Method and apparatus for discriminating the colour of road markings

0001This invention relates to a method of and apparatus for discriminating the colour of road markings, especially but not exclusively in the field of lane departure warning systems. Such a discrimination of the colour of road markings is e.g. known from US 6 038 496.

0002Lane Departure Warning (LDW) and other automotive guidance systems use optical cameras to view the road markings ahead. For economy, it is common practice to use a black and white (monochrome) camera.

0003In some countries, road works or other diversion lanes are marked in coloured paint and the original (generally white) lines are not deleted. Black and white LDW systems may find it hard to discriminate between the two. Distinguishing between white and other colour lines by using absolute intensity is susceptible to local changes in ambient light (such as shadows) and faded paint. It is therefore desirable to be able to detect the difference between different coloured road markings. Herein, "determining" should be read to include distinguishing between two or more different possible colours for a road marking.

0004According to a first aspect of the invention, we provide a method of discriminating between road markings of at least two colours, comprising: <ul id="ul0001" list-style="none"><li>capturing an image of a road and at least one road marking with a monochrome camera having a first colour response in a first area of a field of view of the camera and a second colour response in a second area of the field of view, the second colour response being different from the first colour response and the first area being different to the second area; and</li><li>determining the change in intensity of the captured image of the road marking between the first and second areas.</li></ul>

0005By first and second colour responses, we mean that the camera is more or less sensitive to one or more colours in the first area with respect to the second area.

0006This method therefore provides a simple method of discriminating the colour of a road marking without requiring the use of a colour camera.

0007By monochrome, we mean that the camera is a black and white camera in that it is capable of measuring the intensity but not the hue of an image. The camera may be fitted to a vehicle, and the method may be adapted to determine the colour of a road marking from a vehicle.

0008The method may also detect the edge of the road. The edge of a road may be recognised by a change in intensity due to the change of material from a road surface (for example, asphalt) to that surrounding the road (for example, mud or grass). The difference in colour response may provide more information as to the position of the edge of the road than intensity alone, especially when the difference in intensity over the edge is low.

0009The step of determining the change in intensity of the captured image may comprise the step of comparing the intensity of the portions in the first and second areas and discriminating between the two colours according to the contrast in intensity between the two areas.

0010The step of capturing the image may comprise viewing the image through filter means, wherein sections of the filter means of different absorbance spectra provide the different colour response of the first and second areas. At least one of the sections may be coloured, and may be close to or the same as one of the colours of road markings to be discriminated.

0011From the change in intensity, it may be determined whether the road marking is of similar colour to the coloured section or is of a different colour. A marking may be determined to be of similar colour to the coloured section if the change between the two sections is less than a certain limit, which may be an absolute (that is, the measured intensity changes by a given amount) or a relative (that is, the measured intensity changes by a given ratio) limit.

0012The markings may be lane boundaries marked on the road. In a preferred embodiment, the coloured section is yellow.

0013The method may further include providing the marking colour determination to an automotive guidance system. By automotive guidance system, we may include lane departure warning systems, automatic cruise control systems and the like.

0014The method may further include the step of discriminating between further colours of road marking by providing the camera with at least one further area in the field of view, each of which having a colour response different to the first and second areas. In the case where filter means are provided, the filter means may be provided with at least one further coloured section, which may be a different colour to the coloured section and any other further coloured sections. This enables the method to discriminate, between a plurality of colours, the colour of different colour road markings from measured changes in intensity of the image of the road between the first, second and further areas. The coloured section may be yellow and there may be provided red and blue further coloured sections.

0015According to the second aspect of the invention, there is provided an apparatus arranged to be fitted to a vehicle and capable of discriminating between at least a first colour and a second colour of road markings, comprising: <ul id="ul0002" list-style="none"><li>a monochrome camera arranged to view the road ahead of the vehicle, and</li><li>a processing unit, arranged to capture images from the camera and determine from the captured images the position of road markings;</li><li>in which the camera has a first colour response in a first area of a field of view of the camera and a second colour response in a second area of the field of view, the first colour response being different from the second colour response and the first area being different to the second area; and in which the processing unit is arranged to discriminate between the first and second colour of the road markings from the change in intensity of the captured image of the road markings between the first and second areas.</li></ul>

0016By first and second colour responses, we mean that the camera is more or less sensitive to one or more colours in the first area with respect to the second area.

0017This provides an apparatus that can distinguish between different colour road markings whilst still being able to use a black and white (monochrome) camera.

0018The apparatus may also be arranged to detect the edge of the road. The edge of a road may be recognised by a change in intensity due to the change of material from a road surface (for example, asphalt) to that surrounding the road (for example, mud or grass). The difference in colour response may provide more information as to the position of the edge of the road than intensity alone, especially when the difference in intensity over the edge is low.

0019The camera may be provided with filter means wherein sections of the filter means of different absorbance spectra provide the different colour response of the first and second areas. At least one of the sections may be coloured, and may be close to or the same as one of the colours of road markings to be discriminated.

0020The filter means may be of the form of a background (which may be uncoloured), preferably having a band of colour forming the coloured area. The band may be straight, and may be horizontal. This is advantageous, as most road markings will appear substantially vertical to a vehicle-mounted camera.

