Patent Publication Number: US-2011063421-A1

Title: Stereoscopic image display apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The presently disclosed subject matter relates to a stereoscopic image display apparatus, and more particularly to a stereoscopic image display apparatus by which a stereoscopic image formed of left eye and right eye images having parallax therebetween is displayed so as to be able to be viewed stereoscopically. 
     2. Description of the Related Art 
     In a stereoscopic image display apparatus of this type, parallax between left eye and right eye images needs to be increased in order to enhance the stereoscopic effect and the sense of reality. However, the stereoscopic image display apparatus has a problem that when the binocular parallax exceeds the fusion limit of the viewer, the images cannot be stereoscopically viewed. 
     Here, the left and right eye images having parallax therebetween respectively enter the left and right eyes of the viewer, and the parallax images are fused in the brain, so that the viewer can recognize the images as a stereoscopic image. However, when the binocular parallax between the two parallax images is too large, the two parallax images are not fused and viewed as a double image, which may cause asthenopia. 
     Japanese Examined Application Publication No. 3771964 discloses a technique which detects a focus adjustment state (visual distance) of the eyeballs of the viewer viewing left eye and right eye images and adjusts a shift amount of the left eye and right eye images so that the visual distance becomes a target value, and also discloses a technique which detects a convergence angle of the eyeballs of the viewer viewing left eye and right eye images and adjusts the shift amount of the left eye and right eye images so that the convergence angle becomes a target value. 
     Further, Japanese Patent Application Laid-Open No. 9-74573 discloses a stereoscopic CG image generation apparatus which calculates a binocular fusion range of the viewer based on a screen size of a stereoscopic image display apparatus and on the basis of the visual distance between the screen and the viewer, determines camera parameters so that the whole part of a subject is included in the viewer&#39;s binocular fusion range, and generates a plurality of two-dimensional projection images (natural and easily visible stereoscopic image) by using the determined camera parameters. 
     Japanese Patent Application Laid-Open No. 2004-165709 describes a technique which measures a display time of a stereoscopic image and, when the measured display time of the stereoscopic image exceeds a predetermined time, displays the stereoscopic image as a plane image. Thereby, the viewer is protected so that the level of asthenopia of the viewer does not become excessive. 
     In the stereoscopic display method by which each of images having binocular parallax therebetween is displayed for each of the left and right eyes so that the images can be stereoscopically viewed, an image leakage (crosstalk) is generated between the left and right images depending on a separation degree of the left and right images. This causes degradation in the quality of the image as a stereoscopic image. 
     Japanese Patent Application Laid-Open No. 2001-186549 discloses a stereoscopic display crosstalk amount measuring apparatus which measures an amount of crosstalk from one of the left and right images to the other of the left and right images. Further, Japanese Patent Application Laid-Open No. 2004-312780 describes a technique which reduces the amount of crosstalk. 
     SUMMARY OF THE INVENTION 
     It is known that the binocular fusion limit is significantly different among individuals. However, the invention described in Japanese Examined Application Publication No. 3771964 is just intended to adjust the shift amounts of the left eye and right eye images so that the visual distance and the convergence angle of the eyeballs of a viewer viewing a stereoscopic image become desired values, and does not involve the idea of controlling the parallax between the left eye and right eye images according to a fusion limit of each viewer. 
     In the invention described in Japanese Patent Application Laid-Open No. 9-74573, a stereographic CG (Computer Graphics) image is generated so that the whole part of a subject is included in the binocular fusion range of a viewer, but the stereoscopic CG image is not generated according to the fusion limit which is different for each viewer. In particular, since the fusion limit is different for each viewer, there is a problem that when a stereoscopic image is generated according to the fusion limit of all viewers, the generated stereoscopic image lacks stereoscopic effect and a sense of reality. 
     In the invention described in Japanese Patent Application Laid-Open No. 2004-165709, a stereoscopic image is switched to a plane image on the basis of the measured viewing time of the stereoscopic image in order to cope with asthenopia of a viewer viewing the stereoscopic image. However, the switching of the images is not performed according to an actual level of fatigue. Thus, even when viewing a stereoscopic image causing less fatigue, the viewer cannot experience the stereoscopic effect for a long time, while when viewing a stereoscopic image causing greater fatigue, asthenopia of the viewer may be increased more than expected. 
     In the invention described in Japanese Patent Application Laid-Open No. 2001-186549, the crosstalk of a stereoscopic image in various stereoscopic display methods can be measured, but it is not possible to check whether or not a stereoscopic image, which is properly viewed in a certain stereoscopic display method, can be properly viewed in another stereoscopic display method in which a larger amount of crosstalk is generated. 
     The presently disclosed subject matter has been made in view of the above described circumstances. An object of the presently disclosed subject matter is to provide a stereoscopic image display apparatus which can display, for each viewer, a stereoscopic image not exceeding the fusion limit of the viewer, and can also accurately measure the level of asthenopia of the viewer so as to thereby suppress the level of asthenopia of the viewer to a fixed level or less, and can further check beforehand the stereoscopic image quality based on the amount of crosstalk which is different for each of various stereoscopic display methods. 
     To this end, a stereoscopic image display apparatus according to a first aspect of the presently disclosed subject matter is featured by including: a stereoscopic image display device; an information acquiring device which acquires, beforehand, information about the fusion limit of each viewer; an image acquiring device which acquires left eye and right eye images having parallax therebetween; a parallax control device which controls the parallax between the acquired left eye and right eye images, the parallax control device controlling, on the basis of the acquired information about the fusion limit of the viewer, the parallax between the left eye and right eye images in a range not exceeding at least the fusion limit; and a first display control device which controls the stereoscopic image display device to display a stereoscopic image formed of the left eye and right eye images having parallax controlled on the basis of the left eye and right eye images having the controlled parallax, wherein the information acquiring device includes: an imaging device which photographs the face of the viewer; a pupil width measuring device which detects left and right pupils of the viewer from the face image obtained by the photographing and measures the pupil width between the left and right pupils; a second display control device which outputs, to the stereographic image display device, left eye and right eye images that are images for measurement to measure the fusion limit of the viewer and that have continuously or stepwise changing parallax therebetween; and a device which acquires information about the fusion limit of the viewer on the basis of the viewer&#39;s pupil width that is measured by the pupil width measuring device during the images for measurement are displayed. 
