Patent Publication Number: US-2013229336-A1

Title: Stereoscopic image display device, stereoscopic image display method, and control device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-047195, filed Mar. 2, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to stereoscopic image display. 
     BACKGROUND 
     A stereoscopic image display device for which the user does not use dedicated glasses displays a plurality of images for different viewpoints, and uses an optical element to control light rays. The controlled light rays are guided to both eyes of a viewer, and he/she can recognize a stereoscopic image as long as his/her observation position falls within an appropriate range (to be referred to as a “viewing range” hereinafter). It may be difficult to view a satisfactory stereoscopic image depending on the relative positional relationship between the viewer and the stereoscopic image display device. Furthermore, even if the viewer is within the viewing range at first, he/she may move outside this range. It is, therefore, preferable to change the mode of stereoscopic image display according to the position of the viewer so as to allow stereoscopy. 
     There is well known a technique of using a liquid crystal optical element or birefringent element as the above optical element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a stereoscopic image display device according to an embodiment; 
         FIG. 2  is a view showing a display; 
         FIG. 3  is a view showing an optical element; 
         FIG. 4  is a view showing an example of a change in refractive index of the optical element and the alignment state of liquid crystal molecules; 
         FIG. 5  is a front view showing the display; 
         FIG. 6  is a block diagram showing a calculator and a controller; 
         FIG. 7  is a block diagram showing details of a person&#39;s information acquirer; 
         FIG. 8  is a block diagram showing details of the calculator; 
         FIG. 9  is a view showing the number of parallaxes, the resolution of one parallax image, and a change in viewing range with a change in lens pitch; 
         FIG. 10  is a view showing a case in which a liquid crystal barrier is applied instead of a birefringent element; 
         FIG. 11  is a view for explaining a weight based on an optical path length difference; 
         FIG. 12  is a view for explaining calculation of a weight based on the area of a stereoscopic enable region; 
         FIG. 13  is a view for explaining calculation of a weight based on a light ray density; 
         FIG. 14  is a view showing an example of a map obtained by arranging weight values in a real space coordinate system; 
         FIG. 15  is a block diagram showing details of an image output device; 
         FIG. 16  is a view for explaining display parameters to be controlled for a viewing range; 
         FIG. 17  is a view for explaining the display parameters to be controlled for the viewing range; 
         FIG. 18  is a view for explaining a neighboring viewing range; 
         FIG. 19  is a view for explaining a control operation according to the arrangement of pixels to be displayed; 
         FIG. 20  is a view for explaining an operation of controlling a viewing range by moving, rotating, or deforming the display; 
         FIG. 21  is a view showing light densities when the numbers of parallaxes are different; and 
         FIG. 22  is a view showing a case in which the number of parallaxes is switched in a software manner. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a stereoscopic image display device includes a display element in which a plurality of pixels are arranged in a matrix topology, an optical element coupled to the display element, the optical element having variable optical characteristics. The device also includes an acquirer, calculator, and controller. The acquirer is configured to acquire person&#39;s information including a position of each of at least one person viewing a stereoscopic image. The calculator is configured to calculate, based on the person&#39;s information, a weight representing a quality of stereoscopic viewing for each person. The controller is configured to select optical characteristic parameters corresponding to the weight, and control the optical characteristics of the optical element based on the optical characteristic parameters. 
     An embodiment will be described below with reference to the accompanying drawings. As shown in  FIG. 1 , a stereoscopic image display device according to the embodiment includes a person&#39;s information acquirer  100 , a calculator  200 , a controller  300 , and a display  400 . This device allows a plurality of viewers to view a good quality stereoscopic video at the same time. The device controls display of a stereoscopic image by evaluating the resolution of the stereoscopic image and crosstalk when a birefringent element is used in addition to changing the mode of stereoscopic image display according to the positions of viewers. 
     The acquirer  100 , the calculator  200 , and the controller  300  can be realized by one or more central processing unit (CPU) and memory used in the CPU. 
     A viewer (person) P observes a stereoscopic image and the like displayed on the display  400  by observing a display element  402  through an optical element  401  (see the direction of an arrow ZA in  FIG. 1 ). The display element  402 , for example, displays parallax images observed as a stereoscopic image. The display element  402  has a display surface in which a plurality of pixels are arranged in the first and second directions in a matrix topology. The first direction is, for example, the row direction (the X-axis direction (horizontal direction) in  FIG. 1 ), and the second direction is a direction perpendicular to the first direction, and is, for example, the column direction (the Y-axis direction (vertical direction) in  FIG. 1 ). 
