阅读说明：本技术 一种基于轴向可调级联转镜的三维成像系统和方法 (Three-dimensional imaging system and method based on axially adjustable cascade rotating mirror ) 是由 李安虎 刘兴盛 于 2020-01-21 设计创作，主要内容包括：本发明涉及一种基于轴向可调级联转镜的三维成像系统和方法。该三维成像系统包括相机装置和级联转镜装置,级联转镜装置包括级联转镜、平移机构和旋转机构。该三维成像方法包括以下步骤：标定成像系统的初始位置及其参数；采集多种视角对应的元素图像序列；建立不同元素图像的逆向映射关系；恢复元素图像重合区域的三维信息；在三维空间实现目标形貌的可视化。与现有技术相比,本发明通过级联转镜的轴向平移运动和同步旋转运动,实现任意调整相机的径向成像视角和周向成像视角,从而采集存在丰富视差信息的多视角目标图像序列,可以有效提高三维计算成像的视场范围和分辨能力。(The invention relates to a three-dimensional imaging system and a three-dimensional imaging method based on an axially adjustable cascade rotating mirror. The three-dimensional imaging system comprises a camera device and a cascade rotating mirror device, wherein the cascade rotating mirror device comprises a cascade rotating mirror, a translation mechanism and a rotating mechanism. The three-dimensional imaging method comprises the following steps: calibrating the initial position and parameters of the imaging system; acquiring element image sequences corresponding to multiple visual angles; establishing reverse mapping relations of different element images; recovering three-dimensional information of the element image overlapping area; and realizing the visualization of the target morphology in a three-dimensional space. Compared with the prior art, the invention realizes the random adjustment of the radial imaging visual angle and the circumferential imaging visual angle of the camera through the axial translation motion and the synchronous rotation motion of the cascade rotating mirror, thereby acquiring the multi-visual-angle target image sequence with rich parallax information and effectively improving the visual field range and the resolution capability of three-dimensional calculation imaging.)

1. A three-dimensional imaging system based on an axially adjustable cascade rotating mirror is characterized by comprising a camera device and a cascade rotating mirror device; the cascade rotating mirror device comprises a cascade rotating mirror, a translation mechanism and a rotating mechanism; the cascade rotating mirror comprises a first prism assembly and a second prism assembly, wherein the first prism assembly comprises a first lens barrel and a first wedge prism which are connected with each other, the second prism assembly comprises a second lens barrel and a second wedge prism which are connected with each other, and the first wedge prism and the second wedge prism are coaxially arranged and have opposite wedge surfaces; the translation mechanism is connected with the first lens barrel and the second lens barrel and is used for adjusting the two prism assemblies to translate in the axial direction; the rotating mechanism is connected with the first lens barrel and the second lens barrel and is used for realizing synchronous rotation of the first prism assembly and the second prism assembly; the camera device is opposite to the first prism component.

2. The three-dimensional imaging system based on the axially adjustable cascaded rotating mirror as claimed in claim 1, wherein the translation mechanism comprises a driving motor, at least one guiding rod and a ball screw, the guiding rod and the ball screw are parallel to each other, the driving motor is fixed on the first lens barrel, a polished rod part of the ball screw passes through a mounting hole on the first lens barrel and is connected with the driving motor, a threaded part of the ball screw passes through a threaded hole arranged on the second lens barrel, one end of each wire rod is fixedly connected with the first lens barrel, and the other end of each wire rod passes through a smooth hole on the second lens barrel; the driving motor rotates forwards or backwards to drive the ball screw to rotate, and the second lens barrel realizes axial translation under the driving action of the ball screw and the threaded hole and under the supporting and guiding action of the guide rod and the smooth hole.

3. The three-dimensional imaging system based on the axially adjustable cascade rotating mirror as claimed in claim 1, wherein the rotating mechanism comprises a torque motor, a rotating inner frame and a rolling bearing, the torque motor comprises a torque motor rotor and a torque motor stator, the cascade rotating mirror device further comprises an outer frame, the cascade rotating mirror is fixedly arranged in the rotating inner frame, the rotating inner frame is arranged in the outer frame through a roller bearing, the torque motor stator is connected with the inner wall of the outer frame, and the torque motor rotor is connected with the outer wall of the rotating inner frame; the torque motor drives the rotating inner frame and the joint rotating mirror to rotate in the outer frame.

