Abstract:
A method and system according to the invention comprises the means for or steps of: acquiring, by means of a digital image taking apparatus, a digital image with aberration of a scene, the digital image incorporating at least a first image zone and a second image zone, the first and second image zones having respectively at least a first resolution and at least a second resolution, the first and second resolutions being different; calculating a deployed digital image, by deployment of at least one zone of the digital image with aberration; applying an object detection algorithm in the deployed digital image; and supplying information on each object detected in the deployed digital image.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention concerns in general terms the field of motor vehicle driving assistance. More particularly, the invention concerns a method and device for detecting objects in a digital image of a road scene, such as for example pedestrians or road signs. The invention also concerns a digital apparatus for taking images, such as a camera, adapted to implementation of the above object detection method.  
       BACKGROUND OF THE INVENTION  
       [0002]     Various motor vehicle driving assistance systems are known in the prior art, using one or more cameras or image sensors.  
         [0003]     In these known systems, the cameras are generally dedicated to specific applications. In other words, one camera is necessary for each type of application, for example a first camera for taking images in the visible range and a second camera for taking images in the near infrared range.  
         [0004]     In addition, the detection of an object in the distance, at the center of the image, and a nearby object at the edge of the field is rarely compatible with the use of a single camera.  
         [0005]     This is because an object in the distance and at the center of the image represents only a few pixels compared with an object at the edge of the image and close to the camera lens.  
         [0006]     A small field makes it possible to detect an object present in the distance and at the center of the image, but does not make it possible to detect an object close to the vehicle and slightly to the side. Conversely, a larger aperture angle makes it possible to detect an object on the edges, but does not make it possible to detect an object in the distance, since the object is then represented by a small number of pixels.  
         [0007]     The available solutions recommend, in the above cases, the use of an expensive image sensor having a large number of pixels, or a zoom, to the detriment of the simultaneity of making the images available for various fields.  
         [0008]     The current techniques do not facilitate the design of compact economical devices which are well adapted to the significant constraints of robustness and costs, generally imposed in the automotive field, and capable of supplying to driving assistance systems the information of various types required by the applications installed in these systems.  
         [0009]     The present invention aims to remedy the drawbacks of the prior art disclosed above by providing a method and device of the above mentioned type which allow detection both distant and broad of the objects, and this with a single image sensor that can be produced at a moderate cost.  
       SUMMARY OF THE INVENTION  
       [0010]     According to a first aspect, the present invention provides a method of detecting objects in a digital image comprising the steps of:  
         [0011]     acquiring, by means of a digital image-taking apparatus, a digital image with aberration of a scene, the digital image incorporating at least a first image zone and a second image zone, the first and second image zones having respectively at least a first resolution and at least a second resolution, the first and second resolutions being different;  
         [0012]     calculating a deployed digital image, by deployment of at least one zone of the digital image with aberration;  
         [0013]     applying an object detection algorithm in the deployed digital image; and  
         [0014]     supplying information on each object detected in the deployed digital image.  
         [0015]     According to another particular characteristic of the method of the invention, the first image zone is a central zone of the digital image and the second image zone is an edge zone of the digital image, and the first resolution is chosen so as to be higher than the second resolution.  
         [0016]     According to a particular embodiment, the image acquisition step is carried out by means of a digital image taking apparatus equipped with an anamorphic lens, the aberration introduced into the image is predetermined so as to obtain a first substantially constant angular resolution for a first sector of the optical field of the anamorphic lens corresponding to the first image zone and a second variable angular resolution for a second sector of the optical field of the anamorphic lens corresponding to the second image zone, the first angular resolution being higher than the second angular resolution.  
         [0017]     Correspondingly, the invention also concerns a digital image-taking apparatus comprising an optical lens and an image sensor adapted to an implementation of the method as briefly described above.  
         [0018]     According to a particular characteristic, the image sensor comprises a plurality of pixels distributed over at least one central pixel zone having a first pitch between any two adjacent pixels in the central pixel zone and at least one edge pixel zone having a second pitch between any two adjacent pixels in the second edge pixel zone, the first and second pitches having different values, the first pitch being greater than the second pitch, and the central and edge pixel zones corresponding respectively to the first and second image zones.  
         [0019]     According to another particular characteristic, the value of the first pitch is variable according to the adjacent pixels in question in the central pixel zone. In addition, the value of the second pitch can also be variable according to the adjacent pixels in question in the edge pixel zone.  
