JP6244926B2 - On-vehicle camera parameter generation device and program - Google Patents

Description

  The present invention relates to an in-vehicle camera parameter generation device and a program for generating camera parameters based on the position and orientation of an in-vehicle camera attached to the vehicle so as to capture the outside of the vehicle.

  Conventionally, in a calibration device for an in-vehicle camera, a feature point in a road marking on a road is extracted from an image captured by the in-vehicle camera, and a vanishing point on the image is obtained from a line segment connecting the image positions of the extracted feature points. A technique for calculating camera parameters indicating the height and posture of an in-vehicle camera based on the vanishing point and the known information on road marking is known (for example, see Patent Document 1).

JP 2008-11174 A

  However, in the above-described conventional technology, there is a concern that the reliability of the calculated camera parameter may be low. Specifically, for example, when the image position of a feature point (and thus a road marking) is shifted laterally, it is easy to determine whether the cause is due to the posture of the in-vehicle camera or the direction of the vehicle. Cannot be determined. And in such a situation, if the camera parameters are calculated uniformly as the cause of the image position deviation due to the attitude of the in-vehicle camera, the vehicle direction is actually deviated from the road marking However, there is a problem that calibration is not performed correctly.

  In addition, in the above-described conventional technology, since a series of processes for calculating camera parameters cannot be performed unless a predetermined road marking is present on the road, the opportunity for calibration is limited due to such restrictions. There was a problem that.

  The present invention has been made in view of such circumstances, and can easily generate camera parameters based on the position and orientation of the in-vehicle camera without being limited by the direction of the vehicle or the presence or absence of road markings. The purpose is to provide technology.

  The present invention relates to an in-vehicle camera parameter generation device that generates camera parameters based on the position and orientation of an in-vehicle camera attached to a vehicle so as to image the outside of the vehicle, the feature point extracting unit, the parameter generating unit, .

The feature point extraction means extracts a feature point of a light projecting road surface area projected onto the road surface by a light projecting device provided in the vehicle, from an image captured by the in-vehicle camera.
The parameter generation means generates a camera parameter based on the image position of the feature point extracted by the feature point extraction means.

  In the in-vehicle camera parameter generation device configured as described above, for example, even when the image position of the feature point (and thus the light projecting road surface area) is deviated, the cause is irrelevant to the direction of the vehicle. This is because the positional relationship between the vehicle and the projector is fixed and does not depend on the direction of the vehicle. Therefore, it is possible to perform calibration correctly by uniformly calculating camera parameters based on the position and / or orientation of the in-vehicle camera.

  In addition, since the feature points of the projected road surface area projected on the road surface by the light projecting device provided in the vehicle are extracted, the feature points are extracted even when there is no road marking on the road. And it is possible to generate camera parameters.

Therefore, according to the present invention, camera parameters based on the position and orientation of the in-vehicle camera can be easily generated without being limited by the direction of the vehicle or the presence or absence of road markings.
In addition, the present invention can be distributed on the market as a program. Specifically, it is a program for causing a computer to function as at least the above feature point extraction means and parameter generation means.

  By incorporating this program into one or a plurality of computers, it is possible to obtain the same effect as that achieved by the in-vehicle camera parameter generation device of the present invention. The program of the present invention may be stored in a ROM, flash memory, or the like incorporated in a computer and loaded from the ROM, flash memory, or the like into the computer, or loaded into the computer via a network. May be.

  Further, the above program may be used by being recorded on a recording medium in any form readable by a computer. Examples of the recording medium include a portable semiconductor memory (for example, a USB memory).

It is a block diagram which illustrates the composition of in-vehicle camera parameter generation device 1 and its connection apparatus. (A) is a bird's-eye view which shows the installation example of each sensor 21-24 which comprises the vehicle-mounted sensor 20, (b) is a perspective view which shows the definition of each axial direction and angular direction in a vehicle. It is a flowchart which illustrates the content of the calibration process which the vehicle-mounted camera parameter generation apparatus 1 performs. It is a flowchart which illustrates the content of the pre-process process implemented in S110 and S160 of a calibration process. It is explanatory drawing which illustrates the various light projection methods of the vehicle-mounted light. It is explanatory drawing which illustrates the feature point of the light projection road surface area | region by various light projection methods. It is explanatory drawing which illustrates the correlation of the shift | offset | difference of the image position of a feature point, and the shift | offset | difference of the attitude | position of the vehicle-mounted camera 10. FIG.

