Patent Publication Number: US-2022215582-A1

Title: Conversion parameter calculation method, displacement amount calculation method, conversion parameter calculation device, and displacement amount calculation device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of PCT International Application No. PCT/JP2020/023535 filed on Jun. 16, 2020, designating the United States of America, which is based on and claims priority of Japanese Patent Application No. 2019-186791 filed on Oct. 10, 2019. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to a conversion parameter calculation method, a displacement amount calculation method, a conversion parameter calculation device, and a displacement amount calculation device, which are for calculating a conversion parameter for measuring an actual displacement amount that is an actual value of a displacement indicating movement of an object using images. 
     BACKGROUND 
     Conventionally, an image capturing device has been disclosed that can measure the state of an object in a contactless manner using image data obtained by capturing an image of an object with a camera and a distance measurement value indicating a distance up to the object, which is obtained by measuring the distance using a distance measurement device such as a laser rangefinder (see Patent Literature (PTL) 1). If the object is a bridge, for example, the state indicates a deformation amount of the bridge, for instance. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 5281610 
     SUMMARY 
     Technical Problem 
     However, the image capturing device according to PTL 1 needs to dispose the camera and the distance measurement device such that the optical axis of the camera and the optical axis of the distance measurement device are parallel to each other in order to accurately measure the state of the object. Accordingly, PTL 1 discloses use of an attaching tool for attaching the camera and the distance measurement device such the optical axis of the camera and the optical axis of the distance measurement device are parallel to each other. In this manner, an attaching tool needs to be provided for the image capturing device according to PTL 1 in order to measure the state of an object accurately, and thus it is difficult to accurately and readily measure the actual value of a displacement. 
     In view of this, the present disclosure relates to a conversion parameter calculation method and others with which the displacement amount can be accurately and readily converted into an actual value in measuring a displacement using images. 
     Solution to Problem 
     A conversion parameter calculation method according to an aspect of the present disclosure is a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation method including: obtaining, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; obtaining, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image of the object, the second image capturing device being disposed in a position different from a position of the first image capturing device; obtaining displacement direction information indicating a direction of the displacement of the object in three dimensions; associating a position on the object in the first image data with a position on the object in the second image data; estimating the position of the first image capturing device relative to the position of the second image capturing device, based on the second distance data and a result of associating the positions; calculating first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and calculating the conversion parameter, using the first distance data and the displacement direction information, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     A conversion parameter calculation method according to an aspect of the present disclosure is a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation method including: obtaining, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; obtaining, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image that includes the object and the first image capturing device, the second image capturing device being disposed in a position different from a position of the first image capturing device; obtaining displacement direction information indicating a direction of the displacement of the object in three dimensions; detecting a shape of the first image capturing device in the second image data; estimating the position of the first image capturing device, based on a result of detecting the shape; calculating first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and calculating the conversion parameter, using the first distance data and the displacement direction information, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     A conversion parameter calculation method according to an aspect of the present disclosure is a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation method including: obtaining, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; obtaining, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image that includes the object and the first image capturing device, the second image capturing device being disposed in a position different from a position of the first image capturing device; obtaining displacement direction information indicating a direction of the displacement of the object in three dimensions; detecting a shape of the first image capturing device in the second image data; estimating the position and an orientation of the first image capturing device, based on a result of detecting the shape; calculating first distance data indicating a distance from the first image capturing device to the object, based on the position and the orientation of the first image capturing device and the second distance data; and calculating the conversion parameter, using the first distance data and the displacement direction information, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     A displacement amount calculation method according to an aspect of the present disclosure includes: obtaining the conversion parameter calculated using the above conversion parameter calculation method; obtaining the at least two third image data items; and converting, based on the conversion parameter, the pixel displacement amount between the at least two third image data items into the actual displacement amount. 
     A conversion parameter calculation device according to an aspect of the present disclosure is a conversion parameter calculation device that calculates a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation device including: a first obtainer that obtains, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; a second obtainer that obtains, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image of the object, the second image capturing device being disposed in a position different from a position of the first image capturing device; a third obtainer that obtains displacement direction information indicating a direction of the displacement of the object in three dimensions; a matcher that associates a position on the object in the first image data with a position on the object in the second image data; a position estimator that estimates the position of the first image capturing device relative to the position of the second image capturing device, based on the second distance data and a result of associating the positions; a distance calculator that calculates first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and a parameter calculator that calculates the conversion parameter, using the first distance data and the displacement direction information, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     A conversion parameter calculation device according to an aspect of the present disclosure is a conversion parameter calculation device that calculates a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation device including: a first obtainer that obtains, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; a second obtainer that obtains, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image that includes the object and the first image capturing device, the second image capturing device being disposed in a position different from a position of the first image capturing device; a third obtainer that obtains displacement direction information indicating a direction of the displacement of the object in three dimensions; a position estimator that detects a shape of the first image capturing device in the second image data, and estimates the position of the first image capturing device, based on a result of detecting the shape; a distance calculator that calculates first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and a parameter calculator that calculates the conversion parameter, using the first distance data and the displacement direction information, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     A displacement amount calculation device according to an aspect of the present disclosure includes: a fourth obtainer that obtains the conversion parameter calculated using the above conversion parameter calculation device; a fifth obtainer that obtains the at least two third image data items; and a converter that converts, based on the conversion parameter, the pixel displacement amount between the at least two third image data items into the actual displacement amount. 
     Advantageous Effects 
     According to the conversion parameter calculation method and others according to an aspect of the present disclosure, a displacement amount can be accurately and readily converted into an actual value. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG. 1  illustrates a schematic configuration of a displacement measurement system according to Embodiment 1. 
         FIG. 2  is a block diagram illustrating a functional configuration of the displacement measurement system according to Embodiment 1. 
         FIG. 3  is a flowchart illustrating operation of the displacement measurement system according to Embodiment 1. 
         FIG. 4  is for describing feature point matching by a matcher according to Embodiment 1. 
         FIG. 5  is for describing a method for converting a displacement into an actual size according to Embodiment 1. 
         FIG. 6  is a flowchart illustrating operation of the displacement measurement system according to a variation of Embodiment 1. 
         FIG. 7A  is a first diagram for describing displacement conversion according to the variation of Embodiment 1, with the displacement direction taken into consideration. 
         FIG. 7B  is a second diagram for describing displacement conversion according to the variation of Embodiment 1, with the displacement direction taken into consideration. 
         FIG. 8  is for describing a method for converting a displacement into an actual size according to the variation of Embodiment 1. 
         FIG. 9  illustrates a schematic configuration of a displacement measurement system according to Embodiment 2. 
         FIG. 10  is a block diagram illustrating a functional configuration of the displacement measurement system according to Embodiment 2. 
         FIG. 11  illustrates examples of a marker applied to a first image capturing device according to Embodiment 2. 
         FIG. 12  is a flowchart illustrating operation of the displacement measurement system according to Embodiment 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Circumstances That Have Led to the Present Disclosure 
     With the image capturing device according to PTL 1, a camera and a distance measurement device need to be disposed such that the optical axis of the camera and the optical axis of the distance measurement device are parallel to each other, in order to accurately measure the state of an object, as described above. PTL 1 uses a dedicated attaching tool in order to cause the optical axis of the camera and the optical axis of the distance measurement device to be parallel to each other. 
     However, in displacement measurement using images, desirably, an actual value of a physical quantity such as a displacement amount can be accurately and readily measured. For example, desirably, a displacement can be accurately measured even if the optical axis of the camera and the optical axis of the distance measurement device are not parallel to each other, which need to be parallel in PTL 1 In addition, for example, desirably, the actual value of a displacement can be readily measured without providing a dedicated attaching tool, for instance. 
     Another method for measuring an actual value of a displacement is a method for capturing an image that includes a specific portion, whose length is known, of an object for which displacement measurement is to be performed, and calculating a conversion value for converting one pixel into an actual value, based on the known length and the number of pixels of a portion on the image which corresponds to the specific portion. However, the specific portion is desirably a flat surface when this method is used. Thus, the method has a restriction on a shape of the specific portion of the object, and thus it is difficult to measure the actual value of a displacement at a measurement point on an object having a complicated shape. 
     With this method, the actual value of a displacement of the specific portion can be accurately measured, but it is difficult to accurately measure the actual value of a displacement of a portion other than the specific portion. This is because if the distance from the image capturing device to the specific portion and the distance from the image capturing device to the portion other than the specific portion are different from each other, an appropriate conversion value is different for each of the portions. With this method, the actual value of the displacement cannot be measured, unless the length of the specific portion is known. 
     In view of this, the inventors have made diligent examinations on, for instance, a conversion parameter calculation method for measuring the actual value of a displacement accurately and readily, or stated differently, with less restrictions as to how a camera and a distance measurement device are disposed and on the shape of an object, and have arrived at a conversion parameter calculation method and others as will be described below. 
     A conversion parameter calculation method according to an aspect of the present disclosure is a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation method including: obtaining, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; obtaining, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image of the object, the second image capturing device being disposed in a position different from a position of the first image capturing device; associating a position on the object in the first image data with a position on the object in the second image data; estimating the position of the first image capturing device relative to the position of the second image capturing device, based on the second distance data and a result of associating the positions; calculating first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and calculating the conversion parameter, using the first distance data, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     Accordingly, according to the conversion parameter calculation method, first distance data indicating a distance from the first image capturing device to the object can be calculated based on the estimated position of the first image capturing device, and thus the first image capturing device and the second image capturing device do not need to be disposed such that the optical axis of the first image capturing device and the optical axis of the second image capturing device are parallel to each other. According to the conversion parameter calculation method, the conversion parameter can be calculated based on the first distance data, and thus even if the length of a specific portion of the object is unknown, a conversion parameter for converting a pixel displacement amount into the actual displacement amount can be calculated. 
