CN110596720A - distance measuring system - Google Patents

Abstract

The present invention provides a distance measurement system, including: a transmitting module, used to emit a speckle pattern beam with a first distortion feature; a collection module, including an imaging lens with a second distortion feature and an array pixel unit, used to collect the The reflected speckle pattern light beam; a processing circuit connected to the emission module and the acquisition module for calculating the time-of-flight between the emission light beam and the reflection light beam; the first distortion feature and The second distortion features compensate each other. A distance measurement system is provided. By implementing distortion compensation on hardware, compared with the traditional algorithm-based implementation of distortion compensation, it not only reduces the requirements for the algorithm, but also solves the distortion problem from the source, and the effect is better.

Description

distance measuring system

technical field

The invention relates to the technical field of computers, in particular to a distance measuring system.

Background technique

Time of flight (TOF) method calculates the distance of objects by measuring the flight time of light beams in space. Due to its advantages of high precision and large measurement range, it is widely used in consumer electronics, unmanned vehicles, AR/ VR and other fields.

Distance measurement systems based on time-of-flight principles, such as time-of-flight depth cameras and lidar systems, often include a light source transmitter and a receiver. Calculate the distance of the object by calculating the time required for the light beam to be reflected and received. When using distance measurement systems such as time-of-flight cameras and laser radars for distance sensing, ambient light interference will affect the accuracy of measurement. For example, when the ambient light intensity is high or even floods the beam of the light source, especially for flood lighting, the It is difficult to distinguish the beam of the light source so that a large measurement error occurs.

In the existing technology, background light can be suppressed by adding optical methods such as optical filters and electronic methods such as setting subtraction circuits (the paper CMOS Sensors for Time-Resolved Active Imaging), however, these methods still cannot essentially eliminate or reduce Measurement error caused by ambient light interference. The method of point scanning is beneficial to improve the signal-to-noise ratio, but compared with flood lighting, the measurement resolution is lower. In a word, the existing technology is difficult to balance the signal-to-noise ratio, measurement accuracy and measurement resolution.

Contents of the invention

In order to solve the existing problems, the present invention provides a distance measurement system.

In order to solve the above problems, the technical solution adopted in the present invention is as follows:

A distance measurement system, comprising: a transmitting module, used to emit a speckle pattern beam with a first distortion feature; a collection module, including an imaging lens with a second distortion feature and an array pixel unit, used to collect all reflected The speckle pattern light beam; the processing circuit, connected with the emission module and the acquisition module, is used to calculate the time-of-flight between the emission light beam and the reflection light beam; the first distortion feature and the second distortion feature The two distortion features compensate each other.

In an embodiment of the present invention, the first distortion feature is a barrel distortion feature or a pincushion distortion feature, and the second distortion feature is a pincushion distortion feature or a barrel distortion feature.

In yet another embodiment of the present invention, the emitting module includes an array light source, a projection lens and a spatial light modulator. The sub-light source arrangement on the array light source is a two-dimensional arrangement pattern with a first distortion feature; or, the projection lens has a first distortion feature; or, the diffraction pattern of the spatial light modulator has a first distortion feature; Or, at least two of the sub-light source arrangement on the array light source, the projection lens and the diffraction pattern of the spatial light modulator are designed comprehensively so that the emitting module has the first distortion feature.

In yet another embodiment of the present invention, the emitting module includes an array light source and a projection lens. The sub-light source arrangement on the array light source is a two-dimensional arrangement pattern with a first distortion feature; or, the projection lens has a first distortion feature; or, the sub-light source arrangement on the array light source and the projection lens are Comprehensively designed so that the emission module has the first distortion feature.

The present invention also provides a distance measurement system, including: a transmitting module, including an array light source and a projection lens, for emitting a speckle pattern beam; a collection module, including an imaging lens and an array pixel unit, for collecting the reflected Speckle pattern light beam; processing circuit, connected with the emission module and the acquisition module, used to calculate the time-of-flight between the emission light beam and the reflection light beam; the sub-light source arrangement on the array light source has A two-dimensional arrangement pattern of first distortion features, the projection lens has second distortion features, and the first distortion features and the second distortion features compensate each other. The first distortion feature is a barrel distortion feature or a pincushion distortion feature, and the second distortion feature is a pincushion distortion feature or a barrel distortion feature.

