Method and device for characterising a terminal comprising a light source and at least one camera

Abstract

The invention relates to a method and device for characterising a terminal comprising a light source and at least one camera. This method makes it possible to characterise a terminal comprising a light source and at least one camera. The method comprises the following steps: - illuminating (E10), with the light source, an at least partially reflective surface; - obtaining (E20) an image of the surface acquired with a camera referred to as the "active camera" from among the at least one camera; - calculating (E60), from this image, the position of a point on the surface within a highlighted light spot; - estimating (E90) a relative position of the light source with respect to the position of the active camera (CAM) on the basis of: (i) the position of the point; and (ii) a position of the surface relative to a plane of the terminal.

Description

[0001] Description

[0002] Title of the invention: Method and device for characterizing a terminal comprising a light source and at least one camera

[0003] Previous technique

[0004] The invention relates to the general field of optics.

[0005] More specifically, it aims at a method and a device for characterizing a terminal comprising a light source (often called "flash") and at least one image acquisition module (hereafter called camera).

[0006] The market for image acquisition devices (cameras, projectors, scanners, etc.) was traditionally a specialized vertical market.

[0007] But the gradual replacement of traditional cameras by phones (sometimes called "smartphones" or smartphones) has developed extensively over the past twenty years, largely due to improvements in the quality of cameras integrated into these devices.

[0008] Technological advances in software and hardware, such as increased resolution, improved sensors, the introduction of image stabilization technologies, software optimization for sharper photographic results, and other advanced features, have helped make terminal cameras, such as those in smartphones, increasingly competitive with standalone digital cameras, particularly for the general public.

[0009] The widespread adoption of smartphones and the rise of social networks where instant photo sharing has become common practice have accentuated this phenomenon and many optical applications, beyond photography, have become accessible to users of these devices.

[0010] The market for these terminals has become a consumer market with a considerable supply; these terminals therefore have very different hardware characteristics (sensors, light sources, etc.) depending on the brand and model, making it difficult for software publishers to control all or part of the technical characteristics of the terminals on which their software is deployed.

[0011] The present invention proposes a solution to help characterize such terminals.

[0012] Object and summary of the invention

[0013] More specifically, the invention relates to a method for characterizing a terminal comprising a light source and at least one camera, the method comprising the following steps:

[0014] - lighting, with said light source, of a surface at least partially reflective;

[0015] - obtaining an image of said surface with a so-called "active" camera, at least one of said cameras being said;

[0016] - calculation, from said image, of the position of a point H of said surface included in a highlighted luminous spot;

[0017] - estimation of the relative position of the light source with respect to the position of said active camera (CAM) from:

[0018] (i) the position of point H; and

[0019] (ii) of a position of said surface with respect to a plane of said terminal.

[0020] Correspondingly, the invention relates to a device for characterizing a terminal, this terminal comprising a light source and at least one camera, the device comprising:

[0021] - a module for obtaining an image of a surface at least partially reflective illuminated with said light source, said image being acquired with a camera called an "active camera" from said at least one camera;

[0022] - a calculation module, based on said image, for the position of a point H of said surface included in a highlighted luminous spot;

[0023] - a module for estimating the relative position of the light source with respect to the position of said active camera from:

[0024] (i) the position of point H; and

[0025] (ii) of a position of said surface with respect to a plane of the terminal.

[0026] Thus, and more generally, the present invention proposes a solution for automatically determining the relative position of a camera and a light source on a terminal. From an operational point of view, the solution consists in particular of acquiring, with the terminal's camera, an image of a surface illuminated by the terminal's light source.

[0027] This operation can, for example, be carried out by a human operator or by a robot.

[0028] The surface, at least partially reflective, may in particular be reflective or semi-reflective, including non-matte.

[0029] In this document, a non-matte surface is considered to be one with a gloss at 60° greater than 10, as defined by ISO 2813 and ASTM D523 standards. For further information, those skilled in the art may refer to reference [3] at the end of the document.

[0030] For example, the gloss unit of the reflective surface is greater than 10 GU, as defined by ISO 2813 and ASTM D523 standards.

[0031] The configuration of the terminals characterized by the invention is arbitrary. In most cases, the light source and the camera are in the aforementioned plane of the terminal, with an optical axis of the camera oriented substantially perpendicular to said plane.