     According to the conversion parameter calculation method, the distance from the first image capturing device to the object can be accurately obtained by estimating the first distance data. Thus, according to the conversion parameter calculation method, since the conversion parameter is calculated using the first distance data, a conversion parameter with which a more precise actual displacement amount can be calculated can be obtained. For example, a conversion parameter for calculating a further accurate actual displacement amount can be obtained as compared to the case where second distance data is used. Thus, according to the conversion parameter calculation method according to the present disclosure, since the displacement amount is calculated using a conversion parameter calculated using the conversion parameter calculation method, the displacement amount can be accurately and readily converted into the actual value in displacement measurement using images. 
     A conversion parameter calculation method according to an aspect of the present disclosure is a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation method including: obtaining, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; obtaining, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image that includes the object and the first image capturing device, the second image capturing device being disposed in a position different from a position of the first image capturing device; detecting a shape of the first image capturing device in the second image data; estimating the position of the first image capturing device, based on a result of detecting the shape; calculating first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and calculating the conversion parameter, using the first distance data, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     Accordingly, position information of the first image capturing device can be estimated without being influenced by the position of a feature point or the number of feature points obtained from an image, since position information of the first image capturing device can be estimated using only second image data and distance data. Thus, position information of the first image capturing device can be readily estimated. 
     According to the conversion parameter calculation method, the distance from the first image capturing device to the object can be accurately obtained by estimating the first distance data. Thus, according to the conversion parameter calculation method, since the conversion parameter is calculated using the first distance data, a conversion parameter with which a more precise actual displacement amount can be calculated can be obtained. For example, a conversion parameter for calculating a further accurate actual displacement amount can be obtained as compared to the case where second distance data is used. Thus, according to the conversion parameter calculation method according to the present disclosure, since the displacement amount is calculated using a conversion parameter calculated using the conversion parameter calculation method, the displacement amount can be accurately and readily converted into the actual value in displacement measurement using images. 
     For example, the object has at least two measurement points for measuring the displacement, the at least two measurement points including the measurement point, the calculating of the first distance data includes calculating the first distance data indicating a distance from each of the at least two measurement points to the first image capturing device, to obtain at least two first distance data items, and the calculating of the conversion parameter, using the first distance data and the displacement direction information, includes calculating, for each of the at least two measurement points, a conversion value for the measurement point as the conversion parameter, based on one of the at least two first distance data items indicating the distance from the measurement point. 
     Accordingly, according to the conversion parameter calculation method, a correction value is calculated for each of a plurality of measurement points, and thus an actual displacement amount at each measurement point can be accurately calculated. 
     For example, the calculating of the first distance data includes calculating, for each of surface points on the object, the first distance data from the surface point to the first image capturing device to obtain first distance data items, the surface points including the at least two measurement points, and the calculating of the conversion parameter, using the first distance data and the displacement direction information, includes: calculating, for each of the surface points, a conversion value for the surface point, based on one of the first distance data items indicating the distance from the surface point, to calculate conversion values; and generating a conversion value map as the conversion parameter, based on the conversion values calculated. 
     Accordingly, according to the conversion parameter calculation method, when an actual displacement amount at a position other than the measurement points is to be measured, such an actual displacement amount can be readily measured using the conversion value map. Thus, convenience of the conversion parameter calculation method can be improved. 
     For example, the conversion parameter calculation method further includes obtaining displacement direction information indicating the direction of a displacement of the object, and in calculating the conversion parameter, the conversion parameter is calculated further using the displacement direction information. 
     Accordingly, according to the conversion parameter calculation method, a conversion value can be calculated using displacement direction information when the object is displaced in a direction crossing the image capturing surface of the first image capturing device in a top view of the object, and thus can further accurately measure the actual value of the displacement. 
     For example, the first image capturing device includes a first camera, the second image capturing device includes a second camera that captures the second image data, and a depth sensor that measures the distance indicated by the second distance data, and the first camera has a resolution and a frame rate at least one of which is higher than a resolution and a frame rate of the second camera. 
     Accordingly, based on monochrome image data captured by a monochrome camera, a pixel displacement amount of the object in the monochrome image data can be obtained. Typically, a monochrome camera can capture higher definition images than those captured by a color camera. Thus, a displacement is measured based on a monochrome image captured by a monochrome camera, so that the displacement can be measured highly accurately. In addition, a depth sensor is readily available, and thus a conversion parameter calculation method with high convenience can be provided. 
     A displacement amount calculation method according to an aspect of the present disclosure includes: obtaining the conversion parameter calculated using the conversion parameter calculation method; obtaining the at least two third image data items; and converting the pixel displacement amount between the at least two third image data items into the actual displacement amount. 
     Accordingly, the actual displacement amount can be measured using the conversion parameter calculated using the above conversion parameter calculation method, and thus the actual displacement amount of the object can be measured accurately and readily. 
     A conversion parameter calculation device according to an aspect of the present disclosure is a conversion parameter calculation device that calculates a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation device including: a first obtainer that obtains, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; a second obtainer that obtains, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image of the object, the second image capturing device being disposed in a position different from a position of the first image capturing device; a matcher that associates a position on the object in the first image data with a position on the object in the second image data; a position estimator that estimates the position of the first image capturing device relative to the position of the second image capturing device, based on the second distance data and a result of associating the positions; a distance calculator that calculates first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and a parameter calculator that calculates the conversion parameter, using the first distance data, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     Accordingly, advantageous effects similar to those yielded by the above conversion parameter calculation method can be yielded. Specifically, according to the conversion parameter calculation device, first distance data indicating a distance from the first image capturing device to the object can be calculated based on the estimated position of the first image capturing device, and thus it is unnecessary to dispose the first image capturing device and the second image capturing device such that the optical axis of the first image capturing device and the optical axis of the second image capturing device are parallel to each other. According to the conversion parameter calculation device, the conversion parameter can be calculated based on the first distance data, and thus even if the length of a specific portion of the object is unknown, a conversion parameter for converting a pixel displacement amount into the actual displacement amount can be calculated. 
     According to the conversion parameter calculation device, the distance from the first image capturing device to the object can be accurately obtained by estimating the first distance data. Thus, according to the conversion parameter calculation device, since the conversion parameter is calculated using the first distance data, a conversion parameter with which a more precise actual displacement amount can be calculated can be obtained. For example, a conversion parameter for calculating a further accurate actual displacement amount can be obtained as compared to the case where second distance data is used. Thus, according to the above conversion parameter calculation device, since the displacement amount is calculated using a conversion parameter calculated by the conversion parameter calculation device, the displacement amount can be accurately and readily converted into the actual value in displacement measurement using images. 
     A conversion parameter calculation device according to an aspect of the present disclosure is a conversion parameter calculation device that calculates a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a displacement indicating movement of an object, the conversion parameter calculation device including: a first obtainer that obtains, from a first image capturing device, first image data obtained by the first image capturing device capturing an image of the object; a second obtainer that obtains, from a second image capturing device, second distance data indicating a distance from the second image capturing device to the object, and second image data obtained by the second image capturing device capturing an image that includes the object and the first image capturing device, the second image capturing device being disposed in a position different from a position of the first image capturing device; a position estimator that detects a shape of the first image capturing device in the second image data, and estimates the position of the first image capturing device, based on a result of detecting the shape; a distance calculator that calculates first distance data indicating a distance from the first image capturing device to the object, based on the position of the first image capturing device and the second distance data; and a parameter calculator that calculates the conversion parameter, using the first distance data, the conversion parameter being a parameter for converting, into the actual displacement amount, a pixel displacement amount at a measurement point on the object based on at least two third image data items captured by the first image capturing device at different times. 
     Accordingly, advantageous effects similar to those yielded by the above conversion parameter calculation method can be yielded. Specifically, the conversion parameter calculation device can estimate position information of the first image capturing device using only second image data and distance data, and thus can readily estimate the position information of the first image capturing device. 
     According to the conversion parameter calculation device, the distance from the first image capturing device to the object can be accurately obtained by estimating the first distance data. Thus, according to the conversion parameter calculation device, since the conversion parameter is calculated using the first distance data, a conversion parameter with which a more precise actual displacement amount can be calculated can be obtained. For example, a conversion parameter for calculating a further accurate actual displacement amount can be obtained as compared to the case where second distance data is used. Thus, according to the above conversion parameter calculation device, since the displacement amount is calculated using a conversion parameter calculated by the conversion parameter calculation device, the displacement amount can be accurately and readily converted into the actual value in displacement measurement using images. 
     A displacement amount calculation device according to an aspect of the present disclosure includes: a fourth obtainer that obtains the conversion parameter calculated using the above conversion parameter calculation device; a fifth obtainer that obtains the at least two third image data items; and a converter that converts, based on the conversion parameter, the pixel displacement amount between the at least two third image data items into the actual displacement amount. 
     Accordingly, advantageous effects similar to those yielded by the above displacement amount calculation method can be yielded. 