Hereinafter, an in-vehicle camera parameter generation device 1 as an embodiment of the present invention will be described with reference to the drawings.
The present invention is not construed as being limited in any way by the following embodiments. Moreover, the aspect which abbreviate | omitted a part of following embodiment as long as the subject could be solved is also embodiment of this invention. Moreover, all the aspects which can be considered in the limit which does not deviate from the essence of the invention specified only by the wording described in the claims are embodiments of the present invention. Further, the reference numerals used in the description of the following embodiments are also used in the claims as appropriate, but they are used for the purpose of facilitating the understanding of the invention according to each claim, and the invention according to each claim. It is not intended to limit the technical scope of

<Overall configuration>
As shown in FIG. 1, the in-vehicle camera parameter generation device 1 is composed of one or more known microcomputers having a CPU 2, a ROM 3, a RAM 4, and the like, and the CPU 2 stores them in the ROM 3 and an external memory (for example, a storage device 40). Based on the programmed program, the RAM 4 is used as a work area to execute various processes.

The in-vehicle camera parameter generation device 1 is connected to the in-vehicle camera 10, the in-vehicle sensor 20, the operating device 30, the storage device 40, the in-vehicle light 50, and the notification device 60.
The in-vehicle camera 10 is a camera having an image sensor such as a CCD or a CMOS, and is installed in the vehicle to image the front periphery of the vehicle. And the vehicle-mounted camera 10 outputs the captured image ahead of the vehicle to the vehicle-mounted camera parameter generating device 1 at a predetermined frequency (for example, 60 frames per second). In the present embodiment, the in-vehicle camera 10 is attached to the vehicle so that the road surface in front of the vehicle is reflected in the captured image.

  The in-vehicle sensor 20 is a variety of sensors mounted on the vehicle, and includes at least a plurality of sensors 21 to 24 for determining the level of the vehicle with respect to the road surface. As shown in FIG. 2A, these sensors 21 to 24 are installed in the four parts of the front part, the rear part, the left part, and the right part of the vehicle. A height sensor that detects the height in the vehicle height direction relative to the road surface (distance from the road surface) may be employed. When such a height sensor is employed, the detection values of the sensors 21 to 24 are compared, and if the difference value is equal to or smaller than a predetermined threshold for absorbing an error, the vehicle is horizontal with respect to the road surface. It can be determined that there is.

  In the following, as shown in FIG. 2B, in the vehicle, the vehicle width direction of the vehicle body 5 is the X axis, the vehicle length direction is the Y axis, the vehicle height direction is the Z axis, and the rotation direction with respect to the Z axis is the yaw direction. The rotation direction with respect to the Y axis is referred to as the roll direction, and the rotation direction with respect to the X axis is referred to as the pitch direction.

The operating device 30 is a peripheral device for receiving an input operation by a vehicle occupant (mainly a driver), and includes various switches and a touch panel.
The storage device 40 is configured by, for example, a hard disk drive or other nonvolatile memory, and stores a captured image input from the in-vehicle camera 10 to the in-vehicle camera parameter generation device 1 or various types executed by the in-vehicle camera parameter generation device 1. It is used to store various information generated by the processing (image position information described later).

  In addition, setting information necessary for the in-vehicle camera parameter generation device 1 to perform various processes is stored in the storage device 40 in advance. Although this setting information will be described in detail later, relative position information indicating the relative position (position in the real space) of the feature point that differs for each light projection method of the in-vehicle light 50 with respect to the vehicle body 5 (vehicle), for example, a captured image The setting camera parameter information used in the coordinate conversion from to real space is included. Note that the relative positions of the feature points can be measured in advance before the vehicle is shipped, for example.

  The in-vehicle light 50 is a known headlamp or turn signal lamp mounted on the front portion of the vehicle. Among these, the headlamp irradiates the headlamp with a light projection method according to a low beam or high beam switching instruction. In addition, the blinker lamp irradiates either the left or right signal lamp as a hazard lamp according to the direction indication. These switching instructions, direction instructions, and the like are also input from the in-vehicle camera parameter generation device 1.