[0032] According to the invention, estimating the relative position of the light source with respect to the active camera uses the relative position of the at least partially reflective surface with respect to a plane of the terminal. This relative position can be determined by various means, for example, from the absolute positions of the surface and the terminal plane. Alternatively, this relative position can be obtained by placing markers on these elements and detecting their positions using an optical system. In another embodiment, to estimate the position of the at least partially reflective surface with respect to the terminal plane, the terminal is positioned parallel to this surface, and the distance between the terminal and the surface is measured using, for example, a terminal distance sensor.In yet another embodiment, with an augmented reality system, such as is commonly used to recognize an object, in particular by means of the contours of the object and / or a known pattern on the surface of the object, for example with known dimensions, it is possible to know the relative position of the surface being studied with respect to the plane of said terminal.

[0033] In a particular embodiment of the invention, the position of the light source is obtained by:

[0034] (a) determining a first DCAM line passing through the active camera and point H; and

[0035] (b) by determining the intersection between:

[0036] (i) a second line DF, symmetric to the first line DCAM with respect to a line normal to said surface and passing through point H; and

[0037] (ii) the plan of said terminal.

[0038] This last embodiment implements Snell's law of optics. For more information on image formation techniques using a plane mirror, those skilled in the art can refer to references [1] and [2] given at the end of this document. It is understood that no product is a perfect plane mirror, but if its surface is not matte, at least some of the light emitted by the terminal's light source behaves in a way that follows Snell's law of optics; those skilled in the art refer to this as specular reflection.

[0039] In a particular embodiment, the characterization process further comprises the following steps:

[0040] - determination, in the image, of a connected area comprising a grouping of points that are relatively light compared to the other points in the image;

[0041] - determination of the position, in this image, of a CG reference point of said connected area;

[0042] - determination of the position of point H from the position of the reference point CG of the connected zone and a transformation matrix.

[0043] The connected area can be obtained by at least one thresholding of the image.

[0044] In a particular embodiment, the CG reference point of the connected area is determined as the barycenter of the connected area or by a learning method.

[0045] The invention can be used with a terminal comprising one or more cameras.

[0046] When the terminal has multiple cameras, the invention can be used to determine which camera is active among the plurality of cameras, in other words, the camera that acquires the image of the at least partially reflective surface. Some terminals with multiple cameras are configured to automatically activate a camera to perform an acquisition. The invention makes it possible to automatically determine which camera is activated.

[0047] In this embodiment, the characterization process according to the invention comprises the following steps:

[0048] - obtaining a known position of each of said cameras;

[0049] - determination of said active camera from the estimated position of the light source relative to the active camera and said known positions of each of the cameras.

[0050] In one embodiment, determining the active camera involves selecting the camera whose distance from the light source is closest to the distance between the active camera and the estimated position of the light source. In another embodiment, determining the active camera involves selecting the camera whose position relative to the light source is closest to the position of the active camera relative to the estimated position of the light source.

[0051] In one embodiment of the invention, the known position of at least one of said camera is obtained from:

[0052] (i) a representative image of one face of the terminal and including at least that camera and the light source; or from

[0053] (ii) terminal specifications indicating values ​​for calculating relative positions between said at least one camera and the light source; or from

[0054] (iii) a measurement on the terminal of the relative positions between said at least one camera and the light source.

[0055] The invention offers numerous applications.

[0056] In particular, knowing the relative position of the active camera and the light source can allow:

[0057] - to automatically adjust the brightness of the scene to avoid over-lit or under-lit areas;

[0058] - to illuminate a specific area at a predetermined angle;

[0059] - to avoid illuminating certain specific areas of the product; to avoid specular reflection on certain areas of the product;

[0060] - to automatically adjust the brightness of the scene to define the illuminated area and the direction of the lighting for the purposes of authentication, identification or traceability of products or documents.

[0061] Once determined, the relationship between the position of the light source and the position of the active camera can be used directly by an application on the terminal.

[0062] In a particular embodiment, the characterization process includes a step of recording in a calibration database, an identifier of said terminal in association with a relationship between the estimated position of the light source and the position of said active camera.

[0063] In a particular embodiment, during the registration step in a calibration database, the terminal identifier is also registered in association with information relating to the active camera. For example, in the embodiment where the terminal has several cameras, the information relating to the active camera corresponds to an identifier of that camera among the plurality of cameras or to camera characteristics.

[0064] The identifier of said terminal can be a unique identifier of said terminal or an identifier of the terminal model.