     Note that these general and specific aspects may be implemented using a system, a method, an integrated circuit, a computer program, a computer-readable non-transitory recording medium such as a CD-ROM, or any combination of systems, methods, integrated circuits, computer programs, and recording media. The program may be prestored in a recording medium or may be supplied to a recording medium through a wide area communication network including the Internet. 
     The following specifically describes embodiments with reference to the drawings. 
     Note that embodiments and variations described below each show a general or specific example. The numerical values, shapes, materials, elements, and the arrangement and connection of the elements, steps, and the order of processing the steps, for instance, described in the following embodiments and variations are examples, and thus are not intended to limit the present disclosure. Among the elements in the following embodiments, elements not recited in any of the independent claims are described as arbitrary elements. 
     Note that the diagrams are schematic diagrams, and do not necessarily provide strict illustration. In the drawings, the same numeral is given to a substantially same element, and a redundant description thereof may be omitted or simplified. 
     In the Specification, terms that indicate a relation between elements such as orthogonal and the same, and numerical values do not necessarily have only strict meanings, and also cover substantially equivalent ranges that include a difference of about several percent, for example. 
     In addition, an image is a still image in the following description, but may be a video image. 
     Embodiment 1 
     The following describes a conversion parameter calculation method and others according to Embodiment 1, with reference to  FIG. 1  to  FIG. 5 . 
     1-1. Configuration of Displacement Measurement System 
     First, displacement measurement system  1  that includes conversion parameter calculator  110  that executes the conversion parameter calculation method according to Embodiment 1 is to be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  illustrates a schematic configuration of displacement measurement system  1  according to Embodiment 1.  FIG. 2  is a block diagram illustrating a functional configuration of displacement measurement system  1  according to Embodiment 1. 
     As illustrated in  FIG. 1 , displacement measurement system  1  according to Embodiment 1 is an information processing system that measures an actual value of a displacement of object  60 , using two image capturing devices disposed in different positions. First image capturing device  10  and second image capturing device  20  capture images of object  60  from different viewpoints. Object  60  is a target whose displacement is to be measured, and is, for example, a device such as a motor that is displaced (shakes, for instance) during operation. Yet, object  60  is not limited thereto, and may be, for example, a structure such as an infrastructure structure that is displaced (deforms, for instance) due to stress from the outside. An infrastructure structure may be a bridge where vehicles such as cars and trains travel, for example. Note that the displacement in the Specification includes not only shakes of object  60 , but also deformation of object  60  and a change in position thereof. It can also be said that a displacement indicates movement. 
     As illustrated in  FIG. 1  and  FIG. 2 , displacement measurement system  1  includes first image capturing device  10 , second image capturing device  20 , displacement measurement device  30 , output device  40 , and input device  50 . 
     First image capturing device  10  captures an image for measuring a displacement of object  60 . The image captured by first image capturing device  10  is used to detect the displacement amount (the number of pixels) on an image corresponding to the displacement of object  60 . Note that in the following, an image captured by first image capturing device  10  is also stated as a first image, and a displacement amount on an image corresponding to a displacement of object  60  is also stated as a pixel displacement amount. A pixel displacement amount is an example of a physical quantity in pixels (for example, the number of pixels). Further, a first image may be used to calculate a conversion value later described. 
     First image capturing device  10  is, for example, a monochrome camera. In other words, a first image is a monochrome image, for example. Note that first image capturing device  10  is not limited to a monochrome camera, and may be a color camera. First image capturing device  10  is a digital video camera or a digital still camera that includes an image sensor, for example. Note that first image capturing device  10  is an example of an image capturing device. The monochrome camera or the color camera that first image capturing device  10  includes is an example of a first camera. The first camera has a resolution and a frame rate at least one of which is higher than a resolution and a frame rate of a second camera later described. The first camera can capture an image having a higher resolution than the second camera and/or can capture images at a higher frame rate than the second camera, for example. 
     Second image capturing device  20  captures an image for measuring an actual value of a displacement of object  60 . Second image capturing device  20  is configured to obtain an image of object  60  and a distance from second image capturing device  20  to object  60 . An image captured by second image capturing device  20  and a distance measured by second image capturing device  20  are used to estimate the position of first image capturing device  10 . Note that in the following, an image captured by second image capturing device  20  is also stated as a second image, and an actual value of a displacement of object  60  is also stated as an actual displacement amount. The actual displacement amount is an example of a physical quantity in the real space (a distance corresponding to the number of pixels, for example). The distance from second image capturing device  20  to object  60  is an example of a second distance. 
     Second image capturing device  20  includes an image capturer for capturing a second image and a distance measurer for measuring a distance. The image capturer is a color camera, for example, but may be a monochrome camera. It is sufficient if the image capturer can obtain a grayscale image of object  60 . The distance measurer includes a depth sensor, for example. The depth sensor can measure a distance up to one point or each of multiple points on object  60 , and obtains a distance between second image capturing device  20  and object  60  (a positional relation, for example), based on a time for a laser beam to reflect off object  60  and return. The depth sensor may be a laser imaging detection and ranging (lidar) sensor, for example. Note that the sensor included in the distance measurer is not limited to a depth sensor, and may be a time-of-flight (TOF) sensor, for example. Second image capturing device  20  may be configured to integrally or detachably include the image capturer and the distance measurer, for example. Note that second image capturing device  20  is an example of a measurement device. The monochrome camera or the color camera that second image capturing device  20  includes is an example of a second camera. 
     When a displacement of object  60  is less than or equal to a predetermined value, first image capturing device  10  and second image capturing device  20  may capture images of same object  60  at different times, for example. When a displacement of object  60  is greater than the predetermined value, first image capturing device  10  and second image capturing device  20  may capture images of object  60  synchronously, for example. The predetermined value may be set based on an error tolerance value of a displacement. 
     After being disposed, second image capturing device  20  may capture at least one image and make measurement at least once. When a displacement of object  60  is less than or equal to the predetermined value, image capturing by the image capturer of second image capturing device  20  and measurement by the distance measurer thereof may be conducted at different times, and may be conducted synchronously when a displacement of object  60  is greater than the predetermined value. 
     First image capturing device  10  and second image capturing device  20  may be fixed so that the positions thereof do not change during image capturing. Second image capturing device  20  may be disposed close to first image capturing device  10 . First image capturing device  10  and second image capturing device  20  may be disposed in positions where images that include measurement points on object  60  can be obtained. 
     The first image may be an image having a resolution higher than that of the second image and/or may be an image captured under an image-capturing condition under which the frame rate is higher than that in the condition for the second image. Note that in Embodiment 1, the first image is a monochrome image and the second image is a color image, but are not limited thereto. 
     Displacement measurement device  30  is an information processing device that calculates a conversion value for converting a pixel displacement amount into an actual displacement amount, based on image data obtained from first image capturing device  10  and image data and distance data obtained from second image capturing device  20 , and outputs the actual displacement amount of object  60  obtained using the calculated conversion value. Displacement measurement device  30  may be a server device, for example. Displacement measurement device  30  includes conversion parameter calculator  110 , displacement detector  120 , and displacement amount calculator  130 . 
     Conversion parameter calculator  110  calculates a conversion value for converting a pixel displacement amount into an actual displacement amount, based on image data obtained from first image capturing device  10  and image data and distance data obtained from second image capturing device  20 . Conversion parameter calculator  110  includes first obtainer  111 , second obtainer  112 , matcher  113 , position estimator  114 , distance calculator  115 , and conversion value calculator  116 . 
     First obtainer  111  obtains first image data indicating a first image from first image capturing device  10 . 
     Second obtainer  112  obtains distance and image data from second image capturing device  20 . Second obtainer  112  obtains second image data indicating a second image and distance data, for example. 
     First obtainer  111  and second obtainer  112  are communication interfaces (communication circuits) communicably connected to first image capturing device  10  and second image capturing device  20 , respectively, through wireless or wired communication. 
     Matcher  113  associates a position on object  60  in first image data with a position on object  60  in second image data, by matching feature points in the first image data and the second image data. Matcher  113  associates the positions (for example, pixel positions) on object  60  in the first image data and the second image data. Accordingly, matcher  113  obtains correspondence information indicating a correspondence between the position on object  60  in first image data and the position on object  60  in the second image data. For a method of detecting feature points, any existing technique such as scale invariant feature transform (SIFT) may be used. 
     Position estimator  114  estimates the position of first image capturing device  10 , based on correspondence information, distance data, and one or more internal parameters of first image capturing device  10 . Specifically, position estimator  114  estimates the position of first image capturing device  10  in a coordinate system of second image capturing device  20 . Note that the one or more internal parameters of first image capturing device  10  are assumed to be known. Further, the one or more internal parameters include at least one of, for example, a 3×3 parameter matrix (cameraMatrix), a distortion coefficient (distCoeffs) of a lens, or a focal distance thereof. 
     Distance calculator  115  calculates distance information indicating a distance from first image capturing device  10  to object  60 , based on position information indicating the position of first image capturing device  10  and distance data. Distance calculator  115  calculates a distance from first image capturing device  10  to each of one or more measurement points on object  60 , for example. Distance calculator  115  may calculate a distance up to each of surface points including the measurement points for measuring a displacement of object  60 , for example. Distance calculator  115  may generate a distance map in which distances from first image capturing device  10  to object  60  are arranged. Note that the number of measurement points on object  60  is not limited in particular, and may be two or more. 
