Patent Publication Number: US-2021191146-A1

Title: Information processing apparatus and non-transitory computer readable medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2019-229721 filed Dec. 19, 2019. 
     BACKGROUND 
     (i) Technical Field 
     The present disclosure relates to an information processing apparatus, and a non-transitory computer readable medium. 
     (ii) Related Art 
     To date, various techniques have been proposed to form images in the air. Some of these techniques are now being used in fields such as advertising and gaming (see, for example, Japanese Unexamined Patent Application Publication No. 2007-044241). 
     To be effective, advertisements need to be observed by people. The same is true for advertisement images formed in the air. 
     SUMMARY 
     Aspects of non-limiting embodiments of the present disclosure relate to enabling an image in the air to be represented in diverse ways compared with when the image continues to be represented in a fixed way. 
     Aspects of certain non-limiting embodiments of the present disclosure address the above advantages and/or other advantages not described above. However, aspects of the non-limiting embodiments are not required to address the advantages described above, and aspects of the non-limiting embodiments of the present disclosure may not address advantages described above. 
     According to an aspect of the present disclosure, there is provided an information processing apparatus including a processor configured to, in accordance with the state of a person in the vicinity of an image, instruct that a change be made to the image, the image being an image to be formed in the air, the change being one of a change in the dimensionality of the image and a change in the size of the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates an exemplary configuration of an information processing system according to exemplary embodiments; 
         FIG. 2  is a flowchart illustrating an exemplary process that switches the shapes of aerial images in accordance with Exemplary Embodiment 1; 
         FIGS. 3A and 3B  each illustrate an example of a predetermined area used for a determination at step  2 , of which  FIG. 3A  illustrates a case in which a person is determined to be not located within the predetermined area, and  FIG. 3B  illustrates a case in which a person is determined to be located within the predetermined area; 
         FIGS. 4A and 4B  each illustrate another example of a predetermined area used for the determination at step  2 , of which  FIG. 4A  illustrates a case in which a person is determined to be not located within the predetermined area, and  FIG. 4B  illustrates a case in which a person is determined to be located within the predetermined area; 
         FIGS. 5A to 5C  each illustrate how the shapes of aerial images formed in the air are switched, of which  FIG. 5A  illustrates a case in which no person is located within an area captured by a camera,  FIG. 5B  illustrates a case in which a person is located outside the area used for the determination at step  2 , and  FIG. 5C  illustrates a case in which a person is located inside the area used for the determination at step  2 ; 
         FIGS. 6A to 6C  each illustrate another example of how the shapes of aerial images formed in the air are switched, of which  FIG. 6A  illustrates a case in which no person is located within an area captured by a camera,  FIG. 6B  illustrates a case in which a person is located outside the area used for the determination at step  2 , and  FIG. 6C  illustrates a case in which a person is located inside the area used for the determination at step  2 ; 
         FIG. 7  is a flowchart illustrating an exemplary process that switches the shapes of aerial images in accordance with Exemplary Embodiment 2; 
         FIGS. 8A to 8C  each illustrate how the shapes of aerial images formed in the air are switched, of which  FIG. 8A  illustrates a case in which a person□s gaze is directed not toward an aerial image,  FIG. 8B  illustrates a case in which a person□s gaze is directed toward an aerial image, and  FIG. 8C  illustrates a case in which a person□s gaze is directed toward an aerial image; 
         FIGS. 9A to 9C  each illustrate another example of how the shapes of aerial images formed in the air are switched, of which  FIG. 9A  illustrates a case in which a person□s gaze is directed not toward an aerial image,  FIG. 9B  illustrates a case in which a person□s gaze is directed toward an aerial image, and  FIG. 9C  illustrates a case in which a person□s gaze is directed toward an aerial image; 
         FIGS. 10A to 10C  each illustrate an example in which an aerial image formed in three dimensions is enlarged in volume with the passage of time, of which  FIG. 10A  illustrates an example in which the aerial image is enlarged in volume in one direction,  FIG. 10B  illustrates an example in which the aerial image is enlarged in volume in two directions, and  FIG. 10C  illustrates an example in which the aerial image is enlarged in volume in three directions; 
         FIGS. 11A to 11C  each illustrate a case in which, at the beginning of enlargement in volume, an aerial image is located at the same position as Position P 1  where the aerial image has been previously formed in two dimensions, of which  FIG. 11A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 11B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 11C  illustrates the position and shape of the aerial image at time T 3 ; 
         FIGS. 12A to 12C  each illustrate a case in which, at the beginning of enlargement in volume, an aerial image is located at a position different from Position P 1  where the aerial image has been previously formed in two dimensions, of which  FIG. 12A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 12B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 12C  illustrates the position and shape of the aerial image at time T 3 ; 
         FIGS. 13A to 13D  each illustrate an example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed, of which  FIG. 13A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 13B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 13C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 13D  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 ; 
         FIGS. 14A to 14E  each illustrate another example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed, of which  FIG. 14A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 14B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 14C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 ,  FIG. 14D  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 14E  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 ; 
         FIG. 15  illustrates an example in which an aerial image formed in three dimensions is enlarged in volume with the center of the aerial image fixed in position; 
         FIGS. 16A to 16C  each illustrate an example in which an object constituting an aerial image formed in three dimensions is changed as the aerial image is enlarged in volume, of which  FIG. 16A  illustrates an exemplary object constituting the aerial image with the smallest volume,  FIG. 16B  illustrates an exemplary object constituting the aerial image with the second largest volume, and  FIG. 16C  illustrates an exemplary object constituting the aerial image with the largest volume; 
         FIGS. 17A to 17C  each illustrate an example in which an aerial image formed in three dimensions is reduced in volume with the passage of time, of which  FIG. 17A  illustrates an example in which the aerial image is reduced in volume in one direction,  FIG. 17B  illustrates an example in which the aerial image is reduced in volume in two directions, and  FIG. 17C  illustrates an example in which the aerial image is reduced in volume in three directions; 
         FIGS. 18A to 18C  each illustrate a case in which, at the beginning of reduction in volume, an aerial image is located at the same position as Position P 1  where the aerial image has been previously formed in two dimensions, of which  FIG. 18A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 18B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 18C  illustrates the position and shape of the aerial image at time T 3 ; 
         FIGS. 19Ato 19C  each illustrate a case in which, at the beginning of reduction in volume, an aerial image is located at a position different from Position P 1  where the aerial image has been previously formed in two dimensions, of which  FIG. 19A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 19B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 19C  illustrates the position and shape of the aerial image at time T 3 ; 
         FIGS. 20A to 20D  each illustrate an example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed, of which  FIG. 20A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 20B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 20C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 20D  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 ; 
         FIGS. 21A to 21E  each illustrate another example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed, of which  FIG. 21A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 21B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 21C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 ,  FIG. 21D  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 21E  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 ; 
         FIG. 22  illustrates an example in which an aerial image formed in three dimensions is reduced in volume with the center of the aerial image fixed in position; 
         FIGS. 23A to 23C  each illustrate an example in which an object constituting an aerial image formed in three dimensions is changed as the aerial image is reduced in volume, of which  FIG. 23A  illustrates an exemplary object constituting the aerial image with the largest volume,  FIG. 23B  illustrates an exemplary object constituting the aerial image with the second largest volume, and  FIG. 23C  illustrates an exemplary object constituting the aerial image with the smallest volume; and 
         FIG. 24  is a flowchart illustrating an exemplary process that switches the contents of aerial images in accordance with Exemplary Embodiment 4. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure will be described below with reference to the drawings. 