     According to the first aspect of the presently disclosed subject matter, while left eye and right eye images (images for measurement) having continuously or stepwise changing parallax therebetween are outputted beforehand to the stereoscopic image display device, the fusion limit of the viewer is measured by measuring the pupil width of the viewer viewing the images for measurement. Then, when a stereoscopic image for appreciation is displayed, the parallax of the stereoscopic image (between the left eye and right eye images) is controlled so as not to exceed the viewer&#39;s fusion limit measured beforehand. Thereby, it is possible to stably provide a stereoscopic image, in which the stereoscopic effect and the sense of reality are maximized for each viewer. 
     According to a second aspect of the presently disclosed subject matter, the stereoscopic image display apparatus according to the first aspect further includes: a face recognizing device which recognizes the face of the viewer from the face image acquired by the photographing; and a registering device which registers, in a storage device, the acquired information about the fusion limit of the viewer in association with the recognized face, wherein when the viewer&#39;s face recognized by the face recognizing device is the face which has been registered by the registering device, the information acquiring device acquires the information about the fusion limit by reading out, from the storage device, the information about the fusion limit registered in association with the face. 
     The information about the viewer&#39;s fusion limit is registered in the storage unit in association with the recognized face of the viewer. Thus, when the viewer is individually specified from the viewer&#39;s face image and then the viewer&#39;s fusion limit registered beforehand is read out, there is an advantage that it is not necessary to measure the viewer&#39;s fusion limit each time. 
     According to a third aspect of the presently disclosed subject matter, in the stereoscopic image display apparatus according to one of the first and second aspects, only during a period in which the information about the fusion limit of the viewer is acquired, the power source of the imaging device is turned on so as to enable the imaging device to photograph the viewer. Thereby, power consumption can be suppressed. 
     According to a fourth aspect of the presently disclosed subject matter, the stereoscopic image display apparatus according to one of the first to third aspects, further includes: an integration device which integrates, with respect to a display time of stereoscopic image, an amount of change in the pupil width of the viewer from a prescribed value of pupil width, on the basis of the pupil width of the viewer which is measured by the pupil width measuring device while the viewer views the stereoscopic image displayed on the stereoscopic image display device; a threshold setting device which sets a threshold value used as a reference for determination of a level of asthenopia of the viewer viewing the stereoscopic image; and a stopping device which, when the integrated value exceeds the set threshold value, stops displaying the stereoscopic image performed by the stereoscopic image display device. 
     According to the fourth aspect of the presently disclosed subject matter, since the level of asthenopia of the viewer can be accurately measured, the level of asthenopia of the viewer can be suppressed to a fixed level or less, and also a stereoscopic image, which less causes asthenopia of the viewer, can be viewed by the viewer for a long time. 
     According to a fifth aspect of the presently disclosed subject matter, in the stereoscopic image display apparatus according to the fourth aspect, the stopping device allows only one of the left eye and right eye images to be displayed on the stereoscopic image display device in place of the stereoscopic image formed of the left eye and right eye images. That is, in the fifth aspect of the presently disclosed subject matter, it is configured such that, when the level of asthenopia of the viewer reaches the threshold value set beforehand, only one of the left eye and right eye images (two-dimensional images) is displayed to thereby enable the viewer to recover from asthenopia. 
     According to a sixth aspect of the presently disclosed subject matter, the stereoscopic image display apparatus according to one of the first to fifth aspects, further includes: a crosstalk amount setting device which sets a ratio of the right eye image mixed into the left eye image and which sets a ratio of the left eye image mixed into the right eye image; and an image mixing device which generates left eye and right eye images including the crosstalk by mixing, according to the set ratios, the left eye and right eye images acquired by the image acquiring device, wherein the first control device controls the stereoscopic image display device to display a stereoscopic image formed of the left eye and right eye images including the crosstalk, on the basis of the generated left eye and right images including the crosstalk 
     According to the sixth aspect of the presently disclosed subject matter, the crosstalk can be intentionally generated. Thus, when the content of a specific stereoscopic image is viewed by the other three-dimensional display method, the level of stereoscopic viewing quality can be checked beforehand without using the other three-dimensional display apparatus. 
     According to a seventh aspect of the presently disclosed subject matter, in the stereoscopic image display apparatus according to one of the first to fifth aspects, the stereoscopic image display device includes: an image display device which displays left eye and right eye images by switching the left eye and right eye images alternately; and stereoscopic viewing eyeglasses which can switch, alternately, transmittance of light beams respectively entering the left and right eyes of the viewer, and the first display control device which controls the image display device to display the left eye and right eye images acquired by the image acquiring device alternately at a predetermined period, and which controls the transmittance of the stereoscopic viewing eyeglasses to switch alternately at the predetermined period. 
     According to an eighth aspect of the presently disclosed subject matter, the stereoscopic image display apparatus according to the seventh aspect, further includes: a crosstalk amount setting device which sets a ratio of the right eye image mixed into the left eye image and which sets a ratio of the left eye image mixed into the right eye image; and a phase control device which shifts, based on the ratios set by the crosstalk amount setting device, the switching timing of the left eye and right eye images displayed on the image display device from the switching timing of the transmittance of the stereoscopic viewing eyeglasses. 
