image array containing three values for each pixel of the image element, those values indicating the relative intensities of the red, blue and green components as detected by the pixel. A mapping function is then applied to transform the colour model values into a colour space such as YUV, Adobe RGB, or sRGB for example. The mapped RGB signal is transmitted to the controller unit 22.

The relative luminance (Y') of points within the image array are then determined at Step 78 from linear RGB components. The image points are defined as data point corresponding to physical pixels of the image capturing element 26, in this embodiment the groups of three values referred to above. For example, by using standard ITU-R BT.709 primaries such as will be familiar to the skilled reader, the RGB signal can be converted into a relative image luminance using the following equation: Y = 0.2126 S + 0.7152 G + 0.0722 B It is noted that the image data captured by the image acquisition system 20 is handled by compiling it into an image matrix, i.e. a two-dimensional array of data values. The positions of individual data points in the image matrix correspond to physical locations on the COD sensor, and accordingly the dimensions of the matrix are determined by the size of the CCD sensor in terms of how many pixels it contains. It is likely that the matrix size is actually a multiple of the size of the CCD sensor: for example, as noted above in this embodiment the image data is handled in RGB format, which entails three data points for each physical pixel of the COD sensor.

It is noted that processing steps 74 to 78 may be handled in a variety of ways. For example the processing may be performed entirely in software, or alternatively dedicated hardware such as an ASIC or an FPGA processor may be provided to handle certain stages of the process. In a typical practical implementation, an NTSC/PAL camera such as will be familiar to the skilled person is used to provide a YIQJYUV4200 output. Such a camera can also be configured to calculate luminance values internally and provide these values as a separate output which can be used directly in the subsequent steps of the process. Similarly, any suitable colour model and colour space may be used in the process. The precise method and hardware used is not important, only that luminance values are extracted from the original analogue signal.

In another variation, instead of calculating luminance from linear RGB components, a gamma correction may be applied to the RGB signal so as to adjust for human perception. In this case, luma values are extracted, rather than luminance values, using the following equation: Y' = 0.2126 R' + 0.7152 G' + 0.0722 B' It will be understood that luma values can be used in the same manner as luminance values, and so throughout the remainder of this specification references to luminance' should be taken to also envisage embodiments in which luma' values are used from a gamma corrected signal. Furthermore, the skilled reader will be aware of other measures of brightness that could be used as equivalents to luma' or luminance', and such measures can readily be incorporated into embodiments of the present invention; the priority is that a measure of brightness within a captured image is obtained.

The image matrix is manipulated according to the steps outlined below in order to produce an image mask. As the skilled reader will appreciate, in general, matrix operations cannot act to alter the dimensions of a matrix, although the matrix can be transposed, transformed or inverted, and values within the matrix can be updated.

The maximum possible luminance value represents the brightest data point possible for the image matrix; and the minimum possible Y value represents the least bright data point possible for the image matrix. The RGB values across the entire image are converted using the above equation into a relative luminance data point matrix, which is hereafter referred to as matrix_i.

The process 70 then continues with the controller unit 22 analysing at Step 80 each data point in matrix_i against a rejection cut-off threshold (ROT), in order to identify less luminous regions of the image which the mirror dimming system 12 will not act upon.

The ROT isa calibration parameter which enables the system 12 to be tuneable and take into account variations within the image acquisition system 20 such as ambient luminance conditions; design specifications; and user preferences. As indicated in Figure 4, the ROT can be statically defined, or alternatively the mirror dimming system 12 can integrate different data sources (sensors or other controllers) to allow for a dynamic variation of the ROT.

There are two particular factors that influence the value that the ROT should take: the performance of the image capturing element 26 in different ambient light conditions; and perceived human discomfort. The behaviour of the image capturing element 26 can be determined in advance, and control of the ROT value is tailored to account for this. Human discomfort is measured across a representative sample of people, as is explained further later with reference to Figure 5.

Accordingly, the controller unit 22 determines at Step 82 for each data point in matrix_i whether the relative luminance falls below the RCT. If a data point is above the RCT, it is retained at Step 84, that is to say, the data point value is not altered. If below the ROT, the data point is updated at Step 86 to take a minimum saturation value, which is defined as the minimum possible value allowed for any data point encoded in matrix_i. This value corresponds to the lowest possible luminance that can be acquired by the image acquisition system 20.

The retained data points and the updated data points are aggregated at Step 88 to form a post rejection matrix, which is hereafter referred to as matrix_2.