The present invention further provides a distance measurement system, including: a transmitting module, including an array light source, a projection lens, and a spatial light modulator, for emitting a speckle pattern beam; an acquisition module, including an imaging lens and an array pixel unit, for acquiring The reflected speckle pattern light beam; a processing circuit connected to the emission module and the collection module for calculating the time-of-flight between the emission light beam and the reflection light beam; The sub-light source arrangement and/or the diffraction pattern of the spatial light modulator is a two-dimensional arrangement pattern with a first distortion feature; the projection lens has a second distortion feature, and the first distortion feature and the second distortion feature Compensate each other. The first distortion feature is a barrel distortion feature or a pincushion distortion feature, and the second distortion feature is a pincushion distortion feature or a barrel distortion feature.

The beneficial effect of the present invention is: to provide a distance measurement system, by realizing the distortion compensation on the hardware, compared with the traditional scheme of realizing the distortion compensation through the algorithm, not only the requirement for the algorithm is reduced, but also the distortion can be solved from the source problem, the effect is better.

Description of drawings

Fig. 1 is a schematic diagram of an array light source distance measurement system according to an embodiment of the present invention.

Fig. 2 is a schematic diagram of a transmitting module according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of a transmitting module according to another embodiment of the present invention.

Fig. 4 is a schematic diagram of the influence of parallax on imaging according to an embodiment of the present invention.

Fig. 5 is a schematic diagram of an array light source according to an embodiment of the present invention.

Fig. 6 is a schematic diagram of the principle of distortion correction according to an embodiment of the present invention.

Among them, 10-distance measurement system, 11-transmission module, 12-acquisition module, 13-processing circuit, 20-object, 30-emission beam, 40-reflection beam, 111-array light source, 112-spatial light modulator , 121-array pixel unit, 122-imaging lens unit, 201-array light source, 202-projection lens, 203-diffractive optical element, 204-beam, 205-beam, 206-beam, 301-array light source, 302-projection lens , 303-beam, 304-beam, 305-beam, 121-array pixel unit, 401-imaging position, 402-imaging position, 403-pixel, 51-array light source, 511-VCSEL sub-light source, 512-VCSEL sub-light source, 61-the first distortion, 62-the second distortion, 63-the effect after compensation.

Detailed ways

In order to make the technical problems, technical solutions and beneficial effects to be solved by the embodiments of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

It should be noted that when an element is referred to as being “fixed” or “disposed on” another element, it may be directly on the other element or be indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element. In addition, the connection can be used for both fixing and circuit communication.

It is to be understood that the terms "length", "width", "top", "bottom", "front", "rear", "left", "right", "vertical", "horizontal", "top" , "bottom", "inner", "outer" and other indicated orientations or positional relationships are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the embodiments of the present invention and simplifying the description, rather than indicating or implying Any device or element must have a specific orientation, be constructed and operate in a specific orientation and therefore should not be construed as limiting the invention.

In addition, the terms "first" and "second" are used for descriptive purposes only, and cannot be interpreted as indicating or implying relative importance or implicitly specifying the quantity of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of these features. In the description of the embodiments of the present invention, "plurality" means two or more, unless otherwise specifically defined.

The invention provides a distance measurement system, which has stronger ability to resist ambient light and higher resolution.

Fig. 1 is a schematic diagram of an array light source distance measurement system according to an embodiment of the present invention. The distance measurement system 10 includes a transmitting module 11, an acquisition module 12 and a processing circuit 13, wherein the transmitting module 11 provides a transmitting beam 30 to the target space to illuminate the object 20 in the space, and at least part of the transmitting beam 30 is reflected by the object 20 A reflected light beam 40 is formed, at least part of the reflected light beam 40 is collected by the collection module 12, and the processing circuit 13 is respectively connected with the emission module 11 and the collection module 12, and the trigger signals of the emission module 11 and the collection module 12 are synchronized to calculate the light beam The time required to be sent by the emission module 11 and received by the acquisition module 12, that is, the flight time t between the emission beam 30 and the reflection beam 40, further, the distance D of the corresponding point on the object can be calculated by the following formula:

D＝c·t/2 (1)

where c is the speed of light.