     The person&#39;s information acquirer  100  detects the position of the viewer P. This embodiment can be applied when there are a plurality of target viewers, and thus the person&#39;s information acquirer  100  detects the position of each person. The person&#39;s information acquirer  100  outputs person&#39;s information indicating the detected position of each person. The person&#39;s information acquirer  100  may cause a detector such as a camera to detect the position of a person, and then obtain the relative position coordinates (to be referred to as “position coordinates (X P , Y P )” hereinafter) of the viewer, the relative position is defined as the position of the stereoscopic image display device, based on the detection result. Based on the person&#39;s information including the position of each person which has been acquired by the person&#39;s information acquirer  100 , the calculator  200  calculates a weight representing the quality of stereoscopic viewing for each person. The controller  300  selects display parameters with which the total of the weights of the respective persons that have been calculated by the calculator  200  becomes largest, and outputs a multi-view image (that is, parallax images) according to the selected display parameters. The display  400  displays the multi-view image output from the controller  300 . 
     The display  400  is a display device for displaying a stereoscopic image or plan-view image.  FIG. 2  is a schematic view showing the schematic arrangement of the display  400 . The optical element  401  is a birefringent element, the refractive index distribution of which changes depending on the applied voltage. Light rays diverge from the display element  402  toward the optical element  401  side, pass through the optical element  401 , and exit the optical element  401  in a direction according to the refractive index distribution of the optical element  401 . The optical element  401  need only be an element, the refractive index distribution of which changes depending on the applied voltage. As the optical element  401 , a liquid crystal element in which liquid crystal molecules are distributed between a pair of substrates is used. Note that a case in which a liquid crystal element is used as the optical element  401  will be described as an example in this embodiment. The optical element  401 , however, need only be an element, the refractive index distribution of which changes depending on the applied voltage, and is not limited to a liquid crystal element. For example, a liquid lens which is formed by two kinds of liquid including an aqueous solution and oil, a water lens which uses the surface tension of water, or the like may be used as the optical element  401 . The optical element  401  has an arrangement in which a liquid crystal layer  401 C is arranged between a pair of substrates  401 E and  401 D. An electrode  401 A is arranged in the substrate  401 E. An electrode  401 B is arranged in the substrate  401 D. Note that in this embodiment, a case wherein the optical element  401  has an arrangement in which electrodes (the electrodes  401 A and  401 B) are respectively arranged in the substrates  401 E and  401 D will be described. The optical element  401 , however, need only have an arrangement in which it is possible to apply a voltage to the liquid crystal layer  401 C, and the present embodiment is not limited to this. The optical element  401  may have, for example, an arrangement in which an electrode is arranged in one of the substrates  401 D and  401 E. 
       FIG. 3  is a schematic enlarged view showing a portion of the optical element  401 . As shown in  FIG. 3 , liquid crystal molecules  406  are distributed in a dispersion medium  405  in the liquid crystal layer  401 C. A liquid crystal material which is aligned according to the applied voltage is used for the liquid crystal molecules  406 . The liquid crystal material need only have the above characteristics, and a nematic liquid crystal material in which the alignment direction changes depending on the applied voltage can be used. As is well known, the liquid crystal material has a long, narrow shape, and has the anisotropy of the refractive index in the longitudinal direction of the molecule. The strength of the applied voltage and a voltage application time for changing the alignment of the liquid crystal molecules  406  are different depending on the type of liquid crystal molecules  406  and the arrangement of the optical element  401  (that is, the shape and arrangement of the electrodes  401 A and  401 B). A voltage, therefore, is applied to the electrodes  401 A and  401 B (for example, electrodes  401 B 1  to  401 B 3 ) so as to form an electric field having a specific shape at a position, in the liquid crystal layer  401 C, which corresponds to each element pixel of the display element  402 . Then, the liquid crystal molecules  406  are aligned along the electric field in the liquid crystal layer  401 C, and thus the optical element  401  has a refractive index distribution according to the applied voltage. This is because the liquid crystal molecules  406  have refractive index anisotropy according to a polarization state. This is because when the alignment of the liquid crystal molecules  406  changes due to application of a voltage, the refractive index changes in an arbitrary polarization state. For example, the electrodes  401 A and  401 B are arranged in advance so as to form a different electric field at each position corresponding to each element pixel of the display element  402 . Then, a voltage is applied to the electrodes  401 B and  401 A so as to form an electric field having the shape of a lens  403  in a region, in the liquid crystal layer  401 C, which corresponds to each element pixel. The liquid crystal molecules  406  in the liquid crystal layer  401 C are then aligned along the electric field formed according to the applied voltage. In this case, the optical element  401  has a refractive index distribution with the shape of the lens  403 , as shown in  FIG. 3 . Therefore, the optical element  401  has a refractive index distribution with the shape of a lens array in which a plurality of lenses  403  are arranged in a predetermined direction, as shown in  FIG. 2 . 