     Conversion value calculator  116  calculates a conversion value for converting a pixel displacement amount into an actual displacement amount, based on the position of first image capturing device  10 , a distance from first image capturing device  10  to object  60 , and the internal parameter(s) of first image capturing device  10 . Calculation of a conversion value by conversion value calculator  116  is to be described later. Conversion value calculator  116  may calculate a conversion value for each of the surface points, and generate a conversion value map in which the calculated conversion values are arranged, for example. Note that the conversion values and the conversion value map are examples of conversion parameters. Conversion value calculator  116  is an example of a parameter calculator. 
     Note that an example in which conversion parameter calculator  110  is included in displacement measurement device  30  has been described, but the present embodiment is not limited thereto. Conversion parameter calculator  110  may be acquired as a single device. In this case, conversion parameter calculator  110  functions as a conversion parameter calculation device. 
     Note that displacement measurement device  30  is an example of a displacement amount calculation device. 
     Displacement detector  120  detects a pixel displacement amount (the number of pixels) on an image which corresponds to a displacement of object  60 , based on two or more first image data items captured by first image capturing device  10 . Displacement detector  120  detects a pixel displacement amount for each measurement point, for example. 
     Based on a pixel displacement amount and conversion information indicating a conversion parameter, displacement amount calculator  130  calculates an actual displacement amount of object  60  by converting the pixel displacement amount into the actual displacement amount. Displacement amount calculator  130  calculates actual displacement amounts at measurement points on object  60 , based on, for example, conversion values for the measurement points and pixel displacement amounts at the measurement points. 
     Output device  40  obtains presentation information that includes an actual displacement amount from displacement measurement device  30 , and outputs the obtained presentation information. Output device  40  is a display device that displays presentation information as an image, for example. Output device  40  is a liquid crystal display, for example. An image output by output device  40  is checked by a worker. 
     Output device  40  may be a stationary device or may be a device included in a mobile terminal that the worker has. The mobile terminal is not limited in particular as long as the mobile terminal includes output device  40  and can communicate with displacement measurement device  30 , and may be a smartphone or a tablet terminal, for example. If the mobile terminal includes output device  40 , the worker can be informed of an actual displacement amount by checking an image output by output device  40  of the mobile terminal in the vicinity of object  60 . Note that the worker is an example of a user. 
     Displacement measurement system  1  may include a device that outputs sound as output device  40 , together with or instead of the display device. Displacement measurement system  1  may include, as output device  40 , a device that displays presentation information on a target object (for example, a screen) of a projector, for instance. If displacement measurement device  30  is disposed in a remote location, displacement measurement device  30  and output device  40  may be connected to each other via a network. 
     Input device  50  is a user interface that receives, from a worker, predetermined information for use in measurement of a displacement. Input device  50  may receive, from a worker, information for calculating a conversion value, information on a displacement direction of object  60 , or information on an approximate position of first image capturing device  10 , for example. Further, input device  50  may receive input of predetermined information through voice. Note that the approximate position is an estimated value of a position in which first image capturing device  10  is disposed. 
     Input device  50  is acquired with a hardware key (a hardware button), a slide switch, or a touch panel, for instance. Input device  50  may be a stationary device or may be a device included in a mobile terminal that a worker has. 
     1-2. Operation of Displacement Measurement System 
     Next, operation of displacement measurement system  1  is to be described with reference to  FIG. 3  to  FIG. 5 .  FIG. 3  is a flowchart illustrating operation of displacement measurement system  1  according to Embodiment 1. Specifically,  FIG. 3  is a flowchart illustrating operation of displacement measurement device  30 . Steps S 11  to S 16  illustrated in  FIG. 3  show operation of conversion parameter calculator  110 .  FIG. 4  is for describing feature point matching by matcher  113  according to Embodiment 1. Note that  FIG. 4  illustrates an example in which object  60   a  is a piece of furniture. In addition, in order to explain feature points in a readily understandable manner, quick response codes (QR codes (registered trademark)) are applied to some of the feature points. The QR codes (registered trademark) may be provided in correspondence with the measurement points of object  60   a.    
     As illustrated in  FIG. 3 , first obtainer  111  obtains first image data indicating a first image from first image capturing device  10  (S 11 ). First obtainer  111  obtains first image data I 1  as illustrated in (a) of  FIG. 4 , for example. In (a) of  FIG. 4 , the positions (x, y) indicated by circles show pixel positions of feature points on first image data I 1 , for example. The positions of five feature points are illustrated in (a) of  FIG. 4 . Further, QR codes (registered trademark) are captured in first image data I 1 . 
     Note that first obtainer  111  may obtain at least one first image data item in step S 11  to calculate a conversion value. First obtainer  111  may obtain two or more first image data items captured at different times in step S 11  to measure a displacement. 
     When first obtainer  111  obtains a plurality of first image data items, first obtainer  111  outputs at least one first image data item to matcher  113  and two or more first image data items to displacement detector  120 . First obtainer  111  may output, to matcher  113  and displacement detector  120 , different first image data items or first image data items that are at least partially the same. Two or more first image data items output to displacement detector  120  are captured at different times and are examples of third image data. Step S 11  is an example of a the obtaining of the first image data. Step S 11  may be an example of the obtaining of the at least two third image data items in the displacement amount calculation method described later. First obtainer  111  may function as a fourth obtainer that obtains at least two third image data items. 
     Next, second obtainer  112  obtains second image data indicating a second image and distance data indicating a distance from second image capturing device  20  (S 12 ). Second obtainer  112  obtains distance and image data I 2  that indicates second image data and distance data as illustrated in (b) of  FIG. 4 . Distance and image data I 2  includes coordinates (X, Y, Z) indicating a position on object  60   a  relative to the position of second image capturing device  20  (shown by circles in (b) of  FIG. 4 , for example) and pixel values (R, G, B) that indicate colors at the positions. Coordinates (X, Y, Z) are based on the position of second image capturing device  20  (coordinates (0, 0, 0), for example). Distance data is an example of second distance data. 
     Note that distance and image data I 2  may not include color information. It is sufficient if distance and image data I 2  is an image with which feature point matching with first image data I 1  can be performed and which includes coordinates (X, Y, Z) indicating a position on object  60   a  relative to the position of second image capturing device  20 . Also, distance and image data I 2  may be, for example, an image that shows shading of object  60   a  and data that contains coordinates (X, Y, Z) indicating a position on object  60  relative to the position of second image capturing device  20 . Distance and image data I 2  may include a monochrome image or a shading image of object  60  captured. 
     Second obtainer  112  outputs distance and image data I 2  to matcher  113  and position estimator  114 . Note that second image data contains a pixel value (R, G, B) indicating a color, and distance data contains coordinates (X, Y, Z) indicating a position on object  60   a  relative to the position of second image capturing device  20 . Further, step S 12  is an example of the obtaining of the second distance data and the second image data. 
     Next, matcher  113  associates positions on object  60  in first image data I 1  and second image data in distance and image data I 2  (S 13 ). If an explanation is given using first image data I 1  and distance and image data I 2  illustrated in  FIG. 4 , for example, five feature point matching candidates (shown by circles in (a) and (b) of  FIG. 4 ) are extracted, and positions in first image data I 1  and distance and image data I 2  are associated with one another. Matcher  113  associates a pixel position (x, y) in first image data I 1  with coordinates (X, Y, Z) in distance and image data I 2 . Matcher  113  locates, in distance and image data I 2 , a position of a portion of object  60  in pixel position (x, y) in first image data I 1 , to associate the positions, for example. 
     For example, matcher  113  associates a pixel position (x, y) in first image data I 1  with coordinates (X, Y, Z) in distance data corresponding to the pixel position, based on association between the pixel position (x, y) in first image data I 1  and a pixel position (x 1 , y 1 ) in second image data. Matcher  113  relates the pixel position (x, y) in first image data I 1  to the distance data corresponding to the pixel position. 
     Matcher  113  determines five sets of feature points, for example, yet the number of sets of feature points is not limited thereto and appropriately determines the number of sets depending on, for instance, how correspondences are obtained. Matcher  113  may determine four sets of feature points or may determine six sets of feature points, for example. Note that the method of calculating correspondences and the number of feature points are examples, and are not limited to those stated above. In the following, the number of sets of feature points is stated as n. 
     Matcher  113  outputs, to position estimator  114 , correspondence information indicating correspondences of positions between first image data I 1  and second image data. Matcher  113  outputs correspondence information indicating five sets of feature points determined as above, for example. Note that step S 13  is an example of the associating. 
     Next, position estimator  114  estimates the position of first image capturing device  10  relative to the position of second image capturing device  20 , based on correspondence information, distance data, and one or more internal parameter of first image capturing device  10  (S 14 ). If position estimator  114  is informed of correspondences of n sets of feature points in first image data I 1  and distance and image data I 2 , position estimator  114  can estimate the position of first image capturing device  10  in a coordinate system of second image capturing device  20  by solving a perspective-n-point (PnP) problem. Note that the one or more internal parameters of first image capturing device  10  are known. 