     According to a ninth aspect of the presently disclosed subject matter, the stereoscopic image display apparatus according to the seventh aspect, further includes: a crosstalk amount setting device which sets a ratio of the right eye image mixed into the left eye image and which sets a ratio of the left eye image mixed into the right eye image; and a transmittance control device which controls the ratios of the transmittance of the right and left stereoscopic viewing eyeglasses on the basis of the ratios set by the crosstalk amount setting device. 
     A stereoscopic image display apparatus according to a tenth aspect of the presently disclosed subject matter includes: a stereoscopic image display device; an image acquiring device which acquires left eye and right eye images having parallax therebetween; a display control device which controls the stereoscopic image display device to display a stereoscopic image formed of the left eye and right eye images, on the basis of the acquired left eye and right eye images; an imaging device which photographs a face of a viewer viewing the stereoscopic image displayed on the stereoscopic image display device; a pupil width measuring device which detects left and right pupils of the viewer from an image of the face acquired by the imaging device, and which measures a pupil width between the left and right pupils; an integration device which integrates, with respect to a display time of the stereoscopic image, an amount of change in the pupil width of the viewer from a prescribed value of pupil width based on the measured pupil width of the viewer; a threshold setting device which sets a threshold value used as a reference for determination of a level of asthenopia of the viewer viewing the stereoscopic image; and a stopping device which, when the integrated value exceeds the set threshold value, stops displaying the stereoscopic image performed by the stereoscopic image display device. 
     According to an eleventh aspect of the presently disclosed subject matter, in the stereoscopic image display apparatus according to the tenth aspect, the stopping device allows only one of the left eye and right eye images to be displayed on the stereoscopic image display device in place of the stereoscopic image formed of the left eye and right eye images. 
     A stereoscopic image display apparatus according to a twelfth aspect of the presently disclosed subject matter includes: a stereoscopic image display device; an image acquiring device which acquires left eye and right eye images having parallax therebetween; a crosstalk amount setting device which sets a ratio of the right eye image mixed into the left eye image and which sets a ratio of the left eye image mixed into the right eye image; an image mixing device which generates left eye and right eye images including the crosstalk by mixing, according to the set ratios, the left eye and right eye images acquired by the image acquiring device; and a display control device which controls the stereoscopic image display device to display a stereoscopic image formed of the left eye and right eye images including the crosstalk, based on the generated left eye and right images including the crosstalk. 
     A stereoscopic image display apparatus according to a thirteenth aspect of the presently disclosed subject matter includes: a stereoscopic image display device including an image display device which displays left eye and right eye images by switching the left eye and right eye images alternately at a predetermined period, and stereoscopic viewing eyeglasses which can switch, alternately at the predetermined period, transmittance of light beams respectively entering the left and right eyes of a viewer; an image acquiring device which acquires left eye and right eye images having parallax therebetween; a display control device which controls the image display device to display the left eye and right eye images acquired by the image acquiring device alternately at the predetermined period, and which controls the transmittance of the stereoscopic viewing eyeglasses to switch alternately at the predetermined period; a crosstalk amount setting device which sets a ratio of the right eye image mixed into the left eye image and which sets a ratio of the left eye image mixed into the right eye image; and a phase control device which shifts the switching timing of the left eye and right eye images displayed on the image display device from the switching timing of the transmittance of the stereoscopic viewing eyeglasses, based on the ratios set by the crosstalk amount setting device. 
     A stereoscopic image display apparatus according to a fourteenth aspect of the presently disclosed subject matter includes: a stereoscopic image display device including an image display device which displays left eye and right eye images by switching the left eye and right eye images alternately at a predetermined period, and stereoscopic viewing eyeglasses which can switch, alternately at the predetermined period, transmittance of light beams respectively entering the left and right eyes of a viewer; an image acquiring device which acquires left eye and right eye images having parallax therebetween; a display control device which controls the image display device to display the left eye and right eye images acquired by the image acquiring device alternately at the predetermined period, and which controls the transmittance of the stereoscopic viewing eyeglasses to switch alternately at the predetermined period; a crosstalk amount setting device which sets a ratio of the right eye image mixed into the left eye image and which sets a ratio of the left eye image mixed into the right eye image; and a transmittance control device which controls the ratios of the transmittance of the right and left stereoscopic viewing eyeglasses based on the ratio set by the crosstalk amount setting device. 
