METHOD AND DEVICE FOR SHOOTING AN AREA OF INTEREST OF A MOVING SUBJECT
By calculating and predicting the three-dimensional trajectory of a moving subject's area of interest and adjusting focus accordingly, the method effectively captures clear images despite subject movement, addressing the challenges of blurriness and shallow depth of field.
Patent Information
- Application Number
- FR2022014381
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2022-12-23
- Publication Date
- 2024-12-13
- Estimated Expiration
- 2042-12-23
AI Technical Summary
Capturing clear images of a moving subject's area of interest is complicated due to subject movement, which can cause blurry images and shallow depth of field, especially when high resolution and accuracy are required.
A method involving repeated calculation of the three-dimensional position and trajectory of the area of interest using context cameras, combined with a capture camera that adjusts focus based on estimated future positions and a varying focusing factor to compensate for positional inaccuracies.
This approach ensures the capture of sharp images of the moving subject's area of interest by predicting future positions and adjusting focus to mitigate movement-related blurriness, enhancing image clarity and resolution.
Smart Images
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Abstract
Description
Title of the invention: METHOD AND DEVICE FOR TAKING A PICTURE OF AN AREA OF INTEREST OF A MOVING SUBJECT
[0001] The present invention relates to the field of taking a picture of an area of interest located on a moving subject. More particularly, the invention relates to tracking and obtaining a clear picture of the area of interest located on the moving subject.
[0002] In a first example, this may involve recognizing the irises of a person moving in front of the capture system. In a second example, it may also involve reading a QR code affixed to an object, during the handling of the latter. Other examples of technical contexts may benefit from the invention.
[0003] There are systems that allow the capture of the area of interest when the subject is fixed. The capture volume is the area of space considered for the capture of the image of the area of interest. It is the area in which the subject can move while allowing the capture of the area. If the subject leaves the capture volume, it is not considered by the capture system.
[0004] When the subject is fixed, the capture system can operate on the following principle. In a first step, the subject is detected and located within the capture volume. Then, the capture system waits for the subject to stabilize. When the subject is stable, a precise step of locating the area of interest makes it possible to obtain its three-dimensional coordinates. These coordinates then make it possible to position the capture camera, equipped with motors allowing its orientation, in the precise direction of the targeted area of interest. A series of images is then captured until sufficient quality is obtained for the application, typically iris recognition or QR code reading. It should be noted that the envisaged applications require the capture of a good resolution image, whether for iris recognition or QR code reading.
[0005] When the subject is moving, capturing clear images of the area of interest becomes more complicated. Indeed, the movement of the subject can generate blurred images. In addition, due to the resolution and sensitivity required, the captured images have a very low depth of field. An inaccuracy regarding the distance of the area of interest then also produces the capture of a blurred image.
[0006] The invention aims to solve this problem by proposing a method for capturing images of an area of interest carried by a moving subject which quickly and reliably produces at least one clear image of the targeted area of interest.
[0007] According to one aspect of the invention, there is provided a method of capturing images of a area of interest of a moving subject within a capture volume characterized in that the method comprises the following steps: - repeatedly calculating at a first frequency the three-dimensional position of the area of interest in the capture volume by capturing the capture volume by a context camera system, and detecting and locating the area of interest in the capture volume; - estimation at a frequency, identical or different from the first frequency, of a three-dimensional trajectory of the area of interest in the capture volume from the calculated positions; - capturing a series of images of the area of interest by a capture camera at a second frequency by estimating a position of the area of interest at at least one future time t from the estimated three-dimensional trajectory of the area of interest, pointing the capture camera in the direction of the estimated position of the area of interest at time t, adjusting the focus of the capture camera to the distance between the capture camera and the estimated position of the area of interest at time t plus a focus factor varying in an interval centered on zero, and capturing an image by the capture camera at time t.
[0008] In one embodiment, the context camera system comprises two cameras producing stereoscopic images.
[0009] In one embodiment, the context camera system comprises a time-of-flight camera.
[0010] In one embodiment, the focus factor scales linearly between the extreme values of the interval centered on zero.
[0011] In one embodiment, the focus factor evolves sinusoidally between the extreme values of the interval centered on zero.
[0012] In one embodiment, the focus factor evolves with an increment step corresponding to the depth of field between the extreme values of the interval centered on zero.