     Position estimator  114  estimates a position and an orientation of first image capturing device  10  relative to those of second image capturing device  20  by calculating R (rotation) and T (position), using each set of feature points whose reprojection error is less than or equal to a reference value, for example. Position estimator  114  calculates R (rotation matrix) and T (translation vector) that minimize the reprojection error, for example. R (rotation matrix) is an external parameter indicating an orientation of first image capturing device  10 . T (translation vector) is an external parameter indicating a position of first image capturing device  10 . Note that it is sufficient if position estimator  114  estimates at least a position. 
     Matcher  113  and position estimator  114  may execute processing of estimating a position of first image capturing device  10  by robust estimation that decreases influence of an outlier in order to remove an incorrect correspondence of feature points, for example. Matcher  113  and position estimator  114  may estimate a position of first image capturing device  10  by random sample consensus (RANSAC) estimation, median estimation, or M-estimation, for instance, which are examples of robust estimation, for example. RANSAC estimation may be an estimation method based on a reprojection error, for example. A reprojection error is expressed by a square error of a second position (xi, yi) in second image data on which a first position (Xi, Yi, Zi) in distance data obtained using a function for converting one coordinate system into another coordinate system, and a third position (xi 0 , yi 0 ) in first image data which corresponds to the second position. An example of a function for converting one coordinate system into another coordinate system is a projectPoints function. 
     Matcher  113  randomly selects n sets of feature points, and calculates a position of first image capturing device  10 , for example. Matcher  113  calculates the number of sets of features points whose reprojection error is less than or equal to the reference value, out of the n sets. Matcher  113  repeats the above processing for each feature point in an image, and determines n sets of feature points that include the greatest number of sets of feature points whose reprojection error is less than or equal to the reference value. Position estimator  114  may estimate a position of first image capturing device  10 , using the n sets of feature points, for example. Note that n sets may be six sets, for example. Thus, position estimator  114  may estimate a position of first image capturing device  10  by solving the P6P problem. 
     In this manner, matcher  113  and position estimator  114  obtain correspondences of feature points of object  60  between first image data I 1  and the second image data, and estimate a position of first image capturing device  10  using the correspondences. Thus, displacement measurement device  30  according to Embodiment 1 can estimate a position of first image capturing device  10  even if positions and orientations in which first image capturing device  10  and second image capturing device  20  are disposed are different. 
     Position estimator  114  outputs position information indicating the estimated position of first image capturing device  10  to distance calculator  115 . Note that step S 14  is an example of the estimating. 
     Distance calculator  115  calculates a distance from first image capturing device  10  to object  60 , based on position information and distance data (S 15 ). Distance calculator  115  calculates a distance from first image capturing device  10  to object  60 , based on a position (coordinates) of first image capturing device  10  included in position information and coordinates (X, Y, Z) of object  60  based on distance data. It can be said that distance calculator  115  converts a distance up to object  60  measured by the distance measurer of second image capturing device  20  into a distance up to object  60  when viewed from first image capturing device  10 . Accordingly, a position on object  60  when viewed from first image capturing device  10  is obtained. For example, distances up to measurement points on object  60  when viewed from first image capturing device  10  are obtained. Data that includes a distance (a distance from first image capturing device  10  to object  60 ) calculated by distance calculator  115  is an example of first distance data. A distance calculated by distance calculator  115  is an example of a first distance. Distance calculator  115  outputs distance information indicating a distance between first image capturing device  10  and object  60  to conversion value calculator  116 . Note that step S 15  is an example of the calculating of the first distance data. 
     Conversion value calculator  116  calculates a conversion value for converting a pixel displacement amount into an actual displacement amount, based on the position of first image capturing device  10 , distance information, and the internal parameter(s) of first image capturing device  10  (S 16 ). Conversion value calculator  116  calculates a conversion value for each of measurement points of object  60 , for example. The conversion value for a measurement point includes information indicating an actual value corresponding to one pixel at the measurement point, for example. Step S 16  is an example of the calculating of the conversion parameter. Further, step S 16  may be an example of the obtaining of the conversion parameter. 
     Here, processing by conversion value calculator  116  is to be described with reference to  FIG. 5 .  FIG. 5  is for describing a method for converting a displacement into an actual size according to Embodiment 1. Note that  FIG. 5  illustrates an example in which object  60  is displaced in a direction parallel to an image capturing surface (a projection surface) of first image capturing device  10 . Optical center O illustrated in  FIG. 5  indicates a center of lens  11  of first image capturing device  10 , position P 1  (x, y) indicates a position on the image capturing surface corresponding to position M 1  (Y, Y, Z) at a measurement point on object  60  at a first time point, and displacement (Δx 1 , Δy 1 ) on an image indicates a difference between position P 1  and position P 2  on the image capturing surface corresponding to position M 2  at the measurement point at a second time point different from the first time point. A displacement (Δx 1 , Δy 1 ) is represented by the number of pixels on an image. 
     As illustrated in  FIG. 5 , triangle ΔOP 1 P 2  and triangle ΔOM 1 M 2  are in a relation of similarity, and conversion value calculator  116  calculates a conversion value for converting a pixel displacement amount into an actual displacement amount using the relation of similarity, for example. When a distance between an image center (cx, cy) on the image capturing surface of first image capturing device  10  and optical center O is focal distance f, distance L 1  between optical center O and position P 1  is calculated by: 
         L 1=√( f   2   +x   2   +y   2 )  (Expression 1)
 
     The first time point is an initial time point when object  60  is not displaced, for example. 
     Assuming that the actual displacement amount is a displacement (Dx 1 , Dy 1 ), from a similarity relation of triangle ΔOP 1 P 2  and triangle ΔOM 1 M 2 , the following expression holds for distance L 2  from optical center O and position M 1 : 
         L 2 :L 1= Dy 1 :Dy 1= Dx 1 :Δx 1  (Expression 2)
 
     If distance L 2 , that is, the distance from optical center O to position M 1  is known from (Expression 2), displacement (Dx 1 , Dy 1 ) that is the actual displacement amount can be calculated. Distance calculator  115  calculates distance L 2  for each measurement point, since distance L 2  may vary for each measurement point. 
     Conversion value calculator  116  calculates a conversion value, based on (Expression 2) above. A conversion value for calculating displacement Dx 1  is based on (Expression 2) and is L 2 /L 1 . A conversion value for calculating displacement Dy 1  is also based on (Expression 2) and is L 2 /L 1 . 
     Note that the image center (cx, cy) and focal distance f are obtained as internal parameters of first image capturing device  10 . Conversion value calculator  116  may calculate internal parameters of first image capturing device  10  using a chart image. 
     Conversion value calculator  116  outputs conversion information indicating the calculated conversion value to displacement amount calculator  130 . Displacement amount calculator  130  obtains the conversion information from conversion value calculator  116 . Displacement amount calculator  130  functions as a third obtainer that obtains a conversion parameter. 
     Next, displacement detector  120  calculates, from first image data I 1 , a pixel displacement amount by which object  60  is displaced in first image data I 1  (S 17 ). Displacement detector  120  calculates a pixel displacement amount from the displacement of object  60  projected on the image capturing surface of first image capturing device  10 , for example. Displacement detector  120  calculates, for each of measurement points on object  60 , the number of pixels on an image which corresponds to the displacement at the measurement point, for example. Displacement detector  120  outputs the calculated pixel displacement amount to displacement amount calculator  130 . 
     Displacement amount calculator  130  calculates an actual displacement amount, based on a pixel displacement amount and a conversion value (S 18 ). Displacement amount calculator  130  calculates, for each of the measurement points on object  60 , an actual displacement amount at the measurement point by computing a pixel displacement amount at and a conversion value for the measurement point. Accordingly, displacement amount calculator  130  functions as a converter that coverts a pixel displacement amount between at least two third image data items into an actual displacement amount, based on the conversion information. 
     Displacement amount calculator  130  outputs presentation information that includes the calculated actual displacement amount to output device  40  (S 19 ). Output device  40  displays presentation information obtained from displacement amount calculator  130  as an image. Note that step S 19  is an example of the converting of the pixel displacement amount. 
     Note that the operation stated in steps S 11  to S 16  illustrated in  FIG. 3  may be performed each time processing of calculating an actual displacement amount of object  60  is executed or may be performed each time at least one of a disposition position, an orientation, or camera parameters (including the internal parameter(s)) of at least one of first image capturing device  10  or second image capturing device  20  is changed. Conversion value calculator  116  may store calculated conversion values into a storage (not illustrated). When conversion value calculator  116  calculates an actual displacement amount of object  60 , conversion value calculator  116  may read a conversion value from the storage and output the read conversion value to displacement amount calculator  130 . Thus, conversion value calculator  116  may use a conversion value calculated in the past as a conversion value for calculating a current actual displacement amount. Accordingly, the amount of processing by conversion parameter calculator  110  can be decreased. 
     Steps S 11  to S 16  described above are processes executed in the conversion parameter calculation method for calculating a conversion parameter. Steps S 17  to S 19  are processes executed in the displacement amount calculation method for calculating an actual displacement amount. Note that the displacement amount calculation method may include processes of steps S 11  to S 16 . 
     As described above, displacement measurement system  1  according to Embodiment 1 includes first image capturing device  10  and second image capturing device  20  disposed in different positions, and displacement measurement device  30  that includes conversion parameter calculator  110 . Conversion parameter calculator  110  estimates a position of first image capturing device  10 , based on first image data I 1  obtained from first image capturing device  10  and distance and image data I 2  obtained from second image capturing device  20 . Conversion parameter calculator  110  estimates the position of first image capturing device  10 , and thus displacement measurement device  30  can accurately calculate the actual value of displacement even if first image capturing device  10  and second image capturing device  20  initially have different positions and orientations. 