     Note that the refractive index distribution with the shape of the lens array, for example, is along the arrangement direction of the element pixels of the display element  402 . More specifically, the optical element  401  has a refractive index distribution with the shape of the lens array in one or both of the horizontal and vertical directions on the display surface of the display element  402 . Note that it is possible to adjust, based on the arrangement of the optical element  401  (that is, the shape and arrangement of the electrodes  401 A and  401 B), whether the optical element  401  has a refractive index distribution in one or both of the horizontal and vertical directions. Note that voltage conditions such as the strength of a voltage to be applied to the liquid crystal layer  401 C and a voltage application time for attaining the specific alignment of the liquid crystal molecules  406  change depending on the type of liquid crystal molecules  406 , the shape and arrangement of the electrodes  401 A and  401 B, and the like. 
       FIG. 4  is a view showing an example of a change in refractive index of the optical element  401  and the alignment state of the liquid crystal molecules  406 . More specifically, (A) in  FIG. 4  shows an example of the relationship between the refractive index of the optical element  401  and a voltage applied to the electrodes  401 A and  401 B. (B) and (C) in  FIG. 4  show an example of the alignment state of the liquid crystal molecules  406  corresponding to the refractive index of the optical element  401 . 
     In the example shown in  FIG. 4 , when no voltage is applied between the electrodes  401 A and  401 B, the liquid crystal molecules  406  are aligned in the horizontal direction (see (B) in  FIG. 4 ), and a refractive index n has a small value ((A) in  FIG. 4 ). Then, the liquid crystal molecules  406  are aligned in the vertical direction (see (C) in  FIG. 4 ) when the voltage value to be applied to the electrodes  401 A and  401 B increases. The refractive index n of the optical element  401  increases with a change in alignment (see (A) in  FIG. 4 ). In the example shown in  FIG. 4 , therefore, the relationship between the applied voltage and the refractive index of the optical element  401  is indicated by a curve  407 . 
     When the arrangement of the electrodes  401 A and  401 B and conditions under which a voltage is applied to the liquid crystal layer  401 C through the electrodes  401 A and  401 B are adjusted, the optical element  401  has a refractive index distribution with the shape of the lens  403 , as shown in  FIG. 3 . Consequently, the optical element  401  has a refractive index distribution with the shape of the lens array, as shown in  FIG. 2 . 
     Although in this embodiment, a case in which the optical element  401  has the refractive index distribution with the shape of the lens  403  by applying the voltage is described, the present embodiment is not limited to this. The optical element  401  can be configured to have a refractive index distribution with a desired shape by, for example, adjusting conditions under which a voltage is applied to the electrodes  401 A and  401 B, and the arrangement and shape of the electrodes  401 A and  401 B. The voltage application conditions, and the arrangement and shape of the electrodes  401 A and  401 B may be adjusted so that, for example, the optical element  401  has a refractive index distribution with a prism shape. Furthermore, the voltage application conditions may be adjusted so that the optical element  401  has a refractive index distribution with both a prism shape and lens shape. 
     The display  400  according to this embodiment has the above-described arrangement. By controlling a voltage to be applied to the optical element  401 , therefore, the lens shape of the optical element  401  changes, thereby enabling to change the optical characteristics such as the lens pitch and focal length of the optical element  401 . 
       FIG. 5  is a view showing the display  400  when seen from the front side. The display  400  is a device capable of displaying a plurality of parallax images. The parallax images are used to cause the viewer to observe a stereoscopic image, and form the stereoscopic image. The stereoscopic image is obtained by assigning the pixels of the parallax images so that one of the eyes of the viewer P observes one parallax image and the other eye observes another parallax image when the viewer observes the display element  402  through the optical element  401  from the position of his/her viewpoint. That is, a stereoscopic image is generated by rearranging the pixels of each parallax image. Note that one pixel of a parallax image includes a plurality of sub-pixels. The display element  402  is a liquid crystal panel in which a plurality of sub-pixels each having a color component (for example, R, G, or B) are arranged in the first direction (row direction) and the second direction (column direction) in a matrix topology. Examples of the display element  402  are a direct-view two-dimensional display such as an organic EL (Electro Luminescence) display, LCD (Liquid Crystal Display), PDP (Plasma Display Panel), projection-type display, and plasma display. In the example of  FIG. 5 , one pixel includes sub-pixels having R, G, and B components. Sub-pixels respectively having R (red), G (green), and B (blue) components are repeatedly arranged in the order named in the first direction, and sub-pixels having the same color components are arranged in the second direction. The optical element  401  controls the direction in which a light ray exits from each sub-pixel of the display element  402 . In the optical element  401 , an optical aperture for passing light rays linearly extends, and a plurality of such optical apertures are arranged in the first direction. The display element  402  and optical element  401  have a given distance (gap) between them. Since the optical element  401  is arranged so that the extension direction of the optical aperture has a predetermined inclination with respect to the second direction (column direction) of the display element  402 , the positions of the optical aperture and display pixel are different from each other in the row direction, and thus the viewing range (a region where it is possible to observe a stereoscopic image) changes depending on the height. 
