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

Abstract

Method and apparatus for characterizing a terminal comprising a light source and at least one camera. This method allows the characterization of a terminal comprising a light source and at least one camera. It comprises the following steps: - illuminating (E10), with the light source, a surface that is at least partially reflective; - obtaining (E20) an image of said surface acquired with a camera called the "active camera," from among said at least one camera; - calculating (E60), from this image, the position of a point on said surface contained within a highlighted light spot; - estimating (E90) a relative position of the light source with respect to the position of said active camera (CAM) from: (i) the position of said point; and (ii) a position of the surface with respect to a plane of the terminal. Fig. 2

Description

Title of the invention: Method and device for characterizing a terminal comprising a light source and at least one camera Previous technique

[0001] The invention relates to the general field of optics.

[0002] It aims more specifically at a method and a device for characterizing a terminal comprising a light source (often called "flash") and at least one image acquisition module (hereinafter referred to as camera).

[0003] The market for image acquisition devices (cameras, projectors, scanners, etc.) was traditionally a specialized vertical market.

[0004] But the gradual replacement of traditional cameras by telephones (sometimes called "smartphones" or smartphones) has developed extensively over the past twenty years, in particular due to improvements in the quality of the cameras integrated into these devices.

[0005] 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 contributed to making terminal cameras, such as those in smartphones, increasingly competitive with standalone digital cameras, particularly for the general public.

[0006] 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 terminals.

[0007] 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, ...) depending on the brand and model, so that it is difficult for software publishers to control all or part of the technical characteristics of the terminals on which their software is deployed.

[0008] The present invention proposes a solution to help characterize such terminals. Object and summary of the invention

[0009] Thus, 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: - lighting, with said light source, of a surface at least partially reflective; - obtaining an image of said surface with a so-called "active" camera, at least one of said cameras; - calculation, from said image, of the position of a point H of said surface included in a highlighted luminous spot; - estimation of the relative position of the light source with respect to the position of said active camera (CAM) from: (i) the position of point H; and (ii) of a position of said surface with respect to a plane of said terminal.

[0010] 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: - 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; - a calculation module, based on said image, for the position of a point H of said surface included in a highlighted luminous spot; - a module for estimating the relative position of the light source with respect to the position of said active camera from: (i) the position of point H; and

[0011] (ii) of a position of said surface with respect to a plane of the terminal.

[0012] 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.

[0013] This operation can, for example, be carried out by a human operator or by a robot.

[0014] The at least partially reflective surface may in particular be reflective or semi-reflective, in particular non-matte.

[0015] In this document, a non-matte surface is considered to be a surface having a gloss at 60° greater than 10 in accordance with 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.

[0016] For example, the gloss unit of the reflective surface is greater than 10 GU, as defined by ISO 2813 and ASTM D523 standards.

[0017] 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.

[0018] In one embodiment, to estimate the position of the at least partially reflective surface relative to the plane of the terminal, 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.

[0019] In a particular embodiment of the invention, the position of the light source is obtained by: (a) determining a first straight line DCam passing through the active camera and point H; and (b) determining the intersection between: (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 (ii) the plan of said terminal.

[0020] This latter embodiment implements Snell's law of optics. For more information on image formation techniques using a plane mirror, those skilled in the art may refer to references [1] and [2] given at the end of this document. It is understood that any product is not a perfect plane mirror, but if its surface is not matte, at least a portion of the light emitted by the terminal's light source behaves in accordance with Snell's law of optics; those skilled in the art refer to this as specular reflection.

[0021] In a particular embodiment, the characterization process further comprises the following steps: - 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; - determination of the position, in this image, of a CG reference point of said connected area; - determination of the position of point H from the position of the reference point CG of the connected zone and a transformation matrix.

[0022] The connected area can be obtained by at least one thresholding of the image.

[0023] In a particular embodiment, the reference point CG of the connected area is determined as the barycenter of the connected area or by a learning method.

[0024] The invention can be used with a terminal having one or more cameras.

[0025] When the terminal has several cameras, the invention can be used to determine the active camera among the plurality of cameras, in other words, the camera that acquires the image of the at least partially reflective surface. Some terminals having several cameras are configured to activate The invention automatically selects a camera to perform an acquisition. It allows for the automatic determination of which camera is activated.