     According to the presently disclosed subject matter, the fusion limit of each viewer is measured beforehand, and a stereoscopic image is outputted while the parallax of the stereoscopic image is controlled so as not to exceed the fusion limit of the viewer. Thereby, it is possible to provide a stereoscopic image which can be viewed by each viewer in the state where the stereoscopic effect and the sense of reality are maximized for the viewer. Further, the level of asthenopia of the viewer can be accurately measured, and thus the level of asthenopia of the viewer can be suppressed to a fixed level or less. Also, in the case of a stereoscopic image less causing asthenopia, the viewer can view the stereoscopic image for a long time. Further, the crosstalk can be intentionally generated. Thus, when the contents of a specific stereoscopic image is viewed by using another three-dimensional display apparatus, the level of stereoscopic viewing quality can be estimated beforehand without using the other three-dimensional display apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view of a stereoscopic image display apparatus according to the presently disclosed subject matter; 
         FIG. 2  is a block diagram showing an internal configuration of the stereoscopic image display apparatus; 
         FIG. 3  is a schematic diagram of a display apparatus of a parallax barrier system; 
         FIG. 4  is a figure showing a principle of stereoscopic display of the parallax barrier system; 
         FIG. 5  shows plan views of a left eye image (L image) and a right eye image (R image) which have parallax therebetween, and a stereoscopic display image; 
         FIG. 6  is a flow chart showing a flow of overall control of a first embodiment according to the presently disclosed subject matter; 
         FIG. 7  is a flow chart used for explaining a method to acquire information on the fusion limit of a viewer; 
         FIG. 8A  to  FIG. 8C  are schematic diagrams each showing a relationship between positions of pupils in both eyes of a viewer and a 3D image which is displayed on a stereoscopic image display unit and which is used for measuring the fusion limit; 
         FIG. 9  is a schematic diagram showing a relationship between the positions of pupils in both eyes of the viewer and a 3D image for the fusion limit measurement, which illustrates a state where the convergence angle of eyeballs of the viewer reaches a limit; 
         FIG. 10  is a figure showing a relationship between two cameras and a distance to a subject at the time of photographing a stereoscopic image; 
         FIG. 11A  to  FIG. 11C  are figures each showing an image which is acquired when the subject located at a different photographing distance is photographed in the camera arrangement as shown in  FIG. 10 ; 
         FIG. 12  is a flow chart showing a second embodiment according to the presently disclosed subject matter; 
         FIGS. 13A to 13C  are figures used to explain a method for acquiring information representing a level of asthenopia of the viewer viewing a stereoscopic image; 
         FIG. 14  is a block diagram showing an internal configuration of a stereoscopic image display apparatus according to a third embodiment; 
         FIG. 15  is a figure showing an example of a menu screen of a stereoscopic image quality simulation; 
         FIGS. 16A to 16D  are figures showing an original stereoscopic image, and stereoscopic images in each of which the amount of crosstalk is increased; 
         FIG. 17  is a block diagram showing an internal configuration of a stereoscopic image display apparatus of a modification of the third embodiment; 
         FIG. 18  is a graph showing a relationship between a control level and a transmittance of liquid crystal shutter eyeglasses; 
         FIG. 19  is a timing chart which shows the left and right image output timing and the control timing of the liquid crystal shutter eyeglasses at the normal time; 
         FIG. 20  is a timing chart which shows the left and right image output timing and the control timing of the liquid crystal shutter eyeglasses in the case where the amount of crosstalk is increased; and 
         FIG. 21  is a figure showing the control of the transmittance of the liquid crystal shutter eyeglasses in the case where the amount of crosstalk is increased. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of a stereoscopic image display apparatus according to the presently disclosed subject matter will be described with reference to the accompanying drawings. 
     [Configuration of Stereoscopic Image Display Apparatus] 
       FIG. 1  is an external view of a stereoscopic image display apparatus according to the presently disclosed subject matter, and  FIG. 2  is a block diagram showing an internal configuration of the stereoscopic image display apparatus. 
     The stereoscopic image display apparatus  10  includes an imaging unit  20 , a stereoscopic image display unit  30 , a display control unit  40 , a memory control unit  42 , a main memory  44 , a digital signal processing unit  46 , a central processing unit (CPU)  48 , a face recognition processing unit  50 , a media control unit  52 , and a storage unit  54 , and these are connected to each other via a data bus  56  and a control bus  58 . 
     As shown in  FIG. 1 , the imaging unit  20  is arranged at an upper portion of the stereoscopic image display apparatus  10  in order to photograph a face of a viewer. The imaging unit  20 , whose imaging operation is controlled by a command from the CPU  48 , is mainly configured by an imaging lens  22 , the solid-state imaging element  24 , such as a CCD, and an imaging element driving/imaging signal processing unit  26 . 
     An optical image (including the face of the viewer) of the subject imaged on the light receiving surface of the solid-state imaging element  24  via the imaging lens  22  is subjected to photoelectric conversion by the solid-state imaging element  24 , and is read, as an image signal, by the imaging element driving/imaging signal processing unit  26 . The read image signal is subjected, by the imaging element driving/imaging signal processing unit  26 , to analog processing, such as amplification, as well as to A/D conversion, and thereafter is once stored in the main memory  44  via the data bus  56  and the memory control unit  42 . 
     The digital signal processing unit  46  performs processing, such as expansion processing, size conversion processing, and image quality correction processing, to a compressed still or moving image read from a recording medium  60  via the media control unit  52 , in addition to the image quality correction processing, and the like, of the image signal stored in the main memory  44 . 
     The face recognition processing unit  50  detects the face of the viewer from the image which is photographed by the imaging unit  20  and which includes the face of the viewer, and also recognizes the face (specifies the viewer) on the basis of the characteristic amounts of the detected face. 
     The face detection is performed in such a manner that while the position of a predetermined target area is moved within the photographed image including the face of the viewer, the image of the target area is collated with a face image template to check correlation between the image of the target area and the face image template, and that, when the obtained correlation score exceeds a threshold value set beforehand, the target area is detected as a face area. In addition, it is possible to use, as the face detecting method, known methods, such as the face detecting method based on the edge detection or the shape pattern detection, and the face detecting method based on the hue detection, or the skin color detection. 
     Further, the face recognition of the detected face is performed in such a manner that the main component analysis results of the detected face image and characteristic amounts such as a size of face parts such as eyes, a nose, and a mouth, and a space between the face parts, are obtained for each of the viewers and registered in the storage unit  54 , and then the viewer is specified according to the matching degree between the characteristic amounts obtained from the face in the photographed image and the characteristic amounts of the face of the viewer registered in the storage unit  54 . 
     Further, the face recognition processing unit  50  detects the position of the left and right pupils of the viewer viewing a stereoscopic image. Note that a method of utilizing the detected position of the left and right pupils will be described below. 
     Further, the information on the binocular fusion limit of each viewer is registered in the storage unit  54  in association with the characteristic amounts of the face (or in association with the face image) of the viewer. Further, a stereoscopic image for measurement, which is used to measure the fusion limit of a viewer and has continuously or stepwise changing parallax, is stored in the storage unit  54 , and is read at the time of measuring the fusion limit of the viewer, and displayed on the stereoscopic image display unit  30 . Note that the details of the measuring method of the fusion limit of each viewer will be described below. 
     The stereoscopic image display unit  30  is, for example, a type of parallax barrier system, and configured such that, as shown in  FIG. 3  and  FIG. 4 , a barrier  34  provided with vertical slits is arranged on a front side of a flat display apparatus  32  such as a liquid crystal display. The stereoscopic image display unit  30  can display a stereoscopic image having parallax. 
