1.  Field of the Invention
The present invention relates to a method for evaluation, by a motor vehicle, of the characteristics of a front element.  The term “front element” is used to denote any object, which may or may not be moving and which may or may not be living, disposed in front of the vehicle in question, on a road along which it is travelling or at the edge of this road.  For example, it may be another motor vehicle, which may be moving or stationary, or a pedestrian crossing the road.  One essential characteristic of the front element, which the method according to the invention makes it possible to determine with precision, is a distance between said vehicle and the front element in question.  In some embodiments of the method according to the invention, an additional characteristic of the front element which is determined is the height of the front element.  The present invention also relates to any motor vehicle which is able to implement such a method. 
2.  Description of the Related Art
The general field of the invention is that of motoring safety, and more particularly that of the detection of obstacles by a vehicle in question.  In this field, the aim is to determine as rapidly as possible the presence of front elements which are likely to present a risk of collision with the vehicle in question, so as to warn the driver, by means of an audible, visible or other signal, of the proximity of an obstacle and optionally of the nature of this obstacle.  One essential characteristic that has to be determined is therefore in particular the distance separating the vehicle from the front element.  It may also be beneficial to determine other characteristics of the front element, such as the dimensions, and in particular the height, so as to have a greater awareness of the front element in question. 
In the prior art, solutions exist which make use of equipment of the radar type, or of the lidar type, which make it possible to determine with precision certain characteristics of a front element.  However, the cost of such equipment means that motor vehicle manufacturers often rule it out.  Consequently, most of the solutions proposed for determining the characteristics of a front element are based on the use of at least one camera.  Among these solutions, there are firstly those which make use of a single camera; this type of solution is based on a postulate which greatly impairs the precision of the determinations: this postulate consists in considering that any front element arranged at a given distance is characterized by a standard width.  Implicitly, this postulate implies that a front element is necessarily another vehicle.  The determination of the distance between the vehicle implementing such a solution and a pedestrian is consequently highly erroneous, and therefore is of no use.  Furthermore, such a solution relies greatly on the attitude of the vehicle, which can give rise to distance estimates that are as much as double the actual distance. 
Among the solutions proposed in the prior art, there are also those which make use of two cameras, with which stereoscopic calculation rules are applied in order to determine the distance separating the vehicle in question from the detected front element.  One example of such a solution is shown in FIG.  1.  This figure schematically shows a plan view of a motor vehicle 100.  It comprises a first camera 101 and a second camera 102, arranged respectively at a left headlamp and at a right headlamp.  In this figure, like in the following figures which will be used to illustrate the invention, an orthogonal reference frame 103 is considered, which has an origin O referenced to any fixed point on the vehicle, for example located on a front bumper, an abscissa axis (Ox) oriented parallel to a direction of displacement 104 of the vehicle 100, a horizontal applicate axis (Oy) and a vertical ordinate axis (Oz).  The first camera 101 and the second camera 102 are positioned such that they both have an optical axis that is generally parallel to the abscissa axis.  The expression “axes that are generally parallel” refers to the fact that the axes in question are either strictly parallel or have—or their orthogonal projection onto a horizontal plane has—an angle of a few degrees, at most ten.  Furthermore, in the examples of the prior art, the two cameras that are used are arranged such that they have only one offset along the applicate axis.  It is this single offset D which is used to carry out a stereoscopic calculation, making it possible to arrive at an evaluation of the distance between the vehicle in question and any front element 105. 
However, this type of solution has a significant defect: the stereoscopic calculation which is carried out is directly dependent on the very precise positioning of the two cameras; no verification of the stereoscopic calculation is provided.  If there is the slightest variation in the offset along the applicate axis, the error will automatically be passed on to the distance evaluation.  Such variations are frequent since the cameras are subjected to considerable temperature variations due to the fact that they are positioned in the headlamps.  Moreover, at least one of the two cameras used is exclusively reserved for the stereoscopic calculation, with no application to date requiring the presence of two cameras in two headlamps arranged at each side of the front of the vehicle. 
There is therefore a need to provide a method that overcomes one or more of the problems of the prior art.