[0026] In this embodiment, the characterization process according to the invention comprises the following steps: - obtaining a known position of each of said cameras; - 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.

[0027] In one embodiment, determining the active camera consists of selecting the camera whose distance from the light source is closest to the distance between the active camera and said estimated position of the light source.

[0028] In another embodiment, the determination of the active camera consists of selecting the camera whose position relative to the light source is closest to the position of the active camera relative to said estimated position of the light source.

[0029] In one embodiment of the invention, the known position of at least one said camera is obtained from: (i) a representative image of one face of the terminal and including at least that camera and the light source; or from (ii) terminal specifications indicating values ​​enabling calculation of relative positions between said at least one camera and the light source; or from (iii) a measurement on the terminal of the relative positions between said at least one camera and the light source.

[0030] The invention offers numerous applications.

[0031] In particular, knowledge of the relative position of the active camera and the light source can allow: - to automatically adjust the brightness of the scene to avoid over-lit or under-lit areas; - to illuminate a specific area at a predetermined angle; - to avoid illuminating certain specific areas of the product;

[0032] - to avoid specular reflection on certain areas of the product; - 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.

[0033] Once determined, the relationship between the position of the light source and the position of the active camera can be used directly by a terminal application.

[0034] In a particular embodiment, the characterization process includes a step of registering an identifier in a calibration database said terminal in association with a relationship between the estimated position of the light source and the position of said active camera.

[0035] In a particular embodiment, during the step of registering in a calibration database, the terminal identifier is also registered in association with information relating to the active camera. For example, in the embodiment in which the terminal has several cameras, the information relating to the active camera corresponds to an identifier of said camera among the plurality of cameras or to camera characteristics.

[0036] The identifier of said terminal may be a unique identifier of said terminal or an identifier of the terminal model.

[0037] The aforementioned relationship between the estimated position of the light source and the position of said active camera can, for example, be: - a pair comprising the estimated position of the light source and the position of said camera in a coordinate system of the terminal; or - the position of the active camera relative to the light source.

[0038] The invention thus makes it possible to constitute 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.

[0039] In a particular embodiment, the different stages of the characterization process according to the invention are determined by computer program instructions.

[0040] 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.

[0041] 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.

[0042] This program may use any programming language, and be in the form of source code, object code, or code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0043] The invention also relates to a computer-readable information or recording medium, comprising instructions for a computer program as mentioned above.

[0044] 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.

[0045] 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 downloaded onto an Internet-type network.

[0046] 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.

[0047] 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 have in combination all or part of the aforementioned characteristics. Brief description of the drawings

[0048] 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: • [Fig.1] represents a user implementing a method for characterizing a terminal in accordance with a particular embodiment of the invention; • [Fig.2] represents in the form of a flowchart the main steps of a characterization process conforming to a particular embodiment of the invention; • [Fig.3] represents an image of a product surface; • Figure 4 illustrates a method for determining the position of a source luminous of a terminal in a particular embodiment of the invention; • [Fig.5] represents an image of a terminal; • Figure 6 represents the functional architecture of a characterization device conforming to a particular embodiment of the invention; and • Figure 7 represents the hardware architecture of a characterization device according to a particular embodiment of the invention. Detailed description of the invention

[0049] Fig. 1 represents a user illuminating a reflective or semi-reflective surface SR, in particular non-matte, with the light source F of his terminal T.

[0050] In the embodiment of [Fig.1], the terminal T comprises three cameras CAMj to CAM3.

[0051] In this detailed description, we will assume that the surface SR is flat, at least in the illuminated area.

[0052] 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 CAMP cameras of the terminal are in the same plane PT of the terminal T. RT is a frame of reference associated with the terminal T in this plane.

[0053] In the embodiment described here, the origin of this frame RT corresponds to the position of the active camera, denoted CAM. The frame RT comprises three orthonormal axes x, y, z, the x and y axes being in the PT plane of the terminal.

[0054] The active camera CAM can be any of the cameras from CAMi to CAM3, so the origin of the reference point varies.

[0055] In the embodiment described here, an optical axis not shown of the active camera CAM is oriented perpendicular to the plane PT.

[0056] 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.

[0057] In one embodiment, to estimate the position POSsr%pt of the surface SR with respect to the plane PT 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 distance sensor of the terminal.