[0065] The aforementioned relationship between the estimated position of the light source and the position of said active camera can, for example, be:

[0066] - a pair comprising the estimated position of the light source and the position of said camera in a coordinate system of the terminal; or

[0067] - a position of the active camera relative to the light source. The invention thus makes it possible to create a database which groups together, for a plurality of terminals equipped or not with a plurality of cameras, the relationship between the position of the light source and the known and / or estimated position of at least one camera of each of these terminals.

[0068] In a particular embodiment, the different stages of the characterization process according to the invention are determined by computer program instructions.

[0069] Consequently, the invention also relates to a computer program on an information medium, this program being capable of being implemented in a computer, this program comprising instructions adapted to the implementation of the steps of a characterization process as described above.

[0070] The invention also relates to a computer program on an information medium, this program being capable of being implemented in a mobile access network access point or more generally in a computer, this program comprising instructions adapted to the implementation of the steps of a characterization process as described above.

[0071] This program can use any programming language, and be in the form of source code, object code, or code somewhere between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0072] The invention also relates to a computer-readable information or recording medium, and comprising instructions for a computer program as mentioned above.

[0073] The information or recording medium can be any entity or device capable of storing the program. For example, the medium can include a storage means, such as a ROM, for example a CD-ROM or a microelectronic circuit ROM, or an optical, quantum or magnetic recording means, for example a floppy disk or a hard disk drive.

[0074] On the other hand, the information or recording medium can be a transmissible medium such as an electrical or optical signal, which can be transmitted via an electrical or optical cable, by radio, or by other means. The program according to the invention can, in particular, be uploaded to a network such as the Internet.

[0075] Alternatively, the information or recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the process in question.

[0076] It can also be envisaged, in other embodiments, that the management process, the device located at the entrance of the core network, the processing process, the access point and the communication system according to the invention present in combination all or part of the aforementioned characteristics.

[0077] Brief description of the drawings

[0078] Other features and advantages of the present invention will become apparent from the description below, with reference to the accompanying drawings, which illustrate an example of an embodiment without being limiting in any way. In the figures:

[0079] — Figure 1 represents a user implementing a terminal characterization process according to a particular embodiment of the invention; — Figure 2 represents in flowchart form the main steps of a characterization process according to a particular embodiment of the invention;

[0080] — Figure 3 represents an image of a product surface;

[0081] — Figure 4 illustrates a method for determining the position of a light source of a terminal in a particular embodiment of the invention;

[0082] — Figure 5 represents an image of a terminal;

[0083] — Figure 6 represents the functional architecture of a characterization device conforming to a particular embodiment of the invention; and

[0084] — Figure 7 represents the material architecture of a characterization device conforming to a particular embodiment of the invention.

[0085] Detailed description of the invention

[0086] Figure 1 represents a user illuminating a reflective or semi-reflective surface SR, including non-matte, with the light source F of his terminal T.

[0087] In the embodiment of Figure 1, terminal T includes three cameras CAMi to CAM3.

[0088] In this detailed description, we will assume that the SR surface is flat, at least in the illuminated area.

[0089] A frame of reference R is attached to the surface SR. It has an origin O and three orthonormal axes xx, yy, zz, the xx and yy axes of this frame being, in this example, in the plane of the surface SR. Note that in this embodiment, the light source F and the cameras CAM Pof the terminal are in the same plane PT of the terminal T. We call RT a frame associated with the terminal T in this plane.

[0090] In the embodiment described here, the origin of this RT coordinate system corresponds to the position of the active camera, denoted CAM. The RT coordinate system comprises three orthonormal axes x, y, z, with the x and y axes lying in the PT plane of the terminal.

[0091] The active CAM camera can be any of the CAMi to CAM3 cameras, so the origin of the marker varies.

[0092] In the embodiment described here, an unshown optical axis of the active camera CAM is oriented perpendicular to the PT plane.

[0093] It is assumed that the position POSSR%PT of the surface SR with respect to the plane PT of the terminal T is known or determined.

[0094] In one embodiment, to estimate the POSSR%PT position of the surface SR relative to the PT plane of the terminal T, the terminal T is positioned parallel to the surface SR and the distance between the terminal and the surface is measured using a terminal distance sensor.

[0095] Alternatively, the POSSR%PT position is determined from the position of the SR surface in the RT frame using augmented reality type algorithms configured to detect the SR surface and its position in the environment captured by the active CAM camera.