     Exemplary Embodiment 1 
     System Configuration 
       FIG. 1  illustrates an exemplary configuration of an information processing system  1  according to exemplary embodiments. 
     The information processing system  1  illustrated in  FIG. 1  includes an aerial-image forming apparatus  10 , a controller  20 , and a camera  30 . The aerial-image forming apparatus  10  forms an image (to be also referred to as “aerial image” hereinafter) such that the image floats up in the air. The controller  20  controls the aerial-image forming apparatus  10  or other components. The camera  30  captures an image of the area in the vicinity of an aerial image. 
     The term “person” as used in the exemplary embodiments refers to both a person observing an aerial image and a person not observing an aerial image. 
     In the exemplary embodiments, an aerial image is used to present an advertisement or other information. In the exemplary embodiments, advertisements refer to all items of content including information used to draw attention to, for example, a product or a service. In other words, an advertisement may be a portion of an aerial image. In the following description of the exemplary embodiments, an aerial image used for advertising will be also referred to as an advertisement image. It is to be noted, however, that an advertisement is an example of information represented by an aerial image. Information represented by an aerial image may not necessarily be an advertisement. 
     An aerial image may be of any shape, and may have a solid or planar shape. Examples of solid shapes include a sphere, a polyhedron, a cylinder or other such curved solid, and the shape of an object such as a person, an animal, an electrical appliance, or a fruit. 
     Examples of planar shapes include a circle, a polygon, and the shape of an object such as a person, an animal, an electrical appliance, or a fruit. The terms “person” and “animal” as used herein may include imaginary characters or creatures. 
     An aerial image formed in the air may not necessarily be an image defining the surfaces of a solid but may be made up of an image defining the surfaces of a solid and an image corresponding to its interior. In other words, for example, an aerial image may be represented by voxel data or other such data with image attributes given not only to its three-dimensional surfaces but also to its internal structure. 
     The aerial image according to the exemplary embodiments may be a static image or a moving image. 
     The aerial-image forming apparatus  10  according to the exemplary embodiments is an apparatus that directly forms an aerial image in the air. Various methods for forming an aerial image have already been proposed, and some of these methods have been put into practical use. 
     Examples of methods for forming an aerial image include use of a half mirror, use of a beam splitter, use of a minute mirror array, use of a minute lens array, use of a parallax barrier, use of plasma emission, and use of a hologram. An aerial image generated by such a method allows a person to pass through the aerial image. 
     An example of the aerial-image forming apparatus  10  that generates an aerial image that does not allow a person to pass therethrough is a projector that projects an image onto a screen existing in the real space. Other examples of the aerial-image forming apparatus  10  include an apparatus that moves an array of light-emitting elements in the real space at high speed, and uses the resulting persistence of vision effect to allow a person to see an aerial image. 
     The aerial-image forming apparatus  10  according to the exemplary embodiments is capable of forming both a planar aerial image and a solid aerial image in the air. A planar aerial image is an example of an aerial image formed in two dimensions, and a solid aerial image is an example of an aerial image formed in three dimensions. In this regard, cases in which an aerial image is formed in two dimensions also include when an image is formed in only one face of an aerial image formed in three dimensions. In this sense, an aerial image formed in two dimensions also represents one form of an aerial image formed in three dimensions. 
     It is to be noted, however, that in the exemplary embodiments, a planar shape may be any shape recognized by a person as a plane, and does not need to be a plane in the mathematical sense. Likewise, in the exemplary embodiments, a solid shape may be any shape recognized by a person as a solid, and does not need to be a solid in the mathematical sense. For example, a curved surface obtained by bending a rectangular flat plate may be regarded as a plane, or may be regarded as a solid. 
     The controller  20  includes a processor  21 , a memory  22 , a network interface (IF)  23 , and a signal line  24 . The processor  21  executes a program to thereby control generation of aerial image data. The memory  22  stores information such as a program or various data. The network IF  23  is used to achieve communication with the external environment. The signal line  24  is a bus or other such signal line that connects the above-mentioned components of the controller  20 . The controller  20  is an example of an information processing apparatus. 
     The processor  21  is, for example, a CPU. The memory  22  includes, for example, a read only memory (ROM) in which a basic input output system (BIOS) or other information is stored, a random access memory (RAM) used as a work area, and a hard disk device in which a basic program, an application program, or other information is stored. 
     It is to be noted, however, that the above description does not preclude inclusion of the ROM or RAM in a portion of the processor  21 . The processor  21  and the memory  22  constitute a computer. 
     The camera  30  is used to capture an image of a person present in the vicinity of an aerial image. In the exemplary embodiments, the camera  30  is used to acquire, for example, information related to the respective positions of an aerial image and a person, information related to the direction of a person□s gaze, or information related to a person□s facial expression. Such information represents an example of the state of a person in the vicinity of an aerial image. The vicinity of an aerial image may include the aerial image. 
     Examples of information related to the respective positions of an aerial image and a person in this case include an approximate distance between the aerial image and the person, and the direction of the person□s movement. 
     Exemplary methods for measuring distance include, in addition to measuring distance by use of a pair of images captured by a stereo camera, measuring distance by use of a single color image captured by a monocular camera. Techniques for measuring distance by use of a monocular camera have already been put into practical use. If a monocular camera is used, distance is measured from information about blurring that appears in a color image captured through a color filter divided into two colors. 
     For distance measurement, it is also possible to use, for example, a distance-measuring image sensor, or a laser distance meter that measures the time it takes for a radiated beam of light to return after reflecting off a person. 