[0013] In one embodiment, the three-dimensional trajectory of the area of interest is estimated linearly from the last two calculated positions.
[0014] In one embodiment, the three-dimensional trajectory of the area of interest is estimated at a frequency higher than the first frequency.
[0015] According to another aspect of the invention, a computer program is provided comprising instructions adapted to the implementation of each of the steps of the method according to the invention when said program is executed on a computer.
[0016] According to another aspect of the invention, there is provided a means of storing information, removable or not, partially or totally readable by a computer or a microprocessor comprising code instructions of a program computer for the execution of each of the steps of the method according to the invention.
[0017] According to another aspect of the invention, there is provided a device for capturing images of an area of interest of a subject in motion within a capture volume characterized in that the device comprises a processor configured for the following steps: - repeatedly calculating at a first frequency the three-dimensional position of the area of interest in the capture volume by capturing the capture volume by a context camera system, and detecting and locating the area of interest in the capture volume; - estimation at a frequency, identical or different from the first frequency, of a three-dimensional trajectory of the area of interest in the capture volume from the calculated positions; - capturing a series of images of the area of interest by a capture camera at a second frequency by estimating a position of the area of interest at at least one future time t from the estimated three-dimensional trajectory of the area of interest, pointing the capture camera in the direction of the estimated position of the area of interest at time t, adjusting the focus of the capture camera to the distance between the capture camera and the estimated position of the area of interest at time t plus a focus factor varying in an interval centered on zero, and capturing an image by the capture camera at time t.
[0018] In the attached drawings, given as non-limiting examples:
[0019] [Fig.l] represents, according to a principle diagram, an architecture of the capture system according to an embodiment of the invention;
[0020] [Fig.2] represents, according to a three-dimensional schematic view, a hardware architecture of a capture system according to an exemplary embodiment of the invention;
[0021] [Fig.3] represents a software architecture for trajectory estimation and tracking in an exemplary embodiment of the invention;
[0022] [Fig.4] illustrates the principle of position estimation in an exemplary embodiment of the invention;
[0023] [Fig.5] illustrates the capture of images of the area of interest from the estimated positions in one embodiment of the invention;
[0024] [Fig.6] is a schematic block diagram of an information processing device for implementing one or more embodiments of the invention.
[0025] The invention can be applied in different contexts. The exemplary embodiment is placed in the context of the recognition of the irises of a person in motion moving in front of a capture camera. The person can, for example, pass through a security airlock in an airport, or simply move in a free space and be followed by the capture system. Another exemplary embodiment of the invention concerns the capture of a QR code placed on an object that is being moved. This involves capturing the code of the object during its handling, typically in a warehouse. In all cases, the aim is to capture a clear image of an area of interest located on a moving subject.
[0026] [Fig.l] illustrates the architecture of the capture system according to an embodiment of the invention. This figure illustrates the capture volume 101 equipped with a capture camera 102. This capture camera advantageously makes it possible to follow a subject 103 carrying an area of interest 104. To do this, the capture camera 102 is typically motorized to allow its orientation in space. In the exemplary embodiment, it is equipped with two motors, one allowing rotation in the horizontal plane and the other in the vertical plane. The movement of these two motors makes it possible to point the capture camera in any direction within the capture volume. A third motor makes it possible to adjust the focus of the capture camera, that is to say to adapt the shot to the distance from the subject, and more particularly to the distance from the area of interest carried by the subject. The distance here is the distance to the camera, we also speak of depth.
[0027] The capture system is controlled by an information processing device 106 which makes it possible to control the movements of these motors. Typically, but not necessarily, it is also this information processing device 106 which receives and processes the images received from the capture camera 102. The information processing device is typically a computer, a tablet, a smartphone or any other device allowing the execution of a computer program responsible for controlling the camera, acquiring the images and the various steps of the method according to the invention.
[0028] Advantageously, in the reference embodiment, the capture camera 102 is supplemented by one or more context cameras 105. These context cameras make it possible to capture the entire capture volume and to recognize the subject, identify and locate it, then locate the area of interest 104. According to the embodiments, the context cameras 105 are also controlled by the information processing device 106. Alternatively, a dedicated device may be responsible for controlling the context cameras. In this case, this device communicates with the device 106 to perform the capture. In certain embodiments, the context camera(s) may be supplemented or replaced by additional sensors contributing to the location of the subject, such as, for example, a time-of-flight (ToF) sensor, a radar or any other sensor.These context cameras and their possible additional sensors make it possible to calculate the position of the subject and more particularly of the subject's area of interest in three-dimensional space. constituted by the capture volume. In the absence of additional sensors, the subject distance can be calculated by triangulation using at least two context cameras.