     That is, a stereoscopic display image, shown in Portion  3  of  FIG. 5 , in which pixels of a left eye image (L image), shown in Portion  1  of  FIG. 5 , and pixels of a right eye image (R image), shown in Portion  2  of  FIG. 5 , are alternately arranged is synthesized from the L image and the R image that have parallax therebetween as shown in  FIG. 5 . And, when the synthesized stereoscopic image is displayed on the flat display apparatus  32 , as shown in  FIG. 4 , the barrier  34  functions to enable only the L image to be viewed by the left eye and also functions to enable only the R image to be viewed by the right eye. 
     The display control unit  40  controls the stereoscopic image display unit  30  to display the stereoscopic display image. In addition, the display control unit  40  controls to display a stereoscopic display image with the amount of parallax between the L image and the R image adjusted according to a command from the CPU  48 , or controls to display one (two-dimensional (2D) image) of the L image and the R image. Note that it is preferred that the barrier  34  is configured by a liquid crystal, and the like, and that the barrier  34  is made transparent at the time of displaying a 2D image. 
     First Embodiment 
     Next, a first embodiment of a stereoscopic image display apparatus according to the presently disclosed subject matter will be described. 
     As shown in  FIG. 6 , when a stereoscopic image (for example, a slide show of 3D still images and a 3D moving image) is displayed by the stereoscopic image display apparatus, the information on the fusion limit of a viewer is first acquired (step S 10 ). 
     The method for acquiring the information on the fusion limit of the viewer will be described with reference to  FIG. 7 . 
     The CPU  48  turns on the power source of the camera (imaging unit  20 ) mounted on the stereoscopic image display apparatus  10  (step S 20 ), and makes the camera photograph the viewer (including the face of the viewer) (step S 22 ). The face recognition processing unit  50  detects a face image from the photographed image, and performs face recognition on the basis of the characteristic amounts of the detected face image (step S 24 ). 
     When it is determined that the face of the viewer is the one whose fusion limit has already been registered, the CPU  48  reads, from the storage unit  54 , the information on the fusion limit of the corresponding viewer (step S 28 ), and thereafter turns off the power source of the camera (step S 40 ). 
     On the other hand, when it is determined that the face of the viewer is not the one whose fusion limit has already been registered, the CPU  48  reads the 3D image for fusion limit measurement from the storage unit  54 , and makes the 3D image for fusion limit measurement displayed on the stereoscopic image display unit  30  (step S 30 ). 
       FIG. 8A  to  FIG. 8C  are schematic diagrams each showing a relationship between the positions of the pupils of both eyes of the viewer and the 3D image for fusion limit measurement (in which the solid line shows the L image and in which the broken line shows the R image) which is displayed on the stereoscopic image display unit  30 . 
     When there is almost no parallax between the L image and the R image as shown in  FIG. 8B , the observed subject is recognized to be located near the surface of the display surface, and the convergence angle of the eyeballs α b  becomes an angle formed by the left and right lines of sight which intersect each other on the display surface. 
     On the other hand, when parallax for the subject exists in a depth direction as shown in  FIG. 8A , the convergence angle of the eyeballs α a  becomes an angle formed by the left and right lines of sight which intersect each other at the imaginary point located on the deeper side from the display surface. On the contrary, when parallax for the subject exists in a pop-up direction (near side direction) from the display surface as shown in  FIG. 8C , the convergence angle of the eyeballs α c  becomes an angle formed by the left and right lines of sight which intersect each other at the imaginary point located on the front side from the display surface. The relationship between the convergence angles of the eyeballs α a , α b , α c  is expressed as α a &lt;α b &lt;α c . Further, the convergence angles of the eyeballs α a , α b , α c  and the width between the left and right pupils (pupil width) L a , L b , and L c  have a fixed relationship as expressed as L a &gt;L b &gt;L c . 
     When parallax is gradually increased in the pop-up direction of the 3D image for measurement, at a point where the convergence angle of the eyeballs of the viewer reaches a limit, the viewer cannot view the image as a stereoscopic image and views the image as a double image on the display surface as shown in  FIG. 9 . The convergence angle of the eyeballs α b  and the pupil width L b  at this point are the same as those shown in  FIG. 8B  in the case where the parallax is small. 
     The CPU  48  controls to display the 3D image for measurement having gradually changing parallax on the stereoscopic image display unit  30  (step S 30 ), and also measures the pupil width of the viewer by photographing the face of the viewer (step S 32 , S 34 ). 
     Then, the CPU  48  determines, on the basis of the relationship between the amount of parallax of the displayed 3D image for measurement and the pupil width, whether or not the binocular fusion range of the viewer reaches the fusion limit (step S 36 ). The CPU  48  registers the information on the fusion limit at the time of the binocular fusion range reaching the fusion limit (for example, the information on the amount of parallax of the 3D image for measurement, or the information on the convergence angle of the eyeballs, the pupil width, and the like) in the storage unit  54  in association with the face image of the viewer or with the characteristic amounts of the face image (step S 38 ), and turns off the power source of the camera (step S 40 ). 
     Returning to  FIG. 6 , the CPU  48 , which has acquired the information on the fusion limit of the viewers as described above, outputs the information on the fusion limit of each of the viewers to the display control unit  40 . The display control unit  40  outputs the L image and the R image to the stereoscopic image display unit  30  while controlling, on the basis of the information on the fusion limit of each of the viewers, the maximum amount of parallax between the L image and the R image does not exceed the fusion limit of the viewer. 