       FIG. 6  shows the schematic arrangement of the calculator  200  and controller  300 . The calculator  200  includes a weight calculator  202  which receives person&#39;s information  101  including the position of each person that has been acquired by the person&#39;s information acquirer  100 , and a parameter group  201  for determining a lens shape/image, calculates a weight representing the quality of stereoscopic viewing for each person, and outputs the weight of each person and corresponding display parameters  203 . 
     Assume that a weight W represents the quality of stereoscopic viewing. In this case, the weight W is calculated based on the person&#39;s information  101  for each display parameter of the display parameter group  201  associated with a multi-view image (that is, a combination of arrangements of pixels to be displayed) to be displayed on the stereoscopic image display device, and the hardware design and lens shape of the stereoscopic image display device. As the value of the weight W is larger, the stereoscopy is more satisfactory. The weight W reflects at least the position of each person but it can be arbitrarily changed. For example, the weight W may be changed to deal with several viewing modes selectable by the viewer. Note that targets controllable by the display parameters, such as a combination of arrangements of pixels to be displayed, will be described in detail later. 
     In this embodiment, a weight (to be referred to as a “position weight” hereinafter) according to the area of a stereoscopic enable display region, a light ray density, and a predetermined position is calculated. It is, therefore, necessary to be able to acquire position information of each person by some means. Furthermore, in this embodiment, an optical path length difference associated with crosstalk and the resolution of a stereoscopic image is calculated as another position weight. These position weights will be described in detail later. 
     Furthermore, in this embodiment, in addition to the position weight, a weight (to be referred to as an “attribute weight” hereinafter) according to the attributes of each person is calculated. The weight W is calculated by combining the value of the position weight and that of the attribute weight. 
     The controller  300  includes a parameter selector  301  which receives the weight W of each person and the corresponding display parameters  203  and selects display parameters with which the total of the weights of the respective persons that have been calculated by the calculator  200  becomes largest, an image output device  302  which outputs a stereoscopic image according to the display parameters selected by the parameter selector  301 , and a voltage applying device  303  which outputs an applied voltage for changing the optical characteristics of the optical element  401  according to the display parameters selected by the parameter selector  301 . 
     The more detailed arrangement of the stereoscopic image display device according to this embodiment will be described below. 
       FIG. 7  shows the arrangement of the person&#39;s information acquirer  100 . The person&#39;s information acquirer  100  includes a detector  103  which detects the position of each person by receiving a camera image or the like  102  and outputs the person&#39;s information  101  representing the position and attributes of each person, and a tracking device  104  which tracks a change in position of the same person, that is, movement of each person for a predetermined period of time based on the output of the detector  103 . 
     The image used for position detection is not limited to an image from a camera and, for example, a signal provided by radar may be used. In the position detection operation, an arbitrary target such as a face, a head, a person as a whole, or a marker which can be determined as a human may be detected. Examples of the attributes of each person include information such as the name of each person, data for distinguishing between an adult and child, a viewing time, and data indicating whether the viewer is a remote controller holder. These pieces of information are detected by some means, or may be directly input by a viewer or the like. 
     Note that the person&#39;s information acquirer  100  may include a person&#39;s position converter  105  which converts a coordinate value in a camera coordinate system into that in a real space coordinate system with respect to the position information of each person output from the person&#39;s information acquirer  100 . Furthermore, the person&#39;s position converter  105  may be provided in the calculator  200  instead of the person&#39;s information acquirer  100 . 
       FIG. 8  shows the arrangement of the calculator  200 . The calculator  200  includes a weight calculator  202  which receives the person&#39;s information  101  from the person&#39;s information acquirer  100  and the parameter group  201  for determining the lens shape/image, and calculates and outputs the weight of each person and the corresponding display parameters  203 . The weight calculator  202  includes a position weight calculator  202 A which calculates a position weight based on a person&#39;s position  101 A and the parameter group  201 , an attribute weight calculator  202 B which calculates an attribute weight based on person&#39;s attributes  101 B, and a calculator  213  which calculates a sum or product of the calculated position weight and attribute weight. Note that if one of the weights is used, an addition or multiplication operation can be omitted. 