     The estimation or calculation of information such as the positional relationship between an aerial image and a person, the direction of a person□s gaze, or a person□s facial expression is executed by the processor  21 . Switching of Aerial Image Shapes 
       FIG. 2  is a flowchart illustrating an exemplary process that switches the shapes of aerial images in accordance with Exemplary Embodiment 1. The process illustrated in  FIG. 2  is implemented through execution of a program by the processor  21  (see  FIG. 1 ). 
     First, the processor  21  determines whether the current time is within the period during which an aerial image is to be formed (to be referred to as formation period hereinafter) (step  1 ). If a negative determination is made at step  1 , the processor  21  repeats the determination of step  1 . If an aerial image formed in the air exists at the time when a negative determination is made at step  1 , the processor  21  causes the formation of the aerial image to end. 
     A negative determination is made at step  1  if, for example, the time at which the determination is made does not fall within a period set by a contract as a period during which to place an advertisement. 
     If an affirmative determination is made at step  1 , the processor  21  determines whether a person is located within an area previously set with reference to the position of the aerial image (step  2 ). In Exemplary Embodiment 1, the position where a person is located is determined by processing an image captured by the camera  30  (see  FIG. 1 ). Of course, the person□s position is determined through execution of a program by the processor  21 . 
     If an affirmative determination is made at step  2 , the processor  21  instructs that a three-dimensional aerial image be formed (step  3 ). Specifically, the processor  21  forms advertisement data in a three-dimensional shape, and outputs the formed advertisement data to the aerial-image forming apparatus  10  (see  FIG. 1 ). 
     If a negative determination is made at step  2 , the processor  21  instructs that a two-dimensional aerial image be formed (step  4 ). Specifically, the processor  21  forms advertisement data in a two-dimensional shape, and outputs the formed advertisement data to the aerial-image forming apparatus  10 . 
     Although the foregoing description of  FIG. 2  assumes, as a precondition for the determination at step  2 , that the position of a person is determined, it may be determined instead whether some mobile object is located within a predetermined area. This configuration makes it possible to omit the process of determining whether a person appears in an image captured by the camera  30 . Furthermore, this configuration allows an aerial image to be switched to a three-dimensional image also when a baby stroller, a shopping cart, a pet, or other such object is located within a predetermined area. 
     The process illustrated in  FIG. 2  is repeatedly executed by the processor  21 . 
       FIGS. 3A and 3B  each illustrate an example of a predetermined area used for the determination at step  2 .  FIG. 3A  illustrates a case in which a person is determined to be not located within the predetermined area, and  FIG. 3B  illustrates a case in which a person is determined to be located within the predetermined area. 
     The aerial image in  FIGS. 3A and 3B  is formed in the plane defined by the X- and Z-axes. Thus,  FIGS. 3A and 3B  depict a two-dimensional aerial image. Specifically, the aerial image is assumed to be a rectangular aerial image. 
     In Exemplary Embodiment 1, the area used for the determination at step  2  is set with reference to the position where the two-dimensional aerial image is formed. In Exemplary Embodiment 1, the basic shape of an aerial image is assumed to be two-dimensional. If the position where the two-dimensional aerial image is formed moves with time or other factors, the reference position used in setting the predetermined area is also changed. 
     In the area illustrated in  FIGS. 3A and 3B , the aerial image is represented as being seen from above. The area illustrated in  FIGS. 3A and 3B  has a shape obtained by cutting an ellipse in half along the minor axis, with the major axis being located forward of the aerial image. In other words, the minor axis is located within the plane in which the aerial image is formed. 
     In  FIGS. 3A and 3B , L 0  represents the length of the major axis. It is to be noted, however, that the area used for the determination at step  2 , or a marking or other indication of the outer edges of the area is not necessarily provided in the actual space. Therefore, the determination at step  2  is not an exact determination in a strict sense. Even if the determination is not an exact in a strict sense, this does not pose any practical problem. The area used for the determination at step  2  is, for example, set in advance for an image captured from where the camera  30  (see  FIG. 1 ) is installed. 
     In any case, if a person is located outside the area represented by the alternate long and short dash line in each of  FIGS. 3A and 3B , a negative determination is made at step  2 . Conversely, if a person is located inside the area represented by the alternate long and short dash line in each of  FIGS. 3A and 3B , an affirmative determination is made at step  2 . 
     The foregoing description of  FIGS. 3A and 3B  is directed to an example in which as seen from above, the area mentioned above has a shape obtained by cutting an ellipse in half. However, the above-mentioned area may be of any shape. For example, the above-mentioned area may have a sectoral shape, a rectangular shape, or a semi-circular shape. 
     In  FIGS. 3A and 3B , in setting the area used for the determination at step  2 , the side from which the aerial image is to be basically observed is defined as front side. Alternatively, the area used for the determination at step  2  may be set on both the front and back sides relative to the aerial image. This is because a portion of the aerial image is also observable from the back side, unlike when the aerial image is displayed by using a physical display. 
     The side from which the aerial image is to be basically observed refers to the side from which, if the aerial image contains letters, the letters can be correctly read. 
       FIGS. 4A and 4B  each illustrate another example of a predetermined area used for the determination at step  2 .  FIG. 4A  illustrates a case in which a person is determined to be not located within the predetermined area, and  FIG. 4B  illustrates a case in which a person is determined to be located within the predetermined area. In  FIGS. 4A and 4B , features corresponding to those in  FIGS. 3A and 3B  are denoted by the corresponding reference signs. 
     The area illustrated in  FIGS. 4A and 4B  has the shape of an ellipse covering the entire aerial image. 
     Although  FIGS. 3A and 3B  and  FIGS. 4A and 4B  each depict the shape of the above-mentioned area with the aerial image seen from above, the area used for the determination at step  2  may be set also with respect to the Z-axis direction. That is, the area used for the determination at step  2  may not necessarily have a planar shape but may be set as a solid shape. 
       FIGS. 5A to 5C  each illustrate how the shapes of aerial images formed in the air are switched.  FIG. 5A  illustrates a case in which no person is located within an area captured by the camera  30  (see  FIG. 1 ),  FIG. 5B  illustrates a case in which a person is located outside the area used for the determination at step  2 , and  FIG. 5C  illustrates a case in which a person is located inside the area used for the determination at step  2 . 
     In  FIGS. 5A to 5C , P 1  represents the position where a two-dimensional aerial image is formed. In the case of  FIGS. 5A to 5C  as well, the area used for the determination mentioned above is only the area adjacent to one face of the aerial image, which is defined as the area on the observation side from which the aerial image is to be basically observed. 
     In the case of  FIG. 5A , although no one is present in the vicinity of Position P 1 , a two-dimensional aerial image is formed at Position P 1 . In Exemplary Embodiment 1, if it is currently during the formation period in which an aerial image is to be formed, the aerial image is formed irrespective of whether a person is present in the vicinity of Position P 1 . 