  The notification device 60 is a display or a speaker installed in the vehicle interior, and is used in various processes executed by the in-vehicle camera parameter generation device 1 when notification to a vehicle occupant is required by a display image, a message, or the like. .

<Calibration process>
Next, calibration processing of the in-vehicle camera 10 among various processing executed by the CPU 2 of the in-vehicle camera parameter generation device 1 will be described with reference to the flowcharts of FIGS. 3 and 4. The calibration processing referred to in the present embodiment is a parameter indicating the correlation between the coordinates of the point projected on the image plane (image position) and the three-dimensional coordinates (position in real space) of the world coordinate system. This is a process of generating a matrix (camera parameters including at least an external parameter matrix based on the position and orientation of the in-vehicle camera 10) and updating the set camera parameter information stored in the storage device 40 with the generated parameter matrix. This set camera parameter information can be used, for example, when detecting the position of an obstacle ahead of the vehicle.

  Further, the calibration process is performed in either the running state or the stopped state of the vehicle, and may be performed periodically, or may be performed as many times as necessary. Further, when an illuminance sensor is mounted on the vehicle as the in-vehicle sensor 20, even when it is determined that the illuminance around the vehicle is below a predetermined threshold based on the detection value input from the illuminance sensor. Good.

When the calibration process is started, as shown in FIG. 3, the CPU 2 starts a pre-process process necessary for generating camera parameters in S110.
Specifically, when this pre-process is started, as shown in FIG. 4, the CPU 2 determines that the vehicle is horizontal with respect to the road surface based on the detection value input from the in-vehicle sensor 20 in S310. If it is determined whether or not the condition (hereinafter referred to as “horizontal condition”) is satisfied and it is determined that the horizontal condition is satisfied, the process proceeds to S320, and if it is determined that the horizontal condition is not satisfied Execute this step again.

  Next, in S320, the CPU 2 determines an obstacle (others within a preset projection distance based on the captured image input from the in-vehicle camera 10 and the set camera parameter information stored in the storage device 40. Vehicle) or the like (hereinafter referred to as “obstacle condition”) is determined. If it is determined that the obstacle condition is satisfied, the process proceeds to S330 and the obstacle condition is set. If it is determined that the condition is not satisfied, the process returns to S310. In the present embodiment, the light projection distance is determined to be a distance from the farthest point in the area projected on the road surface by the in-vehicle light 50 by various light projection methods (hereinafter referred to as “light projection road surface area”) to the vehicle. Can be defined as the distance plus the margin.

  In S330, the CPU 2 outputs a switching instruction, a direction instruction, and the like to the in-vehicle light 50. For example, as shown in FIG. 5, a light projection process is performed in which the headlamp in the headlamp is irradiated with a low beam and a high beam, and then both the left and right signal lamps in the winker lamp are irradiated.

  In subsequent S340, the CPU executes a feature point extraction process for extracting a feature point of the light projecting road surface area based on the captured image input from the in-vehicle camera 10 for each of the various light projecting methods in the light projecting process in S330. To do. In this feature point extraction processing, a light projecting road surface area (see FIG. 5) is extracted from the difference between the captured image immediately before being projected by the in-vehicle light 50 and the captured image immediately after the light is projected, and the extracted light projecting path surface area is extracted. A feature point (see FIG. 6) is extracted for each. The feature points include, for example, barycentric coordinates obtained by a weighted average of the luminance values of each pixel based on the luminance value of each pixel in the light projection path surface area in the captured image, a pixel position indicating the maximum luminance value, and the like. Used.

  When the pre-process of S310 to S340 (S110) is completed in this way, as shown in FIG. 3, the CPU 2 stores in the storage device 40 in S120 based on the image position of the feature point extracted in S340. A parameter update determination process is performed to determine whether or not the set camera parameter information to be updated is necessary. In this parameter update determination process, the image position of the feature point previously stored in the storage device 40 as image position information in S210 described later is compared with the image position of the feature point extracted this time in S340, and both images are compared. When the position is the same or the image position deviation amount is within the predetermined error range, it is determined that there is no need to update the set camera parameter information, and when the image position deviation amount exceeds the error range, the setting is made. It is determined that the camera parameter information needs to be updated.

  In subsequent S130, the CPU 2 branches the process based on the determination result of the parameter update determination process in S120. That is, in S120, if it is determined that there is no need to update the set camera parameter information, the process returns to S110, and if it is determined that the set camera parameter information needs to be updated, the process proceeds to S140.