4. The three-dimensional imaging system based on the axially adjustable cascaded rotating mirrors as claimed in claim 3, wherein the camera device comprises a camera body and a camera support, and the camera body is mounted on the outer frame through the camera support.

5. An imaging method of the three-dimensional imaging system based on the axially adjustable cascade turning mirror as claimed in any one of claims 1 to 4, characterized by comprising the following steps:

s1, calibrating the initial position of the imaging system and the parameters thereof: establishing a working coordinate system of an imaging system, and determining the main section positions of two wedge prisms in the cascade rotating mirror by an optical calibration method; acquiring internal parameters of a camera and an axial distance between the camera and a cascade rotating mirror by using a camera calibration method;

s2, acquiring element image sequences corresponding to multiple visual angles: changing the axial distance between the cascaded rotating mirrors through a translation mechanism of the cascaded rotating mirror device, simultaneously changing the rotating angle of the cascaded rotating mirrors through a rotating mechanism of the cascaded rotating mirror device, acquiring a series of imaging visual angles which change along the radial direction and the circumferential direction, and respectively acquiring element image sequences of a target under various imaging visual angles;

s3, establishing a reverse mapping relation of different element images: combining the axial distance and the rotation angle of the cascade rotating mirror, and establishing a mapping relation between pixels contained in the element image and target points corresponding to the element image on a reconstruction plane by using a back projection method;

s4, restoring the three-dimensional information of the element image overlapping region: aiming at all pixels in the overlapping area of the element image sequence, calculating the real depth information of the corresponding target point according to the principle that the same-name image points meet the consistent gray distribution on a reconstruction plane, and further recovering the position information of the target point in other two dimensions;

s5, realizing visualization of the target morphology in a three-dimensional space: and calculating the gray information of each target point according to the gray of the image points with the same name in all the element images, and realizing the visualization of the interested area and even the partial shielding area of the target surface.

6. The imaging method of the three-dimensional imaging system based on the axially adjustable cascaded rotating mirrors of claim 5, wherein the step S1 specifically comprises:

s11, establishing a working coordinate system O-XYZ of the three-dimensional imaging system, wherein an origin O is fixed at the optical center position of the camera, a Z axis is overlapped with the optical axis direction of the camera, an X axis and a Y axis are both orthogonal to the Z axis and respectively correspond to the row scanning direction and the column scanning direction of the camera image sensor;

s12, calibrating the main section position of the cascaded rotating mirror device and the axial alignment relation between the camera and the cascaded rotating mirror by adopting an auto-collimation method or an optical interference method, and calibrating the internal parameters of the camera and the distortion coefficient of the lens by adopting a Zhang-Zhengyou calibration method, a direct linear transformation method or a two-step calibration method.

7. The imaging method of the three-dimensional imaging system based on the axially adjustable cascaded rotating mirrors of claim 5, wherein the step S2 specifically comprises:

s21, under the condition that the initial corner orientation of the cascaded rotating mirror is kept unchanged, a translation mechanism is used for driving the cascaded rotating mirror to perform axial translation motion by a given step length, and a camera is triggered to acquire image information of a target after each axial distance is changed;

s22, driving the cascaded rotating mirrors to synchronously rotate at a given step angle by using the rotating mechanism, adjusting the axial spacing of the cascaded rotating mirrors by using the translation mechanism under the condition of keeping a new corner position unchanged, enabling the axial spacing to sequentially reach the series of spacings related in the step S21, and triggering the camera to acquire image information of a target under the condition of each axial spacing;

and S23, repeating the step S22, sequentially acquiring imaging visual angles generated by the cascade rotating mirrors in different rotating angle directions and different axial intervals, and respectively acquiring multi-visual-angle element image sequences of the target by using the camera.