     In the case of  FIG. 5B , a person is located in the vicinity of Position P 1 . In this case, however, a distance Ll, which is the distance in the Y-axis direction between the person and the aerial image, is greater than a distance L 0  that defines the outer boundary of the area used for the determination at step  2  (see  FIG. 2 ). Thus, the aerial image formed in the area remains two-dimensional. 
     In the case of  FIG. 5C , a distance L 2 , which is the distance between a person and the position where the two-dimensional aerial image is formed, is less than the distance L 0  that defines the outer boundary of the area used for the determination at step  2 . Thus, the aerial image is switched to a three-dimensional image. 
     That is, in Exemplary Embodiment 1, if a person enters inside an area set in the vicinity of an aerial image that is being formed in two dimensions, the processor  21  (see  FIG. 1 ) switches the shape of the aerial image from two-dimensional to three-dimensional. This change in shape draws the attention of the person, thus increasing the likelihood of the aerial image being observed by the person. This means that the chance of the advertisement being observed by the person may be increased. 
       FIGS. 6A to 6C  each illustrate another example of how the shapes of aerial images formed in the air are switched.  FIG. 6A  illustrates a case in which no person is located within an area captured by the camera  30  (see  FIG. 1 ),  FIG. 6B  illustrates a case in which a person is located outside the area used for the determination at step  2 , and  FIG. 6C  illustrates a case in which a person is located inside the area used for the determination at step  2 . In  FIGS. 6A to 6C , features corresponding to those in  FIGS. 5A to 5C  are denoted by the corresponding reference signs. 
     In  FIGS. 6A to 6C , the image formed in three dimensions is located at a position different from the position in  FIGS. 5A to 5C . In  FIGS. 5A to 5C , the far side of the three-dimensional aerial image as seen from the person includes the plane in which the two-dimensional image is formed, or the far side of the three-dimensional aerial image is located near the plane. 
     However, in  FIGS. 6A to 6C , the three-dimensional aerial image is formed at a position closer to the person than the position illustrated in  FIGS. 5A to 5C  is. In other words, if a three-dimensional aerial image is to be formed in  FIGS. 6A to 6C , the three-dimensional aerial image appears immediately near the person. This is expected to increase the person□s interest in the aerial image. 
     Exemplary Embodiment 2 
     The following describes Exemplary Embodiment 2 that uses the information processing system  1  (see  FIG. 1 ) illustrated in  FIG. 1 . 
     Exemplary Embodiment 2 also uses the information processing system  1  illustrated in  FIG. 1 . It is to be noted, however, that in Exemplary Embodiment 2, the process used in switching the shapes of aerial images formed in the air differs from the process in Exemplary Embodiment 1. 
       FIG. 7  is a flowchart illustrating an exemplary process that switches the shapes of aerial images in accordance with Exemplary Embodiment 2. The process illustrated in  FIG. 7  is implemented through execution of a program by the processor  21  (see  FIG. 1 ). In  FIG. 7 , features corresponding to those in  FIG. 2  are denoted by the corresponding reference signs. 
     First, the processor  21  determines whether the current time is within the period during which an aerial image is to be formed (i.e., formation period) (step  1 ). This process is the same as that in Exemplary Embodiment 1. That is, the processor  21  repeats the determination of step  1  as long as the result of determination at step  1  is negative. 
     It is to be noted, however, that if an affirmative determination is made at step  1 , the processor  21  determines whether the gaze of a person captured by the camera  30  (see  FIG. 1 ) is directed toward the aerial image (step  11 ). In Exemplary Embodiment 2, the processor  21  determines the direction of gaze of a person recognized from an image captured by the camera  30 . 
     In this regard, the memory  22  (see  FIG. 1 ) according to Exemplary Embodiment 2 stores the following pieces of information: the position where the camera  30  is mounted; the direction in which the camera  30  captures an image; and Position P 1  where a two-dimensional aerial image is formed. 
     Based on the pieces of information mentioned above, the processor  21  determines whether the gaze of a person recognized within an image captured by the camera  30  is directed toward an aerial image formed as a two-dimensional image. 
     The determination of the direction of gaze by the processor  21  is not an exact determination in a strict sense. Even if the determination is not exact in a strict sense, this does not pose any practical problem. This is because the control according to Exemplary Embodiment 2 is performed for purposes such as making a person notice an aerial image or increasing the interest of a person in an aerial image. Indeed, even if a person□s gaze is not strictly directed toward where an aerial image is formed, there is a possibility that, as the aerial image transforms from being two-dimensional to three-dimensional, the person may notice the aerial image. Further, there is no inconvenience involved for both a person who does not notice the aerial image and the provider of the advertisement. 
     If the person□s gaze is directed toward the aerial image, the processor  21  obtains an affirmative result at step  11 . In this case, the processor  21  instructs that a three-dimensional aerial image be formed (step  3 ). 
     If the person□s gaze is directed not toward the aerial image, the processor  21  obtains a negative result at step  11 . In this case, the processor  21  instructs that a two-dimensional aerial image be formed (step  4 ). 
       FIGS. 8A to 8C  each illustrate how the shapes of aerial images formed in the air are switched.  FIG. 8A  illustrates a case in which a person□s gaze is directed not toward an aerial image,  FIG. 8B  illustrates a case in which a person□s gaze is directed toward an aerial image, and  FIG. 8C  illustrates a case in which a person□s gaze is directed toward an aerial image. 
     In  FIGS. 8A to 8C  as well, P 1  represents the position where a two-dimensional aerial image is formed. 
     In the case of  FIG. 8A , the direction of gaze is opposite to the direction toward an aerial image. If a person is not looking at an aerial image as in this case, the aerial image is formed in two dimensions. 
     In contrast, in the case of  FIG. 8B , the direction of gaze is toward an aerial image. In this case, the processor  21  switches the aerial image to a three-dimensional shape as illustrated in  FIG. 8C . This change in shape draws the attention of the person, thus increasing the likelihood of the aerial image being observed by the person. This means increased chance of the advertisement being observed by the person. 
       FIGS. 9A to 9C  each illustrate another example of how the shapes of aerial images formed in the air are switched.  FIG. 9A  illustrates a case in which a person□s gaze is directed not toward an aerial image,  FIG. 9B  illustrates a case in which a person□s gaze is directed toward an aerial image, and  FIG. 9C  illustrates a case in which a person□s gaze is directed toward an aerial image. In  FIGS. 9A to 9C , features corresponding to those in  FIGS. 8A to 8C  are denoted by the corresponding reference signs. 