     The position weight calculator  202 A calculates a position weight based on a stereoscopic image resolution  204 , an optical path length difference  205 , a stereoscopic enable display region  206 , a light ray density  207 , a predetermined position weight  208 , or the like. The stereoscopic image resolution  204  is a weight associated with a resolution depending on a change in lens pitch of the optical element  401 . The optical path length difference  205  is a weight associated with a crosstalk amount depending on a change in focal length. More specifically, the optical path length difference  205  corresponds to a difference (|f−dm|) between a lens focal length f of the optical element  401  and a length dm of a straight line connecting a person&#39;s position with the lens position of the optical element  401  from the lens position to the display element  402 . The weights based on the stereoscopic image resolution  204  and optical path length difference  205  will be described in detail later. 
     The area of the stereoscopic enable display region  206  is determined based on the position of each person (that is, the relative position with respect to the display screen of the stereoscopic image display device) and a multi-view image. As the area is larger, the value of the position weight is larger. The light ray density  207  is determined based on the number of viewpoints and the distance from the display screen of the stereoscopic image display device. As the light ray density  207  is higher, the position weight is larger. As for the predetermined position weight  208 , a position where the viewer normally views an image is assigned with a weight larger than those for other positions. 
     The position weight calculator  202 A calculates and outputs a sum or product of the weight values which have been respectively calculated for the stereoscopic image resolution  204 , optical path length difference  205 , stereoscopic enable display region  206 , light ray density  207 , predetermined position weight  208 , and the like. Note that if one of the weights is used, calculation of a sum or product of the position weights can be omitted. In addition to them, a term which can represent a weight associated with the appearance may be added. 
     The attribute weight calculator  202 B calculates an attribute weight based on an attribute value such as a viewing time or start order  209 , a specific person  210 , a remote controller holder  211 , or a positional relationship between persons  212 . As for the viewing time or start order  209 , a person who is viewing for a long time or a person who has started viewing earlier is assigned with a larger weight value to have high priority. Similarly, the specific person  210  or remote controller holder  211  is assigned with a larger weight value to have high priority. As for the positional relationship between persons  212 , a person, among all viewers, who is in front of the display or is closer to the display, is assigned with a larger weight value. The attribute weight calculator  202 B calculates and outputs a sum or product of the weight values which have been respectively calculated for the viewing time or start order  209 , the specific person  210 , the remote controller holder  211 , the positional relationship between persons  212 , and the like. Note that if one of the weights is used, an addition or multiplication operation can be omitted. In addition to them, a term which can represent a weight associated with the attributes of a viewer may be added. 
     Furthermore, the calculator  213  calculates a sum or product of the value of the position weight output from the position weight calculator  202 A and the value of the attribute weight output from the attribute weight calculator  202 B. 
     Note that it is necessary to at least calculate a position weight except when parameters are selected based on only the information of the specific person  210 . The weight of each person is calculated for each of a plurality of display parameters included in the parameter group  201  for determining the lens shape/image. Furthermore, a weight is calculated for all the persons in principle (except when parameters are selected based on only the information of the specific person  210 ). 
     The weight based on the stereoscopic image resolution  204  will be described with reference to  FIG. 9 . 
     When the lens pitch of the optical element  401  changes with respect to a reference, the number of parallaxes, the resolution of one parallax image, and the viewing range change. When the lens pitch of the optical element  401  changes to be larger than the reference (θ 1 &gt;θ), the number of display elements for one lens increases. At this time, if the number of display pixels for one parallax image for one lens is constant, the number of parallaxes increases. The resolution of one parallax image can be represented by H×V/N where H represents the number of pixels in the horizontal direction, V represents the number of pixels in the vertical direction, and N represents the number of parallaxes. The resolution indicates the number of pixels for one parallax, which corresponds to the stereoscopic image resolution  204 . If the lens pitch of the optical element  401  becomes larger than the reference, the number of parallaxes increases as described above, thereby decreasing the resolution of one parallax image. As is apparent from  FIG. 9 , the viewing range expands from a region  10 B to a region  10 A. On the other hand, if the lens pitch of the optical element  401  becomes smaller than the reference (θ 2 &lt;θ), the number of display elements for one lens decreases, thereby reducing the number of parallaxes. The resolution of one parallax image, therefore, increases. As is apparent from  FIG. 9 , the viewing range reduces from the region  10 B to a region  10 C. 
     As for such the variable stereoscopic image resolution  204 , for example, a larger weight is assigned as the stereoscopic image resolution  204  is higher (that is, the number of parallaxes is smaller and the lens pitch is smaller). To the contrary, a smaller weight is assigned as the stereoscopic image resolution  204  is lower (that is, the number of parallaxes is larger and the lens pitch is larger). More specifically, a weight may be calculated according to equation (1), (2), or (3). 
     (1) Use of Resolution R of One Parallax Image 
     Let R max  be the resolution of a panel. Then, a weight w 1  when the resolution of one parallax image is R is calculated according to: 
         w   1   =R/R   max   (1)
 