8. The imaging method of the three-dimensional imaging system based on the axially adjustable cascaded rotating mirrors according to claim 5, wherein in step S3, the reconstruction plane is a virtual plane perpendicular to the Z-axis direction of the working coordinate system and used for approximating the real depth position of the target, and the inverse mapping relationship between the pixel position contained in any element image and its corresponding target point on the reconstruction plane can be determined by an analytic geometry method, and is represented as:

wherein d is_iIs the axial distance, theta, of the cascaded rotating mirrors after i relative translational movements_jIs the corner orientation of the cascaded rotating mirror after j times of synchronous rotation motion, (x)_ji,y_ji) Representing the coordinates of the image points, M, contained in the acquired image at the current axial spacing and angular range_iAnd T_iRespectively a scaling factor and a displacement factor, z, of the backprojection process_rIs the axial distance of the reconstruction plane relative to the camera, (x)_r,y_r) Is the projected position of the object point on the reconstruction plane.

9. The imaging method of the three-dimensional imaging system based on the axially adjustable cascaded rotating mirrors according to claim 5, wherein the step S4 specifically comprises:

s41, selecting an image from the element image sequence as a reference image, and searching the homonymy image point matching relationship between other images and the reference image according to the reverse mapping relationship obtained in the step S3;

s42, according to the principle that corresponding image points with the same name meet the consistent gray level distribution when the reconstruction plane is at the real depth, optimally solving the depth information of target points corresponding to all the image points with the same name;

and S43, combining the reverse mapping relation obtained in the step S3 and the target depth information obtained in the step S42 to calculate the position information of any target point in the x direction and the y direction.

10. The imaging method of the three-dimensional imaging system based on the axially adjustable cascaded rotating mirrors of claim 9, wherein in the step S5, the gray scale information of any target point is calculated by the gray scale mean of corresponding image points with the same name in all the elemental images, so the gray scale distribution rule of the reconstructed target image is expressed as:

wherein the content of the first and second substances,respectively representing positions in a reconstruction planeThe target point of (a) is,representing the identified location of the image point of the same name within the corresponding elemental image according to the location of the reconstruction plane.

Technical Field

The invention relates to the field of machine vision, in particular to a three-dimensional imaging system and a three-dimensional imaging method based on an axially adjustable cascade rotating mirror.

Background

The three-dimensional computational imaging is a technology for reconstructing three-dimensional information of a space target by combining an imaging system and a computing method, and has important application value in the fields of biomedical observation, unmanned autonomous navigation, geographic topography exploration and the like. The three-dimensional imaging method reported at present mainly adopts basic principles such as laser ranging, ultrasonic sensing, holography, stereoscopic vision and the like, wherein the stereoscopic vision has the unique advantages of high efficiency, wide spectrum, full-field perception, environmental adaptability and the like, so that a particularly flexible and effective solution can be provided for large-range three-dimensional computational imaging.

The following prior studies propose several typical three-dimensional computational imaging systems and methods based on the stereoscopic vision principle:

in the prior research (r.schulein, et al. "3D imaging with axial distribution imaging", Optics L meters 34(13):2012 and 2014,2009) disclosed an axial distribution three-dimensional imaging system employing a single camera, which sequentially acquires element image sequences during movement along the optical axis direction, in the prior research (y.piao, et al.three-dimensional imaging and visualization using off-axial distribution imaging, Optics L meters 38(16):3162 and 3164,2013) the limitation that only longitudinal viewing angle information can be acquired for the axial distribution three-dimensional imaging system, the limitation that the camera moves along the non-axial direction to acquire image sequences of lateral and longitudinal viewing angles is proposed, the integration of the acquisition of image sequences with the axial distribution imaging system is further disclosed by the two-dimensional imaging system, namely the integration of the two-dimensional imaging system and the prism field tracking system, the integration of the prism field acquisition system is further disclosed by the optical acquisition system under the requirements of the two-dimensional imaging system, the integration of the prism field tracking system, the acquisition of the prism field tracking system, the integration of the two-dimensional imaging system, the prism acquisition of the imaging system, the prism field tracking system, the two-dimensional imaging system, the prism distribution imaging system, the integration of the imaging system, the prism distribution of the imaging system, the prism acquisition of lateral and the imaging system, the integration of the imaging system, the integration of the imaging system, the integration of the imaging system, the integration of the imaging system, the integration of the integration.