       FIGS. 9A to 9C  differ from  FIGS. 8A to 8C  in where a three-dimensional aerial image is formed. In the case of  FIGS. 8A to 8C , the far side of the three-dimensional aerial image as seen from the person includes the plane in which the two-dimensional image is formed, or the far side of the three-dimensional aerial image is located near the plane. 
     However, in  FIGS. 9A to 9C , the three-dimensional aerial image is formed at a position closer to the person than the position illustrated in  FIGS. 8A to 8C  is. In other words, if a three-dimensional aerial image is to be formed in  FIGS. 9A to 9C , the three-dimensional aerial image appears immediately near the person. This is expected to increase the person□s interest in the aerial image. 
     In Exemplary Embodiment 2, where a person is located has no relation with the area used for the determination at step  2  in Exemplary Embodiment 1. This means that even if a person is located outside the area used for the determination at step  2 , as long as the person□s gaze is directed toward an aerial image, the shape of the aerial image is switched from being two-dimensional to three-dimensional. 
     In Exemplary Embodiment 2, even if Position P 1  where a two-dimensional aerial image is formed, and a person are located far away from each other, the aerial image is switched to a three-dimensional image. This is expected to effectively make the person notice the aerial image. This may lead to an increased chance that the person who has noticed the aerial image approaches the aerial image to check what the aerial image is. 
     In this regard, however, if a person and an aerial image are far too away from each other, even when the person notices the presence of the aerial image, the person is unable to see what the aerial image is from where the person is present. In this case, therefore, the change in the shape of the aerial image may not lead to a greater advertising effect. 
     Accordingly, in Exemplary Embodiment 2, the distance between a person and an aerial image may be added as a condition for the determination at step  11  as in Exemplary Embodiment 1. 
     Specifically, in one exemplary control, even when the gaze of a person is directed toward an aerial image, if the distance between the person and the aerial image is greater than or equal to a predetermined distance, or if the person is not located inside the area used in the determination at step  2  (see  FIG. 2 ), a positive determination is not made at step  11 . 
     With the above-mentioned control, if it is possible for a person to view the content of an aerial image, and the person□s gaze is directed toward the aerial image, then the aerial image may be transformed from being two-dimensional to three-dimensional, thus increasing the person□s attention to the aerial image. 
     In this case, even when the person and the aerial image are close to each other, if the person□s gaze is directed not toward the aerial image, the two-dimensional shape of the aerial image may be maintained. This is because changing the shape of the aerial image from two-dimensional to three-dimensional does not change the person□s attention to or interest in the aerial image unless the person is looking in the direction of the aerial image. 
     Exemplary Embodiment 3 
     The following describes an exemplary process that can be used in forming an aerial image in three dimensions. 
     EXAMPLE 1 
       FIGS. 10A to 10C  each illustrate an example in which an aerial image formed in three dimensions is enlarged in volume with the passage of time.  FIG. 10A  illustrates an example in which the aerial image is enlarged in volume in one direction,  FIG. 10B  illustrates an example in which the aerial image is enlarged in volume in two directions, and  FIG. 10C  illustrates an example in which the aerial image is enlarged in volume in three directions. 
     Although the example in each of  FIGS. 10A to 10C  focuses only on the volume of the aerial image, it is also possible to change the content of the aerial image in conjunction with the passage of time or with the enlargement of the aerial image, such as by changing the aerial image in color or brightness. 
       FIGS. 10A to 10C  each illustrate, with time represented on the horizontal axis, how the shape of the aerial image at time T 1  changes at times T 2  and T 3 . In the case of  FIGS. 10A to 10C , the aerial image has a cuboid shape at time T 1 . 
     The example in  FIG. 10A  represents enlargement in the X-axis direction. In this case, as seen from a person, the three-dimensional aerial image appears to be enlarged in the horizontal direction with the passage of time. 
     The example in  FIG. 10B  represents enlargement by the same factor in each of the X- and Y-axis directions. It is to be noted, however, that the enlargement factor may differ between the X-axis direction and the Y-axis direction. The enlargement factor may not necessarily be fixed but may change with the passage of time. The Y-axis direction is the direction of depth as seen from a person. Thus, in the case of  FIG. 10B , the aerial image formed in three dimensions appears to increase in thickness in the depth direction with the passage of time. 
     The example in  FIG. 10C  represents enlargement by the same factor in each of the X-, Y-, and Z-axis directions. Thus, in the example in  FIG. 10C , the aerial image is enlarged while keeping its cuboid shape. It is to be noted, however, that the enlargement factor may differ along each axis. The enlargement factor may not necessarily be fixed but may change with the passage of time. In the case of  FIG. 10C , the aerial image formed in three dimensions appears to be enlarged in a uniform manner with the passage of time. 
     EXAMPLE 2 
     Example 1 above is directed to an example of how an aerial image changes in shape as the aerial image is enlarged in volume with the passage of time. 
     The following describes differences in the direction of enlargement of a three-dimensional aerial image that changes in volume with the passage of time. 
       FIGS. 11A to 11C  each illustrate a case in which, at the beginning of enlargement in volume, an aerial image is located at the same position as Position P 1  where the aerial image has been previously formed in two dimensions.  FIG. 11A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 11B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 11C  illustrates the position and shape of the aerial image at time T 3 . 
     The positioning of the aerial image at time T 1  in  FIGS. 11A to 11C  corresponds to that in  FIG. 5C  or  FIG. 8C . In the case of  FIGS. 11A to 11C , the aerial image formed in three dimensions is enlarged with the passage of time in a direction toward a person. Specifically, the aerial image is enlarged in the Y-axis direction. 
       FIGS. 12A to 12C  each illustrate a case in which, at the beginning of enlargement in volume, an aerial image is located at a position different from Position P 1  where the aerial image has been previously formed in two dimensions.  FIG. 12A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 12B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 12C  illustrates the position and shape of the aerial image at time T 3 . 
     The positioning of the aerial image at time T 1  in  FIGS. 12A to 12C  corresponds to that in  FIG. 6C  or  FIG. 9C . In the case of  FIGS. 12A to 12C , the aerial image formed in three dimensions is enlarged with the passage of time in a direction away from a person, until the aerial image eventually approaches Position P 1  where the aerial image has been previously formed in two dimensions. It is to be noted that in the case of  FIGS. 12A to 12C , the aerial image is not only enlarged in the Y-axis direction, but also moves in the Y-axis direction with the passage of time. Specifically, the aerial image is moved in a direction away from the person. In other words, the aerial image formed in three dimensions is moved in a direction toward Position P 1  where the aerial image is formed in two dimensions. 