Next, the controller unit 22 "inverts" at Step 90 matrix_2 with reference to a maximum saturation value, which is defined as the maximum possible value allowed for any data point encoded in matrix_i, corresponding to the highest possible luminance that can be acquired by the image acquisition system 20.

In this context invert" means flipping the point the values in the matrix. In other words, the step produces a negative of the image, as represented in the matrix, that is, in which the lightest areas of the image appear darkest and the darkest areas appear lightest.

The inversion of matrix_2 is achieved by subtracting each data point of matrix_2 from the maximum saturation value. The output from this step is an "inverted" matrix which is hereafter referred to as matrix_3. It is noted that all data point values in matrix_2 lie between the minimum and maximum saturation values. Therefore, all data point values in matrix_3 lie in a range defined between 0 and the difference between the maximum and minimum saturation values, with all values of the data points in matrix_3 being inversely related to the corresponding values in matrix_2. In this way, the dimming effect implemented by the mirror dimming system 12 is not digital, but is instead tuneable in response to the luminous intensity of each of the data points, provided they are above the RCT. Conversely, pads of the image having a relative luminance below the RCT are not dimmed at all.

Therefore, varying the RCT has the effect of changing the extent to which bright areas of the image are dimmed, and also the level of dimming applied relative to the intensity of the glare. If the RCT is set relatively high, only the brightest points of glare will be dimmed by the resulting mask; if the RCT is reduced, many more of the bright parts of the image are dimmed.

For example, in a common scenario in which the headlights of a following vehicle are reflected in the rear view mirror and cause glare, through appropriate control of the RCT, the mirror dimming system 12 operates to dim only the headlights in the reflected image.

Therefore, all other detail in the image continues to be displayed to the driver, for example the rest of the following vehicle or objects to the side of it. Furthermore, as the dimming is proportionate to the brightness in the image, the brightest part of the headlight, corresponding to the location of a light bulb, is dimmed more than the outer regions of the headlight. Accordingly, the headlights are not simply removed from the reflected image: their brightness is reduced to an acceptable level, thereby avoiding creating blackened portions in the reflected image. In this example, the result is that the driver can see that the headlights are present, but does not get dazzled by them. This is beneficial, since headlights are often the clearest indication that a vehicle is following when driving at night, and so to black them out completely is undesirable.

Following the determination of matrix_3, the controller unit 22 then applies at Step 92 a display correction function (DCF) to matrix_3, whereby all data points in matrix_3 are treated as independent variables of the display correction function. The resulting matrix of data points will be hereafter referred to as matrix_4.

In effect, the data contained in matrix_3 is analogous to a negative of the image captured by the image capturing element 26, albeit with a brightness threshold applied by way of the ROT. The DOF calibrates this into an image which takes into account the characteristics of the display element 14 of the mask display unit 18 and ambient conditions, along with user preferences such as the position at which the mask display unit 18 is angled, along with user input to the ROT. The DOF can be statically defined, or the mirror dimming system 12 can integrate different data sources to allow for a dynamic adjustment of the DCF, for example to account for ambient light conditions.

In this embodiment, the DCF is configured such that data points in matrix_3 corresponding to a matrix_i data point value above the ROT will have a resulting relative luminance equal to data point regions corresponding to a value equal to ROT. The effect of this is that all areas of the reflected image having higher brightness than the ROT are dimmed such that their brightness is equal to the ROT; the DOF configures the mask such that brightness throughout the reflected image is capped' to the RCT.

The image matrix, matrix_4, produced using the DCF performs the function of dimming bright areas effectively without removing any more image detail than is necessary. Matrix_4 therefore represents an initial form of mask information; the higher the relative luminance for a data point in matrix_i, the higher the resulting display saturation for data point in matrix_4.

For an embodiment where a transparent display surface technology is used as a masking means, an increase in saturation reduces the transparency of the transparent display surface in front of the reflective mirror surface, thereby dimming those portions of the display element 14. In an alternative embodiment in which reflective display technology is used, an increase in saturation reduces the reflectiveness of the display, providing the same dimming effect.

At this stage it is noted that, due to the way in which the data has been processed, matrix_4 is "inverted" with respect to the original image captured by the image capturing element 26, and therefore the corresponding reflected image in the display element 14. Further processing is also required, however, to take into account differences in position and orientation between the image acquisition system 20 and the mask display unit 18. Inverted means that a negative image is produced, that is, a total inversion, in which light areas appear dark and vice versa.