[0058] Alternatively, the position POSsr%pt is determined from the position of the surface SR in the frame RT using augmented reality type algorithms configured to detect the surface SR and its position in the environment captured by the active camera CAM.

[0059] In [Fig.1] a highlighted luminous spot TLSR is shown on the surface SR produced by the light source F of the terminal and visible by the active camera CAM.

[0060] With reference to [Fig.2], we will now in particular describe how the POSF3D position of the light source F in the frame R can be estimated.

[0061] More specifically, [Fig.2] represents in flowchart form the main steps of a characterization process according to a particular embodiment of the invention.

[0062] 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.

[0063] During a step E20, an IMG image of the SR surface is obtained with an active CAM camera of the terminal T, the illumination of the SR surface of the product P with the light source F of the terminal T producing a TLSR light spot which appears highlighted in the IMG image on the SR surface.

[0064] In a known way, all points of the image can be represented by their two-dimensional coordinates in the image and by an intensity value, for example in grey level.

[0065] In this IMG image, and as shown in [Fig. 3], points p corresponding to the highlighted light spot TLSR appear relatively bright compared to the other points in the IMG image. These points appear, for example, with a high light intensity, e.g., saturated...

[0066] In the embodiment described here, the characterization process includes a step E30 of determining, in 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 by the active camera CAM.

[0067] In the embodiment described here, the characterization process includes a step of determining a reference point CG of the connected zone ZC.

[0068] 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.

[0069] In another particular embodiment, the reference CG point of the connected area ZC is determined by a learning method E40b. This method may 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.

[0070] The position POSCg2d of the reference point CG of the connected zone ZC is thus determined.

[0071] To simplify the description, the reference point CG of the connected zone is hereinafter referred to as the "center of the connected zone" and the point H of the surface SR is referred to as the "center of the halo".

[0072] In the embodiment described here, the position POShsd of the center of the halo H in the frame R is determined during a step E60 from said position POSCg2d 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.

[0073] For example, a projection matrix and a transformation matrix can be used, such as those respectively defined by “Projection matrix” and “Model View” (“Eye” corresponding to the camera and “Object” corresponding to the SR surface) defined 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 further information on transformations, those skilled in the art may refer to reference [4].

[0074] Other projection methods known to the person in the trade of computer vision, virtual reality, and 3D reconstruction can be used to convert a point of the 2D planar image into a point of 3D space and vice versa.

[0075] In a particular embodiment, this transformation matrix MP can define a projection as defined in document [5].

[0076] As described below, the POSH3d position of the center of the halo H in the frame R is used to estimate the POSF3D position of the light source F in this frame.

[0077] It is recalled that the position of the active camera CAM defines the origin of the reference frame RT attached to the terminal and that the position POScamsd of the active camera in the reference frame R can be obtained by a change of reference frame.

[0078] In the embodiment described here, with reference to [Fig.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.

[0079] The position and orientation of the surface of the product being known in the frame R, they can be obtained in the frame RT of the terminal by a change of frame.

[0080] An angle 9 is then determined between this line DCam and the normal DN to the reflective surface SR passing through the center of the halo H.

[0081] Then, during a step E80, the intersection between: is determined (i) a line DF symmetric to the line DCam with respect to the line DN. DF also forms an angle 9 with the normal line DN, in accordance with Sneel's law. In the embodiment, the lines DCAM, DF and DN are in the same plane, called the “plane of incidence”; and (ii) the plane PT of said terminal T.

[0082] This intersection defines the estimated position POSF3D of the light source F in the frame R.

[0083] In a step E90, the position POSF3D of the light source F is estimated in the frame RT whose origin is the active camera CAM, which amounts to estimating the relative position of the light source F with respect to said active camera CAM. This estimation of the position POSF3D of the light source F is obtained from the position POSH3d of said point H, the center of the halo, and the position POSsr%pt of the surface SR with respect to a plane PT of the terminal T.

[0084] The characterization process includes a step E95 of recording, in a BDCal calibration database, an ID identifier of the terminal T associated with a POSf%Cam relationship between the POSF3D 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 implemented when the terminal T has only one camera.

[0085] In this embodiment, the method obtains, during step E100, known positions of each of said CAMP cameras of terminal T with respect to the light source F.

[0086] 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.

[0087] 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.