[0029] This estimation of the position of the area of interest is used for positioning control and adjustment of the capture camera's shooting. However, acquiring the images from the context cameras, analyzing them to produce the three-dimensional positioning of the area of interest, calculating the motor commands for positioning the capture camera at the calculated position, and acquiring the images by the capture camera takes a certain amount of time. The capture system is therefore subject to latencies between calculating the position and capturing the images of the area of interest. Due to the continuous movement of the subject, these latencies, and an inherent imprecision in the positioning calculation, obtaining a clear capture image of the area of interest is a difficulty.
[0030] In this document, it is proposed to control the capture camera based, not on the calculation of the position of the area of interest made by the context cameras, but on an estimate of this position at the time of the actual capture of this area of interest by the capture camera. This estimate of the position is therefore an estimate of a future position of the area of interest at the time of its estimation. It is made from the calculated effective positions which make it possible to estimate the trajectory of the subject and therefore to obtain an estimate of the position in the future, at the time when the capture camera will proceed to capture the image.
[0031] Advantageously, the three-dimensional position of this position estimate is modified by a focusing factor in the dimension representing the distance to the capture camera of the area of interest. This focusing factor represents the addition of a value evolving linearly between a negative minimum and a positive maximum around the zero value and changes between each shot. By slightly adjusting the focus of the image captures in this way, the chances of obtaining at least one sharp image over a set of captures can be increased.
[0032] Advantageously, the motors with which the capture camera is equipped are of the brushless type, in particular with direct drive, which allow continuous movement of the motor. This type of brushless motor allows smooth and continuous tracking by the capture camera of the movements of the subject without interfering with the shots. When the motors are in direct drive, the absence of a reduction gear specific to these motors eliminates the associated operating clearances. This would not be possible with stepper motors controlled using a stepper control method sometimes used in known systems.
[0033] [Fig.2] illustrates the hardware architecture of a capture system according to an exemplary embodiment of the invention. The system is mainly controlled by the processor main 201. This processor is typically operated by a traditional operating system such as Linux (RTM), Windows (RTM) or MacOs (RTM). These operating systems are not strictly real-time, which can affect the accuracy of the estimates and will need to be taken into account.
[0034] The system also has a coprocessor 202, the main characteristic of which is to be real-time, that is to say to be capable of executing certain tasks at precise predetermined times, and this in a guaranteed manner. The real-time coprocessor 202 is in charge of triggering 211 the context cameras 203, of triggering 212 the capture camera 204, of controlling 213 the focus motor 205 of the capture camera, of controlling 214 the aiming motors 206 which allow the orientation of the capture camera, and finally of controlling 215 the illumination 207 of the area of interest in a manner synchronized with the shots of the capture camera. The images 209 from the context cameras 203 and the images 210 from the capture camera 204 are transmitted for processing to the main processor 201.
[0035] The real-time coprocessor 202 is responsible for real-time tasks. It triggers the taking of images from the context cameras at regular intervals, for example at a frequency of 15 Hz. It controls the movements of the aiming motors, for example with position and / or speed control. It controls the movements of the focusing motor. It triggers the taking of images on the capture camera at the appropriate time, and it activates the illumination of the area of interest synchronously with the taking of the image by the capture camera.
[0036] The main processor 201 receives a stream of context images. On these context images it detects the presence of the subject and in this case it locates the area of interest in the capture volume in three dimensions. On the basis of these three-dimensional coordinates, the main processor estimates the trajectory of the area of interest, then sends the description of this trajectory to the coprocessor 202.
[0037] The coprocessor 202 controls the position and speed of the aiming motors 206, so as to reach the trajectory sent to it by the main processor 201 as soon as possible.
[0038] When the aiming motors have "locked" the ideal trajectory, the coprocessor 202 triggers the taking of an image of the area of interest periodically.
[0039] The more often the coprocessor 202 receives trajectory updates, the closer it will come to the actual trajectory of the area of interest.