         [0020]     According to another particular embodiment, the lens is of the anamorphic type and comprises a plurality of microlenses associated respectively with a plurality of pixels of the image sensor and having, between any two adjacent microlenses, a pitch of given value less than the pitch of the pixels.  
         [0021]     According to another particular embodiment, the anamorphic lens comprises at least one toric lens.  
         [0022]     According to yet another particular embodiment, the apparatus according to the invention comprises an optical lens and an image sensor and is characterised in that the lens is of the anamorphic type and comprises a plurality of microlenses distributed in first and second types to which the first and second focal distances respectively correspond, the microlenses being associated respectively with a plurality of pixels of the image sensor.  
         [0023]     According to a particular characteristic of the above embodiment, the microlenses and pixels are disposed according to different constant pitches, the pitch of the pixels being less than the pitch of the microlenses.  
         [0024]     According to another particular characteristic, the microlenses of the first type are situated in at least one central zone of the anamorphic lens and the microlenses of the second type are situated in at least one edge zone of the anamorphic lens.  
         [0025]     According to yet another particular characteristic, the digital image taking apparatus according to the invention also comprises an infrared filter disposed at the level of the image plane or an intermediate image plane of the apparatus.  
         [0026]     Preferably, the infrared filter comprises at least two zones in which distinct filterings are provided, a first zone corresponding to the top part of the image and in which the infrared rays undergo substantially no filtering and a second zone corresponding to the bottom part of the image and in which the infrared rays are substantially entirely filtered.  
         [0027]     In addition, the infrared filter can be designed so as to provide a filtering of the infrared rays which increases-progressively from top to bottom of the digital image.  
         [0028]     According to the applications of the invention, the image sensor can be of the CCD or CMOS type.  
         [0029]     Correspondingly, the invention also concerns a device for detecting objects in a digital image, adapted to be installed in a motor vehicle. The device comprises a digital image-taking apparatus and a processing unit which are suitable for implementation of the method of the invention as briefly described above.  
         [0030]     According to another particular characteristic, the device according to the invention also comprises a display screen and the processing unit comprises means able to correct the digital image in order to produce a corrected digital image reproducing the scene seen by the digital image-taking apparatus, the corrected digital image being intended for display on the screen.  
         [0031]     Preferably, the device also comprises means for supplying information on each object detected.  
         [0032]     The invention also concerns an information storage means, and a computer program whose execution allows implementation of the method according to the invention.  
         [0033]     Other aspects and advantages of the present invention will emerge more clearly from a reading of the description of particular embodiments and designs which follow, this description being given by way of non-limiting example and made with reference to the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  shows the general structure of a preferred embodiment of the object detection device according to the invention;  
         [0035]      FIG. 2  is a theoretical diagram showing an example of the distribution of angular resolutions in digital image-taking apparatus according to the invention;  
         [0036]      FIGS. 3A and 3B  show respectively an example of a scene and an image with corresponding aberration as obtained in accordance with the invention;  
         [0037]      FIG. 4  is an algorithm showing various steps of the object detection method according to the invention;  
         [0038]      FIG. 5  shows a hardware configuration of a processor unit included in an object detection device according to the invention;  
         [0039]      FIGS. 6A and 6B  show first and second particular embodiments of a digital image-taking apparatus according to the invention;  
         [0040]      FIG. 6C  shows a case of angular limitation to be prevented in a digital image-taking apparatus according to the invention; and  
         [0041]      FIG. 7  shows, in a simplified form, an example of infrared filtering introduced in a particular embodiment of the digital image-taking apparatus according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     With reference to  FIG. 1 , a particular embodiment of the object detection device according to the invention comprises essentially a camera  10 , a processor unit  11  in which image processing software modules  110  and  111  are installed, and a display screen  12 .  
         [0043]     In accordance with the invention, the camera  10  is able to supply images IM comprising a given aberration. To this end, the camera  10  can be provided with an anamorphic lens.  
         [0044]     The aberration introduced into the images IM by the camera  10  is intended to allow a detection of close and distant objects present in the scene seen by the camera  10 , from images IM supplied by the camera  10 .  
         [0045]     Typically, the aberration introduced is calculated by a person skilled in the art when the optical lens of the camera  10  is designed, according in particular to the envisaged application of the invention.  
         [0046]     One example of the effect of an optical lens determined so as to introduce an aberration as required by the invention is now described with reference to  FIG. 2  and  FIGS. 3A and 3B .  