     This equation gives a larger weight value as the resolution R for one parallax is higher. As long as the equation is satisfied, any method other than equation (1) may be used. 
     (2) Use of Number N of Parallaxes 
     Let N max  be the maximum number of parallaxes. Then, a weight w 1  for the number N of parallaxes is calculated according to: 
         w   1 =1− N/N   max   (2)
 
     This equation gives a larger weight value as the number N of parallaxes is smaller. As long as the equation is satisfied, any method other than equation (2) may be used. 
     (3) Use of Lens Pitch p 
     Let p max  be the maximum lens pitch. Then, a weight w 1  for the lens pitch p is calculated according to: 
         w   1 =1− p/p   max   (3)
 
     This equation gives a larger weight value as the lens pitch p is smaller. As long as the equation is satisfied, any method other than equation (3) may be used. 
     In equation (1), (2), or (3), the fraction portion is an important term. The weight is obtained by comparing each parameter with a maximum possible value of the parameter. Note that a Gaussian distribution with a fraction as an argument may be used. Instead, a sigmoid function with the resolution R, the number N of parallaxes, and the lens pitch p as arguments may be used. 
       FIG. 10  is a view showing a case in which a liquid crystal barrier is applied as an optical element. A liquid crystal barrier can be used as the optical element  401  instead of an element (birefringent element) with a variable refractive index distribution. As shown in  FIG. 10 , it is possible to use a liquid crystal barrier  408  to change the pitch of an optical aperture (corresponding to a lens)  11  which transmits light, as shown in (a) to (c) of  FIG. 10 . 
       FIG. 11  is a view for explaining the weight based on the optical path length difference. As described above, it is possible to change the focal length of the optical element  401 . When the focal length is changed, an in-focus direction and an in-focus distance change. Consequently, the crosstalk amount, that is, the degree of satisfaction of the “appearance” of a stereoscopic image, changes. As the crosstalk amount is smaller, the “appearance” of the stereoscopic image is more satisfactory. It is difficult to calculate the crosstalk amount itself. Let dm be the optical path length of a line segment connecting the position of the viewer P with the principal point of the lens from the principal point of the lens to a display pixel, and Δ be the difference between the focal length f of the lens and the optical path length dm. When Δ is close to 0, the focal point of the lens exists at the position of the display pixel, the light reaches the position of the viewer P most effectively, and thus the crosstalk amount reduces. To the contrary, when Δ has a value away from 0, the focal point exists behind (or in front of) the display pixel, light around the display pixel also reaches the position of the viewer P, and thus the crosstalk amount increases. A weight, therefore, is calculated according to the value of Δ using the optical path length difference Δ=|f−dm| as an amount reflecting crosstalk, as indicated by: 
     
       
         
           
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             = 
             
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                 f 
                 - 
                 
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                   m 
                 
               
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               d 
               m 
             
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               g 
               
                 sin 
                  
                 
                     
                 
                  
                 
                   θ 
                   Lm 
                 
               
             
           
         
       
       
         
           
             
               θ 
               Lm 
             
             = 
             
               arc 
                
               
                   
               
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               cos 
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                 L 
                 
                   
                     