Accordingly, the controller unit 22 applies at Step 94 a mathematical rotational transform to matrix_4 so as to adjust the mask to correspond to the angular displacement of the display element 14 and the driver's point of view. Additionally the matrix is re-scaled to correspond to the surface dimensions of the display element 14, which in an automotive context are typically smaller than the angle of coverage' of the image capturing element 26.

This step therefore matches the mask to the image that will ultimately be viewed by the driver. The more closely the mask can be matched to this image, the more effective the glare reduction will be. Therefore, further parameters may be taken into account during the transformation step, according to the availability of the requisite data, including: the precise image acquisition system 20 position; the driver's head position; the properties of the display surface, such as convexity and focal point; and the driver's point of view, including the zenith and azimuth from a centre point of the display surface.

Once the transformation is complete, the controller unit 22 encodes at Step 96 the resulting matrix to create a control signal which instructs the mirror mask display 16 to create a mask which is output on the display as described above.

It should be noted that the process is conducted continuously in real-time, such that the mask updates to counteract glare almost instantaneously as it arises.

Figure 5 shows graphically how an adjustment range for the ROT can be determined. The figure includes four horizontal lines which each represent ranges of values.

The uppermost line 100 represents a range of possible values of lux from a light source, lux being defined as luminous flux incident on a surface per unit area. This ranges from 0, corresponding to no light, to infinity.

Below that, the next line 102 represents a range of lux values that are considered comfortable by a representative sample of users, ranging from 0 to a finite maximum lux value. This line 102 is shorter than the uppermost line 100, which is indicative of the fact that the comfortable range falls within and is smaller than the entire range of possible lux values.

The next line 104 represents a range of possible luminance values that can be captured by the image capturing element 26. This line 104 is shorter than the uppermost line 100, but longer than the line 102 representing comfortable lux values. This indicates that the image capturing element 26 captures light up to a finite maximum luminance, and that the maximum is higher than the maximum luminance that users are on average comfortable to look at.

The lowermost line 106 shows that a dynamic range within which the ROT is adjustable is set such that an upper limit of the range coincides with the maximum luminance that users are comfortable with, and the lower limit of the range falls somewhere between this value and zero. By setting the upper limit to coincide with a user's limit of comfort, the ROT always acts to ensure that the masking that is applied has the effect that no part of the image is brighter than the user can cope with, i.e. glare. The lower limit is held above 0, because an ROT value of 0 would result in complete masking of the image.

The size of the adjustment range for the RCT is calibrated to account for the range of lux values that the image capturing element 26 is able to capture. To this end, a function for dynamically calculating ROT values is determined, the function being arranged to account for the characteristics of the image capturing element 26, varying ambient conditions, as well as the maximum lux that the average user is comfortable with. It is noted that the user's comfort limits may vary depending on ambient conditions as the user's eyes adjust to compensate for changing light conditions.

As well as aiding dynamic calculation of RCT values, the upper and lower limits for the ROT also apply to user adjustment of the ROT, such that the user does not accidentally adjust the ROT to an undesirable level.

Turning finally to Figure 6, a representation of a mask 108 to be superimposed on a reflected image 110 is shown. The figure shows four areas 112, 114, 116, 118 of the mask 108 corresponding to four data points of differing candela (cd), or luminous intensity.

At the top of the figure, a high candela region of the image 110 having data point values above the ROT results in a severely darkened area 112, to provide strong dimming.

Below this, another region having data point values above the ROT, but with a slightly lower candela has a correspondingly slightly lighter area 114 of masking applied to it. This demonstrates the ability of the mirror dimming system 12 to tune the level of dimming that is applied throughout the reflected image 110 appropriately to the candela of the image 110 at each point.

The final two data points have no dimming applied, as in each case the data point candela is equal to or less than the RCT. This demonstrates the application of the RCT in ensuring that regions of the image having a candela which will not cause dazzling are not dimmed unnecessarily.

Figure 6 also demonstrates the dynamic nature of the mirror dimming system 12, in that the mask 108 that is applied is closely matched to the pattern of glare detected. This is in stark contrast with prior art arrangements in which blanket dimming is applied across pre-defined regions of a mirror, or to the entire mirror. The system 12 therefore offers a high dimming resolution.

In the above described embodiments, the only parts of the display that are dimmed are those pads experiencing glare; all other areas of the display are unaffected and continue to present valuable information to the driver. Where dimming is applied, it is optimised so as to act to normalise the undesirably bright areas of the image, rather than simply black out portions of the image containing glare. Accordingly, image information is not lost; indeed, by normalising areas of glare whilst leaving other pads of the image unaltered, the mirror dimming system 12 acts to add information to the image, as the source of glare may become identifiable.