     Even though the shape of an aerial image formed in three dimensions is the same between  FIGS. 11A to 11C  and  FIGS. 12A to 12C  at each point in time, the position where enlargement of the aerial image begins or the direction in which the aerial image is enlarged differs between  FIGS. 11A to 11C  and  FIGS. 12A to 12C . This may make it possible to give a different impression to the person observing the aerial image. 
     EXAMPLE 3 
     In the case of Example 2, an aerial image formed in three dimensions is enlarged in volume with the passage of time. The following describes an example in which enlargement of an aerial image in volume is controlled in relation to where a person is located. 
       FIGS. 13A to 13D  each illustrate an example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed.  FIG. 13A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 13B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 13C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 13D  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 . Position P 1  in this case is where the aerial image is formed in two dimensions. 
     In  FIGS. 13A to 13D  as well, the aerial image is transformed so as to be enlarged in volume. In this case, however, the distance between the person and Position P 1 , rather than time, is used in controlling the volume of the aerial image. 
     For example, in the case of  FIG. 13A  in which the distance between the person and Position P 1  is L 11 , the aerial image formed in three dimensions is positioned midway between Position P 1  and the person. This positioning corresponds to that in  FIG. 6C  or  FIG. 9C . 
     When, upon noticing the aerial image, the person moves closer to the aerial image, the volume of the aerial image is enlarged in accordance with the distance between the person and Position P 1 . In the case of  FIGS. 13A to 13D , when the distance is L 12 (&lt;L 11 ), the aerial image has a larger volume than when the distance is L 11 , and when the distance is L 13 (&lt;L 12 ), the aerial image has a larger volume than when the distance is L 12 . 
     As with the examples mentioned above, the change in the volume of the aerial image at this time may be enlargement in one direction or enlargement in two directions. The example in each of  FIGS. 13A to 13D  represents enlargement in three directions. The enlargement factor may be fixed, or may be varied in accordance with the distance between the person and Position P 1 . 
     When the person moves to a position where a distance L 14 (&lt;L 13 ) between the person and Position P 1  is less than a predetermined distance, the aerial image is controlled to have a two-dimensional shape as illustrated in  FIG. 13D . 
     By changing the volume of an aerial image as illustrated in  FIGS. 13A to 13D , a person interested in or intrigued by the aerial image may be directed toward a specific position. In  FIGS. 13A to 13D , the person is directed toward Position P 1  where the aerial image is formed in two dimensions, thus increasing the advertising effect. 
     The enlargement of the aerial image in volume may be stopped once the distance between the person and Position P 1  becomes less than a predetermined distance. 
       FIGS. 14A to 14E  each illustrate another example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed.  FIG. 14A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 14B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 14C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 ,  FIG. 14D  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 14E  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 . In  FIGS. 14A to 14E , features corresponding to those in  FIGS. 13A to 13D  are denoted by the corresponding reference signs. 
     In the case of  FIGS. 14A to 14E , enlargement of the aerial image is stopped on the condition that a distance Ll 3 A, which is the distance between the person and Position P 1 , has become less than a predetermined distance L 10 . Thus, the volume of the aerial image in  FIG. 14C  is the same as the volume of the aerial image in  FIG. 14D . 
     A larger volume of an aerial image does not necessarily increase a person□s interest or attention. Further, there is a limit on the physical size in which an aerial image can be formed. For these reasons, the control as illustrated in  FIGS. 14A to 14E  is also provided. 
     EXAMPLE 4 
     In the case of Examples 1 to 3 above, as an aerial image formed in three dimensions is enlarged, the center of the aerial image also moves. Alternatively, however, it is also possible to enlarge an aerial image with the center position of the aerial image fixed. 
       FIG. 15  illustrates an example in which an aerial image formed in three dimensions is enlarged in volume with the center of the aerial image fixed in position. 
     The aerial images illustrated in  FIG. 15  each have the shape of a cube.  FIG. 15  illustrates three aerial images with different volumes. In this regard, the smallest cube corresponds to the aerial image at time T 1  in  FIGS. 10A to 12C  or the aerial image when the distance described above with reference to  FIGS. 13A to 14E  is L 11 . The second largest cube corresponds to the aerial image at time T 2  in  FIGS. 10A to 12C  or the aerial image when the distance described above with reference to  FIGS. 13A to 14E  is L 12 . The largest cube corresponds to the aerial image at time T 3  in  FIGS. 10A to 12C  or the aerial image when the distance described above with reference to  FIGS. 13A to 14E  is L 13 . 
     As described above, with the center of an aerial image fixed in position, the aerial image is enlarged in volume as time passes or as a person approaches the aerial image. This may also make it possible to increase the person□s attention to or interest in the advertisement represented as the aerial image. 
     EXAMPLE 5 
     In the case of Examples 1 to 4 above, an object constituting an aerial image formed in three dimensions remains unchanged as the aerial image is enlarged in volume. Alternatively, however, it is also possible to control the object constituting the aerial image such that the content represented by the object evolves as the aerial image is enlarged in volume. 
       FIGS. 16A to 16C  each illustrate an example in which an object constituting an aerial image formed in three dimensions is changed as the aerial image is enlarged in volume.  FIG. 16A  illustrates an exemplary object constituting the aerial image with the smallest volume,  FIG. 16B  illustrates an exemplary object constituting the aerial image with the second largest volume, and  FIG. 16C  illustrates an exemplary object constituting the aerial image with the largest volume. 
     In  FIGS. 16A to 16C , the object evolves from an egg to a chick and then to a chicken. Another example of evolution of an object is when an object evolves from an egg to a tadpole and then to a frog. Of course, these examples are only illustrative. 
     The way in which an object changes as the object is enlarged in volume is not limited to evolution. For example, if the object is a living thing, the posture or facial expression of the same object may change. It is to be noted, however, that such changes also represent one form of a moving image. 
     EXAMPLE 6 
       FIGS. 17A to 17C  each illustrate an example in which an aerial image formed in three dimensions is reduced in volume with the passage of time.  FIG. 17A  illustrates an example in which the aerial image is reduced in volume in one direction,  FIG. 17B  illustrates an example in which the aerial image is reduced in volume in two directions, and  FIG. 17C  illustrates an example in which the aerial image is reduced in volume in three directions. 
     Although the example in each of  FIGS. 17A to 17C  focuses only on the volume of an aerial image, it is also possible to change the content of the aerial image in conjunction with the passage of time or with the reduction of the aerial image, such as by changing the aerial image in color or brightness. 
       FIGS. 17A to 17C  each illustrate, with time represented on the horizontal axis, how the shape of the aerial image at time T 1  changes at times T 2  and T 3 . In the case of  FIGS. 17A to 17C , the aerial image has a cuboid shape at time T 1 . 