         [0047]     In order to simplify the description of the invention, the aberration is here considered in a single dimension, namely along the X axis in  FIG. 2  and along the horizontal axis in  FIG. 3B .  
         [0048]     With reference to  FIG. 2 , the reference  20  corresponds to the image sensor, for example of the CCD or CMOS type, and the reference  21  to an anamorphic lens.  
         [0049]     The image sensor  20  comprises in a conventional manner a matrix composed of pixels Pi whose pitch is depicted in  FIG. 2  as having a given constant value.  
         [0050]     The anamorphic lens  21  is shown here in the form of a single lens. It should be noted however that the anamorphic lens  21  can also, in accordance with the invention, be produced in the form of a plurality of microlenses obtained by the use of micro-optic techniques, as described below with reference to  FIGS. 6A, 6B  and  6 C.  
         [0051]     In the embodiment in  FIG. 2 , the anamorphic lens  21  comprises a lens of the toric type. Naturally, in other embodiments, the lens  21  can comprise several toric lenses. Such a toric lens makes it possible to introduce a variable angular resolution in the image IM and causes an effect of compression of the edges of the image.  
         [0052]     As is clear in  FIG. 2 , the field of the camera  10  is divided into angular sectors SC and SB.  
         [0053]     The angular sector SC at the centre of the field has an angular pitch Pc for a pixel zone ZC. The pitch Pc is here approximated as being substantially constant.  
         [0054]     The angular sectors SB on the edges of the field have an angular pitch Pv for pixel zones ZB which increases from the central zone of the field towards the edges thereof.  
         [0055]     Naturally, with this type of lens  21  with a toric lens, there is strictly speaking no discontinuity or step in the variation in the angle of pitch, but a continuous transition from the angular pitch Pc towards the angular pitch Pv and vice versa.  
         [0056]     The aberration introduced by the anamorphic lens  21  has the effect of favoring, in terms of number of pixels, the central angular sector SC compared with the edge angular sectors ZB. In other words, for the angular sector SC, the ratio of the angular opening of the sector to the number of pixels in the corresponding pixel zone is greater than that obtained for the angular sectors SB.  
         [0057]     An example of the effect introduced on the image IM by the anamorphic lens is shown in  FIG. 3B , for a scene seen by the camera  10 , which is that shown in  FIG. 3A .  
         [0058]     The image of  FIG. 3A  is that which would normally be reproduced by the camera  10  if the latter were equipped with a spherical lens.  
         [0059]     As shown in  FIG. 3B  the central zone ZC of the image IM does not comprise any aberration and keeps correct proportions for the objects present in the zone. In addition, the high number of pixels in the zone ZC allows sufficient resolution for the detection of distant objects. The good legibility of the signalling panel  30  in the image IM of  FIG. 3B , compared with the image in  FIG. 3A , will in particular be remarked.  
         [0060]     The nature of the aberration introduced in the edge area ZB is shown in a simplified manner in  FIG. 3B  by the deformation of the person  31 . In this example, the person  31  undergoes a compression deformation along the horizontal axis. It should be noted here that the aberration introduced in the edge areas ZB by the anamorphic lens must be determined when the lens is calculated so that it does not prevent correct detection of the objects in the zones ZB.  
         [0061]     With reference now more particularly to  FIGS. 1 and 4 , the camera  10  delivers the images IM to software modules  110  and  111  of the processing unit  11 .  
         [0062]     The software module  110  fulfils the function of detection of objects in the image IM.  
         [0063]     The software module  111  fulfils an image correction function which is necessary in particular when a display on the screen  12  of the scene seen by the camera  10  is required.  
         [0064]      FIG. 4  shows processing steps E 1  to E 6  carried out by the processing unit  11 .  
         [0065]     At step E 1 , the camera  10  produces an image IM which corresponds to a scene seen by it. The image IM, which comprises an aberration by compression introduced by the anamorphic lens of the camera  10 , is supplied to the software modules  110  and  111 .  
         [0066]     At step E 2 , a deployed image IMD is calculated from the image with aberration IM. The operation performed at step E 2  makes it possible to obtain an image IMD in which the objects present have normal proportions, that is to say those that they have in reality. The deployment of the compressed image zones carried out at step E 2  on the image IM makes it possible to obtain an image IMD in which the aberration appears as being eliminated to a major extent.  