  In S140, the CPU 2 notifies the vehicle occupant by the notification device 60 that the setting camera parameter information needs to be updated. This notification may include specific instruction content for the driver necessary for updating the set camera parameter information (for example, pressing a specific switch while the vehicle is stopped).

  In subsequent S150, the CPU 2 determines whether or not an input operation for updating the set camera parameter information (hereinafter referred to as “occupant instruction”) has been received via the operation device 30, and when it is determined that an occupant instruction has been received. In step S160, if it is determined that an occupant instruction has not been received, this step is re-executed.

  In S160, CPU2 extracts the feature point of a light projection road surface area | region about each of the various light projection methods by the vehicle-mounted light 50 by performing the pre-process process of S310-S340 similarly to previous S110.

  In subsequent S170, the CPU 2 compares the image position of the feature point previously stored as image position information in the storage device 40 in S210, which will be described later, with the image position of the feature point extracted in S340. It is determined whether or not the deviation of the position is caused by only the deviation of the attitude of the in-vehicle camera 10 among the deviations of the position and attitude of the in-vehicle camera 10, and is judged to be caused only by the deviation of the attitude. In this case, the process proceeds to S180, and if it is determined that it is due to the deviation of both the position and the attitude, the process proceeds to S190.

  Specifically, the image position information and the current captured image are compared for the image positions of two feature points extracted from the captured image, the distance between the feature points is equal, and the image position of each feature point is shown in FIG. When the vehicle camera 10 is shifted in the lateral direction as shown in a), the vehicle camera 10 is shifted in the yaw direction, and when it is shifted in the vertical direction as shown in FIG. 10 postures are determined to be shifted in the pitch direction. Further, when the distance between the feature points is equal and the line segments connecting the feature points intersect as shown in FIG. 7C (when they have an angle), the posture of the in-vehicle camera 10 is shifted in the roll direction. Judge that

  In S180, the CPU 2 executes a parameter correction process for correcting the camera parameter indicated by the set camera parameter information stored in the storage device 40. In this parameter correction process, for at least two feature points extracted from the captured image, the camera position is determined based on the image position and the known position in the real space indicated by the relative position information stored in the storage device 40, respectively. Of the parameters, a parameter matrix based on the attitude of the in-vehicle camera 10 is calculated.

  On the other hand, in S190, the CPU 2 determines each of at least four feature points extracted from the captured image based on the image position and the known position in the real space indicated by the relative position information stored in the storage device 40. Of the camera parameters, a parameter matrix based on the position and orientation of the in-vehicle camera 10 is calculated.

  As a method for calculating the parameter matrix, for example, “Z. Zhang,“ A Flexible New Technique for Camera Calibration ”, IEEE Transactions on Pattern Analysis and Machine Intelligence. Vol. 22, No. 11, pp. 1330-1334, The technique disclosed in “November 2000” can be used.

  Subsequently, in S200, the CPU 2 updates the set camera parameter information stored in the storage device 40 with the parameter matrix calculated in S180 or S190. In S210, the CPU 2 performs the pre-process (S160) of S310 to S340. The image position information indicating the image position of the feature point extracted in this manner is stored in the storage device 40, and the process returns to S310.

<Effect>
As described above, in the in-vehicle camera parameter generation device 1, in the calibration process, the feature points of the projected road surface area projected on the road surface by the in-vehicle light 50 provided in the vehicle from the image captured by the in-vehicle camera 10. Is extracted (S340), and camera parameters are generated based on the image positions of the extracted feature points (S170 to S200).

  In the in-vehicle camera parameter generation device 1 configured as described above, for example, even when the image position of the feature point (and thus the light projecting road surface area) is deviated, the cause is irrelevant to the direction of the vehicle. By uniformly calculating camera parameters based on the position and / or orientation of the camera 10, calibration can be performed correctly.

  Further, since the feature points of the projected road surface area projected on the road surface by the in-vehicle light 50 provided in the vehicle are extracted, the feature points are extracted even when there is no road marking on the road. And it is possible to generate camera parameters.

  Therefore, according to the in-vehicle camera parameter generation device 1, camera parameters based on the position and orientation of the in-vehicle camera 10 can be easily generated without being limited by the direction of the vehicle or the presence or absence of road markings.