The emission module 11 includes an array light source 111 and a spatial light modulator 112 . The array light source 111 can be an array composed of light emitting diodes (LEDs), edge emitting lasers (EEL), vertical cavity surface emitting lasers (VCSEL) and other light sources. Preferably, the array light source 111 is formed by generating multiple VCSEL light sources on a single semiconductor substrate. With the formed VCSEL array light source chip, the light beam emitted by the array light source 111 may be visible light, infrared light, ultraviolet light, and the like. The array light source 111 emits a light beam outward under the control of the processing circuit 13. For example, in one embodiment, the array light source 111 emits a pulsed light beam at a certain frequency under the control of the processing circuit 13, which can be used in the direct time-of-flight method (Direct TOF ) measurement, the frequency is set according to the measurement distance, for example, it can be set to 1MHz to 100MHz, and the measurement distance is several meters to hundreds of meters; in one embodiment, the amplitude of the light beam emitted by the light source 111 is controlled by the processing circuit 13. Modulated to emit continuous wave beams such as square wave beams and sine wave beams, which can be used in indirect time-of-flight (Indirect TOF) measurements. It can be understood that it may be a part of the processing circuit 13 or a sub-circuit independent of the processing circuit 13 to control the light source 111 to emit relevant light beams, such as a pulse signal generator.

The spatial light modulator 112 receives the carrier light beam from the light source 111 and performs spatial modulation on the carrier light beam, that is, modulates the distribution of the carrier light beam in space to form a non-flood carrier light beam with uneven intensity distribution and emit it outward. Compared with the traditional flood beam, due to the non-uniform intensity distribution of the non-flood beam, in the case of the same light source power, the area with higher intensity distribution will have higher anti-interference performance to ambient light; in addition, when projecting In the case of the same field of view, due to the inhomogeneity of the intensity distribution, that is, to achieve the same anti-interference performance of ambient light, flood lighting requires higher power consumption. In some embodiments, the spatial light modulator 112 is also used to expand the received carrier beam to expand the field of view.

The processing circuit 13 can be an independent dedicated circuit, such as a dedicated SOC chip, FPGA chip, ASIC chip, etc., or can include a general-purpose processor, such as when the depth camera is integrated into smart terminals such as mobile phones, TVs, and computers. A processor in the terminal may serve as at least a part of the processing circuit 13 .

The acquisition module 12 includes an array pixel unit 121 and an imaging lens unit 122 . The imaging lens unit 122 receives and guides at least part of the non-flood carrier beam reflected back by the object onto at least part of the array pixel unit 121 . In one embodiment, the array pixel unit 121 may be an array pixel unit composed of a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS), an avalanche diode (AD), a single photon avalanche diode (SPAD), etc., and the array pixel The array size of the unit 121 represents the resolution of the depth camera, such as 320x240 and so on. Generally, a readout circuit (not shown in the figure) composed of one or more of devices such as a signal amplifier, a time-to-digital converter (TDC), and an analog-to-digital converter (ADC) is also connected to the array pixel unit 121. out).

In some embodiments, the distance measurement system 10 may also include a color camera, an infrared camera, an IMU and other devices, and the combination of these devices can realize more abundant functions, such as 3D texture modeling, infrared face recognition, SLAM and other functions.

Fig. 2 is a schematic diagram of a transmitting module according to an embodiment of the present invention. The emission module 11 includes an array light source 201, a projection lens 202, and a diffractive optical element (DOE) 203. The array light source 201 emits a pulse, square wave or sine wave modulated light beam under the power time modulation of the processing circuit 13, and the light beam passes through the projection lens 202. After being collimated or focused, it enters the DOE203, and the DOE203 performs spatial modulation on the incident beam, that is, diffraction. In one embodiment, the DOE 203 splits the incident light beam and emits a plurality of light beams 204 , 205 and 206 into the target space, such as tens of thousands of light beams, and each light beam forms a spot on the surface of the object 20 . In one embodiment, the DOE203 will form a speckle pattern in a regular arrangement (meaning that the angular offset of each spot is evenly distributed, and the regular arrangement is incident on the surface of a 3D object, and the arrangement will be reconstructed) by diffracting the incident light beam. In one embodiment, the DOE203 will form a speckle pattern by diffracting the incident light beam, that is, the arrangement of the spots in the speckle pattern has a certain degree of randomness.