                       L 
                       2 
                     
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     As the optical path length difference Δ is smaller, a weight for the “appearance” of the stereoscopic image is set to be larger. To the contrary, as the optical path length difference Δ is larger, a weight for the “appearance” of the stereoscopic image is set to be smaller. 
     The optical path length difference Δ is preferably calculated for each lens. For example, Δ may represent a weighted average for an optical path length difference Δi of a lens i. The weight in this case may be constant, or may be changed to have a smaller value toward the edge of the screen. It is possible to use the thus calculated optical path length difference Δ to calculate a weight w 2  associated with the “appearance” of the stereoscopic image, according to: 
         w   2 =exp(Δ/σ 2 )
 
     where the weight w 2  conforms to the Gaussian distribution. 
     Alternatively, let Δ max  be the maximum value of the optical path length difference Δ. Then, the weight w 2  can be calculated according to: 
         w   2 =1−Δ/Δ max  
 
     where Δ is 0 or larger. 
     Calculation of the weight based on the area of the stereoscopic enable display region  206  will be described next with reference to  FIG. 12 . The view of the “appearance” can be geometrically obtained, and calculated. A pattern  21  clipped by lines respectively connecting the viewer P with both sides of the display  400  at a viewing range setting distance coincides with the “appearance” of the display. In the example of  FIG. 12 , a region  22  in the pattern  21  is a stereoscopic enable region and a region  23  is a stereoscopic disable region. It is possible to calculate, as a weight, the ratio of the stereoscopic enable region  22  to the area of the whole region of the pattern  21 . If, for example, the ratio of the stereoscopic enable region  22  is 100%, stereoscopy is possible in the whole region, and a value “1” is set as a maximum value. 
     Calculation of the weight based on the light ray density  207  will be described next with reference to  FIG. 13 . Let N be the number of parallaxes, 2θ be the divergence angle of light rays, Z be the distance from the display  400  to the viewer P, and d be the difference between the eyes of the viewer P. Then, it is possible to calculate the weight based on the light ray density  207  according to: 
     
       
         