  In the calibration process, a parameter matrix (camera parameter) is calculated based on the image position of the feature point extracted by the feature point extraction process (S340) and the known position of the feature point with respect to the vehicle in real space. The parameter calculation process is executed (S190).

  That is, since the position of the feature point of the projected road surface area projected onto the road surface by the in-vehicle light 50 can be measured in advance, the known position of the feature point with respect to the vehicle (in real space) The camera parameter based on the position and orientation of the in-vehicle camera 10 can be directly obtained by the relationship between the position) and the image position.

  In the calibration process, the light projection method of the in-vehicle light 50 mounted on the vehicle as the light projection device is switched (S330), and a plurality of feature points that are different depending on each light projection method are extracted (S340).

  According to such a configuration, it is possible to extract feature points using the existing in-vehicle light 50 without preparing a dedicated projector for generating camera parameters. A plurality of feature points can be easily extracted by changing the projection angle or the like. Thus, since the camera parameter can be generated based on a plurality of feature points, the calculation accuracy of the camera parameter can be improved by a simple method.

  In the calibration process, image position information indicating the image position of the feature point used when generating the camera parameter is stored (S210), and the image position of the feature point extracted by the feature point extraction process (S340); Based on the comparison result with the image position of the feature point indicated by the image position information, parameter correction processing for correcting the camera parameter is performed (S180, S190).

  According to such a configuration, the relationship between the known position of the feature point in the real space and the image position is corrected by correcting the camera parameter based on the deviation of the image position of the feature point (and thus the light projecting road surface area). As compared with the case where the camera parameter is calculated every time based on the above, the processing load of the apparatus can be reduced.

  In the calibration process, the horizontal condition (S310) is that the vehicle is horizontal with respect to the road surface, and the obstacle condition is that there is no obstacle within a predetermined distance based on the projected road surface area (S320). When the horizontal condition and the obstacle condition are satisfied, feature point extraction by the feature point extraction process (S340) is permitted.

  According to such a configuration, the relationship between the known position of the feature point in the real space and the image position can be reliably ensured, and the feature point can be easily extracted, so that the calculation accuracy of the camera parameter is further improved. be able to.

  Further, in the calibration process, a parameter update determination process (S120) for determining whether or not camera parameter generation (update) is necessary based on the image position of the feature point extracted by the feature point extraction process (S340). ) When it is determined that the camera parameter needs to be updated, the vehicle occupant is notified (S140).

  According to such a configuration, for example, when there is no or small shift in the image position of the feature point (and thus the light projection path surface area), the parameter calculation process (S190) and the parameter correction process (S180) can be omitted. Thus, the processing burden on the apparatus can be reduced, and when these processes are necessary, the calibration process of the in-vehicle camera 10 can be restarted at a suitable timing according to the passenger's instruction, for example, by notifying the passenger. it can.

<Other embodiments>
As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment, In the range which does not deviate from the summary of this invention, it is possible to implement in various aspects.

  For example, in the in-vehicle camera parameter generation device 1 of the above embodiment, the in-vehicle camera 10 is attached to the vehicle so as to image the front of the vehicle, but the present invention is not limited to this. And so on.

  Moreover, in the vehicle-mounted camera parameter production | generation apparatus 1 of the said embodiment, although illustrated about the case where a height sensor is employ | adopted as the some sensors 21-24 which comprise the vehicle-mounted sensor 20, it is not limited to this, For example, A gyro sensor for detecting the attitude of the vehicle is adopted, and the detection value of the gyro sensor is monitored. When the detection value does not continuously fluctuate (is stable), the vehicle is level with respect to the road surface. You may determine that there is.

  Moreover, in the vehicle-mounted camera parameter generation device 1 of the above embodiment, the vehicle-mounted light 50 mounted on the vehicle is used as the light projecting device. However, the present invention is not limited to this. The device may be separately installed in the vehicle.

  In the calibration process of the above embodiment, it is determined in S120 that the setting camera parameter information needs to be updated when the image position deviation exceeds the error range. However, the present invention is not limited to this. is not. For example, a parameter matrix is calculated in the same manner as in S180 or S190, the parameter matrix (camera parameter) is compared with the camera parameter indicated by the set camera parameter information currently stored in the storage device 40, and the difference between the two is compared. May exceed the error range, it may be determined that the setting camera parameter information needs to be updated.