In some embodiments, the DOE203 can also be replaced by a mask, which contains a two-dimensional pattern that modulates the incident beam into a non-flooded beam, so that the incident beam can be spatially modulated through the mask to form a two-dimensional code Pattern carrier beam.

Fig. 3 is a schematic diagram of a transmitting module according to another embodiment of the present invention. The emission module 11 includes an array light source 301 and a projection lens 302. The projection lens 302 can be designed in a single-chip or multi-chip form as required. The light beam emitted by the array light source 301 passes through the projection lens 302 and then emits light beams 303, 304, and 305 to the target space, and finally A projection pattern consistent with the light source arrangement pattern on the array light source 301 , that is, a speckle pattern, is formed. The processing circuit 13 performs sequential power modulation on the array light source 301 to emit pulse, square wave or sine wave modulated light beams. In one embodiment, the array light source 301 is a VCSEL array light source.

Fig. 4 is a schematic diagram of the influence of parallax on imaging according to another embodiment of the present invention. Fig. 4 only exemplarily shows a part of the array pixel unit 121, including a total of 32 pixels in 4 rows (R1, R2, R3, R4) and 8 columns (C1, C2, C3, C4, C5, C6, C7, C8) 403. The light beam reflected by the target will be imaged on the array pixel unit 121 through the imaging lens 122. In this embodiment, assuming that a single light beam will be imaged on about 2x2=4 pixels, the 2x2=4 pixels can be called is a macro pixel, and a single macro pixel corresponds to a spot formed by a single beam. It should be understood that the "imaging" here can refer to the integration of incident light signals by a CCD or CMOS image sensor with a common photon integration function, or it can refer to the counting of incident light signals by a SPAD image sensor.

In the present application, as shown in FIG. 1 , the transmitter module 11 and the collection module 12 in the range imaging system 10 are arranged off-axis, and there is a certain parallax between the two, so when the transmitter module 11 emits a light beam 30 to the target 20 , when the distance of the target 20 changes, the parallax will cause the imaging position of the light beam on the array pixel unit 121 to deviate. Take any light beam as an example for illustration. If the measurement range of the distance imaging system is [D1, D2], when the target 20 is at the position D1, the imaging position corresponding to the beam is 401; when the target is at the position D2, the imaging position corresponding to the beam is The position is 402, considering the problem of imaging magnification, the size of the spot will also change slightly. Therefore, when performing actual measurement, it is often necessary to determine the deviation range of the light spot in advance according to the measurement range, so as to guide the distance analysis and calculation performed by the processor later.

Fig. 5 is a schematic diagram of an array light source according to an embodiment of the present invention. The array light source 51 includes a semiconductor substrate and a plurality of VCSEL sub-light sources 511 and 512 disposed on the substrate. Regardless of the emission module 11 shown in FIG. 2 or FIG. 3 , the arrangement density of the sub-light sources in the array light source 51 determines the pattern density of the beam 30 projected by the final emission module 11, thereby further affecting the resolution of depth calculation. . However, due to factors such as the manufacturing process, the diameter of a single sub-light source, and the mutual influence of multiple sub-light sources, it is impossible for the two sub-light sources 511 to be infinitely close together.

In order to further increase the resolution, in this embodiment, the array light source 51 is arranged as an array light source arranged in a regular crossing pattern, that is, a first sub-light source array composed of a plurality of first sub-light sources 511 and a plurality of second sub-light sources 512 The formed second sub-light source array is formed by intersecting. In the figure, the first sub-light source 511 and the second sub-light source 512 are drawn in two different forms for the sake of distinction, not to limit the actual properties of the two sub-light sources. Assuming that the minimum physical distance between the current two light sources during manufacture is D, the distances between the nearest two light sources along the horizontal and vertical directions are x, y respectively after crossing each other, and x<D, y<D, that is, the cross-arranged horizontal , The vertical interval is smaller. When the projected light beam formed by the array light source is further used for depth calculation, the resolution of the formed depth image has been improved, that is, compared with the regular array light source with both horizontal and vertical spacings D, the cross-set The array light source can improve the resolution horizontally and vertically.