0021The filter means may be a filter, which may comprise a single filter component. Alternatively, the filter may comprise a plurality of filter components.

0022The camera may have a lens, and the filter means may be provided between the lens and the image to be captured. Alternatively, the filter may be provided between the lens and a sensor element. The coloured section of the filter means may cover an area greater than one adjacent pixel in the image as captured. Indeed, it may cover an area greater than 10 or 100 pixels in the image as captured. The filter means may cover a majority of the field of view.

0023In a preferred embodiment, the coloured area is yellow.

0024The camera may be provided with at least one further area in the field of view, each of which having a colour response different to the first and second areas. In the case where filter means are provided, a plurality of coloured areas may be provided, which may each have at least one boundary in the field of view of the camera. The coloured sections may be of different colours to one another. The processing unit may be adapted to determine the colour of the road markings from the change in intensity of the captured image of the road markings over the, or each, boundary. There may be coloured sections coloured yellow, red and blue.

0025The apparatus may form part of an automotive guidance system, such as a lane departure warning system or an automatic cruise control system.

0026The apparatus may be arranged to carry out the method of the first aspect of the invention.

0027According to a third aspect of the invention, there is provided a vehicle fitted with the apparatus of the second aspect of the invention.

0028There now follows, by way of example, an embodiment of the present invention, described with reference to the accompanying drawings, in which: <ul id="ul0003" list-style="none"><li><b>Figure 1</b> shows a vehicle fitted with a lane departure warning sensor according to the present invention;</li><li><b>Figure 2</b> shows a first embodiment of the filter used in the present invention;</li><li><b>Figure 3</b> shows a view of lane markings as captured through the filter of Figure 2;</li><li><b>Figures 4a and 4b</b> show the measured intensities along lines Y-Y' and W-W' of Figure 3, respectively; and</li><li><b>Figure 5</b> shows an alternate embodiment of the filter used in the present invention.</li></ul>

0029The vehicle 100 depicted in Figure 1 of the accompanying drawings has been fitted with a lane departure warning system. This comprises a monochrome video camera 102 connected to a processing unit 103, which is arranged to capture images from the video camera 102. The processing unit then analyses the images to determine the position of the lanes as is known in the prior art, such as in PCT publication number WO 03/087724 published 23 October 2003, and to give a user of the vehicle an indication of when they are leaving a marked lane.

0030In order that the monochrome video camera 102 is able to distinguish between white and yellow lane markings, the camera 102 is fitted with a filter 104, which is depicted in more detail in Figure 2 of the accompanying drawings. This comprises a generally clear background 110 overlaid with a horizontal yellow filter section 112.

0031Accordingly, when the processing unit 103 captures images of lanes, such as the exemplary image of Figure 3 of the accompanying drawings, the processing unit 103 may be able to determine whether the lane markings are yellow or white by comparing the measured intensity of the captured image of the relevant lane marking at either side of the yellow filter section 112 and within the yellow filter section 112. This can be determined by measuring the intensity along each detected lane marking as it crosses the yellow filter section 112. This can be seen in more detail in Figures 4a and 4b of the accompanying drawings.

0032Figures 4a and 4b of the accompanying drawings show the measured intensity along the two lane markings 115 and 117 depicted in Figure 3 of the accompanying drawings. Figure 4a depicts the measured intensity along line Y-Y' along yellow line 115, and Figure 4b depicts the measured intensity along line W-W' along white line 117. As can be seen in Figure 4a, the intensity measured within the yellow filter section 112 is the same as measured in the clear background 110 and as such the intensity does not greatly change, indicating a yellow lane marking 115. However, Figure 4b shows the effect that the yellow filter section 112 has on white line 117. A significant drop in measured intensity occurs over the yellow filter area 112, thereby indicating the presence of a white lane marking 115.

0033From the difference in intensities within and without the yellow filter section 112, the processing unit 103 can determine whether the lane markings are white or yellow. In a preferred embodiment, the processing unit determines that the lane marking is yellow if the difference in the measured intensities is less than a predetermined ratio.

0034The processing unit can then act on its determination. For example, yellow lane markings may take precedence over white lane markings, such that the lane departure warning system preferentially follows yellow lane markings rather than white lane markings. Alternatively, the system may simply display which lane markings are which to the user.

0035It is appreciated that different territories, states and countries may have different coloured lane markings. In an alternative embodiment, the filter 104 above is replaced with the alternative filter 150 shown in Figure 5 of the accompanying drawings. A clear background 151 is provided as before, but in this embodiment a plurality (in this case, three) different colour filter sections 152a, 152b, 152c are provided each separated by an area of clear background 151. Any number of colour filter sections may be provided, the colour of each corresponding with a lane marking which it is desired to differentiate from other, generally white, lane markings. In the present embodiment, colour filter section 152a may be yellow, colour filter section 152b may be blue and colour filter section 152c may be red.

0036In the same way as in the first embodiment, the changes in intensity along each detected lane marking over the different colour filter sections 152 may be determined. A small change in intensity (typically measured as a ratio) over a given colour filter section 152 implies that the lane marking is of that colour, whilst no significant changes in intensity over any of the colour filter sections 152 implying a white marking.