     As described above, according to displacement measurement system  1 , less restrictions are imposed on the positions and orientations of first image capturing device  10  and second image capturing device  20 , and thus it is unnecessary to finely adjust the positions and orientations of first image capturing device  10  and second image capturing device  20 . Accordingly, time taken to dispose the image capturing devices can be shortened at the measurement site. As compared to the case where the actual displacement amount of object  60  is measured by the multi-viewpoint method, less image capturing devices are disposed, and thus can be readily disposed and collected at the measurement site. 
     As described above, conversion parameter calculator  110  calculates a conversion value for converting the pixel displacement amount into the actual displacement amount, using the estimated position of first image capturing device  10 , for example. Accordingly, conversion parameter calculator  110  can measure the actual displacement amount of object  60  even if a scale marker cannot be applied onto object  60  or the actual length of a specific portion of object  60  is unknown. 
     As described above, conversion parameter calculator  110  calculates a conversion value for each of measurement points on object  60 , for example. Accordingly, an operator can select a measurement point, without paying attention to differences in distance between first image capturing device  10  and the measurement points. Stated differently, the degree of freedom of selecting a measurement point is enhanced with conversion parameter calculator  110 . 
     Displacement measurement device  30  measures the actual displacement amount of a displacement of object  60 , using two or more first image data items captured by first image capturing device  10 . Accordingly, displacement measurement device  30  can measure the actual displacement amount even when the displacement of object  60  is based on vibrations having such a cycle that the vibrations cannot be measured using a laser displacement gauge, for instance. 
     Note that the present embodiment has described an example in which a physical quantity in the real space is the actual displacement amount, but the physical quantity is not limited thereto. The physical quantity in the real space is not limited in particular as long as the quantity can be obtained from image data, and may be a length or an area, for example. Further, the physical quantity in the real space may be a speed at which a displacement is made, for instance, based on the actual displacement amount between a plurality of frames. In this case, in the calculating of the conversion parameter (corresponding to S 16 ), a conversion parameter for converting the physical quantity in pixels at a measurement point on object  60  based on at least two third image data items captured by first image capturing device  10  at different times into a physical quantity in the real space is calculated using first distance data. Further, in this case, displacement measurement device  30  described in the present embodiment can also be considered as a physical quantity measurement device. 
     A conversion parameter calculation method for calculating a conversion parameter for calculating a physical quantity in such a real space is described below. For example, a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, an actual displacement amount that is an actual value of a physical quantity in a real space of object  60  may include: obtaining, from first image capturing device  10 , first image data I 1  obtained by first image capturing device  10  capturing an image of object  60  (corresponding to S 11 ); obtaining, from second image capturing device  20 , second distance data indicating a distance from second image capturing device  20  to object  60 , and second image data obtained by second image capturing device  20  capturing an image of object  60 , second image capturing device  20  being disposed in a position different from a position of first image capturing device  10  (corresponding to S 12 ); associating a position on object  60  in first image data I 1  with a position on object  60  in the second image data (corresponding to S 13 ); estimating the position of first image capturing device  10  relative to the position of second image capturing device  20 , based on the second distance data and a result of associating the positions (S 14 ); calculating distance data indicating a distance from first image capturing device  10  to object  60 , based on the position of first image capturing device  10  and the second distance data (S 15 ); and calculating a conversion value (an example of the conversion parameter), using the first distance data, the conversion value being a value for converting, into a physical quantity in a real space, a physical quantity in pixels at a measurement point on object  60  based on third image data captured by first image capturing device  10  (corresponding to S 16 ). 
     For example, conversion parameter calculation device  110  that calculates a conversion parameter for measuring, using images, an actual physical quantity that is an actual value of a physical quantity in a real space of object  60  may be acquired. For example, conversion parameter calculation device  110  may include: first obtainer  111  that obtains, from first image capturing device  10 , first image data obtained by first image capturing device  10  capturing an image of object  60 ; second obtainer  112  that obtains, from second image capturing device  20 , second distance data indicating a distance from second image capturing device  20  to object  60 , and second image data obtained by second image capturing device  20  capturing an image of object  60 , second image capturing device  20  being disposed in a position different from a position of first image capturing device  10 ; matcher  113  that associates a position on object  60  in the first image data with a position on object  60  in the second image data; position estimator  114  that estimates the position of first image capturing device  10  relative to the position of second image capturing device  20 , based on the second distance data and a result of associating the positions; 
     distance calculator  115  that calculates first distance data indicating a distance from first image capturing device  10  to object  60 , based on the position of first image capturing device and the second distance data; and conversion value calculator  116  that calculates the conversion parameter, using the first distance data, the conversion parameter being a parameter for converting, into the physical quantity in the real space, a physical quantity in pixels at a measurement point on object  60  based on third image data captured by first image capturing device  10 . 
     Variation of Embodiment 1 
     The following describes a conversion parameter calculation method and others according to this variation, with reference to  FIG. 6  to  FIG. 8 . Note that the following description focuses on differences from Embodiment 1, and the same signs are assigned to equivalent elements to those of Embodiment 1, so that a description thereof may be omitted or simplified. The configuration of the displacement measurement system according to this variation is similar to displacement measurement system  1  according to Embodiment 1, and a description thereof is omitted. 
     Displacement measurement system  1  according to this variation has a feature that even if the displacement direction of object  60  is different from the direction parallel to the image capturing surface (a projection surface) of first image capturing device  10 , the actual displacement amount can be accurately calculated.  FIG. 6  is a flowchart illustrating operation of displacement measurement system  1  according to this variation. 
     As illustrated in  FIG. 6 , conversion value calculator  116  obtains displacement direction information of object  60  (S 21 ). Conversion value calculator  116  may obtain displacement direction information via input device  50 , for example. When object  60  is a motor, for example, displacement direction information may indicate a displacement direction based on a driving direction of a driver of the motor. For example, when the displacement direction of object  60  can be predicted based on the design, displacement direction information may indicate the predicted displacement direction. Further, for example, when object  60  is a bridge, for instance, and receives stress from vehicles, for instance, the displacement direction information may indicate a direction in which the stress is applied (for example, a vertical direction). The displacement direction information may indicate a direction relative to the image capturing surface of first image capturing device  10 , for example. Further, the displacement direction indicated by displacement direction information is not limited to a single direction and may be two or more directions. Step S 21  is an example of the obtaining of the displacement direction information. 
     Note that the displacement direction information is not limited to the one obtained via input device  50 . Displacement direction information may be determined based on, for example, two or more distance and image data items I 2  (second image data, for example). In this case, second obtainer  112  obtains, from second image capturing device  20 , two or more distance and image data items I 2  captured at different times, and outputs obtained two or more distance and image data items I 2  to conversion value calculator  116  (not illustrated). Conversion value calculator  116  may obtain displacement direction information by identifying a displacement direction based on the position (X, Y, Z) of a predetermined measurement point included in each of two or more distance and image data items I 2 . Since two or more distance and image data items I 2  are used, when a displacement is a vibration having a short cycle, for instance, it is difficult to accurately measure the actual displacement amount, yet the displacement direction can be obtained. 
     The orientation of a surface that includes a displacement measurement point on object  60  may be obtained from distance and image data I 2 , and the displacement direction may be defined relative to the orientation of this surface (the direction normal or parallel to the surface, for instance). 
     Note that displacement direction information may be obtained based on first image data I 1 . QR codes (registered trademark) may be applied to object  60 , as illustrated in (a) of  FIG. 4 , for example. In addition, the QR codes (registered trademark) may store therein information regarding the displacement direction of object  60 . Conversion value calculator  116  may obtain the displacement direction of object  60 , based on a QR code (registered trademark) included in first image data I 1 . In this case, the displacement direction may be a direction relative to the surface of object  60  onto which the QR code (registered trademark) is applied, for example. 
     Note that as long as displacement direction information can be obtained based on first image data I 1 , the present embodiment is not limited to a QR code (registered trademark) being applied to object  60 . For example, an augmented reality (AR) marker may be applied to object  60 . Based on an AR marker included in first image data I 1 , for example, conversion value calculator  116  may obtain displacement direction information associated with the AR marker. The displacement direction information may be displayed, being superimposed on first image data I 1 , for example. 
     Conversion value calculator  116  calculates a conversion value for converting a pixel displacement amount into an actual displacement amount, based on the position of first image capturing device  10 , distance information, the internal parameter(s) of first image capturing device  10 , and displacement direction information (S 22 ). Stated differently, conversion value calculator  116  calculates a conversion value using displacement direction information, in addition to those used in Embodiment 1. Note that step S 22  is an example of the calculating of the conversion parameter. 
     Here, displacement conversion using displacement direction information is to be described with reference to  FIG. 7A  and  FIG. 7B .  FIG. 7A  is a first diagram for describing displacement conversion according to this variation, with the displacement direction taken into consideration. Further,  FIG. 7B  is a second diagram for describing displacement conversion according to this variation, with the displacement direction taken into consideration.  FIG. 7A  and  FIG. 7B  are bird&#39;s-eye views of object  60   b  from above. The shape of object  60   b  is a square, for convenience. 