As an illustrative example of the benefit of this feature, reference is drawn to a night-time driving scenario in which the headlights of following vehicles present numerous sources of glare. In this situation, the mirror dimming system 12 acts to identify the precise location of each headlight in the reflected image, and to dim the respective points in isolation from the remainder of the image. Therefore, despite numerous and widespread points of glare in the reflected image, the mirror dimming system 12 is able to dim all sources of glare effectively whilst leaving most of the image untouched. Therefore, the display continues to present valuable information to the driver. Furthermore, since the dimming is applied intelligently, the driver is able to identify each headlight within the reflected image, enabling the driver to determine the positions of the following vehicles. This also enables the driver to categorise the light sources, which may allow for identification of vulnerable road users such as cyclists or motorcyclists.

An additional benefit is that, as the mirror dimming system 12 is able to apply dimming to varying extents, it can compensate for ambient light conditions through appropriate adjustment of the DCF and ROT. For example, strong dimming may be applied at night time when the risk of dazzling is high, whereas reduced dimming may be applied during low-light conditions. In this way, the displayed image is optimised to retain the highest level of information possible in all conditions.

In contrast, in the prior art arrangements discussed previously, the lack of refinement in the application of dimming necessarily leads to most, if not all of the display being blacked out in response to multiple sources of glare, and irrespective of ambient conditions.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims.

For example, in another embodiment the image processing means (22) may include an application specific integrated circuit (ASIO) arranged to extract a relative luminance (Y) array from the processed image signal prior to transmitting to the controller unit

Claims

Claims 1. A dimming system for a vehicle mirror, the system comprising: image capturing means arranged to acquire a captured image that corresponds to a reflected image of a scene appearing in the mirror; processing means arranged to analyse the captured image and to determine an image mask; and masking means arranged to implement the image mask determined by the processing means, wherein the image mask is operable to reduce the brightness of one or more selected zones in the reflected image in proportion to the brightness of a respective one or more zones in the captured image.
2. A dimming system for a vehicle display system, the display system comprising an electronic display, image capturing means arranged to acquire a captured image of a scene, and processing means arranged to analyse the captured image to produce a presented image that is displayed to a user on the display; wherein the dimming system comprises masking means arranged to analyse the captured image to determine an image mask, and to implement the image mask; and wherein the image mask is operable to reduce the brightness of one or more selected zones in the presented image in proportion to the brightness of a respective one or more zones in the captured image.
3. A dimming system according to claim 2, wherein the display of the vehicle display system is of a type selected from the group comprising: OLED screen; LED screen; LCD screen.
4. A dimming system according to claim 2 or claim 3, wherein the dimming system and the display system are integrated, and wherein the processing means comprises the image masking means.
5. A dimming system according to any one of claims 1 to 3, wherein the masking means comprises transparent display technology.
6. A dimming system according to any one of claims 1 to 3, wherein the masking means comprises reflective display technology.
7. A dimming system according to claim 5 or claim 6, wherein the image mask comprises an image arranged to be superimposed over the reflected image or the presented image to reduce the brightness of the selected zones.
8. A dimming system according to any preceding claim, wherein the processing means is arranged to select the respective one or more zones in the captured image if the brightness in those zones is above a threshold.
9. A dimming system according to claim 8, wherein the threshold is pre-determined.
10. A dimming system according to claim 8, wherein the processing means is arranged to adjust the threshold dynamically in response to environmental conditions.
11. A dimming system according to any one claims 8 to 10, wherein the threshold is useradjustable.
12. A dimming system according to any preceding claim, wherein the image capturing means is a vehicle-mounted camera.
13. A dimming system according to claim 12, wherein the camera is associated with a parking assist system.
14. A method of dimming glare in a vehicle mirror, the method comprising: capturing an image that corresponds to a reflected image of a scene appearing in the mirror; analysing the captured image to determine an image mask; and implementing the image mask; wherein the image mask is operable to reduce the brightness of one or more selected zones in the reflected image in proportion to the brightness of a respective one or more zones in the captured image.
15. A method of dimming glare in a vehicle display system, the display system comprising an electronic display, image capturing means arranged to acquire a captured image of a scene, and processing means arranged to analyse the captured image to produce a presented image that is displayed to a user on the display; wherein the method comprises: analysing the captured image to determine an image mask; and implementing the image mask; wherein the image mask is operable to reduce the brightness of one or more selected zones in the presented image in proportion to the brightness of a respective one or more zones in the captured image.