         [0067]     This calculation step E 2  has the advantage, for the supply of the image IMD, of then applying only a single object search algorithm A for detecting objects determined in this image IMD, such as road signs or obstacles.  
         [0068]     During step E 3   a , algorithm A is executed, which searches for given objects in the whole of the image IMD.  
         [0069]     At the conditional step E 3   b , when an object sought is detected in the image IMD by the algorithm A, detection information INF on the objects detected are supplied as an output by the software module  110 .  
         [0070]     It should be noted that, in this particular embodiment, steps E 2 , E 3   a , E 3   b  and E 4  are implemented in the module  110 .  
         [0071]     At step E 4  the information INF is for example converted into audible and/or visual information intended for the driver of the motor vehicle in which the device of the invention is installed. In other applications of the invention, the information INF is transmitted to one or more other systems equipping the vehicle, such as for example a driving system or systems controlling functions of the vehicle.  
         [0072]     At step E 5 , when a display of the scene is required, a step E 6  is executed, which corresponds to the image correction processing for display effected by the software module  111 . The module  111  supplies corrected images IMA in which the aberration present in the images IM is eliminated. The images IMA are displayed on the screen  12 . In a variant, it is the images IMD calculated by the module  110  rather than the images IM, as shown in  FIG. 1 , which are supplied to the module  111  in order to produce the images IMA.  
         [0073]     As shown in  FIG. 5 , the processing unit  11  has a conventional architecture and comprises a central processing unit CPU  50  such as a microprocessor, a read-only memory ROM or EEPROM  51 , a random-access memory RAM  52 , a storage memory  53 , for example of the FLASH type, interfaces  54  and an internal communication buss  55 . In another embodiment of the invention, the processor unit  11  is also equipped with a man-machine communication means, such as a keypad, through which the driver can select various operating modes.  
         [0074]     The processing unit  11  executes one or more programs PROG which enable the implementation of the method according to the invention.  
         [0075]     In the configuration in  FIG. 5 , the executable code of the programs PROG is housed partly or wholly in the ROM memory  51 .  
         [0076]     The executable code of the programs PROG can also be loaded partly in the storage memory  53 , via the interfaces  54 , from, for example, a diskette introduced into a disk drive or through a communication link connected for example to a microcomputer used for configuring the processing unit  11 .  
         [0077]     Naturally, the diskette from which the programs PROG are loaded can be replaced by a compact disc CD-ROM or a memory card. In more general terms, any information storage means that can be read by a computer or microprocessor, integrated or not in the processing unit  11 , possibly removable, is adapted to store the programs PROG in whole or in part.  
         [0078]     The central unit  50  controls the execution of the instructions or portions of code for the programs PROG, the instructions being stored in the ROM memory  51  and/or the storage memory  53  and/or the other information storage means indicated above.  
         [0079]     When the processing unit  11  is powered up, the programs PROG stored in a non-volatile memory, such as the ROM memory  51  or the storage memory  53 , are transferred in part or in whole into the volatile memory RAM  52 , which will then contain the executable code transferred from the programs PROG as well as various registers for storing variables and parameters necessary for implementing the method according to the invention.  
         [0080]     It should also be noted that the processing unit  11  can take the form of a programmed apparatus. This programmed apparatus then contains the executable code of the programs PROG in a fixed form in an application specific integrated circuit (ASIC).  
         [0081]     With reference essentially to  FIGS. 6A and 6B , first and second particular embodiments of the camera  10  according to the invention are now described. These particular embodiments of the camera  10  are obtained by the use of micro-optic techniques.  
         [0082]     In accordance with these particular embodiments, the camera  10  comprises in particular an optical lens formed by a plurality of microlenses and an image sensor of the CCD or CMOS type.  
         [0083]     As shown in  FIG. 6A , the first embodiment of the camera  10  comprises an optical lens  60 A in the form of a plurality of microlenses substantially identical and having the same focal distance.  
         [0084]     A lens of this type is described in the article “Artificial apposition compound eye fabricated by micro-optics technology” by Jacques Duparré, Peter Dannberg, Peter Schreiber, Andreas Bãuer and Andreas Tunnermann, published in Applied Optics, Vol 43, No 22, 1 Aug. 2004.  