[0096] Figure 1 shows a highlighted light spot TLSR on the surface SR produced by the terminal's light source F and visible to the active camera CAM. With reference to Figure 2, we will now describe in detail how the POSF3D position of the light source F in the frame R can be estimated.

[0097] More specifically, Figure 2 represents in flowchart form the main steps of a characterization process according to a particular embodiment of the invention.

[0098] During a step E10 of the characterization process, the surface SR of the product P is illuminated with the light source F of the terminal T.

[0099] During an E20 step, an IMG image of the SR surface is obtained with an active CAM camera of terminal T, the illumination of the SR surface of product P with the light source F of terminal T producing a TLSR light spot which appears highlighted in the IMG image on the SR surface.

[0100] As is known, all points in the image can be represented by their two-dimensional coordinates in the image and by an intensity value, for example in grey level.

[0101] In this image (IMG), and as shown in Figure 3, points p corresponding to the highlighted light spot (TLSR) appear relatively bright compared to other points in the image (IMG). These points appear, for example, with high light intensity, saturated...

[0102] In the embodiment described here, the characterization process includes a step E30 for determining, within the IMG image, a connected area ZC comprising a grouping of these points p, for example by a thresholding method or a learning method. The connected area ZC corresponds to the image of the highlighted light spot TLSR visible to the active camera CAM.

[0103] In the embodiment described here, the characterization process includes a step of determining a reference point CG of the connected zone ZC.

[0104] In a particular embodiment, the reference point CG is determined during a step E40a as being the barycenter of the points p of the connected zone ZC.

[0105] In another particular embodiment, the reference CG point of the connected area ZC is determined by a learning method E40b. This method can consist of training a neural network in a supervised manner to learn to recognize reference points in a connected area by providing it with input images in which the position of the CG center of the connected area is known.

[0106] The POSCGZD position of the CG reference point of the connected zone ZC is thus determined.

[0107] To simplify the description, the reference point CG of the connected area is hereafter referred to as the "center of the connected area" and the point H of the surface SR is referred to as the "center of the halo".

[0108] In the embodiment described here, the POSHSD position of the center of the halo H in the frame R is determined during a step E60 from the POSCGZD position of the center CG of the connected zone ZC and a transformation matrix MP. More precisely, the point H is the transformed center CG by the matrix MP.

[0109] For example, a projection matrix and a transformation matrix can be used, such as those defined respectively by "Projection matrix" and "Model View" ("Eye" corresponding to the camera and "Object" corresponding to the SR surface) in the "OpenGL Projection Matrix" document, available at https: / / www.songho.ca / opengl / gl_transform.html. It should be noted that the projection matrix described in this document is used to project three-dimensional objects in 3D space onto a two-dimensional screen. This projection matrix defines the camera properties, such as perspective, field of view, and depth of field. For more information on transformations, those skilled in the art can refer to reference [4].

[0110] Other projection methods known to people in the fields of computer vision, virtual reality, and 3D reconstruction can be used to convert a point in a 2D planar image to a point in 3D space and vice versa.

[0111] In a particular embodiment, this transformation matrix MP can define a projection as defined in document [5].

[0112] As described below, the POSHSD position of the center of the halo H in the frame R is used to estimate the POSFSD position of the light source F in this frame.

[0113] It is recalled that the position of the active camera CAM defines the origin of the RT reference frame attached to the terminal and that the POSCAMSD position of the active camera in the R reference frame can be obtained by a change of reference frame.

[0114] In the embodiment described here, with reference to Figure 4, a first straight line DCAM passing through the active camera CAM and the center of the halo H is determined during a step E70.

[0115] Since the position and orientation of the product surface are known in the R frame, they can be obtained in the RT frame of the terminal by a change of frame.

[0116] We then determine an angle 0 between this line DCAM and the normal DN to the reflective surface SR passing through the center of the halo H.

[0117] Then, during step E80, the intersection between is determined:

[0118] (i) a line DF symmetric to the line DCAM with respect to the line DN. DF also forms an angle 0 with the normal line DN, in accordance with Sneel's law. In this embodiment, the lines DCAM, Dp, and DN are in the same plane, called the "plane of incidence"; and

[0119] (ii) the PT plan of said terminal T.

[0120] This intersection defines the estimated position POSFSD of the light source F in the frame R.