           
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     That is, assume that the ratio between the width len of light rays at the position of the eyes of the viewer P and the distance d between the eyes is the weight based on the light ray density  207 . If the width len of the light rays is smaller than the distance d between the eyes, the value of the weight based on the light ray density  207  is set to “1”. 
       FIG. 14  shows an example of a map M obtained by arranging the weight values calculated by the calculator  200  in the real space coordinate system. 
       FIG. 15  shows the arrangement of the image output device. The image output device  302  receives the weight of each person and the corresponding display parameters  203  which have been input from the calculator  200 . As the weight of each person and the corresponding display parameters  203 , one or multiple outputs may be provided. For example, if one output is provided, a largest total of the weights of the respective persons, a largest weight of a specific person, or a larger value of the average and median of the weights of the respective persons may be used. Alternatively, priority levels are assigned to viewers according to attribute weights, and then a largest total of the weights of respective persons with a given priority level or higher, or a larger value of the average and median of the weights may be used. If multiple outputs are provided, a largest value of the weight values of the respective persons may be used. 
     A determiner  310  of the image output device  302  determines whether the weight value as described above is equal to or larger than a predetermined reference value. If multiple outputs are provided, it is determined whether the weight values of all the persons (or the number N or more of persons) are equal to or larger than the predetermined reference value. Alternatively, priority levels may be assigned to viewers according to attribute weights, and then the determination may be made for only persons with a given priority level or higher. In either case, the display parameters  203  corresponding to the weight equal to or larger than the reference value are selected. The image output device  302  may include a blender/selector  311  for blending past display parameters  214  with the selected display parameters to slowly change the image based on the past display parameters  214 , or changing a scene so that the change is difficult to perceive or switching the image when the image frequently moves, as processing for improving the visibility in image switching. Similarly, the image output device  302  may also include a blender/selector  312  for, for example, blending a past image  216  with a multi-view image (stereoscopic image)  215  according to the display parameters output from the blender/selector  311  to slowly change the image based on the past image  216 . The blend processing preferably absorbs a first-order delay and the like. 
     Note that it is also possible to obtain a multi-view image (stereoscopic image) according to the selected display parameters by physically changing the position and orientation of the display  400 , as will be described later. 
     If the determiner  310  determines that the weight value is smaller than the reference value, a two-dimensional image, a black image (non-display), or an achromatic image  212  is displayed (2D display) to prevent inappropriate stereoscopy. A criterion for performing 2D display is, for example, that the total of the weights is small, there is a person who cannot view an image, or display is dependent on the viewing experience of a specific person. In this case, the image output device  302  may further include an information display  313  which guides a person to a position where stereoscopy is available, or warns that stereoscopy is available. 
     As for the parameter group  201  for determining an image, an example of a control operation using the respective display parameters will now be described. The display parameters include parameters to be controlled for a viewing range and those to be controlled for a light ray density. The display parameters to be controlled for the viewing range include the shift of an image, the pitch between pixels, the gap between a lens and a pixel, and rotation, deformation, or movement of a display. The display parameters to be controlled for the light ray density include the gap between a lens and a pixel, and the number of parallaxes. 
     The display parameters to be controlled for the viewing range will be described with reference to  FIGS. 16 and 17 . If, for example, a displayed image is shifted to the right side, a region where satisfactory stereoscopy is possible, that is, the “viewing range” changes from a viewing range A to a viewing range B, as shown in  FIG. 16 . This is because a light ray L moves to the left side in (c) of  FIG. 17 , and thus the viewing range also moves to the left side to obtain the viewing range B, as is apparent by comparing (a) with (c) in  FIG. 17 . 
     As is apparent by comparing (a) with (b) in  FIG. 17 , if the gap between the optical element  401  and the display element  402  is made smaller, the viewing range A changes to a viewing range C as shown in  FIG. 16 . In this case, although the viewing range becomes closer, the light ray density reduces. 
     Note that as shown in  FIG. 17 , parallax images are sequentially arranged in the display element  402 . The parallax images indicate images viewed from different viewpoints and, for example, correspond to images obtained by shooting the viewer P by a plurality of cameras  106 , as shown in  FIG. 17 . A light ray from the display element  402  (sub-pixel) exits through the lens (optical aperture)  401 . It is possible to geometrically obtain the shape of the viewing range using θ and η shown in  FIG. 17 . 
     A neighboring viewing range will be described with reference to  FIG. 18 . A viewing range B next to a viewing range A where the viewer mainly views an image is formed by a combination of (the left-most pixel, a lens on the right side of the left-most lens) and (a pixel on the left side of the right-most pixel, the right-most lens). It is also possible to further move the viewing range B to the left or right side. 
     A control operation according to the arrangement (display pitch) of pixels to be displayed will be described with reference to  FIG. 19 . It is possible to control the viewing range by shifting the positions of the display element  402  and optical element (lens)  401  from each other by a relatively larger amount toward the edge (right or left edge) of the screen. If the shift amount of the relative positions of the display element  402  and optical element  401  becomes larger, the viewing range changes from a viewing range A to a viewing range B, as shown in  FIG. 19 . To the contrary, if the shift amount of the relative positions of the display element  402  and optical element  401  becomes smaller, the viewing range changes from the viewing range A to a viewing range C, as shown in  FIG. 19 . As described above, it is possible to control the width or closeness of the viewing range with the display parameters associated with the arrangement (pixel pitch) of the pixels. Note that a location with a largest width of the viewing range will be referred to as a “viewing range setting distance”. 
     An operation of controlling a viewing range by moving, rotating, or deforming the display  400  will be described with reference to  FIG. 20 . As shown in (a) of  FIG. 20 , it is possible to change a default viewing range A to a viewing range B by rotating the display  400 . Similarly, it is possible to change the default viewing range A to a viewing range C by moving the display  400 , or to change the default viewing range A to a viewing range D by deforming the display  400 . Thus, the viewing range can be controlled by changing the display parameters to move, rotate, or deform the display  400 . 
     As for the display parameters to be controlled for a light ray density, light densities when the numbers of parallaxes are different will be described with reference to  FIG. 21 . 
     If, for example, the number of parallaxes shown in (a) of  FIG. 21  is 6, the number of light rays for giving parallaxes is larger for a person  31  who is relatively closer to the display  400  than a person  30 , thereby making stereoscopy satisfactory for him or her. If the number of parallaxes shown in (b) of  FIG. 21  is 3, it is smaller than the number of parallaxes shown in (a) of  FIG. 21  and the light rays are less dense, thereby making stereoscopy at the same distance difficult. It is possible to calculate the light ray density of light rays coming from the respective pixels of the display  400  based on an angle θ determined based on a lens and gap, the number of parallaxes, and the position of a person. 
     A case in which the number of parallaxes is switched in a software manner will be described. In the above-described embodiment, a case in which when changing the lens pitch of the optical element  401 , the number of parallaxes changes, and accordingly, the resolution of the stereoscopic image also changes has been described. The resolution can be switched not only by changing the lens pitch of the optical element  401  but also by switching the number of parallaxes in a software manner. For example, (a) of  FIG. 22  shows six parallaxes, that is, a case in which the number of parallaxes is “6”. By connecting two neighboring pixels, it is possible to change to three parallaxes, that is, to change the number of parallaxes to “3”, as shown in (b) of  FIG. 22 . When switching the number of parallaxes in a software manner, a weight may be calculated similarly to a case in which the lens pitch of the optical element  401  is changed. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 
     The acquirer  100 , the calculator  200 , and the controller  300  in this embodiment may be distributed by storing as a program allowing the computer to execute in a recording medium such as a magnetic disk (flexible disk, hard disk etc.), an optical disk (CD-ROM, DVD etc.) or a semiconductor memory and so on.