  Further, in the calibration process of the above embodiment, in S160, the feature points of the light projecting road surface area are extracted for each of the various light projecting methods using the in-vehicle light 50 as in S110. However, the present invention is not limited to this. Instead, it is only necessary to extract feature points for at least one light projection method (for example, low beam). In this case, when the parameter calculation process is performed in S190, a feature point for further different light projection methods may be extracted.

  DESCRIPTION OF SYMBOLS 1 ... Car-mounted camera parameter generation apparatus, 2 ... CPU, 3 ... ROM, 4 ... RAM, 5 ... Vehicle body, 10 ... Car-mounted camera, 20 (21-24) ... Car-mounted sensor, 30 ... Operating equipment, 40 ... Memory | storage device, 50 ... car light, 60 ... notification device.

Claims (8)

An in-vehicle camera parameter generation device (1) for generating camera parameters based on the position and orientation of an in-vehicle camera (10) attached to the vehicle so as to image the outside of the vehicle,
Feature point extraction means (2, S340) for extracting the feature points of the light projecting road surface area projected onto the road surface by the light projecting device (50) provided in the vehicle from the image taken by the in-vehicle camera;
Parameter generation means (2, S170 to S200) for generating the camera parameter based on the image position of the feature point extracted by the feature point extraction means;
Position storage means (2, S210) for storing image position information indicating an image position of the feature point used when the camera parameter is generated by the parameter generation means;
With
The parameter generation means is based on a comparison result between the image position of the feature point extracted by the feature point extraction means and the image position of the feature point indicated by the image position information stored by the position storage means. An in-vehicle camera parameter generation device that performs correction processing (S170 to S180) for correcting the camera parameter. An in-vehicle camera parameter generation device (1) for generating camera parameters based on the position and orientation of an in-vehicle camera (10) attached to the vehicle so as to image the outside of the vehicle,
Feature point extraction means (2, S340) for extracting the feature points of the light projecting road surface area projected onto the road surface by the light projecting device (50) provided in the vehicle from the image taken by the in-vehicle camera;
Parameter generation means (2, S170 to S200) for generating the camera parameter based on the image position of the feature point extracted by the feature point extraction means;
When the horizontal condition is that the vehicle is horizontal with respect to the road surface, the obstacle condition is that there is no obstacle within a predetermined distance based on the projected road surface area, and the horizontal condition and the obstacle condition are satisfied Permission means (S310 to S320) for permitting extraction of the feature points by the feature point extraction means;
An in-vehicle camera parameter generation device comprising: An in-vehicle camera parameter generation device (1) for generating camera parameters based on the position and orientation of an in-vehicle camera (10) attached to the vehicle so as to image the outside of the vehicle,
Feature point extraction means (2, S340) for extracting the feature points of the light projecting road surface area projected onto the road surface by the light projecting device (50) provided in the vehicle from the image taken by the in-vehicle camera;
Parameter generation means (2, S170 to S200) for generating the camera parameter based on the image position of the feature point extracted by the feature point extraction means;
A determination unit (S120) for determining whether or not generation of the camera parameter by the parameter generation unit is necessary based on an image position of the feature point extracted by the feature point extraction unit;
Notification means (S140) for notifying the vehicle occupant when the determination means determines that the generation of the camera parameter is necessary;
An in-vehicle camera parameter generation device comprising: The parameter generation unit calculates the camera parameter based on the image position of the feature point extracted by the feature point extraction unit and the known position of the feature point in real space with respect to the vehicle. The in-vehicle camera parameter generation device according to any one of claims 1 to 3, wherein (S190) is performed. 2. The feature point extraction unit switches a light projection method of an in-vehicle light mounted on the vehicle as the light projecting device, and extracts a plurality of the feature points that differ depending on each light projection method. The in-vehicle camera parameter generation device according to any one of claims 4 to 4. A program for causing a computer to function as at least the feature point extraction unit , the parameter generation unit, and the position storage unit according to claim 1. A program for causing a computer to function as at least the feature point extraction means , the parameter generation means, and the permission means according to claim 2 . A program for causing a computer to function as at least the feature point extraction unit , the parameter generation unit , the determination unit, and the notification unit according to claim 3 .