[0121] In step E90, the POSFSD position of the light source F is estimated in the RT frame of reference, whose origin is the active camera CAM. This amounts to estimating the relative position of the light source F with respect to said active camera CAM. This estimation of the POSFSD position of the light source F is obtained from the POSHSD position of said point H, the center of the halo, and the POSSR%PT position of the surface SR with respect to a plane PT of the terminal T.

[0122] The characterization process includes a step E95 of recording, in a calibration database BDCAL, an ID identifier of the terminal T associated with a POSF%CAM relationship between the POSFSD position of the light source F and the position of the active camera CAM. In a particular embodiment of the invention, when the terminal T has several CAMp cameras, the characterization process further includes steps E100 to E120 to determine which of these cameras is the active camera. These steps are not performed when the terminal T has only one camera.

[0123] In this embodiment, the process obtains, during step E100, known positions of each of said CAM cameras Pof terminal T with respect to the light source F. These known relative camera / light source positions can be obtained from a technical specification document of the terminal or any document that indicates values ​​for calculating these relative positions.

[0124] These relative positions can also be determined from at least one representative image of the terminal on which the camera(s) and light source appear.

[0125] Figure 5 represents an IT image of a terminal. In this example, the image contains information about the terminal's length (XX) and width (YY). From these measurements and the dimensions of the IT image, it is possible to deduce the image's scale or resolution, and then its actual POSCAM position. P %F of the CAM camera P relative to the light source F.

[0126] This is the actual POSCAM position. P%F can be obtained for each of the CAM cameras P In the example shown in Figure 5, the actual POSCAM position P %F can be constituted by the coordinates dXp, dYp of the light source F in a frame similar to the RT frame with the camera CAM as its origin P and x, y axes identical to the x, y axes of the RT coordinate system.

[0127] During a step E110, the characterization process determines the so-called "estimated" position and noted POSCAMVOF of the active camera CAM relative to the light source F from the position POSCAMSD of the active camera CAM (origin of the RT frame) and the estimated position POSFSD of the light source F in the RT frame obtained in step E90.

[0128] During this step E110, the active camera CAM is determined to be the CAM camera Pwhose known position relative to the light source is closest to said estimated position of the light source F relative to the position of said active camera (origin of the reference frame RT), calculated from the known positions of each of said cameras CAM P of terminal T with respect to the light source F obtained in step 100. Alternatively, instead of comparing positions, the active camera is determined to be the camera whose distance to the light source is closest to the distance between the active camera and the estimated position of the light source.

[0129] In the embodiment described here, the characterization process includes a step E120 of recording, in a calibration database BDCAL, an ID identifier of the terminal T in association with a relation POSF%CAM between the position POSFSD of the light source F and the known position POSCAMSD of the camera CAM.

[0130] This POSF%CAM relationship between the estimated POSFSD position of the light source F and the POSCAMSD position of the camera CAM can, for example, be:

[0131] - a pair comprising these positions, for example in the RT frame of the terminal; or

[0132] - a relative POSCAM-F position of active CAM camera with respect to said light source F.

[0133] Figure 6 represents a DIS device for characterizing a terminal according to a particular embodiment of the invention.

[0134] This DIS device includes a MOD-OBT module for obtaining an image (IMG) acquired with the camera of a terminal T of a surface SR that is at least partially reflective and illuminated by a light source F from this terminal T. This device includes a MOD-CALC calculation module. This module is configured to calculate, from this IMG image, the POSHSD position of a point H on the surface SR that lies within a highlighted light spot.

[0135] This MOD-CALC calculation module is configured to estimate a POSFSD position of the light source F from:

[0136] (i) the POSHSD position of point H;

[0137] (ii) of a POSCAMSD position of the active camera and;

[0138] (iii) of a POSSR%PT position of said surface SR with respect to a plane PT of terminal T.

[0139] In one embodiment of the invention, the MOD-CAL calculation module is configured to obtain the POSFSD position of the light source F by:

[0140] (a) determining a first straight line DCAM passing through the active CAM camera and point H; and

[0141] (b) by determining the intersection between:

[0142] (i) a second line DF, symmetric to the first line DCAM with respect to a line DN normal to said surface and passing through point H; and

[0143] (ii) the PT plan of the terminal.

[0144] In the embodiment described here, the MOD-OBT acquisition module is configured to record, in a calibration database BDCAL, an ID identifier of the terminal T and information relating to the active camera (CAM) in association with a POSF%CAM relationship between the estimated position POSFSD of the light source F and the position POSCAMSD of the active camera CAM.