In one embodiment, the first sub-light source array and the second sub-light source array can be independently controlled to be turned on independently or all of them in different applications.

In addition to affecting the resolution due to the limitation of manufacturing physical size, the imaging effect caused by parallax also affects the resolution to a certain extent. As shown in Figure 4, after the baseline between the emission module 11 and the collection module 12 and the measurement range of the distance measurement system 10 are determined, the interval of the deviation value of the facula caused by the parallax is also determined, in order to avoid adjacent Two light spots are misidentified. For example, assuming that the light spots deviate from the horizontal interval of 8 pixel values, if the interval between two horizontal pixels corresponds to the interval on the array pixel unit 121 is less than 8 pixels, it will cause misidentification. There will be at least two light spots on an interval, which will cause measurement errors. In order to avoid misidentification, there is a minimum limit (determined by parallax, measurement range, etc.) on the lateral interval between adjacent sub-light sources. The horizontal resolution can be improved through the cross light source setting shown in Figure 5. In addition, due to the staggered horizontal setting, the vertical resolution is also improved under the premise of the limit of the manufacturing process.

Fig. 6 is a schematic diagram of the principle of distortion correction according to an embodiment of the present invention. In the above embodiments, the influence of system distortion is not considered. In an actual distance measurement system, distortions, such as barrel distortion and pincushion distortion, will be brought about by the projection lens and/or imaging lens. For structured light technology, distortion can increase the irrelevance of structured light images, so it is often allowed, but for time-of-flight distance measurement, especially for the off-axis system in this application, larger distortion is not allowed. As shown in Figure 4, if there is a large distortion, the deviation of the light spot due to parallax will no longer be along the horizontal direction, and there will also be a vertical deviation. When the vertical deviation is too large, such as exceeding the row interval corresponding to the macro pixel, the It will lead to a large error in the measurement. In order to reduce distortion, this embodiment provides a principle of distortion compensation, that is, by designing two opposite distortion features in the distance measurement system to achieve the purpose of compensation, that is, the first (positive) distortion 61 and the second (Negative) distortion 62, the effect after compensation is shown in 63, for example, if barrel distortion is called positive distortion, pincushion distortion is negative distortion.

In one embodiment, the array light source and/or DOE and projection lens in the emitting module 11 are respectively designed to be positively and negatively distorted so as to reduce the degree of distortion of the projected pattern. For example, in the embodiment shown in FIG. 2 and FIG. 3 , if the projection lens 202 has a barrel (pillow) distortion, then one way may be to design the arrangement of the sub-light sources on the array light source 201 as a two-dimensional pillow (barrel) distortion. Arrangement pattern, another way can design the diffraction pattern of DOE203 as a pillow (barrel) distortion two-dimensional diffraction pattern. Of course, it is also possible to simultaneously design the sub-light source arrangement on the array light source 201 and the diffraction pattern of the DOE 203 to compensate for the distortion effect caused by the projection lens. It can be understood that the light source modem here is a DOE, and may also be other devices with the same or similar functions, as in the following embodiments.

In one embodiment, the emitting module 11 and the collecting module 12 are respectively designed to have positive and negative distortion characteristics to reduce the degree of distortion of the projected pattern. If the imaging lens 122 in the collection module 12 has a barrel (pillow) shape distortion, then the speckle pattern emitted by the emission module 11 is designed to be a speckle pattern with a pillow (barrel) shape distortion feature, which can also be realized in a variety of ways. Design of pillow (barrel) distorted speckled pattern. For the embodiment of the emission module shown in Figure 2: one is to directly design the arrangement of the sub-light sources on the array light source 201 as a two-dimensional arrangement pattern with a pillow (barrel) distortion feature; the other is to design the projection lens as a pillow (barrel)-shaped distortion lens; the third is to design the diffraction pattern of the DOE into a pillow (barrel)-shaped distortion two-dimensional diffraction pattern (for the case where the emission module has a DOE); the fourth is to arrange the pattern of the array light source, projection lens, At least two of the DOE diffraction patterns are designed in combination to project a pincushion (barrel) distorted spot pattern. For the embodiment of the emission module shown in Figure 3: one is to directly design the arrangement of the sub-light sources on the array light source 301 as a two-dimensional arrangement pattern with a pillow (barrel) distortion feature; the other is to design the projection lens as a pillow (barrel)-shaped distortion lens; the third is to comprehensively design the arrangement pattern of the array light source and the projection lens to project a pillow (barrel)-shaped distorted spot pattern.