[0040] The advantage of synchronization between the main processor and the coprocessor is that the sending of commands by the main processor does not have a real-time constraint, which makes it possible to use a non-real-time multi-tasking operating system. In other alternative embodiments, a single real-time processor takes over handles all the tasks shared here between the central processor and the real-time coprocessor.
[0041] [Fig.3] illustrates the software architecture of trajectory estimation and tracking in an exemplary embodiment of the invention.
[0042] The exemplary embodiment is based on the use of two context cameras used in stereoscopic mode to enable three-dimensional localization of objects detected in the image. It is recalled here that other embodiments may use other techniques, alternative or in addition to stereoscopic visualization, such as time-of-flight cameras, or even a three-dimensional vision system using structured light.
[0043] The main processor 201 executes a first module 301 responsible for locating the area of interest, for example the subject's eyes in the case of iris recognition within the context images. The recognition and localization algorithm used is considered known and is not the subject of this document.
[0044] A second module 302 executed by the main processor 201 is responsible for calculating the three-dimensional position of the area of interest located by the first module. These positions are stored, at least the last positions. The number of stored positions will depend on the algorithm used to estimate the trajectory; it is at least two positions.
[0045] A third module 303 executed by the main processor 201 is responsible for estimating the trajectory of the area of interest. This estimation is typically made by linear approximation of the stored positions. The simplest embodiment simply estimates the coordinates of a straight line passing through the last two stored positions. In this case, a locally rectilinear trajectory is estimated, which may be sufficient. In more complex embodiments, it is possible to calculate a polynomial modeling of the trajectory based on a number of positions greater than two. Such a polynomial modeling makes it possible to obtain a more precise trajectory estimation than the locally rectilinear trajectory.
[0046] The trajectory is estimated as a function of time. To do this, the received context images are temporally marked with a time label (lime.si amp in English) upon receipt. These time labels make it possible to associate a time index n with the positions calculated in three dimensions. The time index n, corresponding to an instant t, therefore corresponds to the three-dimensional position Pn = (Y^, ïn) • We thus obtain a series of indexed positions. It is then It is also possible to calculate a speed associated with this position as soon as at least two positions have been stored. This series may include indexes for which there are no associated positions. This may be due to a transmission problem. context images, to an inability to recognize the area of interest in certain context images for example. These missing positions must be taken into account when estimating the trajectory, particularly for estimating the speed.
[0047] For example, for a locally rectilinear trajectory estimation based on two stored positions Pnl - (X^, Yn_}, Z^), and Pn = (Xn, Yn, Zn), it is possible to estimate the speed of the subject at time t corresponding to the time index n: V„=(VX„ VY„ VZ,;)=(X„-X„.„ Yn-Y„.tZ„-Z„J. If the position ^, is missing, it is possible to use the position Pn-2 in a similar way, it is then necessary to divide the speed obtained by two to take into account the missing position. It is then possible to estimate a position at a time T greater than the current time t, from the current position at time t by applying the speed over a time (7' -t).
[0048] The trajectory estimation module 303 transmits the successive estimates to a trajectory tracking module 304 executed by the real-time coprocessor 202. This trajectory tracking module is responsible for estimating the position of the area of interest, at any time, on the basis of the last trajectory estimate transmitted by the module 303. In practice, the transmission frequency of the successive estimates is generally higher than the capture frequency of the context images. For example, the trajectory estimates are transmitted at a frequency of 1 kHz in an exemplary embodiment of the invention.
[0049] This knowledge of an estimate of the position of the area of interest at any time makes it possible to control the module 305 for controlling the aiming motors. This is how the capture camera 204 performs continuous monitoring of the area of interest.
[0050] A module 306 also receives the estimate of the position of the area of interest. This module is responsible for calculating a focus factor which is added to the Z position, i.e. to the distance between the capture camera and the area of interest. This focus factor evolves, for example, linearly between a negative minimum and a positive maximum. The distance between the capture camera and the estimate of the position of the area of interest, corrected by the focus factor, allows the control of the module 307 which controls the focus motor of the capture camera.