[0145] In the embodiment described here, the DIS characterization device has the hardware architecture of a computer, as schematically represented in Figure 7.

[0146] It includes, in particular, a processor 5, read-only memory 6, random access memory 7, non-volatile memory 8 and means of communication 9.

[0147] The read-only memory 6 of the DIS device constitutes a recording medium according to the invention, readable by the processor 5 and on which is recorded a computer program PG according to the invention, comprising instructions for the execution of steps of a characterization process according to the invention.

[0148] The PG program defines in particular the MOD-COM acquisition module and the MOD-CALC calculation module of the DIS device based on the hardware elements 5-9 of the DIS device.

[0149] This computer program PG, when executed by processor 5, contains instructions for:

[0150] - to obtain an image, acquired with an active camera of a terminal, of an image of a surface at least partially reflective illuminated with a light source from that terminal;

[0151] - calculate, from this image, the position of a point H of said surface included in a highlighted luminous spot;

[0152] - to obtain the position of the light source from:

[0153] (i) of the position of point H;

[0154] (ii) of an active camera position; and

[0155] (iii) of a position of said surface relative to a plane of said terminal. The DIS characterization device may be incorporated into the terminal T. Alternatively, at least some of its modules may be in a remote server.

[0156] For example, the MODJDBT module is incorporated into the T terminal and the MOD-CALC module is integrated into a remote server to calculate the position of point H and estimate the position of the light source.

[0157] Of course, these examples are given only for illustrative purposes and are not exhaustive in themselves.

[0158] References:

[0159] [1]: Image formed by a plane mirror: https: / / www.cbsetuts.com / formation-of-image-in-a-plane-mirror /

[0160] [2] Images formed by plane mirrors: https: / / phys.libretexts.org / Bookshelves / University_Physics / Book%3A_University_Physics_(OpenStax) / Uni versity_Physics_III_- _Optics_and_Modern_Physics_(OpenStax) / 02%3A_Geometric_Optics_and_Image_Formation / 2.02%3A_I mages_Formed_by_Plane_Mirrors

[0161] [3]: Measuring the gloss of a surface: https: / / labomat.eu / fr / faq-brillance / 771-glossmetre-quel-produit-convient-a-mon-application.html

[0162] [4]: Change of reference frame: https: / / fr.wikipedia.org / wiki / Changement_de_repère

[0163] [5] https: / / registry.khronos.Org / OpenGL-Refpages / gl2.l / xhtml / gluPerspective.xml

Claims

DEMANDS 1. Method for characterizing a terminal (T) comprising a light source (F) and at least one camera (CAM P ), the process comprising the following steps: - lighting (E10), with said light source (F), of a surface at least partially reflective (SR); - obtaining (E20) an image (IMG) of said surface (SR) with a camera called an "active camera" (CAM) among said at least one camera (CAM P ); - calculation (E60), from said image (IMG), of the position (POSHSD) of a point H of said surface (SR) included in a luminous spot (TLSR) which appears highlighted in said image (IMG); - estimation (E90) of a relative position (POSFSD) of the light source (F) with respect to the position of said active camera (CAM) from: (i) of the position (POSHSD) of said point H; and (ii) of a position (POSSR%PT) of said surface (SR) with respect to a plane (PT) of said terminal (T).

2. A characterization method according to claim 1, wherein the light source (F) and said at least one camera (CAM) P ) are in said plane (PT) of the terminal (T), an optical axis of said at least one camera (CAM P ) being oriented perpendicularly to said plane (PT).

3. Characterization method according to claim 1 or 2, wherein said surface (SR) is reflective or semi-reflective.

4. A characterization method according to any one of claims 1 to 3, wherein the position (POSFSD) of the light source (F) is obtained (E90) by: (a) determining (E70) a first straight line DCAM passing through the active camera (CAM) and said point H; and (b) by determining (E80) the intersection between: (i) a second line DF symmetric to the first line DCAM with respect to a line (DN) normal to said surface (SR) and passing through said point H; and (ii) the plan (PT) of said terminal (T).

5. A characterization method according to any one of claims 1 to 4, further comprising the following steps: - determination (E30), in said image (IMG), of a connected zone (ZC) comprising a grouping of points relatively light compared to the other points of said image (IMG); - determination (E50) of the position (POSCGZD), in said image (IMG), of a reference point CG of said connected zone (ZC); - determination (E60) of said position (POSHSD) of point H of said surface (SR) from said position (POSCGZD) of the reference point CG of the connected zone (ZC) and of a transformation matrix (MP).