The distortion correction scheme shown in Figure 6 achieves distortion compensation on the hardware. Compared with the traditional algorithm-based distortion compensation scheme, it not only reduces the requirements for the algorithm, but also solves the distortion problem from the source, and the effect is better.

It can be understood that when the distance ranging system of the present invention is embedded in a device or hardware, corresponding structural or component changes will be made to meet the requirements, and its essence still adopts the distance ranging system of the present invention, so it should be regarded as scope of protection.

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be assumed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art to which the present invention belongs, without departing from the concept of the present invention, several equivalent substitutions or obvious modifications can be made, and the same performance or use should be considered as belonging to the protection scope of the present invention.

Claims (10)

1. A distance measuring system, characterized in that, comprising: An emission module, configured to emit a speckle pattern light beam having a first distortion feature; A collection module, including an imaging lens with a second distortion feature and an array pixel unit, for collecting the reflected speckle pattern light beam; a processing circuit, connected to the emission module and the acquisition module, for calculating the time-of-flight between the emission beam and the reflection beam; The first distortion feature and the second distortion feature compensate each other. 2. The distance measurement system according to claim 1, wherein the first distortion feature is a barrel distortion feature or a pincushion distortion feature, and the second distortion feature is a pincushion distortion feature or a barrel distortion feature . 3. The distance measurement system according to claim 1, wherein the transmitting module comprises an array light source, a projection lens and a spatial light modulator. 4. The distance measurement system according to claim 3, wherein the sub-light source arrangement on the array light source is a two-dimensional arrangement pattern with a first distortion feature; Or, the projection lens has a first distortion characteristic; Or, the diffraction pattern of the spatial light modulator has a first distortion feature; Or, at least two of the sub-light source arrangement on the array light source, the projection lens and the diffraction pattern of the spatial light modulator are designed comprehensively so that the emitting module has the first distortion feature. 5. The distance measurement system according to claim 1, wherein the emission module comprises an array light source and a projection lens. 6. The distance measurement system according to claim 5, wherein the sub-light source arrangement on the array light source is a two-dimensional arrangement pattern with a first distortion feature; Or, the projection lens has a first distortion characteristic; Or, the sub-light source arrangement on the array light source and the projection lens are comprehensively designed so that the emission module has the first distortion feature. 7. A distance measuring system, characterized in that, comprising: Emitter module, including array light source and projection lens, used to emit speckle pattern beam; A collection module, including an imaging lens and an array pixel unit, for collecting the reflected speckle pattern light beam; a processing circuit, connected to the emission module and the acquisition module, for calculating the time-of-flight between the emission beam and the reflection beam; The sub-light source arrangement on the array light source is a two-dimensional arrangement pattern with a first distortion feature, the projection lens has a second distortion feature, and the first distortion feature and the second distortion feature compensate each other. 8. The distance measuring system according to claim 7, wherein the first distortion feature is a barrel distortion feature or a pincushion distortion feature, and the second distortion feature is a pincushion distortion feature or a barrel distortion feature . 9. A distance measuring system, characterized in that, comprising: The emission module includes an array light source, a projection lens, and a spatial light modulator for emitting a speckle pattern beam; A collection module, including an imaging lens and an array pixel unit, for collecting the reflected speckle pattern light beam; a processing circuit, connected to the emission module and the acquisition module, for calculating the time-of-flight between the emission beam and the reflection beam; The sub-light source arrangement on the array light source and/or the diffraction pattern of the spatial light modulator is a two-dimensional array pattern with a first distortion feature; the projection lens has a second distortion feature, and the first distortion feature and The second distortion features compensate each other. 10. The distance measurement system according to claim 9, wherein the first distortion feature is a barrel distortion feature or a pincushion distortion feature, and the second distortion feature is a pincushion distortion feature or a barrel distortion feature .