[0088] Figure 5 represents an IT image of a terminal, this image containing, in this example, information relating to the length XX and the width YY of the terminal. From these measurements and the dimensions of the IT image, it is possible to deduce a scale or resolution of the image, and then a real position POSCamp%f of the camera CAMp relative to the light source F.

[0089] This actual position POSCamp%f can be obtained for each of the CAMP cameras. In the embodiment example of [Fig.5], the actual position POSCamp%f can be constituted by the coordinates dXp, dYp of the light source F in a frame similar to the RT frame having the CAMP camera as its origin and x, y axes identical to the x, y axes of the RT frame.

[0090] During a step El 10, the characterization process determines the so-called "estimated" position, denoted POSCam%f, of the active camera CAM relative to the light source F, starting from the position POScamsd of the active camera CAM (origin of the reference frame RT) and the estimated position POSF3D of the light source F in the reference frame RT obtained in step E90.

[0091] During this step El 10, the active camera CAM is determined to be the camera CAMP whose 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 CAMP cameras of terminal T relative to the light source F obtained in step 100. Alternatively, instead of comparing the positions, the active camera is determined to be 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.

[0092] In the embodiment described here, the characterization process includes a step El20 of recording, in a BDCal calibration database, an ID identifier of the terminal T in association with a POSf%Cam relation between the POSF3D position of the light source F and the known POScamsd position of the camera CAM.

[0093] This POSf%Cam relationship between the estimated POSF3D position of the light source F and the POScamsd position of the camera CAM can, for example, be: - a pair comprising these positions, for example in the RT frame of the terminal; or - a relative position POScam f of active camera CAM with respect to said light source F.

[0094] Figure 6 represents a DIS device for characterizing a terminal according to a particular embodiment of the invention.

[0095] This DIS device includes a MOD-OBT module for obtaining an IMG image acquired with the camera of a terminal T of a surface at least partially reflective SR illuminated with a light source F of this terminal T.

[0096] This device includes a MOD-CALC calculation module. This module is configured to calculate, from this IMG image, the POSh3d position of a point H on the surface SR included in a highlighted luminous spot.

[0097] This MOD-CALC calculation module is configured to estimate a POSF3D position of the light source F from: (i) of the position POSh3d of point H;

[0098] (ii) of a POScamsd position of the active camera and;

[0099] (iii) of a POSSr%pt position of said surface SR with respect to a plane PT of the terminal T.

[0100] In one embodiment of the invention, the MOD-CAL calculation module is configured to obtain the POSF3D position of the light source F by: (a) determining a first straight line DCam passing through the active camera CAM and point H; and (b) by determining the intersection between: (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 (ii) the PT plan of the terminal.

[0101] In the embodiment described here, the MOD-OBT acquisition module is configured to record, in a BDCal calibration database, 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 POSF3D of the light source F and the position POScamsd of the active camera CAM.

[0102] In the embodiment described here, the DIS characterization device has the hardware architecture of a computer, as schematically represented in [Fig.7].

[0103] It includes in particular a processor 5, a read-only memory 6, a random-access memory 7, a non-volatile memory 8 and communication means 9.

[0104] 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.

[0105] 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.

[0106] This computer program PG, when executed by processor 5, includes instructions for: - 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; - calculate, from this image, the position of a point H of said surface included in a highlighted luminous spot; - to obtain the position of the light source from: (i) of the position of point H;

[0107] (ii) of an active camera position; and

[0108] (iii) of a position of said surface with respect to a plane of said terminal.

[0109] The DIS characterization device can be incorporated into the terminal T. Alternatively, at least some of its modules can be in a remote server.

[0110] For example, the MOD_OBT module is incorporated in the terminal T 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.