For this reason, previous studies (liaohu et al, an axially distributed three-dimensional imaging method based on double wedge prisms, publication No.: CN109597275A, published: 2019, 4/9) have proposed yet a new three-dimensional imaging method, which is also based on a camera and two wedge prisms, but acquires a multi-view elemental image sequence of a target by only an axial translational movement of one of the prisms. However, this method only enables the imaging angle of view of the camera to be varied along a particular radial direction, and the translational range of motion of the prism and the resulting effective parallax is limited to some extent by the size of the imaging system. That is, in terms of the spatial resolution capability of the three-dimensional imaging system, it is necessary that the axial translation amount of the prism is large enough, and the parallax of the image acquired by the camera at the corresponding view angle is obvious, so that the calculated three-dimensional information is accurate, and therefore, the three-dimensional information can be realized by the imaging system with a large volume.

Disclosure of Invention

The present invention is directed to overcoming the above-mentioned deficiencies of the prior art and providing a three-dimensional imaging system and method based on axially adjustable cascaded rotating mirrors.

The purpose of the invention can be realized by the following technical scheme:

a three-dimensional imaging system based on an axially adjustable cascade rotating mirror comprises a camera device and a cascade rotating mirror device; the cascade rotating mirror device comprises a cascade rotating mirror, a translation mechanism and a rotating mechanism; the cascade rotating mirror comprises a first prism assembly and a second prism assembly, wherein the first prism assembly comprises a first lens barrel and a first wedge prism which are connected with each other, the second prism assembly comprises a second lens barrel and a second wedge prism which are connected with each other, and the first wedge prism and the second wedge prism are coaxially arranged and have opposite wedge surfaces; the translation mechanism is connected with the first lens barrel and the second lens barrel and is used for adjusting the two prism assemblies to translate in the axial direction; the rotating mechanism is connected with the first lens barrel and the second lens barrel and is used for realizing synchronous rotation of the first prism assembly and the second prism assembly; the camera device is opposite to the first prism component.

Furthermore, the translation mechanism comprises a driving motor, at least one guide rod and a ball screw, the guide rod and the ball screw are parallel to each other, the driving motor is fixed on the first lens barrel, a polished rod part of the ball screw penetrates through a mounting hole on the first lens barrel and is connected with the driving motor, a threaded part of the ball screw penetrates through a threaded hole arranged on the second lens barrel, one end of each wire guide rod is fixedly connected with the first lens barrel, and the other end of each wire guide rod penetrates through a smooth hole on the second lens barrel; the driving motor rotates forwards or backwards to drive the ball screw to rotate, and the second lens barrel realizes axial translation under the driving action of the ball screw and the threaded hole and under the supporting and guiding action of the guide rod and the smooth hole.

Furthermore, the rotating mechanism comprises a torque motor, a rotating inner frame and a rolling bearing, the torque motor comprises a torque motor rotor and a torque motor stator, the cascade rotating mirror device further comprises an outer frame, the cascade rotating mirror is fixedly arranged in the rotating inner frame, the rotating inner frame is arranged in the outer frame through a roller bearing, the torque motor stator is connected with the inner wall of the outer frame, and the torque motor rotor is connected with the outer wall of the rotating inner frame; the torque motor drives the rotating inner frame and the cascade rotating mirror to rotate in the outer frame.

Further, the camera device comprises a camera body and a camera support, wherein the camera body is mounted on the outer frame through the camera support.

An imaging method of the three-dimensional imaging system based on the axially adjustable cascade rotating mirror comprises the following steps:

s1, calibrating the initial position of the imaging system and the parameters thereof: establishing a working coordinate system of an imaging system, and determining the main section positions of two wedge prisms in the cascade rotating mirror by an optical calibration method; acquiring internal parameters of a camera and an axial distance between the camera and a cascade rotating mirror by using a camera calibration method;

s2, acquiring element image sequences corresponding to multiple visual angles: changing the axial distance between the cascaded rotating mirrors through a translation mechanism of the cascaded rotating mirror device, simultaneously changing the rotating angle of the cascaded rotating mirrors through a rotating mechanism of the cascaded rotating mirror device, acquiring a series of imaging visual angles which change along the radial direction and the circumferential direction, and respectively acquiring element image sequences of a target under various imaging visual angles;

s3, establishing a reverse mapping relation of different element images: combining the axial distance and the rotation angle of the cascade rotating mirror, and establishing a mapping relation between pixels contained in the element image and target points corresponding to the element image on a reconstruction plane by using a back projection method;

s4, restoring the three-dimensional information of the element image overlapping region: aiming at all pixels in the overlapping area of the element image sequence, calculating the real depth information of the corresponding target point according to the principle that the same-name image points meet the consistent gray distribution on a reconstruction plane, and further recovering the position information of the target point in other two dimensions;

s5, realizing visualization of the target morphology in a three-dimensional space: and calculating the gray information of each target point according to the gray of the image points with the same name in all the element images, and realizing the visualization of the interested area and even the partial shielding area of the target surface.