     For example, in the case of a 3D still image, when one of the L image and the R image is used as a reference image, and when a parallax image is to be generated from the deviation amount (parallax amount) of respective corresponding points in the reference image and the other image, the parallax image is generated in a manner that, among the parallax amounts of corresponding points, the maximum parallax amount is adjusted so as not to exceed the fusion limit of the viewer and that the parallax amount of corresponding points other than that whose parallax amount is the maximum are also adjusted according to the adjusted maximum value. Further, when the amount of processing is large as in the case of a 3D moving image, parallel movement of the L image and the R image may be performed so as to prevent the maximum parallax amount between the L image and the R image from exceeding the fusion limit of the viewer. 
     Second Embodiment 
     Next, a second embodiment of a stereoscopic image display apparatus according to the presently disclosed subject matter will be described. 
     A stereoscopic image is photographed by a method shown in  FIG. 10 . Two cameras for respectively photographing L and R images are arranged in a state where the optical axes of the two cameras are arranged in parallel with each other or arranged to form a slight (convergence) angle, and a subject is simultaneously photographed by the two cameras to obtain a stereoscopic image. 
     The parallax of the obtained stereoscopic image is changed according to the conditions such as an interval between the two cameras (base line length), the angle (convergence angle) formed by the optical axes, and the distance to the subject. For example, when the photographing distance is different (distances a, b and c) in the camera arrangement shown in  FIG. 10 , the images shown in  FIG. 11A  to  FIG. 11C  are obtained by photographing the subject at the respective distances. 
     In the case of the distance b, since the interunit of the optical axes of the cameras, which respectively photograph the L image and the R image, substantially coincides with the position of the subject, parallax between the photographed L and R images is small, and a synthesized image for stereoscopic display is obtained as shown in Portion  3  of  FIG. 11B .  FIG. 8B  shows the position of the pupil of both eyes of the viewer in the case where the viewer views the stereoscopic image photographed under the condition of distance b. Although the viewer independently views the L and R images by the respective left and right eyes, the parallax between the L and R images is small, and hence the viewer recognizes the subject position at almost the same position as the display surface of the stereoscopic image display apparatus  10 . In the case where the difference between the physical display surface and the display position recognized by the viewer is small in this way, it is estimated that the viewing of the images causes comparatively less asthenopia. 
     The subject position (pupil width L b  at this time) is used as a prescribed value  1  (L 1 ) in the stereoscopic display control as will be described below. 
     Next, the cases where the distance to the subject is different from the distance in the above described case will be described. When the subject located at the distance a and the subject located at the distance c are photographed under the condition of the camera arrangement shown in  FIG. 10 , larger parallaxes are generated between the L image and the R image unlike the case of the distance b (Portion  3  of  FIG. 11A  and Portion  3  of  FIG. 11C ). 
       FIG. 8A  and  FIG. 8C  show the pupil width of the viewer in the cases where the viewer views the image at the distance a and the distance b. 
     In the case of  FIG. 8A , due to the parallax between the L and R images, the viewer feels as if the subject is positioned on the deeper side from the display surface. On the contrary, in the case of  FIG. 8C , the viewer feels as if the subject is positioned on the nearer side from the display surface. At this time, the subject position recognized by the viewer is different from the position in the physical display surface, and the viewer feels a stereoscopic effect. However, a long time viewing of such images causes asthenopia. 
     When stereoscopic images as shown in  FIG. 8A  and  FIG. 8C  are viewed, the pupil widths L a  and L c  of the viewer are changed by the parallax of the image shown in the figures. The pupil width L a  in the case where the subject is recognized to be positioned on the deeper side from the display surface is larger than the pupil width L b  in the case where the subject is recognized to be positioned on the display surface. The pupil width L c  in the case where the subject is recognized to be positioned on the front side from the display surface is smaller than the pupil width L b  in the case where the subject is recognized to be positioned on the display surface. 
     In the second embodiment according to the presently disclosed subject matter, the pupil width between the pupils of both eyes of the viewer viewing a stereoscopic image is measured and calculated, and compared with the prescribed value L 1 , whereby the level of fatigue, that is, the level of asthenopia, of the viewer viewing the image is calculated. It is possible to prevent asthenopia of the viewer in a manner that the level of fatigue is accumulated during the viewing time, and that the stereoscopic display is stopped at the time when the accumulated value S representing the level of fatigue exceeds a prescribed value  2  (S 1 ). 
       FIG. 12  is a flow chart showing a second embodiment according to the presently disclosed subject matter. Here, the case where stereoscopic images  1 ,  2 ,  3 ,  4 , . . . are successively displayed as shown in  FIG. 13A  will be described. 
     The CPU  48  controls the stereoscopic image display unit  30  to display the image  1  at the start of the viewing of a stereoscopic image (step S 50 ). Further, the CPU  48  controls the imaging unit  20  to photograph the face of the viewer, and obtains the pupil width Lx of the viewer. Then, the CPU  48  calculates an absolute value |Lx−L 1 | of the difference between the obtained pupil width Lx and the prescribed value L 1  (step S 52 ,  FIG. 13B ). 
     Then, the CPU  48  calculates, as an increment of fatigue ΔS, the product of the absolute value |Lx−L 1 | of the difference and the display time t of the image  1  (step S 54 ). 
     Then, the CPU  48  calculates the accumulation value S by accumulating the increment of fatigue ΔS (step S 56 ,  FIG. 13C )). Note that the initial value of the accumulation value S is set to 0. 
     Then, the CPU  48  determines whether or not the accumulation value S representing the level of fatigue exceeds the prescribed value S 1  set beforehand (step S 58 ). At the time when the level of fatigue exceeds the prescribed value S 1 , the CPU  48  stops the 3D display, and switches from the 3D display to the 2D display in the present embodiment (step S 60 ). The switching from the 3D display to the 2D display is performed by displaying only one of the L image and the R image. 
     When the level of fatigue does not exceed the prescribed value S 1 , the CPU  48  determines whether or not termination of the 3D display is instructed by the viewer (step S 62 ). When termination of the 3D display is not instructed by the viewer, the CPU  48  proceeds to step S 50 , so as to continue the 3D display. 