[0051] Figure 4 illustrates the principle of position estimation in an exemplary embodiment of the invention. The vertical axis represents the position of the area of interest, while the horizontal axis represents time. The position is represented schematically by a value on the vertical axis; it is actually composed of the three components giving the three-dimensional position of the area of interest. The positions in area 401 represent the calculated positions of the area of interest from the context images at times Ttt_3, Tn_2, TTn; these known positions are used to calculate the estimate of position 403 at time T^. This estimate is typically obtained by linear modeling of the trajectory from the known positions 401. To obtain a reliable trajectory estimate, it is necessary to sample the calculation of the positions at a sufficient frequency. This frequency will depend on the speed of the subject's movement and therefore depend on the intended application. For example, in the case of recognizing the irises of a person moving while walking, a frequency of 15 context images and therefore 15 estimated positions calculated per second has proven sufficient.
[0052] [Fig.5] illustrates the capture of images of the area of interest from the positions estimated in one embodiment of the invention. In this figure, the vertical axis represents the Z component of the position, i.e. the distance between the capture camera and the area of interest. The horizontal axis always represents time. Curve 504 represents the estimated trajectory of the area of interest. Points 501 on this curve 504 represent the different values of the Z component of the position estimates, therefore corresponding to points 403 of [Fig.4].
[0053] As already described, these positions 501 are only estimates and are therefore subject to a certain inaccuracy. This inaccuracy and the need to obtain a sharp, high-resolution image of the area of interest, an image which has a low depth of field, mean that the images obtained directly from the position estimates are often blurred. To counter this inaccuracy, it is proposed to add the focus factor Åz to the Z component of the estimated position of the area of interest. This focus factor is variable from one shot to the next. For example, the focus factor 5Z is varied in an interval [-Az, Az] centered on the zero value, which makes it possible to compensate for errors which statistically have a zero mean value. The maximum value of the focus factor Az depends on the inaccuracy of the system comprising the context cameras and the calculation of the position estimate.The step size of the focus factor must be less than the depth of field of the images captured by the capture camera. The more precise the estimation, the lower the maximum value of the focus factor can be. In an exemplary embodiment, the step size of the focus factor is of the order of one centimeter, while the maximum value of the focus factor is between seven and ten centimeters. Statistically, over all the shots taken by the capture camera, for some of these images, the focus factor compensates for the inaccuracy of the estimation of the distance from the camera to the area of interest, and makes it possible to obtain a sharp image.
[0054] According to the embodiments, several ways of varying the focusing factor in the interval [-Az, Az] can be used. A first alternative consists of varying the focusing factor linearly between -Az and Az, according to a sawtooth profile. A second alternative provides for a sinusoidal evolution of the focus factor over the interval. According to a third alternative, the focus factor is chosen randomly over the interval at each shot. According to a fourth alternative, the focus factor varies such that two consecutive factors are spaced apart by an amount corresponding to the camera's depth of field.
[0055] Curve 503 represents the shots from the capture camera. The height of the points represents the focus distance used when shooting, that is, the Z estimate of the position corrected by the focus factor. The frequency of shots from the capture camera is not necessarily identical to the frequency of position calculations, and therefore of shooting by the context cameras. Indeed, the estimated trajectory makes it possible to calculate a position estimate at any time and therefore to arbitrarily choose the shooting time. The higher the shooting frequency, the more quickly a clear and usable image of the area of interest is obtained. The limit of the shooting frequency is determined by the time required to adjust the focus of the capture camera to the new value and by the time required to capture the image.In practice, for the application of capturing iris images, a shooting frequency of 30 frames per second gives good results.
[0056] [Fig. 6] is a schematic block diagram of an information processing device 600 for implementing one or more embodiments of the invention. This device typically corresponds to the device 160 of [Fig. 1]. The information processing device 600 may be a peripheral such as a microcomputer, a workstation or a mobile telecommunications terminal. The device 600 comprises a communication bus connected to:
[0057] - a central processing unit 601, such as a microprocessor, denoted CPU;
[0058] - a random access memory 602, noted RAM, for storing the executable code of the method for implementing the invention as well as the registers adapted to record variables and parameters necessary for implementing the method according to embodiments of the invention; the memory capacity of the device can be supplemented by an optional RAM memory connected to an expansion port, for example;
[0059] - a read-only memory 603, noted ROM, for storing computer programs for the implementation of the embodiments of the invention;
[0060] - a network interface 604, denoted NET, is normally connected to a network of communication over which digital data to be processed are transmitted or received. The network interface 604 may be a single network interface, or composed of a set of different network interfaces (e.g., wired and wireless, interfaces or different wired or wireless interface types). Data packets are sent over the network interface for transmission or are read from the network interface for reception under the control of the software application running in the processor 601;
[0061] - a user interface 605 for receiving inputs from a user or for display information to a user;
[0062] - a storage device 606 as described in the invention and noted HD;
[0063] - an input / output module 607 for receiving / sending data from / to external devices such as hard drive, removable storage media or others.