6. Characterization method according to claim 5 wherein said connected zone (ZC) is obtained by at least one thresholding (E30) of said image (IMG).

7. Characterization method according to claim 5 or 6 wherein said reference point CG of the connected zone (ZC) is determined (E40a) as being the barycenter of said connected zone (ZC) or (E40b) by a learning method.

8. A characterization method according to any one of claims 1 to 7, wherein said terminal (T) comprises a plurality of cameras (CAMs P ), said process further comprising the following steps: - obtaining (E100) a known position (POSCAM) P %F) of each of said cameras (CAM P ) ; - determination (El 10) of said active camera (CAM) from said estimated position (POSFSD) of the light source (F) relative to the active camera (CAM) and the known positions (POSCAM) P%F) of said cameras (CAM P ).

9. A characterization method according to claim 8, wherein said known position (POSCAM) P %F) of at least one said camera (CAM P ) is obtained from: (i) of an image (IT) representative of a face of the terminal and comprising said at least one camera (CAM P ) and said light source (F); or from (ii) terminal specifications indicating values ​​for calculating relative positions between said at least one camera and the light source (F); or from (iii) a measurement on the terminal (T) of the relative positions between said at least one camera (CAM P ) and the light source (F).

10. A characterization method according to claim 8 or 9, wherein the determination of the active camera consists of selecting: - the camera (CAMp) whose known position relative to the light source is closest to the position of said active camera (CAM) relative to said estimated position (POSFSD) of the light source (F); or - the camera (CAMp) whose distance to the light source is closest to the distance between the active camera (CAM) and the estimated position (POSFSD) of the light source (F).

11. A characterization method according to any one of claims 1 to 10 comprising a step (E95, E120) of recording in a calibration database (BDCAL) an identifier (ID) of said terminal (T) in association with a relationship (POSF%CAM) between the estimated position (POSFSD) of the light source (F) and the position (POSCAMSD) of said active camera (CAM).

12. Characterization method according to claim 11, wherein at the step (E95, E120) of recording in a calibration database (BDCAL), the identifier (ID) of said terminal (T) is recorded in association with information relating to active camera (CAM).

13. Characterization method according to claim 11 or 12, characterized in that said relationship (POSF%CAM) between the estimated position (POSFSD) of the light source (F) and the position (POSCAM3D) of said active camera (CAM) is: - a pair comprising the estimated position (POSFSD) of the light source and the position (POSCAMSD) of the active camera in a reference frame (RT) of the terminal; or - a position (POSCAM-F) of the active camera (CAM) relative to said light source (F).

14. Device (DIS) for characterizing a terminal (T), this terminal (T) comprising a light source (F) and at least one camera (CAM P), the device comprising: - a module (MOD-OBT) for obtaining an image (IMG) of a surface at least partially reflective (SR) illuminated (E10) with said light source (F), said image being acquired (E20) with a camera called "active camera" among said at least one camera (F), ; - a calculation module (MOD-CALC) for calculating, from said image (IMG), the position (POSHSD) of a point H of said surface (SR) included in a luminous spot (TLSR) which appears highlighted in said image (IMG); - a module (MOD-CALC) for estimating (E90) a relative position (POSFSD) of the light source (F) with respect to the position of said active camera (CAM) from: (i) of the position (POSHSD) of said point H; and (ii) of a position (POSSR%PT) of said surface (SR) with respect to a plane (PT) of said terminal (T).

15. Computer program (PG) comprising, when said program is executed by a computer, instructions for: - obtain an image (IMG) of a surface at least partially reflective (SR) acquired (E20) with a camera (F) of a terminal (T), called an "active camera", said surface being illuminated (E10) with a light source (F) of this terminal (T); - calculate, from said image (IMG), the position (POSHSD) of a point H of said surface (SR) included in a highlighted luminous spot (TLSR); - estimate (E90) a relative position (POSFSD) of the light source (F) with respect to the position of said active camera (CAM) from: (i) of the position (POSHSD) of said point H; and (ii) of a position (POSSR%PT) of said surface (SR) with respect to a plane (PT) of said terminal (T).

16. Computer-readable recording medium (13) on which a computer program according to claim 15 is recorded.

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