     On the other hand, when the 3D display is stopped and switched to the 2D display, the viewer gradually recovers from asthenopia. Thus, the accumulation value S representing the level of fatigue is gradually reduced as shown in  FIG. 13C  (step S 64 ). 
     For example, when the accumulation value S representing the level of fatigue reaches the prescribed value S 1 , the 3D display is stopped. Then, the accumulation value S representing the level of fatigue is reduced according to the following expression using a function f (t) which expresses the level of fatigue recovery according to the elapsed time t from the stop of the 3D display, and which is obtained beforehand. 
         S=S 2 −f ( t )  [Expression 1]
 
     Then, the CPU  48  determines whether or not the accumulation value S calculated as described above reaches a prescribed value S 2 , (step S 66 ). When the accumulation value S reaches the prescribed value S 2 , the CPU  48  recognizes that asthenopia is sufficiently reduced, and proceeds to step S 50  via step S 62  so as to return the display to the 3D display. 
     On the other hand, when the calculated accumulation value S is larger than the prescribed value S 2 , the CPU  48  recognizes that asthenopia is not sufficiently reduced, and shifts to step S 60  via step S 68  so as to continue the 2D display. 
     Note that the level of asthenopia of the viewers viewing the stereoscopic display is different for each of the viewers, and hence the prescribed value S 1  which is specified as the threshold value of the level of asthenopia may be suitably set for each of the viewers. Further, it is preferred that the prescribed value S 2  for determining the recovery from asthenopia is also suitably set. 
     Further, in the present embodiment, the images (images  1 ,  2 , . . . ) successively 3D displayed are still images, but the present embodiment can also be applied to the case of a 3D moving image. In this case, an accumulation value representing the level of fatigue is calculated by accumulating the product of the display time and the absolute value of the difference between the prescribed value L 1  and the pupil width measured every frame of the moving image or measured every fixed time. 
     Further, in the present embodiment, the absolute value of the difference between the measured pupil width and the prescribed value L 1  are used as it is, but the present embodiment is not limited to this. Only the difference in case where the pupil width is smaller than the prescribed value L 1 , may also be used. Alternatively, weighting may be performed on the difference in case where the pupil width is smaller than the prescribed value L 1 , and the difference in the case where the pupil width is larger than the prescribed value L 1 . 
     Third Embodiment 
     Next, a third embodiment of a stereoscopic image display apparatus according to the presently disclosed subject matter will be described. 
       FIG. 14  is a block diagram showing an internal configuration of a stereoscopic image display apparatus according to the third embodiment. Note that the same portions as those in the block diagram shown in  FIG. 2  are indicated by the same reference numerals and characters, and the detailed explanation thereof is omitted. 
     As shown in  FIG. 14 , this stereoscopic image display apparatus  10 ′ is mainly configured by additionally including an operation unit  62 , a user interface (UI) control unit  68 , and a left-and-right image addition processing unit  70 , as compared with the stereoscopic image display apparatus  10  shown in  FIG. 2   
     The L and R images for left and right eyes which images are inputted from an external input and output unit (I/O) via a signal input unit  64 , or the L and R images which are inputted from the recording medium  60  via the media control unit  52 , are subjected to, for example, processing of image quality such as contrast and resolution, and processing of field angle, or processing of display position, and the like, by the digital signal processing unit  46 , and are presented to the viewer by the stereoscopic image display unit  30  (a left eye image display unit  30 A and a right eye image display unit  30 B) via the display control unit  40  (a left eye image control unit  40 A, and a right eye image control unit  40 B) and the left-and-right image addition processing unit  70 . As described above, since the left eye image and the right eye image are exclusively and respectively viewed by viewer&#39;s left and right eyes, the images are stereoscopically sensed by the viewer. 
     At this time, it is ideal that the left and right images are independently presented to the viewer&#39;s visual sense. However, for some stereoscopic display methods used in the stereoscopic image display unit  30 , crosstalk is generated in the left and right images, so that the left and right images, into which the other side of the images is mixed, are perceived by the viewer. 
     The quality of a stereoscopic image is evaluated by the parameters such as the stereoscopic effect and the sense of reality, in addition to the parameters such as the resolution sense, the chromaticness and the contrast which are similarly used for the 2D image evaluation. Among them, the amount of crosstalk greatly influences the quality of the stereoscopic image. 
     Thus, in the stereoscopic image display apparatus  10 ′ according to the present embodiment, image information respectively sent to the left eye image display unit  30 A and the right eye image display unit  30 B are subject to weighted addition by the left-and-right image addition processing unit  70  so as to increase the amount of crosstalk, and the quality of the stereoscopic image in the case where crosstalk is increased can be simulated. 
     When the quality of the stereoscopic image is simulated, the stereoscopic image quality simulation menu, for example, as shown in  FIG. 15 , is displayed on the stereoscopic image display unit  30  via the UI control unit  68  on the basis of the operation in the operation unit  62 , and then a software button for selecting an mount of crosstalk in the menu screen is operated to instruct a desired amount of crosstalk. 
     In the example shown in  FIG. 15 , the stereoscopic image quality simulation menu includes a button  62 A for selecting the stereoscopic image display unit  30  of the present apparatus, a button  62 B for selecting a 3D liquid crystal display to be viewed by the naked eye, and a button  62 C for selecting a stereoscopic print on the surface of which a lenticular lens sheet is stuck. A user selects one of the buttons, thereby enabling the user to instruct an amount of crosstalk corresponding to the selected button. Note that the method for increasing the amount of crosstalk is not limited to the case where the kind and the like of apparatus is selected as shown in  FIG. 15 , a numerical value of the amount of crosstalk may also be inputted. 