Further, the step S1 specifically includes:

s11, establishing a working coordinate system O-XYZ of the three-dimensional imaging system, wherein an origin O is fixed at the optical center position of the camera, a Z axis is overlapped with the optical axis direction of the camera, an X axis and a Y axis are both orthogonal to the Z axis and respectively correspond to the row scanning direction and the column scanning direction of the camera image sensor;

s12, calibrating the main section position of the cascaded rotating mirror device and the axial alignment relation between the camera and the cascaded rotating mirror by adopting an auto-collimation method or an optical interference method, and calibrating the internal parameters of the camera and the distortion coefficient of the lens by adopting a Zhang-Zhengyou calibration method, a direct linear transformation method or a two-step calibration method.

Further, the step S2 specifically includes:

s21, under the condition that the initial corner orientation of the cascaded rotating mirror is kept unchanged, a translation mechanism is used for driving the cascaded rotating mirror to perform axial translation motion by a given step length, and a camera is triggered to acquire image information of a target after each axial distance is changed;

s22, driving the cascaded rotating mirrors to synchronously rotate at a given step angle by using the rotating mechanism, adjusting the axial spacing of the cascaded rotating mirrors by using the translation mechanism under the condition of keeping a new corner position unchanged, enabling the axial spacing to sequentially reach the series of spacings related in the step S21, and triggering the camera to acquire image information of a target under the condition of each axial spacing;

and S23, repeating the step S22, sequentially acquiring imaging visual angles generated by the cascade rotating mirrors in different rotating angle directions and different axial intervals, and respectively acquiring multi-visual-angle element image sequences of the target by using the camera.

Further, in step S21, the given step size of the axial distance between the cascaded rotating mirrors should take into account the limitation of the imaging resolution of the camera, thereby determining the minimum step size λ of the axial distance adjustment_minExpressed as:

where f is the focal length of the camera lens, μ is the actual physical size of the pixel, z is the axial distance of the camera from the target, n is the index of refraction of the prism material, α is the wedge angle of the prism.

Further, in the step S22, the minimum step angle of the synchronous rotation motion is determined by considering the limitation of the imaging resolution of the camera for the given step angle of the synchronous rotation motion of the cascaded mirrors_minExpressed as:

wherein d is_minIs the minimum axial spacing between the cascaded rotating mirrors.

Further, in step S3, the reconstruction plane is a virtual plane perpendicular to the Z-axis direction of the working coordinate system and used for approximating the real depth position of the target, and the inverse mapping relationship between the pixel position included in the arbitrary elemental image and the corresponding target point on the reconstruction plane can be determined by an analytic geometry method, and is represented as:

wherein d is_iIs the axial distance, theta, of the cascaded rotating mirrors after i relative translational movements_jIs the corner orientation of the cascaded rotating mirror after j times of synchronous rotation motion, (x)_ji,y_ji) Representing the coordinates of the image points, M, contained in the acquired image at the current axial spacing and angular range_iAnd T_iRespectively a scaling factor and a displacement factor, z, of the backprojection process_rIs the axial distance of the reconstruction plane relative to the camera, (x)_r,y_r) Is the projected position of the object point on the reconstruction plane.

Further, the step S4 specifically includes:

s41, selecting an image from the element image sequence as a reference image, and searching the homonymy image point matching relationship between other images and the reference image according to the reverse mapping relationship obtained in the step S3;

s42, according to the principle that corresponding image points with the same name meet the consistent gray level distribution when the reconstruction plane is at the real depth, optimally solving the depth information of target points corresponding to all the image points with the same name;

and S43, combining the reverse mapping relation obtained in the step S3 and the target depth information obtained in the step S42 to calculate the position information of any target point in the x direction and the y direction.