     As illustrated in  FIG. 7A  and  FIG. 7B , when object  60   b  is viewed from above, if the displacement direction of object  60   b  is not parallel to the image capturing surface of first image capturing device  10 , only the displacement of object  60   b  in a direction parallel to the image capturing surface out of the displacement thereof is projected onto the image capturing surface. In  FIG. 7A , the displacement direction is a direction that connects the upper left and the lower right on the drawing. In this case, for example, if the displacement at a measurement point on object  60   b  is A 1 , the displacement at a measurement point on object  60   b  projected on the image capturing surface is A 2 . Displacement A 2  is a component in a direction parallel to the image capturing surface when displacement A 1  is made. 
     In  FIG. 7B , the displacement direction is a direction that connects the lower left and the upper right on the drawing. In this case, for example, if the displacement at a measurement point on object  60   b  is B 1 , the displacement at a measurement point on object  60   b  projected on the image capturing surface is B 2 . Displacement B 2  is a component in a direction parallel to the image capturing surface when displacement B 1  is made. For example, if displacement A 1  and displacement B 1  have directions orthogonal to each other and have the same displacement amount, the direction and the magnitude of displacements A 2  and B 2  on the image capturing surface are the same. Note that displacements A 1  and B 1  are actual displacement amounts, and are actual values, for example. In addition, displacements A 2  and B 2  are pixel displacement amounts, and are represented by the numbers of pixels, for example. 
     In this manner, displacements A 1  and B 1  whose actual displacement directions are different from each other may be detected as the displacements in the same direction on the image capturing surface. Thus, the displacement in a direction horizontal to the image capturing surface can be measured, yet the displacement in a direction perpendicular to the image capturing surface cannot be measured. 
     In view of this, in this variation, conversion value calculator  116  obtains displacement direction information of object  60  in step S 21 , and calculates in step S 22  a conversion value for converting the pixel displacement amount of object  60  in first image data I 1  into the actual displacement amount, using the displacement direction information, as described above. The conversion value here includes information indicating an actual value corresponding to one pixel and the displacement direction. The conversion value may include information indicating actual values (Xa, Yb, Zc) each corresponding to one pixel, for example. Actual value Xa indicates an actual value of a displacement in the X-axis direction in a three-dimensional orthogonal coordinate system in the real space when a shift is made by one pixel on the image capturing surface. Actual value Yb indicates an actual value of a displacement in the Y-axis direction in the three-dimensional orthogonal coordinate system in the real space when a shift is made by one pixel on the image capturing surface. Actual value Zc indicates an actual value of a displacement in the Z-axis direction in the three-dimensional orthogonal coordinate system in the real space when a shift is made by one pixel on the image capturing surface. Actual values (Xa, Yb, Zc) are examples of conversion parameters. Note that a method for calculating a conversion value is not limited to those described above. 
     Here, processing by conversion value calculator  116  is to be described with reference to  FIG. 8 .  FIG. 8  is for describing a method for converting a displacement into an actual size according to this variation. 
     As illustrated in  FIG. 8 , assuming that position P 1  on the image capturing surface corresponding to position M 1  (X, Y, Z) at a measurement point on object  60  at a first time point is (x, y), distance L 1  from optical center O to position P 1  can be calculated by (Expression 1). The first time point is an initial time point when object  60  is not displaced, for example. Further, assuming that a difference (displacement) between position P 1  and position P 3  on the image capturing surface corresponding to position M 3  at a measurement point at a second time point different from the first time point is (Δx 2 , Δy 2 ), a first conversion value can be calculated by (Expression 2). The first conversion value here is a value with which the actual displacement amount of a displacement in a direction parallel to the image capturing surface of first image capturing device  10  can be calculated. The first conversion value is a value with which out of a displacement (Dx 2 , Dy 2 , Dz 2 ) in the actual value, a displacement (Dx 2 , Dy 2 ) can be calculated from the displacement (Δx 2 , Δy 2 ) on an image, for example. 
     Conversion value calculator  116  converts the calculated first conversion value into a second conversion value that is three-dimensional information, based on the calculated first conversion value and displacement direction information. Conversion value calculator  116  calculates the second conversion value for converting a pixel displacement amount that is two-dimensional information indicating a displacement on the image capturing surface into an actual displacement amount that is three-dimensional information. Conversion value calculator  116  may calculate the second conversion value based on the first conversion value and an angle between the image capturing surface of first image capturing device  10  and the displacement direction. The second conversion value is a value with which the displacement (Dx 2 , Dy 2 , Dz 2 ) in the actual value can be calculated from the displacement (Δx 2 , Δy 2 ) on the image, for example. Note that the method used by conversion value calculator  116  to calculate the second conversion value is not limited to the above. 
     As described above, conversion parameter calculator  110  of displacement measurement system  1  according to this variation further calculates a conversion value for converting a pixel displacement amount into an actual displacement amount, using displacement direction information indicating a displacement direction of object  60 . 
     According to this, displacement measurement system  1  can calculate a conversion value using displacement direction information when object  60  is displaced in a direction crossing the image capturing surface of first image capturing device  10  in a top view of object  60 , and thus can accurately measure the actual value of the displacement. 
     Embodiment 2 
     2-1. Configuration of Displacement Measurement System 
     A configuration of displacement measurement system la according to Embodiment 2 is to be described with reference to  FIG. 9  to  FIG. 11 .  FIG. 9  illustrates a schematic configuration of displacement measurement system la according to Embodiment 2.  FIG. 10  is a block diagram illustrating a functional configuration of displacement measurement system la according to Embodiment 2. Displacement measurement device  30   a  according to Embodiment 2 differs from displacement measurement device  30  according to Embodiment 1 in that second obtainer  112  obtains, from second image capturing device  20 , second image data that includes first image capturing device  10  and distance data, and position estimator  114  estimates position information of first image capturing device  10  using only the second image data and the distance data. Note that the following description focuses on differences from Embodiment 1, and the same signs are assigned to equivalent elements to those of Embodiment 1, so that a description thereof may be omitted or simplified. 
     As illustrated in  FIG. 9 , displacement measurement system  1   a  according to Embodiment 2 is an information processing system that measures an actual value of a displacement of object  60 , using two image capturing devices disposed in different positions. First image capturing device  10  and second image capturing device  20  capture images of object  60  from different viewpoints. 
     As illustrated in  FIG. 9  and  FIG. 10 , displacement measurement system  1   a  includes first image capturing device  10 , second image capturing device  20 , displacement measurement device  30   a,  output device  40 , and input device  50 . 
     Second image capturing device  20  captures an image that includes first image capturing device  10  and object  60 . Second image capturing device  20  may capture an image that includes first image capturing device  10  in such a manner that the shape of first image capturing device  10  can be detected or if a marker is attached to the casing of first image capturing device  10 , may capture an image that includes the marker. Second image capturing device  20  captures an image that includes at least one surface out of a plurality of surfaces that define first image capturing device  10 , for example. 
     Second image capturing device  20  may capture an image in a position and/or an orientation in which first image capturing device  10  and object  60  do not overlap. Second image capturing device  20  may be disposed behind first image capturing device  10 , for example. 
     Displacement measurement device  30   a  differs from displacement measurement device  30  according to Embodiment 1 in that matcher  113  is not included. 
     First obtainer  111  obtains first image data indicating a first image that includes object  60  from first image capturing device  10 . First obtainer  111  outputs the obtained first image data to displacement detector  120 . 
     Second obtainer  112  obtains second image data indicating a second image that includes first image capturing device  10  and object  60  and distance data from second image capturing device  20 . 
     Position estimator  114  detects first image capturing device  10  from the distance data and/or the second image data. Since at least one of the appearance, the size, or the shape of first image capturing device  10  is known in advance, position estimator  114  may detect first image capturing device  10  by two-dimensional or three-dimensional pattern matching. Position estimator  114  may detect first image capturing device  10 , based on at least one of the casing, the lens of first image capturing device  10 , or a marker attached onto the casing of first image capturing device  10  as illustrated in  FIG. 11 , for instance. 
       FIG. 11  illustrates examples of marker  200  applied to first image capturing device  10  according to Embodiment 2. Part (a) of  FIG. 11  illustrates a state in which marker  200  is applied onto the top surface of first image capturing device  10 . Part (b) of  FIG. 11  illustrates a state in which marker  200  is applied onto the side surface of first image capturing device  10 . At least one marker is applied to first image capturing device  10 . 
     Position estimator  114  determines the three-dimensional position coordinates of the optical center, based on the detected position of first image capturing device  10 . The position of the optical center of first image capturing device  10  relative to the casing thereof, for instance, can be determined based on the design value with respect to a combination of the second camera and the lens thereof, or can be obtained in advance by optical calibration. Position estimator  114  outputs, to distance calculator  115 , position information that includes position coordinates of first image capturing device  10  when the obtained optical center is a reference (origin). Note that the rest of the configuration and the operation of displacement measurement system  1   a  are the same as those in Embodiment 1. Embodiment 2 has an advantage that position estimator  114  can estimate position information of first image capturing device  10  without being influenced by the position of a feature point or the number of feature points obtained from an image, since position estimator  114  estimates position information of first image capturing device  10  using only second image data and distance data. 
     2-2. Operation of Displacement Measurement System 
     Next, operation of displacement measurement system  1   a  is to be described with reference to  FIG. 12 .  FIG. 12  is a flowchart illustrating operation of displacement measurement system  1   a  according to Embodiment 2. Specifically,  FIG. 12  is a flowchart illustrating operation of displacement measurement device  30   a.    