[0064] The executable code may be stored in a non-volatile memory 603, for example a flash memory or a read-only memory, on the storage device 606 or on a removable digital medium such as for example a disk. According to a variant, the executable code of the programs may be received by means of a communication network, via the network interface 604, in order to be stored in one of the storage means of the communication device 600, such as the storage device 606, before being executed.
[0065] The central processing unit 601 is adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to one of the embodiments of the invention, instructions which are stored in one of the aforementioned storage means. After power-up, the CPU 601 is capable of executing instructions from the main RAM memory 602, relating to a software application. Such software, when executed by the processor 601, causes the execution of the described methods.
[0066] In this embodiment, the apparatus is a programmable apparatus that uses software to implement the invention. However, alternatively, the present invention may be implemented in hardware (e.g., in the form of an application-specific integrated circuit (ASIC) or in the form of a field-programmable gate array (FPGA)).
[0067] Naturally, to satisfy specific needs, a person skilled in the art of the invention may apply modifications to the preceding description.
[0068] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications that fall within the scope of the present invention will be apparent to a person skilled in the art.
Claims
Claims
1. Method for capturing images of an area of interest of a moving subject within a capture volume, characterized in that the method comprises the following steps: - repeated calculation at a first frequency of the three-dimensional position of the area of interest in the capture volume by capturing the capture volume by a context camera system, and detection and localization of the area of interest in the capture volume; - estimation at a frequency, identical to or different from the first frequency, of a three-dimensional trajectory of the area of interest in the capture volume from the calculated positions;- capturing a series of images of the area of interest by a capture camera at a second frequency by estimating a position of the area of interest at at least one future time t from the estimated three-dimensional trajectory of the area of interest, pointing the capture camera in the direction of the estimated position of the area of interest at time t, adjusting the focus of the capture camera to the distance between the capture camera and the estimated position of the area of interest at time t plus a focus factor varying in an interval centered on zero, and capturing an image by the capture camera at time t.;
2. Method according to claim 1, characterized in that the context camera system comprises two cameras producing stereoscopic images.
3. Method according to claim 1 or 2, characterized in that the context camera system comprises a time-of-flight camera.
4. Method according to any one of claims 1 to 3, characterized in that the focusing factor evolves linearly between the extreme values of the interval centered on zero.
5. Method according to any one of claims 1 to 3, characterized in that the focusing factor evolves sinusoidally between the extreme values of the interval centered on zero.
6. Method according to any one of claims 1 to 3, characterized in that the focusing factor evolves with an increment step corresponding to the depth of field between the extreme values of the interval centered on zero.
7. Method according to any one of claims 1 to 6, characterized in that the three-dimensional trajectory of the area of interest is estimated linearly from the last two calculated positions.
8. Method according to any one of claims 1 to 7, characterized in that the three-dimensional trajectory of the area of interest is estimated at a frequency higher than the first frequency.
9. Computer program comprising instructions adapted to the implementation of each of the steps of the method according to any one of claims 1 to 8 when said program is executed on a computer.
10. Information storage means, removable or not, partially or totally readable by a computer or a microprocessor comprising code instructions of a computer program for the execution of each of the steps of the method according to any one of claims 1 to 8.
11. Device for capturing images of an area of interest of a subject in motion within a capture volume, characterized in that the device comprises a processor configured for the following steps: - repeated calculation at a first frequency of the three-dimensional position of the area of interest in the capture volume by capturing the capture volume by a context camera system, and detection and localization of the area of interest in the capture volume; - estimation at a frequency, identical to or different from the first frequency, of a three-dimensional trajectory of the area of interest in the capture volume from the calculated positions;- capturing a series of images of the area of interest by a capture camera at a second frequency by estimating a position of the area of interest at at least one future time t from the estimated three-dimensional trajectory of the area of interest, pointing the capture camera in the direction of the estimated position of the area of interest at time t, adjusting the focus of the capture camera to the distance between the capture camera and the estimated position of the area of interest at time t plus a focus factor varying in an interval centered on zero, and capturing an image by the capture camera at time t.;