     The example in  FIG. 17A  represents reduction in the X-axis direction. In this case, as seen from a person, the three-dimensional aerial image appears to be reduced in the horizontal direction with the passage of time. 
     The example in  FIG. 17B  represents reduction by the same factor in each of the X- and Y-axis directions. It is to be noted, however, that the reduction factor may differ between the X-axis direction and the Y-axis direction. The reduction factor may not necessarily be fixed but may change with the passage of time. The Y-axis direction is the direction of depth as seen from the person. Thus, in the case of  FIG. 17B , the aerial image formed in three dimensions appears to decrease in thickness in the depth direction with the passage of time. 
     The example in  FIG. 17C  represents reduction by the same factor in each of the X-, Y-, and Z-axis directions. Thus, in the example in  FIG. 17C , the aerial image is reduced while keeping its cuboid shape. It is to be noted, however, that the reduction factor may differ along each axis. The reduction factor may not necessarily be fixed but may change with the passage of time. In the example in  FIG. 17C , the aerial image formed in three dimensions appears to be reduced in a uniform manner with the passage of time. 
     EXAMPLE 7 
     Example 6 above is directed to an example of how an aerial image changes in shape as the aerial image is reduced in volume with the passage of time. 
     The following describes differences in the direction of reduction of a three-dimensional aerial image that changes in volume with the passage of time. 
       FIGS. 18A to 18C  each illustrate a case in which, at the beginning of reduction in volume, an aerial image is located at the same position as Position P 1  where the aerial image has been previously formed in two dimensions.  FIG. 18A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 18B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 18C  illustrates the position and shape of the aerial image at time T 3 . 
     The positioning of the aerial image at time T 1  in  FIGS. 18A to 18C  corresponds to that in  FIG. 5C  or  FIG. 8C . In the case of  FIGS. 18A to 18C , the aerial image formed in three dimensions is reduced with the passage of time in a direction away from a person. Specifically, the aerial image is reduced in the Y-axis direction. 
       FIGS. 19A to 19C  each illustrate a case in which, at the beginning of reduction in volume, an aerial image is located at a position different from Position P 1  where the aerial image has been previously formed in two dimensions.  FIG. 19A  illustrates the position and shape of the aerial image at time T 1 ,  FIG. 19B  illustrates the position and shape of the aerial image at time T 2 , and  FIG. 19C  illustrates the position and shape of the aerial image at time T 3 . 
     The positioning of the aerial image at time T 1  in  FIGS. 19A to 19C  corresponds to that in  FIG. 6C  or  FIG. 9C . In the case of  FIGS. 19A to 19C , the aerial image formed in three dimensions is reduced with the passage of time in a direction toward a person. It is to be noted that in the case of  FIGS. 19A to 19C , the aerial image is not only reduced in the Y-axis direction, but also moves in the Y-axis direction with the passage of time. Specifically, the aerial image is moved in a direction toward the person. In other words, the aerial image formed in three dimensions is moved in a direction away from Position P 1  where the aerial image is formed in two dimensions. 
     Even through the shape of an aerial image formed in three dimensions is the same between  FIGS. 18A to 18C  and  FIGS. 19A to 19C  at each point in time, the position where reduction of the aerial image begins or the direction in which the aerial image is reduced differs between  FIGS. 18A to 18C  and  FIGS. 19A to 19C . This may make it possible to give a different impression to the person observing the aerial image. 
     EXAMPLE 8 
     In the case of Example 7, an aerial image formed in three dimensions is reduced in volume with the passage of time. The following describes an example in which reduction of an aerial image in volume is controlled in relation to where a person is located. 
       FIGS. 20A to 20D  each illustrate an example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed.  FIG. 20A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 20B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 20C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 20D  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 . Position P 1  in this case is where the aerial image is formed in two dimensions. 
     In  FIGS. 20A to 20D  as well, the aerial image is transformed so as to be reduced in volume. In this case, however, the distance between the person and Position P 1 , rather than time, is used in controlling the volume of the aerial image. 
     For example, in the case of  FIG. 20A  in which the distance between the person and Position P 1  is L 11 , the aerial image formed in three dimensions is positioned midway between Position P 1  and the person. This positioning corresponds to that in  FIG. 6C  or  FIG. 9C . 
     When, upon noticing the aerial image, the person moves closer to the aerial image, the aerial image is reduced in volume in accordance with the distance between the person and Position P 1 . In the case of  FIGS. 20A to 20D , when the distance is L 12 (&lt;L 11 ), the aerial image has a smaller volume than when the distance is L 11 , and when the distance is L 13 (&lt;L 12 ), the aerial image has a smaller volume than when the distance is L 12 . 
     As with the examples mentioned above, the change in the volume of the aerial image at this time may be reduction in one direction or reduction in two directions. The example in each of  FIGS. 20A to 20D  illustrates reduction in three directions. The reduction factor may be fixed, or may be varied in accordance with the distance between the person and Position P 1 . 
     When the person moves to a position where the distance L 14 (&lt;L 13 ) between the person and Position P 1  is less than a predetermined distance, the aerial image is controlled to have a two-dimensional shape as illustrated in  FIG. 20D . 
     By changing the volume of an aerial image as illustrated in  FIGS. 20A to 20D , a person interested in or intrigued by the aerial image may be directed toward a specific position. In  FIGS. 20A to 20D , the person is directed toward Position P 1  where the aerial image is formed in two dimensions, thus increasing the advertising effect. 
     The reduction of the aerial image in volume may be stopped once the distance between the person and Position P 1  becomes less than a predetermined distance. 
       FIGS. 21A to 21E  each illustrate another example of how to direct a person toward Position P 1  where a two-dimensional aerial image is formed.  FIG. 21A  illustrates the shape of an aerial image immediately after being formed in three dimensions,  FIG. 21B  illustrates the shape of the aerial image with the person moving closer to Position P 1 ,  FIG. 21C  illustrates the shape of the aerial image with the person moving further closer to Position P 1 ,  FIG. 21D  illustrates the shape of the aerial image with the person moving further closer to Position P 1 , and  FIG. 21E  illustrates the shape of the aerial image with the person moving further closer until the person is just in front of Position P 1 . In  FIGS. 21A to 21E , features corresponding to those in  FIGS. 20A to 20D  are denoted by the corresponding reference signs. 
     In the case of  FIGS. 21 to 21E , reduction of the aerial image is stopped on the condition that the distance Ll 3 A between the person and Position P 1  has become less than the predetermined distance L 10 . Thus, the volume of the aerial image in  FIG. 21C  is the same as the volume of the aerial image in  FIG. 21D . 