     As illustrated in  FIG. 12 , first obtainer  111  obtains first image data indicating a first image from first image capturing device  10  (S 11 ). First obtainer  111  obtains first image data I 1  as illustrated in (a) of  FIG. 4 , for example. Step S 11  is an example of the obtaining of the first image data. 
     Next, second obtainer  112  obtains second image data indicating a second image and distance data indicating a distance from second image capturing device  20  (S 31 ). Second obtainer  112  obtains second image data that includes both object  60  and first image capturing device  10 , and distance data that indicates a distance, from second image capturing device  20 . Second obtainer  112  outputs the second image data and the distance data obtained to position estimator  114 . Step S 31  is an example of the obtaining of the second distance data. 
     Position estimator  114  detects first image capturing device  10  from the distance data and/or the second image data. Position estimator  114  detects first image capturing device  10  in the second image data, from the distance data and/or the second image data, for example (S 32 ). Position estimator  114  detects the shape of first image capturing device  10  in the second image data by the above method, for example. 
     Position estimator  114  estimates the three-dimensional position coordinates of the optical center, based on the detected position of first image capturing device  10 . It can also be said that position estimator  114  estimates the position of first image capturing device  10  relative to the position of second image capturing device  20 , based on the detected position of first image capturing device  10  (S 33 ). In other words, position estimator  114  estimates the position of first image capturing device  10 , without using correspondence information of two images. Position estimator  114  outputs, to distance calculator  115 , position information that includes position coordinates of first image capturing device  10  when the obtained optical center is a reference (origin). Note that the position information includes distance data obtained by second obtainer  112 . 
     The processing of step S 15  and thereafter is the same as that in Embodiment 1, and thus a description thereof is omitted. 
     In this manner, according to displacement measurement system  1   a , position estimator  114  estimates position information of first image capturing device  10  using second image data that includes first image capturing device  10  and distance data, and thus can estimate position information of first image capturing device  10  without being influenced by the position of a feature point or the number of feature points in two images. 
     Note that the present embodiment has described an example in which a physical quantity in the real space is the actual displacement amount, but the physical quantity is not limited thereto. The physical quantity in the real space is not limited in particular as long as the quantity can be obtained from image data, and may be a length or an area, for example. Further, the physical quantity in the real space may be the speed at which a displacement is made, for instance, based on the actual displacement amount between a plurality of frames. In this case, in the calculating of the conversion parameter (corresponding to S 16 ), a conversion parameter for converting the physical quantity in pixels at a measurement point on object  60 , based on at least two third image data items captured by first image capturing device  10  at different times into a physical quantity in the real space is calculated using first distance data. Further, in this case, displacement measurement device  30   a  described in the present embodiment can also be considered as a physical quantity measurement device. 
     For example, a conversion parameter calculation method for calculating a conversion parameter for measuring, using images, a physical quantity in a real space of an object may include: obtaining, from first image capturing device  10 , first image data I 1  obtained by first image capturing device  10  capturing an image of object  60  (corresponding to S 11 ); obtaining, from second image capturing device  20 , second distance data indicating a distance from second image capturing device  20  to the object, and second image data obtained by second image capturing device  20  capturing an image that includes object  60  and first image capturing device  10 , second image capturing device  20  being disposed in a position different from a position of first image capturing device  10  (corresponding to S 31 ); detecting a shape of first image capturing device  10  in the second image data (corresponding to S 32 ); estimating the position of first image capturing device  10 , based on a result of detecting the shape (corresponding to S 33 ); calculating first distance data indicating a distance from first image capturing device  10  to object  60 , based on the position of first image capturing device  10  and the second distance data (corresponding to S 15 ); and calculating a conversion value (an example of the conversion parameter), using the first distance data, the conversion value being a value for converting, into an actual physical quantity in the real space, a pixel physical quantity at a measurement point on object  60  based on third image data captured by first image capturing device  10  (corresponding to S 16 ). 
     For example, conversion parameter calculation device  110  that calculates a conversion parameter for measuring, using images, a physical quantity in a real space of an object may be acquired. For example, conversion parameter calculation device  110  may include: first obtainer  111  that obtains, from first image capturing device  10 , first image data obtained by first image capturing device  10  capturing an image of object  60 ; second obtainer  112  that obtains, from second image capturing device  20 , second distance data indicating a distance from second image capturing device  20  to object  60 , and second image data obtained by second image capturing device  20  capturing an image that includes object  60  and first image capturing device  10 , second image capturing device  20  being disposed in a position different from a position of first image capturing device  10 ; position estimator  114  that detects a shape of first image capturing device  10  in the second image data, and estimates the position of first image capturing device  10 , based on a result of detecting the shape; distance calculator  115  that calculates first distance data indicating a distance from first image capturing device  10  to object  60 , based on the position of first image capturing device  10  and the second distance data; and conversion value calculator  116  that calculates the conversion parameter, using the first distance data, the conversion parameter being a parameter for converting, into the actual physical quantity in the real space, a pixel physical quantity at a measurement point on object  60  based on third image data captured by first image capturing device  10 . 
     Other Embodiments 
     The above has described the conversion parameter calculation method and others according to one or more aspects of the present disclosure, based on Embodiment 1, Embodiment 2, and the variation (hereinafter, also referred to as the embodiments and others), yet the present disclosure is not limited to the embodiments and others. The scope of the one or more aspects of the present disclosure may also encompass embodiments resulting from applying various modifications that those skilled in the art could think of to the embodiments and others, and embodiments resulting from combining elements in different embodiments, as long as the resultant embodiments do not depart from the scope of the present disclosure. 
     For example, the embodiments and others have described examples in which the first image capturing device and the second image capturing device are image capturing devices fixed on the ground, yet the image capturing devices are not limited thereto. At least one of the first image capturing device or the second image capturing device may be provided in an aircraft such as a drone, for example. 
     The embodiments and others have described examples in which the first obtainer and the second obtainer are communicators, but the obtainers are not limited thereto. At least one of the first obtainer or the second obtainer may be, for example, a connector to which a recording medium is connected. For example, a connector may be a universal serial bus (USB) port to which a USB terminal is connected, a security digital (SD) slot into which an SD card is inserted, or an optical drive into which an optical disc is inserted, for instance. 
     The order in which the steps in the flowcharts are executed is an example used to specifically explain the present disclosure, and thus may be a different order from the above. In addition, some of the steps may be executed simultaneously (in parallel) with other steps. 
     Split of functional blocks in the block diagrams is an example, and thus a plurality of functional blocks may be acquired as a single functional block, a single functional block may be split into a plurality of blocks, or some functions may be transferred to another functional block. Single hardware or software may process similar functions of a plurality of functional blocks, in parallel or by time division. 
     The conversion parameter calculation device according to the embodiments and others may be acquired as a single device or by a plurality of devices connected to one another. If a conversion parameter calculation device is acquired by a plurality of devices, elements included in the conversion parameter calculation device may be divided into the plurality of devices in any manner. 
     The embodiments and others have described examples in which the displacement measurement device does not include the output device, or in other words, the displacement measurement device and the output device are separately provided, yet the displacement measurement device may include the output device. In this case, the output device functions as an outputter (for example, a display) that is a part of the displacement measurement device. 
     Further, the method for communication between devices included in the displacement measurement systems according to the embodiments and others are not particularly limited. The devices may communicate with one another in a wireless or wired manner. The devices may communicate with one another in both wireless and wired manners. 
     Further, some or all of the elements included in the calculation devices according to the embodiments and others may be configured by a single system large scale integration (LSI). 
     The system LSI is a super multi-function LSI that is manufactured by integrating a plurality of processors in one chip, and is specifically a computer system configured so as to include a microprocessor, a read only memory (ROM), and a random access memory (RAM), for instance. A computer program is stored in the RAM. The system LSI accomplishes its functions through the operation of the microprocessor in accordance with the computer program. 
     Noted that although a system LSI is mentioned here, the integrated circuit can also be called an IC, an LSI, a super LSI, or an ultra LSI, depending on the difference in the degree of integration. Furthermore, the method of circuit integration is not limited to LSIs, and implementation through a dedicated circuit or a general-purpose processor is also possible. A field programmable gate array (FPGA) that allows programming after LSI manufacturing or a reconfigurable processor that allows reconfiguration of the connections and settings of the circuit cells inside the LSI may also be used. 
     In addition, depending on the emergence of circuit integration technology that replaces LSI due to progress in semiconductor technology or other derivative technology, such technology of course may be used to integrate the functional blocks. Possibilities in this regard include the application of biotechnology, for instance. 
     All or some of the various processes described above may be performed by hardware such as an electronic circuit or software. Note that the processing by the software is implemented by the processor included in the displacement measurement device executing a program stored in the memory. 
     An aspect of the present disclosure may be a computer program that causes a computer to execute distinctive steps included in the conversion parameter calculation method. Further, an aspect of the present disclosure may be a non-transitory computer-readable storage medium that stores therein such a program. For example, such a program may be stored in a recording medium, and the recording medium may be distributed. For example, by installing the distributed program in a device that includes another processor and causing the processor to execute the program, the device can be caused to perform the above processing. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is widely applicable to a device, for instance, that measures an actual value of a displacement in measuring a displacement of an object using images.