     The left-and-right image addition processing unit  70  includes multipliers  72 A and  72 B which multiply the inputted image by a prescribed ratio (1−k) (0≦k&lt;1), multipliers  74 A and  74 B which multiply the inputted image by a prescribed ratio k, and adders  76 A and  76 B which add the multiplication results. The prescribed ratio k is set according to the result of the selection of the 3D display device, which is performed by the operation unit  62 . 
     Assuming that the image signals of the L and R images, which are respectively inputted into the multipliers  72 A and  74 A and the multipliers  72 B and  74 B, are respectively expressed as S L  and S R , the L image calculated by the multipliers  72 A and  74 B and the adder  76  A in the left-and-right image addition processing unit  70 , and the R image calculated by the multipliers  72 B and  74 A and the adder  76 B in the left-and-right image addition processing unit  70 , are expressed by the following expression. 
         L  image=(1− k )· S   L   +k·S   R  
 
         R  image=(1− k )·S R   +k·S   L   [Expression 2]
 
     The L and R images, in each of which the amount of crosstalk is increased by the left-and-right image addition processing unit  70 , are respectively sent to and displayed on the left eye image display unit  30 A and the right eye image display unit  30 B. Thereby, the viewer can simulate the quality of the stereoscopic image having an increased amount of crosstalk by viewing the left eye image display unit  30 A and the right eye image display unit  30 B. For example, it is possible to simulate a stereoscopic image having an increased amount of crosstalk at a ratio of: k=0 when a stereoscopic image displayed by the present apparatus; k=0.1 when a stereoscopic image is displayed by the naked eye liquid crystal; and k=0.25 when a stereoscopic image is stereoscopically printed. 
     When the L and R images inputted into the left-and-right image addition processing unit  70  are the images shown in  FIG. 16A  (images in which the region of pixel value  10  and the region of pixel value  90  exist), and when the prescribed ratio k is 5%, the pixel values  90  and  10  in the central portion in which the L and R images are mixed with each other, become  86  and  14 , respectively ( FIG. 16B ). Further, when the prescribed ratio k is 20%, the pixel values  90  and  10  in the central portion in which the L and R images are mixed with each other, become  72  and  28 , respectively ( FIG. 16C ). 
     Note that in the prior art described in Japanese Patent Application Laid-Open No. 2004-312780, the offset addition processing and the subtraction processing of the opposite image are performed in order to improve the crosstalk. Thus, the contrast is sacrificed as shown in  FIG. 16D , and the simulation to increase the amount of crosstalk cannot be performed. 
     Modification of Third Embodiment 
     Next, a modification of the third embodiment of the stereoscopic image display apparatus according to the presently disclosed subject matter will be described. 
       FIG. 17  is a block diagram showing an internal configuration of a stereoscopic image display apparatus according to the modification of the third embodiment. Note that the same portions as those of the block diagram shown in  FIG. 14  are designated by the same reference numerals and characters, and the detailed explanation thereof is omitted. 
     As shown in  FIG. 17 , this stereoscopic image display apparatus  10 ″ is different mainly in the stereoscopic image display system from the stereoscopic image display apparatus  10 ′ shown in  FIG. 14 . The stereoscopic image display apparatus  10 ″ includes, in place of the left-and-right image addition processing unit  70 , a plane sequential converting unit  80  of left and right images, a left-and-right switching control unit  82 , and liquid crystal shutter eyeglasses  90 . 
     It is configured such that the L and R images are respectively added to contact points  80 A and  80 B of the plane sequential converting unit  80  from the left eye image control unit  40 A and the right eye image control unit  40 B, and such that a contact piece  80 C controlled by the left-and-right switching control unit  82  is alternately switched and connected to one of the contact points  80 A and  80 B so that the L and R images are alternately outputted to a display apparatus  30 ′, such as a CRT, and a liquid crystal display apparatus. 
     Further, the liquid crystal shutter eyeglasses  90  are eyeglasses provided with liquid crystal shutters for left eye and right eye (left eye and right eye liquid crystal shutters). The left-and-right switching control unit  82  alternately controls the transmittance of the left eye and right eye liquid crystal shutters in synchronization with the switching of the images in the plane sequential converting unit  80 . 
       FIG. 18  shows an example of the characteristic of transmittance of the liquid crystal shutter eyeglasses  90 . The transmittance of the liquid crystal shutter eyeglasses  90  is increased according to the control level inputted from the left-and-right switching control unit  82 . When the control level is 0, the transmittance becomes substantially 0, while when the control level is X, the transmittance becomes substantially 30%. 
     In the case of the normal stereoscopic display in which the amount of crosstalk is not controlled, the image output to the display apparatus  30 ′ and the control timing of the liquid crystal shutters of the liquid crystal shutter eyeglasses  90  develop the timing chart shown in  FIG. 19 . That is, the transmittance of the left eye liquid crystal shutter is maximized at the time when the left eye image is outputted, while the transmittance of the right eye liquid crystal shutter is maximized at the time when the right eye image is outputted. 
     On the other hand, in the case of controlling the crosstalk, the crosstalk is controlled by performing the phase control in which the control timing of the liquid crystal shutter eyeglasses  90  is delayed (shifted) with respect to the output timing of the left eye and right eye images as shown in  FIG. 20 . 
     Further, it is also possible to obtain the same effect by controlling the transmittance of the liquid crystal shutter eyeglasses  90  as shown in  FIG. 21 , instead of delaying (shifting) the control timing of the liquid crystal shutter eyeglasses  90 . That is, in the case where the prescribed ratio k is set as k=0.1, the transmittance of the liquid crystal shutter eyeglasses  90  is alternately switched between 3% and 27%, instead of switching the transmittance of the liquid crystal shutter eyeglasses  90  between 0% and 30%. 
     [Others] 
     The presently disclosed subject matter is not limited to the above described embodiments, and may be implemented by suitably combining each of the embodiments. Further, it goes without saying that various modifications are possible within the scope and spirit of the presently disclosed subject matter.