Further, in the step S41, the axial distance d between the cascaded rotating mirrors is selected₀＝d_minThe angular orientation is theta₀If the image acquired at 0 ° is taken as the reference image, the corresponding image point matching relationship between the other images and the reference image is expressed as:

further, in step S42, a reconstruction plane that best matches the gray scale distribution of the region near the corresponding image point is found using a window matching algorithm such as Sum of Absolute Difference (SAD), Mean Absolute Difference (MAD), and Normalized Cross Correlation (NCC), and the like, taking the SAD algorithm having a window size of (2 σ +1) × (2 σ +1) as an example, the best reconstruction plane position is expressed as

Wherein (u)_ji,v_ji) Represents (x)_ji,y_ji) Corresponding pixel position, I_jiDenotes an axial spacing of d_iThe angular orientation is theta_jThe problem belongs to a nonlinear optimization problem and can be solved by adopting the existing methods such as an optimal gradient descent algorithm, a differential evolution algorithm, an L evenberg-Marquardt algorithm and the like.

Further, in step S5, the gray scale information of any target point may be calculated from the average gray scale value of all the corresponding image points with the same name in the elemental image, so that the gray scale distribution rule of the reconstructed target image is represented as:

wherein the content of the first and second substances,respectively representing positions in a reconstruction planeThe target point of (a) is,representing the identified location of the image point of the same name within the corresponding elemental image according to the location of the reconstruction plane.

Compared with the prior art, the invention has the following advantages:

1. according to the invention, by adding the rotating mechanism in the cascaded rotating mirror device, the radial imaging visual angle and the circumferential imaging visual angle of the camera can be adjusted at will through the axial translation motion and the synchronous rotation motion of the cascaded rotating mirror, so that a multi-visual-angle target image sequence with rich parallax information is collected, and the field range and the resolution capability of three-dimensional computational imaging can be effectively improved. Compared with the traditional double-wedge prism device which can only adjust axial translation, the imaging view field of the double-wedge prism device is not limited by the distance between the wedge prism shafts any longer, and the miniaturization of the system volume can be realized.

2. The invention combines the corner position and the axial distribution parameter of the cascade rotating mirror, establishes the pixel mapping relation of the element image sequence by using an analytic geometry method, and can avoid the stereo matching process with large calculation amount and mutual restriction of efficiency and precision.

3. The invention finds the reconstruction plane position closest to the real depth through a simple gray distribution consistency criterion, recovers the three-dimensional information of the target surface and the gray distribution thereof, and can improve the precision and the reliability of three-dimensional imaging with lower operation cost.

4. The invention can realize multi-view image acquisition and three-dimensional calculation imaging only by means of the relative displacement and the azimuth change of the cascade rotating mirror without requiring the camera to carry out any motion, and can obviously improve the dynamic response characteristic of a three-dimensional imaging system.

5. The invention adopts an arrangement scheme that the cameras and the cascade rotating mirrors are axially aligned, allows an imaging system to have good compactness and economy, and can provide a potential technical approach for the expansion and application in the fields of pattern recognition, dynamic monitoring, contour and appearance measurement, shielded target visualization and the like.

Drawings

FIG. 1 is a perspective schematic view of a three-dimensional imaging system.

Fig. 2 is a schematic structural diagram of a three-dimensional imaging system.

Fig. 3 is a flow chart diagram of a three-dimensional imaging method.

Fig. 4a is an imaging visual axis principle diagram of the axial translation motion of the cascaded rotating mirror.

Fig. 4b is a schematic view of the imaging visual axis principle of the synchronous rotary motion of the cascaded rotating mirrors.

Fig. 5 is a projection optical path of an arbitrary target in a three-dimensional imaging system and an imaging principle diagram thereof.

Fig. 6 is a basic block diagram of an optimal reconstruction plane optimization algorithm for a three-dimensional information recovery process.

Reference numerals: 11-camera body, 12-camera mount, 21-first barrel, 22-first wedge prism, 31-second barrel, 32-second wedge prism, 41-drive motor, 42-coupler, 43-ball screw, 44-nut, 45-guide bar, 46-sliding bearing, 51-torque motor stator, 52-brush, 53-torque motor rotor, 54-rotating inner frame, 55-rolling bearing, 56-first bearing retainer, 57-second bearing retainer, 61-first end cap, 62-outer frame, 63-second end cap, 64-third end cap.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.