         [0085]     In the above article by J Duparré et al., the microlenses of the lens are arranged at a constant pitch and are associated with pixels of an image sensor which are arranged also according to a constant pitch. The pitch of the pixels is less than the pitch of the microlenses so that a pixel in question sees an angle of field that is greater and greater as the pixel in question moves away from the axis of the lens. However, an angular limitation in terms of aperture is imposed by the introduction of opaque walls between the microlenses. These opaque walls are intended to prevent the appearance of phantom images. The angular resolution obtained in the embodiment described in this article is constant.  
         [0086]     In the embodiment in  FIG. 6A , the camera  10  according to the invention is differentiated from the design in the article by J Duparré et al in particular by the fact that a break in the pitch of the pixels of the image sensor  61 A is introduced.  
         [0087]     Thus, in the central region ZC, the pitch of the pixels has a first value. In the edge regions ZB, the pitch of the pixels has a second value higher than the first value. In accordance with the invention, different angular resolutions are therefore provided for the central zone ZC and the edge zones ZB.  
         [0088]     As shown in  FIG. 6B , the second embodiment of the camera  10  according to the invention comprises an optical lens  60 B formed from a plurality of microlenses distributed in two different types and an image sensor  61 B.  
         [0089]     The microlenses of the optical lens  60 B and the pixels of the image sensor  61 B have different constant pitches. The constant pitch of the pixel is less than the constant pitch of the microlenses.  
         [0090]     A first type of microlens contains the lenses of the central zone ZC which all have the same focal distance F 1 . A second type of microlens contains the lenses in the edge zones ZB which all have the same focal distance F 2  less than F 1 . This break in the focal distance of the microlenses allows the introduction, in accordance with the invention, of different angular resolutions for the central zone ZC and the edge zones ZB.  
         [0091]     Because of the constant pitch of the pixels of the image sensor  61 B, it is possible, in this second embodiment, to use a standard image sensor, which has an advantage with regard to cost.  
         [0092]     It should be noted here that the optical lens  60 B and the image sensor  61 B must be designed, that is to say the constant pitches of the microlenses and pixels must be chosen, so as to avoid reaching an angular limitation for the pixels in the central zone of the image.  
         [0093]     As shown by way of illustration in  FIG. 6C , this angular limitation occurs for a pixel Pi when the latter is situated in a transition zone between two adjacent microlenses.  
         [0094]     In the embodiments of the camera  10  described above with reference to  FIGS. 6A and 6B , the variation in the resolution in the image was essentially described as comprising steps or levels corresponding to the image zones ZC and ZB, following on from a variation of the same type in the pitch of the pixels or the microlenses. It should be noted here that the invention is not limited under any circumstances to this type of variation. This is because, in some applications, the lens and/or image sensor of the camera can be designed so as to introduce into the image a continuous variation in the resolution, for example by a variation corresponding to the pitch of the pixels or microlenses. A large number of different resolution values can then be present in the image.  
         [0095]     As shown also in  FIGS. 6A and 6B , the camera  10  according to the invention can be equipped with an infrared (IR) filter  62 .  
         [0096]     As shown in  FIG. 7 , the IR filter  62  is designed so as to procure a different IR filtering according to the zone of the image. This distinct IR filtering characteristic according to the zone of the image allows the use of the camera  10  both for applications functioning with wavelengths in the visible range and for applications functioning with wavelengths in the near infrared range.  
         [0097]     Preferably, the IR filter  62  is placed on the image plane or on an intermediate image plane so as not to scramble the image produced by the camera  10 .  
         [0098]      FIG. 7  shows by way of example an IR filter  62  divided into two distinct zones  70  and  71 .  
         [0099]     The zone  70  is situated in the top half of the image and does not have IR filtering. This is because, in this zone  70 , the IR information present is not dazzling by day for the camera  10  and does not require being filtered. The IR information therefore remains present in this zone  70  and can be used at night, for example for a night vision application.  
         [0100]     The zone  71  is situated in the bottom half of the image and has IR filtering. In this zone  71 , there exists a risk of dazzling of the camera  10  by the IR information present. This risk is eliminated by the IR filtering provided in this IR zone  71 . The elimination of the IR information in the zone  71  is not detrimental at night because the zone  71  is illuminated by the vehicle headlights and does not require to be covered by an IR image.  
         [0101]     Obviously, other embodiments are possible for the IR filter  62 , these depending essentially on the applications of the invention. Thus the IR filter  62  can take the form of a filter varying progressively from a zone opaque to IR rays to a zone transparent to IR rays, for example with a 100% filtering at the bottom of the image and a 0% filtering at the top of the image. It should be noted that the variation in the IR filtering is not necessarily linear.