[0111] Of course, these examples are given only as illustrations and are not exhaustive in themselves. List of documents cited

[0112] [1] : Image formed by a plane mirror: https: / / www.cbsetuts.com / formation-of- image-in-a-plane-mirror /

[0113] [2] Images formed by plane mirrors: https: / / phys.libretexts.org / Bookshelves / University_Physics / Book%3A_University_Physics_(OpenStax) / University_Ph ysics_III_-_Optics_and_Modem_Physics_(OpenStax) / 02%3A_Geometric_Opt ics_and_Image_Formation / 2.02%3A_Images_Formed_by_Plane_Mirrors

[0114] [3] : Measurement of surface gloss: https: / / labomat.eu / fr / faq-brillance / 771- glossmetre-quel-produit-suitite-a-mon-application.html

[0115] [4]: ​​Change of reference frame: https: / / fr.wikipedia.org / wiki / Changement_de_repère

[0116] [5] https: / / registry.khronos.Org / OpenGL-Refpages / gl2.l / xhtml / gluPerspective.xml

Claims

Demands

1. A method for characterizing a terminal (T) comprising a light source (F) and at least one camera (CAM), the method comprising the following steps: - illuminating (E10), with said light source (F), a surface at least partially reflective (SR); - obtaining (E20) an image (IMG) of said surface (SR) with a camera called an “active camera” (CAM) from said at least one camera (CAM); - calculating (E60), from said image (IMG), the position (POShsd) of a point H of said surface (SR) included in a highlighted light spot (TLSR); - estimating (E90) a relative position (POSF3D) of the light source (F) with respect to the position of said active camera (CAM) from: (i) the position (POShsd) of said point H; and (ii) 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 (CAMP) are in said plane (PT) of the terminal (T), an optical axis of said at least one camera (CAMP) being oriented perpendicular to said plane (PT).

3. A 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 (POSF3D) 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) determining (E80) the intersection between: (i) a second straight line DF symmetric to the first straight line DCam with respect to a straight line (DN) normal to said surface (SR) and passing through said point H; and (ii) the plane (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 (POSCg2dX 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 (POSCg2d) of the reference point CG of the connected zone (ZC) and a transformation matrix (MP).

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

7. A 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 (CAMP), said method further comprising the following steps: - obtaining (E100) a known position (POSCamp%f) of each of said cameras (CAMP); - determining (E10) said active camera (CAM) from said estimated position (POSF3D) of the light source (F) relative to the active camera (CAM) and the known positions (POSCamp %F) of said cameras (CAMP).

9. A characterization method according to claim 8, wherein said known position (POSCamp%f) of at least one said camera (CAMP) is obtained from: (i) an image (IT) representative of a face of the terminal and comprising said at least one camera (CAMP) and said light source (F); or from (ii) terminal specifications indicating values ​​enabling calculation of 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 (CAMP) 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 (POSF3D) of the light source (F); or - the camera (CAMP) whose distance from the light source is closest to the distance between the active camera (CAM) and the estimated position (POSF3D) of the light source (F).

11. A characterization method according to any one of claims 1 to 10 comprising a step (E95, El20) of recording in a calibration database (BDCalX) an identifier (ID) of said terminal (T) in association with a relationship (POSf%Cam) between the estimated position (POSF3D) of the light source (F) and the position (POScamsd) of said active camera (CAM).

12. A 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 the active camera (CAM).

13. A characterization method according to any one of claims 11 or 12, characterized in that said relationship (POSf%Cam) between the estimated position (POSF3D) of the light source (F) and the position (POScamsd) of said active camera (CAM) is: - a pair comprising the estimated position (POSF3D) of the light source and the position (POScamsd) of the active camera in a frame (RT) of the terminal; or - a position (POScam F) of the active camera (CAM) with respect to said light source (F).

14. Device (DIS) for characterizing a terminal (T), said terminal (T) comprising a light source (F) and at least one camera (CAMP), 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 (POSH3d) of a point H of said surface (SR) included in a highlighted light spot (TLSR); - an estimation module (MOD-CALC) (E90) of a relative position (POSF3D) of the light source (F) with respect to the position of said active camera (CAM) from: (i) the position (POSh3d) of said point H; and (ii) a position (POSSR%PT) of said surface (SR) with respect to a plane (PT) of said terminal (T).

15. A computer program (PG) comprising, when said program is executed by a computer, instructions to: - obtain an image (IMG) of a surface at least partially reflective (SR) acquired (E20) with a camera (F) of a terminal (T), called the "active camera", said surface being illuminated (E10) with a light source (F) of this terminal (T); - calculate, from said image (IMG), the position (POSh3d) of a point H of said surface (SR) contained within a highlighted light spot (TLSR); - estimate (E90) a relative position (POSF3D) of the light source (F) with respect to the position of said active camera (CAM) from: (i) the position (POSh3d) of said point H; and (ii) 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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