     Reducing an aerial image too much in volume makes seeing of the aerial image itself difficult. For this reason, the control illustrated in  FIGS. 21A to 21E  is also provided. 
     EXAMPLE 9 
     In the case of Examples 6 to 8 above, as an aerial image formed in three dimensions is reduced, the center of the aerial image also moves. Alternatively, however, it is also possible to reduce an aerial image with the center position of the aerial image fixed. 
       FIG. 22  illustrates an example in which an aerial image formed in three dimensions is reduced in volume with the center of the aerial image fixed in position. 
     The aerial images illustrated in  FIG. 22  each have the shape of a cube.  FIG. 22  illustrates three aerial images with different volumes. In this regard, the largest cube corresponds to the aerial image at time T 1  in  FIGS. 17A to 19C  or the aerial image when the distance described above with reference to  FIGS. 20A to 21E  is L 11 . The second largest cube corresponds to the aerial image at time T 2  in  FIGS. 17A to 19C  or the aerial image when the distance described above with reference to  FIGS. 20A to 21E  is L 12 . The smallest cube corresponds to the aerial image at time T 3  in  FIGS. 17A to 19C  or the aerial image when the distance described above with reference to  FIGS. 20A to 21E  is L 13 . 
     As described above, with the center of an aerial image fixed in position, the aerial image is reduced in volume as time passes or as a person approaches the aerial image. This may also make it possible to increase the person□s attention to or interest in the advertisement represented as the aerial image. 
     EXAMPLE 10 
     In the case of Examples 6 to 9 above, an object constituting an aerial image formed in three dimensions remains unchanged as the aerial image is reduced in volume. Alternatively, however, it is also possible to control the object constituting the aerial image such that the content represented by the object evolves backward as the aerial image is enlarged in volume. 
       FIGS. 23A to 23C  each illustrate an example in which an object constituting an aerial image formed in three dimensions is changed as the aerial image is reduced in volume.  FIG. 23A  illustrates an exemplary object constituting the aerial image with the largest volume,  FIG. 23B  illustrates an exemplary object constituting the aerial image with the second largest volume, and  FIG. 23C  illustrates an exemplary object constituting the aerial image with the smallest volume. 
     In  FIGS. 23A to 23C , the object evolves backward from being a chicken to a chick and then to an egg. Another example of backward evolution of an object is when an object evolves backward from being a frog to a tadpole and then to an egg. Of course, the above examples are only illustrative. 
     The way in which an object changes as the object is reduced in volume is not limited to backward evolution. For example, if the object is a living thing, the posture or facial expression of the same object may change. It is to be noted, however, that such changes also represent one form of a moving image. 
     Exemplary Embodiment 4 
     With reference to Exemplary Embodiment 4, the following describes a case in which the emotion of a person seeing or observing an aerial image is inferred from the person□s facial expression, and the content of the aerial image is changed accordingly. 
     Exemplary Embodiment 4 also uses the information processing system  1  (see  FIG. 1 ) illustrated in  FIG. 1 . 
       FIG. 24  is a flowchart illustrating an exemplary process that switches the contents of aerial images in accordance with Exemplary Embodiment 4. The process illustrated in  FIG. 24  is implemented through execution of a program by the processor  21  (see  FIG. 1 ). 
     First, the processor  21  recognizes, from an image captured by the camera  30  (see  FIG. 1 ), the facial expression of a person located in the vicinity of an aerial image (step  21 ). In Exemplary Embodiment 4, the following three kinds of facial expressions are assumed to be recognized: disappointed, angry, and pleased. Of course, the specific kinds of facial expressions to be recognized, or the number of such facial expressions are only illustrative. 
     Subsequently, the processor  21  determines the recognized facial expression. In the case of  FIG. 24 , the processor  21  determines whether the person is recognized to be disappointed (step  22 ). 
     If an affirmative determination is made at step  22 , the processor  21  instructs that an advertisement about recreation be output to cheer up the person (step  23 ). 
     If a negative determination is made at step  22 , the processor  21  determines whether the person is recognized to be angry (step  24 ). 
     If an affirmative determination is made at step  24 , the processor  21  instructs that an advertisement for directing the person to a quiet place be output to make the person calm down (step  25 ). 
     If a negative determination is made at step  24 , the processor  21  instructs that a predetermined advertisement be output (step  26 ). In this regard, a case in which a negative determination is made at step  24  in  FIG. 24  is when the person is recognized to be pleased from the person□s facial expression. 
     By combining the control according to Exemplary Embodiment 4, in which the content of an advertisement is changed by using information identified from a person□s facial expression, with the processes according to the above-mentioned exemplary embodiments, information is presented in diverse ways to a person who sees or observes an aerial image. 
     Other Exemplary Embodiments 
     Although exemplary embodiments of the present disclosure have been described above, the technical scope of the present disclosure is not limited to the above exemplary embodiments. It is apparent from the description of the claims that various modifications or improvements of the exemplary embodiments also fall within the technical scope of the present disclosure. 
     For example, although the aerial-image forming apparatus  10  (see  FIG. 1 ) and the controller  20  (see  FIG. 1 ) have been described in the above exemplary embodiments as being independent from each other, the aerial-image forming apparatus  10  and the controller  20  may be integrated with each other. 
     The controller  20  according to the above exemplary embodiments may be a so-called computer or an information terminal such as a smartphone, or may be a server installed on the Internet. 
     Although the exemplary embodiments above are directed to the case in which an aerial image is used for an advertisement, the content of an aerial image is not limited to an advertisement. 
     Although the exemplary embodiments above are directed to the case in which the basic shape of an aerial image is two-dimensional, the basic shape may be three-dimensional. 
     Although the exemplary embodiments above are directed to the case in which a two-dimensional image is formed as an aerial image, a two-dimensional image may be displayed on a planar display with a physical display surface, such as a liquid crystal display or an organic electro-luminescence (EL) display. In such a case, the processor  21  may control the switching between formation of an aerial image by the aerial-image forming apparatus  10  that forms an aerial image, and displaying of an image on the planar display. 
     In the embodiments above, the term “processor” refers to hardware in a broad sense. Examples of the processor include general processors (e.g., CPU: Central Processing Unit) and dedicated processors (e.g., GPU: Graphics Processing Unit, ASIC: Application Specific Integrated Circuit, FPGA: Field Programmable Gate Array, and programmable logic device). 
     In the embodiments above, the term “processor” is broad enough to encompass one processor or plural processors in collaboration which are located physically apart from each other but may work cooperatively. The order of operations of the processor is not limited to one described in the embodiment above, and may be changed. 
     The foregoing description of the exemplary embodiments of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalents.