Patent Publication Number: US-2002003510-A1

Title: Image display apparatus and method for vehicle

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an image display apparatus and an image display method for a vehicle that allow passengers on the vehicle to view a plurality of still-frame images provided along the moving direction of the vehicle with stability.  
       [0003] 2. Description of the Related Art  
       [0004] In general station name displays and various types of advertisements are provided in a train or underground (subway) station, on the wall of a tunnel and so on. Since such displays are fixed to the wall, it is difficult for the passengers on a vehicle such as a train to view the displays with stability. An image display system has been therefore developed for placing a plurality of still image panels whose contents are the same or slightly different from one another along the moving direction of a train and intermittently illuminating the panels in unison in accordance with the movement of the train. The system allows the passengers on the vehicle to view the string of the still image panels as a still-frame image or a moving image with stability. Such a system is disclosed in, for example, Japanese Patent Publication Hei 7-117654 (1995) (Japanese Patent Application Laid-open Hei 5-27197 (1993)), Japanese Patent Application Laid-open Hei 5-40448 (1993), Japanese Patent Application Laid-open Hei 5-224617 (1993), Japanese Patent Application Laid-open Hei 2-201489 (1990), Japanese Patent Application Laid-open Hei 2-256090 (1990), and Japanese Patent Application Laid-open Hei 3-36515 (1991).  
       [0005] In such a system of related art, assuming that the speed of a vehicle passing through the system is maintained at a predetermined velocity, or in order to simplify the system configuration, whether the vehicle enters the section where the system is provided is only detected but the velocity of the vehicle is not detected. In such a case, however, the position in which the image is displayed when viewed by the passengers horizontally drifts and becomes unstable if the velocity of the vehicle changes from the estimated velocity.  
       [0006] In order to overcome the problem, a velocity sensor may be provided at one point in the system installation section (at the forward end, for example). Based on the velocity information obtained by the sensor, timing of still image panel illumination may be controlled. In the method illumination of the still frame panels is turned on and off with the optimal timing in accordance with the velocity as long as the velocity of the vehicle is detected. As a result, the image display position when viewed by the passengers on the vehicle is stabilized.  
       [0007] In the method, however, the velocity information is not obtained by the sensor in the period between the point in time at which the rearmost part of the vehicle passes over the sensor and the point at which the rearmost part passes over the backward end of the system installation section. In the period the operation is controlled based on the last velocity data obtained by the sensor. Therefore, if the velocity of the vehicle changes during the period between the point at which the rearmost part of the vehicle passes over the sensor and the point at which the rearmost part passes over the backward end of the system installation section, it is impossible to change on/ off timing of illumination of the still image panels in response to the change in velocity. Consequently, the image display position when viewed by the passengers on the vehicle horizontally drifts and becomes unstable.  
       SUMMARY OF THE INVENTION  
       [0008] It is an object of the invention to provide an image display apparatus and an image display method for a vehicle that achieve stable image display in accordance with the velocity of the vehicle throughout the period in which the vehicle passes through the system installation section.  
       [0009] An image display apparatus for a vehicle of the invention comprises: a plurality of image display means placed in a row at specific intervals along a moving direction of the vehicle, each being capable of instantaneously displaying a still-frame image; a first velocity detection means, placed near the foremost part of the row of the image display means, for detecting a velocity of the vehicle; a second velocity detection means, placed near the rearmost part of the row of the image display means, for detecting a velocity of the vehicle; and a means for controlling timing of display of the still-frame images by the image display means, based on the velocity obtained by the first velocity detection means and the velocity obtained by the second velocity detection means. The image display means may be each capable of instantaneously displaying still-frame images forming a moving image in time sequence. The image display means may be each capable of instantaneously displaying still-frame images based on image information given as electrical information.  
       [0010] Another image display apparatus for a vehicle of the invention comprises: a plurality of image display means placed in a row at specific intervals along a moving direction of the vehicle, each being capable of instantaneously displaying a still-frame image; a first velocity detection means, placed near the foremost part of the row of the image display means, for detecting a velocity of the vehicle; a second velocity detection means, placed near the rearmost part of the row of the image display means, for detecting a velocity of the vehicle; a switch means for switching information to output from first velocity information obtained by the first velocity detection means to second information obtained by the second velocity detection means when the difference between the first velocity information and second velocity information falls within a specific range in accordance with a movement of the vehicle; and a means for generating and outputting a signal for controlling timing of displaying the still-frame images by the image display means, based on the first velocity information or the second velocity information selected and outputted by the switch means. The image display apparatus may further comprise a means for determining whether the velocity information outputted from the first and second velocity detection means is valid or not so that the velocity information that is determined as valid by the means for determining is only inputted to the switch means to effect switching. The image display means may be each capable of instantaneously displaying still-frame images forming a moving image in time sequence. The image display means may be each capable of instantaneously displaying still-frame images based on image information given as electrical information.  
       [0011] An image display method for a vehicle of the invention includes the steps of: placing a plurality of image display means in a row at specific intervals along a moving direction of the vehicle, each being capable of instantaneously displaying a still-frame image; placing a first velocity detection means for detecting a velocity of the vehicle near the foremost part of the row of the image display means and placing a second velocity detection means for detecting a velocity of the vehicle near the rearmost part of the row of the image display means; and controlling timing of display of the still-frame images by the image display means, based on the velocity obtained by the first velocity detection means and the velocity obtained by the second velocity detection means.  
       [0012] Another image display method for a vehicle of the invention includes the steps of: placing a plurality of image display means in a row at specific intervals along a moving direction of the vehicle, each being capable of instantaneously displaying a still-frame image; placing a first velocity detection means for detecting a velocity of the vehicle near the foremost part of the row of the image display means and placing a second velocity detection means for detecting a velocity of the vehicle near the rearmost part of the row of the image display means; switching information to output from first velocity information obtained by the first velocity detection means to second information obtained by the second velocity detection means when the difference between the first velocity information and second velocity information falls within a specific range in accordance with a movement of the vehicle, and outputting the second information; and generating and outputting a signal for controlling timing of displaying the still-frame images by the image display means, based on the first velocity information or the second velocity information outputted.  
       [0013] According to the image display apparatus or method of the invention, timing of display of the still-frame images by the image display means is controlled, based on the velocity obtained by the first velocity detection means placed near the foremost part of the row of the image display means and the velocity obtained by the second velocity detection means placed near the rearmost part of the row of the image display means. As a result, timing control of still-frame image display is achieved in accordance with the velocity of the vehicle throughout the period in which the vehicle passes through the section where the image display means are placed.  
       [0014] According to the other image display apparatus or method of the invention, information to output is switched from first velocity information obtained by the first velocity detection means placed near the foremost part of the row of the image display means to second information obtained by the second velocity detection means placed near the rearmost part of the row of the image display means when the difference between the first velocity information and the second velocity information falls within a specific range, and the second information is outputted. A signal for controlling timing of displaying the still-frame images by the image display means is generated, based on the first velocity information or the second velocity information outputted. That is, switching of the velocity information is performed and the signal for controlling timing is generated from the velocity information selected through switching. As a result, the interval of display timing instructed by the signal remains constant before and after the switching of information.  
       [0015] Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a block diagram of an image display apparatus for a vehicle of an embodiment of the invention.  
     [0017]FIG. 2 is a block diagram for illustrating the main configuration of each display terminal shown in FIG. 1.  
     [0018]FIG. 3 is a block diagram for illustrating the main configuration of the main controller shown in FIG. 1.  
     [0019]FIG. 4 is a block diagram for illustrating the main configuration of the incoming velocity detector shown in FIG. 1.  
     [0020]FIG. 5 is a view for illustrating the velocity detection principle of the incoming velocity detector.  
     [0021]FIG. 6 illustrates incoming velocity detection pulses outputted from the incoming velocity detector.  
     [0022]FIG. 7 is a table for describing the parameters required for practically arranging the image display apparatus shown in FIG. 1, giving specific values.  
     [0023]FIG. 8A to FIG. 8I illustrates the way a train passes through the system installation section.  
     [0024]FIG. 9 is a flowchart of the operation of the incoming velocity detector shown in FIG. 4.  
     [0025]FIG. 10 illustrates the way a moving image is viewed by the passengers on the train.  
     [0026]FIG. 11 is a block diagram for illustrating the main configuration of an incoming velocity detector of an image display apparatus for a vehicle of another embodiment of the invention.  
     [0027]FIG. 12 is a flowchart of the operation of the incoming velocity detector shown in FIG. 11.  
     [0028]FIG. 13 is a table for illustrating the operation of the switch of the incoming velocity detector in accordance with passing of the train.  
     [0029]FIG. 14 is a block diagram of a velocity sensor unit of an incoming velocity detector of an image display apparatus for a vehicle of another example of the invention.  
     [0030]FIG. 15A and FIG. 15B illustrate detection signals outputted from the two receptors of the velocity sensor unit shown in FIG. 14.  
     [0031]FIG. 16A to FIG. 16C are views for describing an effect in the embodiment shown in FIG. 11.  
     [0032]FIG. 17 is a view for describing the effect in the embodiment shown in FIG. 11. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0033] Preferred embodiments of the invention will now be described in detail with reference to the accompanying drawings.  
     [0034] First Embodiment  
     [0035]FIG. 1 is a schematic diagram of an image display apparatus for a vehicle of an embodiment of the invention. In the embodiment the image display apparatus is provided in a tunnel through which a train passes, for example. An image display method for a vehicle of an embodiment of the invention implemented by the image display apparatus of the embodiment will be described as well. As shown, an image display apparatus  1  for a vehicle comprises: a plurality of display terminals  11 - 1  to  11 -n placed at specific intervals on the sidewall inside a tunnel along the moving direction of a train  5 ; an incoming velocity detector  12  provided near the foremost part of the installation section of the apparatus (where the train enters the tunnel); an outgoing velocity detector  13  provided near the rearmost part of the installation section of the apparatus (where the train goes out of the tunnel); and a main controller  14  for controlling the entire apparatus. In the embodiment the distance between the incoming velocity detector  12  and the outgoing velocity detector  13  (overall system length D) is equal to or shorter than overall length L of the train  15  (about 100 meters, for example). The display terminals  11 - 1  to  11 -n correspond to ‘image display means’ of the invention. The incoming velocity detector  12  corresponds to a ‘first velocity detection means’ of the invention. The outgoing velocity detector  13  corresponds to a ‘second velocity detection means’ of the invention.  
     [0036] The display terminals  11 - 1  to  11 -n each have the configuration as shown in FIG. 2 described later and instantaneously display various types of still-frame images. Instantaneous display means display performed in a period as short as one thirtieth of a second, for example. The display terminals  11 - 1  to  11 -n are connected to the main controller  14  through a data and control line  21 . Through the data and control line  21  the display terminals  11 - 1  to  11 -n receive various types of still-frame image data and undergo control of the main controller  14 . In addition, the display terminals  11 - 1  to  11 -n receive light-emission timing pulses  24  from the incoming velocity detector  12 .  
     [0037] The main controller  14  has the configuration as shown in FIG. 3 described later and holds various types of still-frame image data. When necessary, the main controller  14  supplies still-frame images identical with one another or still-frame images different from one another to the display terminals  11 - 1  to  11 -n through the data and control line  21 . The main controller  14  is connected to the incoming velocity detector  12  as well and controls the operation of the detector  12 .  
     [0038] The incoming velocity detector  12  has the configuration as shown in FIG. 4 described later and continuously detects the velocity of the train  15  and generates incoming velocity detection pulses  22  (not shown in FIG. 1) having a cycle responsive to the detected velocity. The outgoing velocity detector  13  continuously detects the velocity of the train  15  and generates outgoing velocity detection pulses  23  having a cycle responsive to the detected velocity and sends the pulses  23  to the incoming velocity detector  12 . As will be described later, the incoming velocity detector  12  selects either the pulses  22  generated by itself or the pulses  23  inputted from the outgoing velocity detector  13  and outputs the selected pulses as timing pulses  24 . Part of the incoming velocity detector  12  that mainly performs the selection processing described above and the timing pulses  24  correspond to ‘means for controlling timing’ of the invention.  
     [0039]FIG. 2 illustrates the configuration of each of the display terminals  111  to  11 -n. Although the ith display terminal  11 -i is shown (where i=any of 1 to n), the other display terminals  11 - 1  to  11 -n each have a similar configuration. The display terminal  11 -i comprises: a liquid crystal display (LCD) panel  111  for displaying a still-frame image; a backlight  112  for illuminating the LCD panel  111 ; a backlight controller  113  for controlling the illuminating operation of the backlight  112 ; a frame memory  114  capable of retaining still-frame image data for one frame; an input/ output interface  115  for exchanging still-frame image data and control data with the data and control line  21  and for performing input processing of the timing pulses  24 ; a central processing unit (CPU)  116  for controlling the display terminal  11 -i as a whole; and a memory unit  117  made up of a read only memory (ROM) retaining a program for operation of the CPU  116 , a random access memory (RAM) used as a work memory for the CPU  116  and so on. The LCD panel  111  is placed on the sidewall inside the tunnel in the position corresponding to the vertical position of a window of the train  15 .  
     [0040] In the display terminal  11 -i having such a configuration, compressed still-frame image data for one frame sent from the main controller  14  through the data and control line  21  undergoes signal processing such as expansion by the input/ output interface  115  and is stored in the frame memory  114  and further supplied to the LCD panel  111  under the control of the CPU  116 . The input/ output interface  115  receives the timing pulses  24  from the incoming velocity detector  12 , performs signal processing on the pulses such as waveform shaping, and supplies the pulses to the CPU  116 . The CPU  116  controls the backlight controller  113  based on the inputted timing pulses  24 . The backlight controller  113  controls on and off of the backilight  112  (intermittent instantaneous light-emission control) in synchronization with the timing pulses  24 . As a result, a still-frame image corresponding to the still-frame image data supplied from the frame memory  114  is instantaneously displayed on the LCD panel  111  in an intermittent manner with timing synchronized with the timing pulses  24 .  
     [0041]FIG. 3 illustrates the configuration of the main controller  14 . The main controller  14  comprises: a hard disk drive (HDD)  141  for retaining still-frame image data and moving image data; an HDD controller  142  for controlling recording and replaying of data in the HDD  141 ; a buffer memory  143  for preliminarily storing image data replayed from the HDD  141  or image data to record in the HDD  141 ; an input/output interface  144  for exchanging image data and control data with the data and control line  21 ; a CPU  145  for controlling the main controller  14  as a whole; and a memory unit  146  made up of a ROM retaining a program for operation of the CPU  145 , a RAM used as a work memory for the CPU  145  and so on.  
     [0042] In the main controller  14  having such a configuration, still-frame image data or moving image data supplied from an external device not shown through the data and control line  21  undergoes specific signal processing by the input/output interface  144  under the control of the CPU  145 . The data is then preliminarily stored in the buffer memory  143  and recorded in the HDD  141  by the HDD controller  142  under the control of the CPU  145 . If necessary, the CPU  145  performs control of replaying still-frame image data from the HDD  141  by the HDD controller  142  and storing the data in the buffer memory  143  and then sending the data from the input/output interface  144  through the data and control line  21  to the display terminals  11 - 1  to  11 -n. In this case, to show the passengers on the train  15  (FIG. 1) a still-frame image, the same still-frame image data is sent to all the display terminals  11 - 1  to  11 -n. To show the passengers on the train  15  a moving image, still-frame image data items different from one another for ‘n’ frames forming a sequential moving image are each sent to the respective display terminals  11 - 1  to  11 -n.  
     [0043]FIG. 4 is a schematic view of the incoming velocity detector  12  together with the train  15 . The velocity detector  12  comprises a velocity sensor unit  120 , a switch  125  and a switch controller  126 . The velocity sensor unit  120  includes: two cameras  121 a and  121 b each for shooting an image of a window  15 a of the train  15  running; an image processor  122  for performing specific image processing on the two image data items inputted from the cameras  121 a and  121 b; a velocity computation section  123  for determining the velocity of the train  15  based on the result obtained by the image processor  122 ; and a pulse generator  124  for generating and outputting the incoming velocity detection pulses  22  whose cycle corresponds to the computation result obtained by the velocity computation section  123 . The switch  125  selects either the incoming velocity detection pulses  22  outputted from the pulse generator  124  of the velocity sensor unit  120  or the outgoing velocity detection pulses  23  outputted from the outgoing velocity detector  13  (FIG. 1) and outputs the selected pulse as the timing pulses  24 . The switch controller  126  observes the input start timing of the velocity detection pulse  23  from the velocity detector  13 , and controls the switch  125  to switch from position  1  (the detection pulse  22 ) to position  2  (the detection pulse  23 ) at the instant of detecting input of the specific number of the detection pulses  23  (such as three pulses).  
     [0044] As shown, the cameras  121   a  and  121   b  are placed with the space narrower than the width of the window  15   a  of the train  15  and sequentially shoot the side of the train  15  running. The images taken by the cameras are superimposed on one another at the image processor  122 . As shown in FIG. 5, for example, an image  128   a  of the window  15   a  taken by the camera  121   a  is superimposed on an image  128   b  of the same window  15   a  taken by the camera  121   b  in a single image frame  128 . The image processor  122  detects distance ‘d’ between the two images. The velocity computation section  123  finds out the velocity of the train  15  based on detected distance d. The pulse generator  124  generates and outputs the incoming velocity detection pulses  22 , as shown in FIG. 6, having a cycle corresponding to the computation result. Although the velocity sensor unit  120  operates the cameras  121   a  and  121   b  and performs shooting and detects the velocity only when a sensor not shown is detecting the presence of the train  15 , shooting may be constantly performed.  
     [0045] The outgoing velocity detector  13  includes a velocity sensor unit similar to the velocity sensor unit  120  shown in FIG. 4 of the incoming velocity detector  12 . The velocity detector  13  detects the velocity of the train  15 , outputs the velocity detection pulses  23  having a cycle corresponding to the detected velocity, and sends the pulses to the velocity detector  12 .  
     [0046] Reference is now made to FIG. 7 for describing the parameters required for practically arranging the image display apparatus  1 , giving specific values. In general a moving image manipulated by a television set of the National Television System Committee (NTSC) system is made up of thirty still-frame images per second. Each of the still-frame images is the image of each instant of a continuous moving image changing with time. If a person observes the still-frame images at the display rate of thirty images per second, the person recognizes the images as a continuous moving image due to the afterimage effect. Utilizing the principle, the apparatus of the invention splits a moving image of three seconds, for example, into ninety still-frame images and displays the images each on the LCD panel  111  of the respective display terminals  11 - 1  to  11 -n in the order started from the terminal  11 - 1  on the incoming side of the train  15 . The backlights  112  are instantaneously turned on in an intermittent manner in response to the velocity of the train  15 . As a result, the passenger on the train  15  observes a moving image of three seconds in a specific fixed position through the window.  
     [0047] Assuming that a moving image is made up of thirty still-frame images per second as described above, the relationship among velocity A per hour and velocity B per second of the train, interval C among the display terminals  11 - 1  to  11 -n, overall system length D, and train passing period E may be as shown in FIG. 7, where overall length L of the train  15  is 200 meters, for example. Interval C among the display terminals  11 - 1  to  11 -n is determined by B/30. Overall system length D is determined by C×90+5. Period E required for the train to pass through the system is determined by (D+train length)/B. For example, if the train velocity is 60 km per hour, interval C is 55.6 cm, overall system length D is 55 m, and train passing period E is 15.3 seconds.  
     [0048] Reference is now made to FIG. 8A to FIG. 8I through FIG. 10 for describing the operation of the image display apparatus for a vehicle having the configuration thus described. FIG. 8A to FIG. 8I illustrate the way the train  15  passes through the system installation section. FIG. 9 illustrates the operation of the incoming velocity detector  12  shown in FIG. 4. FIG. 10 illustrates the way a moving image is observed by the passengers on the train  15 . In the following description the display terminals  11 - 1  to  11 -n are assumed to have received still-frame image data items from the main controller  14  and retain the data items in the respective frame memories  114 .  
     [0049] Referring to FIG. 8A to FIG. 8I, the way the train  15  passes through the system installation section will now be described. In FIG. 8A to FIG. 8I the train  15  enters the system installation section at the left of the drawing and goes to the right. At the point of FIG. 8A the train  15  has not reached the system installation section yet and the incoming velocity detector  12  does not detect the train  15 . When the train  15  comes to the point of FIG. 8B, the velocity detector  12  detects the entry of the train  15  and starts detecting the velocity thereof. The train  15  further advancing through the point of FIG. 8C and reaching the point of FIG. 8D, the outgoing velocity detector  13  detects the train  15  and starts detecting the velocity thereof. The train  15  further advancing through the point of FIG. 8E and reaching the point of FIG. 8F, the velocity detector  12  terminates detection of the velocity of the train  15 . From then on the velocity is detected by the velocity detector  13  only. Therefore, the velocity of the train  15  is detected by both of the detectors  12  and  13  in the period between the point of FIG. 8D and the point of FIG. 8F. The train  15  further advancing through the point of FIG. 8G and reaching the point of FIG. 8H, the velocity detector  13  terminates detection of the velocity of the train  15 . The train  15  then leaves the system installation section as shown in FIG. 8I.  
     [0050] Referring to FIG. 9, the main operation of the incoming velocity detector  12  will now be described. When the train  15  comes to the point of FIG. 8B, the velocity sensor unit  120  of the velocity detector  12  starts the operation of velocity detection. The image processor  122  performs image processing on the images taken by the cameras  121   a  and  121   b  and determines distance d shown in FIG. 5. The velocity computation section  123  determines the velocity of the train  15  from distance d obtained at the image processor  122  and sends the result to the pulse generator  124 . Based on the velocity data obtained, the pulse generator  124  determines the timing interval of image display (the cycle of intermittent display) in the display terminals  11 - 1  to  11 -n. The pulse generator  124  then outputs the incoming velocity detection pulses  22  whose pulse interval (pulse cycle) is equal to the timing interval. If the velocity of the train  15  is higher than the reference velocity (the estimated velocity), the pulse generator  124  determines the pulse interval shorter than the length corresponding to the reference velocity. If the velocity of the train  15  is lower than the reference velocity, the pulse generator  124  determines the pulse interval longer than the length corresponding to the reference velocity.  
     [0051] In the initial state the switch  125  shown in FIG. 4 of the velocity detector  12  is set to position  1 . Consequently, if the detection pulses  22  whose cycle (pulse interval) corresponds to the velocity of the train  15  are outputted from the velocity sensor unit  120  and inputted to the switch  125  (Y of step S 101  of FIG. 9), the switch  125  starts outputting the detection pulses  22  as they are as the timing pulses  24  (step S 102 ).  
     [0052] The timing pulses  24  are supplied to the display terminals  11 - 1  to  11 -n (FIG. 1). In the display terminals  11 - 1  to  11 -n, the backlights  112  turn on and off in unison in synchronization with the timing pulses  24 . Intermittent instantaneous display is thereby started in unison in all the display terminals  11 - 1  to  11 -n.  
     [0053] The train  15  then advancing to the point of FIG. 8D, the velocity sensor unit (not shown) of the outgoing velocity detector  13  starts the operation of velocity detection and outputs the outgoing velocity detection pulses  23  whose pulse interval corresponds to the velocity of the train  15 . The operation of velocity detection is similar to that of the velocity sensor unit  120  of the velocity detector  12  and description thereof is omitted. The detection pulses  23  outputted from the velocity detector  13  are sent to the velocity detector  12  (FIG. 4) and then inputted to the switch controller  126  and the switch  125 .  
     [0054] The switch controller  126  of the velocity detector  13  monitors whether the velocity detection pulses  23  are inputted (step S 103 ). If the switch controller  126  detects input of the pulses  23  (Y of step S 103 ), the switch controller  126  waits until the inputted pulses reach the specific number (N of step S 104 ). When the inputted pulses reach the specific number (three, for example) (Y of step S 104 ), the switch controller  126  sends a switching signal  127  to the switch  125 . On receiving the switch signal  127 , the switch  125  switches the input from position  1  to position  2  (step S 105 ). From then on the velocity detection pulses  23  are outputted as they are as the timing pulses  24 . Accordingly, the timing pulses  24  generated based on the velocity detected by the velocity detector  13  are supplied to the display terminals  111  to  11 -n (FIG. 1) from then on. In response to the timing pulses  24  intermittent instantaneous display is performed in all the display terminals  11 - 1  to  11 -n in unison.  
     [0055] The switch controller  126  continues monitoring whether the velocity detection pulses  23  are inputted (step S 106 ). If the switch controller  126  detects input of the pulses  23  is terminated (Y of step S 106 ), the switch controller  126  outputs the switching signal  127  and returns the switch  125  to position  1  (step S 107 ).  
     [0056] As thus described, the velocity detection pulses  22  and  23  outputted from the velocity detectors  12  and  13  are switched as the train  15  advances and the pulses are used as the timing pulses  24 . Display timing control of the display terminals  11 - 1  to  11 -n is thereby performed.  
     [0057] Reference is now made to FIG. 10 for describing the way a moving image is observed by the passenger on the train  15 . In FIG. 10 the horizontal direction indicates the arrangement of the display terminal  11 -i. The vertical direction indicates the rising point (instantaneous display point) ‘t(j)’ (where ‘j’ is an integer) of the timing pulses  24  shown in FIG. 6. At the intersection of the vertical and horizontal, still-frame image P(i) displayed on the LCD panel  111  (FIG. 2) at each point is indicated. As shown, the LCD panel  111  of the i th  display terminal  11 -i instantaneously displays the same still-frame image P(i) at points to), t(j), t(j+1), t(j+2) and so on. Since the display points correspond to the velocity of the train  15 , consecutive still-frame images [P 1 ], [P 2 ], [P 3 ] and so on are displayed in the same position through the window when viewed by the passenger on the train  15 . The still-frame images are observed as a sequential moving image due to the afterimage effect. In this case, the velocity detector  13  continues detecting the velocity of the train  15  after the rearmost part of the train  15  passes over the velocity detector  12 . As a result, display timing of the display terminals  11 - 1  to  11 -n changes in response to a change in velocity that may occur after the rearmost part of the train  15  passes over the velocity detector  12 . Therefore horizontal drifts of the moving image when viewed by the passenger are prevented. In FIG. 10, if the contents of still-frame images P(i) are all identical, the image observed by the passenger on the train  15  is a still-frame image.  
     [0058] According to the image display apparatus for a vehicle of the embodiment thus described, the incoming velocity detector  12  and the outgoing velocity detector  13  are each provided at the ends of the system installation section. The velocity of the train  15  is detected throughout the period in which the train  15  passes through the system installation section. Display timing of the display terminals  11 - 1  to  11 -n is controlled in accordance with the detected velocity. As a result, even if the velocity of the train  15  changes while the train  15  passes through the system installation section, no drifts of the image viewed by the passenger on the train will result. That is, the image is displayed in a stable position in response to the velocity of the train  15  throughout the period in which the train  15  passes through the system installation section. For example, the apparatus is suitable for commercials that requires high-quality image display since the image is displayed in a stable position when viewed by the passengers.  
     [0059] Second Embodiment  
     [0060] Another embodiment of the invention will now be described.  
     [0061]FIG. 11 is a schematic view of an incoming velocity detector  32  of an image display apparatus for a vehicle of the second embodiment of the invention together with the train  15 . The velocity detector  32  comprises a velocity sensor unit  320 , two velocity range filters  325  and  326 , a switch  327 , a comparator  328 , and a pulse generator  329 . The velocity sensor unit  320  has a configuration similar to that of the velocity sensor unit  120  shown in FIG. 4 except that the sensor unit  320  does not include the pulse generator  124 . The sensor unit  320  includes: two cameras  321   a  and  321   b  each for shooting an image of the window  15   a  of the train  15  running; an image processor  322  for performing specific image processing on the two image data items inputted from the cameras  321   a  and  321   b;  a velocity computation section  323  for determining the velocity of the train  15  based on the result obtained by the image processor  322  and outputting the result as incoming velocity data  42 .  
     [0062] The velocity range filter  325  outputs the velocity data  42  as it is only when the value of the velocity data  42  outputted from the velocity sensor unit  320  falls within a specific range. The filter  325  outputs ‘0’ when the velocity data  42  falls outside the range. The velocity range filter  326  outputs outgoing velocity data  43  as it is only when the value of the velocity data  43  outputted from an outgoing velocity detector  33  falls within a specific range. The filter  326  outputs ‘0’ when the velocity data  43  falls outside the range. The filters  325  and  326  are provided for using valid data only so as to prevent a malfunction due to noise and so on. Valid data is an item of data having a validity or a significance in contrast to invalid data resulting from noise and so on. The velocity range filters  325  and  326  correspond to ‘means for determining’ of the invention.  
     [0063] The switch  327  selects either the incoming velocity data  42  outputted from the velocity sensor unit  320  and inputted to position  1  through the filter  325  or the outgoing velocity data  43  outputted from the outgoing velocity detector  33  (FIG. 1) and inputted to position  2  through the filter  326 . The switch  327  then supplies the selected data to the pulse generator  329 .  
     [0064] The comparator  328  compares the value of the velocity data  42  having passed the filter  325  to the value of the velocity data  43  having passed the filter  326 . When the difference between the values falls within a specific range, the comparator  328  controls the switch  327  to switch from position  1  (the velocity data  42 ) to position  2  (the velocity data  43 ). The switch  327  and the comparator  328  of the incoming velocity detector  32  mainly correspond to ‘switch means’ of the invention.  
     [0065] Based on the velocity data  42  or  43  inputted from the switch  327 , the pulse generator  329  generates the timing pulses  24  (FIG. 6) whose cycle corresponds to the velocity data. The pulse generator  329  corresponds to ‘means for generating and outputting a signal’ of the invention.  
     [0066] The outgoing velocity detector  33  (FIG. 1) includes a velocity sensor unit similar to the velocity sensor unit  320  of the incoming velocity detector  32  shown in FIG. 11. The velocity detector  33  detects the velocity of the train  15  and outputs the detected velocity as the velocity data  43 . The velocity data  43  is inputted to the velocity range filter  326  of the incoming velocity detector  32 .  
     [0067] Referring to FIG. 12, the main operation of the incoming velocity detector  32  shown in FIG. 11 will now be described. When the train  15  comes to the point of FIG. 8B, the velocity sensor unit  320  of the velocity detector  32  starts the operation of velocity detection. The image processor  322  performs image processing on the images taken by the cameras  321   a  and  321   b  and determines distance d shown in FIG. 5. The velocity computation section  323  determines the velocity of the train  15  from distance d obtained at the image processor  322  and outputs the result as the velocity data  42 .  
     [0068] The velocity range filter  325  determines whether the value of the inputted velocity data  42  falls within the specific range. If the value falls within the range (Y of step S 201  of FIG. 12), the filter  325  outputs the value as it is. Since the switch  327  of the velocity detector  32  is set to position  1  in the initial state, the velocity data  42  outputted from the velocity sensor unit  320  and passing through the filter  325  is inputted to the pulse generator  329  through the switch  327 .  
     [0069] Based on the inputted velocity data  42 , the pulse generator  329  determines the timing interval of image display (the cycle of intermittent display) in the display terminals  11 - 1  to  11 -n (step S 202 ). The pulse generator  329  then starts outputting the timing pulses  24  whose pulse interval is equal to the timing interval (step S 203 ). As in the foregoing embodiment, if the velocity of the train  15  is higher than the reference velocity (the estimated velocity), the pulse generator  329  determines the pulse interval shorter than the length corresponding to the reference velocity. If the velocity of the train  15  is lower than the reference velocity, the pulse generator  329  determines the pulse interval longer than the length corresponding to the reference velocity.  
     [0070] The timing pulses  24  are supplied to the display terminals  11 - 1  to  11 -n (FIG. 1). In the display terminals  11 - 1  to  11 -n, the backlights  112  start turning on and off in unison in synchronization with the timing pulses  24 . Intermittent instantaneous display is thereby started in unison in all the display terminals  11 - 1  to  11 -n.  
     [0071] The train  15  then advancing to the point of FIG. 8D, the velocity sensor unit (not shown) of the outgoing velocity detector  33 , too, starts the operation of velocity detection and outputs the outgoing velocity detection data  43 . The operation of velocity detection is similar to that of the velocity sensor unit  320  of the velocity detector  32  and description thereof is omitted. The velocity data  43  outputted from the velocity detector  33  is sent to the velocity detector  32  (FIG. 11) and then inputted to the filter  326 .  
     [0072] The filter  326  determines whether the value of the inputted velocity data  43  falls within the specific range. If the value falls within the range (Y of step S 204 ), the filter  126  outputs the value as it is.  
     [0073] The comparator  328  compares the value of the velocity data  42  outputted from the filter  325  to the value of the velocity data  43  outputted from the filter  326  (step S 205 ). When the difference between the values falls within a specific range (plus and minus 1 km per hour, for example) (Y of step S 206 ), the comparator  328  sends a switching signal  330  to the switch  327 . On receiving the switching signal  330 , the switch  327  switches input from position  1  to position  2  (step S 207 ). From then on the timing pulses  24  are generated and outputted based on the velocity data  43 . Accordingly, the timing pulses  24  generated based on the velocity data  43  detected by the velocity detector  33  are supplied to the display terminals  11 - 1  to  11 -n (FIG. 1) from then on. In response to the timing pulses  24  intermittent instantaneous display is performed in all the display terminals  11 - 1  to  11 -n in unison.  
     [0074] The comparator  328  continues monitoring whether the outgoing velocity data  43  is inputted (step S 208 ) after the input of the incoming velocity data  42  is terminated. If the comparator  328  detects input of the data  43  is terminated (Y of step S 208 ), output of the timing pulses  24  is terminated (step S 209 ). The comparator  328  outputs the switching signal  330  and returns the switch  327  to position  1  (step S 210 ).  
     [0075] As thus described, the velocity data  42  is switched to the velocity data  43  as the train  15  advances. Based on the switched velocity data the timing pulses  24  are generated and supplied to the display terminals  11 - 1  to  11 -n. Display timing control is performed in response to the timing pulses  24 .  
     [0076]FIG. 13 is a table specifically indicating the operation of the switch  327  (FIG. 11) in accordance with passing of the train  15 . The velocity of the train  15  passing is assumed to be  60  km per hour. As shown, when the train  15  reaches the state of FIG. 8B, the incoming velocity data  42  changes from ‘0 km’ to ‘60 km’. When the train  15  reaches the state of FIG. 8F, the velocity data  42  changes from ‘60 km’ to ‘0 km’. When the train  15  reaches the state of FIG. 8D, the outgoing velocity data  43  changes from ‘0 km’ to ‘60 km’. When the train  15  reaches the state of FIG. 8H, the velocity data  43  changes from ‘60 km’ to ‘0 km’. The switch  327  is set to position  1  and outputs the velocity data  42  when the train  15  reaches the states of FIG. 8A to FIG. 8C. The switch  327  is then switched from position  1  to position  2  at any point in the period between the states of FIG. 8D and FIG. 8F. The output thereof is thereby changed from the velocity data  42  to the velocity data  43 . As a result, the output of the switch  327  is maintained at ‘60 km’ in the period between the states of FIG. 8D and FIG. 8F. The switch  327  is maintained at position  2  and outputs the velocity data  43  in the period between the states of FIG. 8G and FIG. 8H. The switch  327  is again switched from position  2  to position  1  in the state of FIG. 8I when the train  15  completely passes through the system installation section.  
     [0077] According to the image display apparatus for a vehicle of the second embodiment of the invention thus described, as in the foregoing first embodiment, the incoming velocity detector  32  and the outgoing velocity detector  33  are each provided at the ends of the system installation section. The velocity of the train  15  is detected throughout the period in which the train  15  passes through the system installation section. Display timing of the display terminals  11 - 1  to  11 -n is controlled in accordance with the detected velocity. As a result, even if the velocity of the train  15  changes while the train  15  passes through the system installation section, no drifts of the image viewed by the passenger on the train will result. That is, the image is displayed in a stable position in response to the velocity of the train  15  throughout the period in which the train  15  passes through the system installation section.  
     [0078] In the image display apparatus of the foregoing first embodiment, the velocity detection pulses  22  outputted from the velocity detector  12  are directly switched to the velocity detection pulses  23  outputted from the velocity detector  13  to output as the timing pulses  24 . Consequently, the pulse phase may change at the point of switching as shown in FIG. 16A to FIG. 16C. This is because the pulse phases are not necessarily equal although the pulse cycles are equal. FIG. 16A illustrates the incoming velocity detection pulses  22 . FIG. 16B illustrates the outgoing velocity detection pulses  23 . FIG. 16C illustrates the timing pulses  24 .  
     [0079] If the pulse phase changes after switching point ‘tc’ from the velocity detection pulses  22  to the velocity detection pulses  23 , display timing in the display terminals  11 - 1  to  11 -n changes. Consequently, as shown in FIG. 17, for example, the display position of the image appears to shift to the right or the left at a specific point (switching point tc) for the passenger on the train  15 . As shown in FIG. 17, passenger A observes with time still-frame images P(i-1), P(i), P(i+1), P(i+2), and so on in sequence at each of display points t(j), t(j+1), t(j+2), t(j+3), and so on, in order started from the upper part towards the lower part of FIG. 17. Since display point t(j+2) immediately after switching point tc is earlier than the original display point (before switching), the display position of still-frame image P(i+1) at display point t(j+2) appears to shift to the right of FIG. 17.  
     [0080] In the image display apparatus of the second embodiment, switching of pulses is not performed in contrast to the foregoing image display apparatus. Instead, switching is performed on the velocity data outputted from the velocity sensor unit  320  and so on (that is, the velocity data  42  and  43 ) and then the timing pulses  24  are generated. As a result, the pulse phase will not change at the point of switching. The image display apparatus of the second embodiment therefore prevents the phenomenon that the image display position appears to shift to the right or the left at some point for the passenger on the train  15 . More stable image display is thus achieved.  
     [0081] The invention is not limited to the foregoing embodiments but may be practiced in still other ways. For example, although the display terminals  11 - 1  to  11 -n made up of the LCD panels are provided as the image display means in the foregoing embodiments, light-emitting diode (LED) panels made up of a plurality of LEDs may be used. Instead of such an electronic display device that performs display based on electrical image information, the image display means may be made up of still-frame images such as paintings or photographs printed on recording media such as paper or film, for example, and illumination devices for illuminating the images.  
     [0082] Although in the foregoing embodiments the switch  125  and the switch controller  126  (or the switch  327  and the comparator  328 ) are incorporated in the incoming velocity detector  12  (or  32 ), the switches and so on may be incorporated in the main controller  14  and the timing pulses  24  may be outputted from the main controller  14 .  
     [0083] Although the two cameras are used in the velocity sensor unit in the foregoing embodiments, any other sensor such as a transmission light sensor, a reflection light sensor, or an ultrasonic sensor utilizing the Doppler effect may be applied.  
     [0084]FIG. 14 illustrates an example of a velocity sensor unit  420  of the incoming velocity detector, using two pairs of transmission light sensors. The velocity sensor unit  420  comprises: a first transmission light sensor made up of an emitter  421   a  and a receptor  421   b  placed to sandwich the passage of the train  15 ; a second transmission light sensor made up of an emitter  421   c  and a receptor  421   d  placed in a similar manner with the specific space from the first transmission light sensor; a signal processor  422  for performing specific signal processing on detection signals  425   a  and  425   b  outputted from the receptors  421   b  and  421   d;  a velocity computation section  423  for determining the velocity of the train  15  based on the output of the signal processor  422 ; and a pulse generator  424  for generating incoming velocity detection pulses  52  based on the output of the velocity computation section  423 . The two transmission light sensor each outputs the pulseshaped detection signals  425   a  and  425   b  as shown in FIG. 15A and FIG. 15B, respectively, when a pair of wheels  156   a  and  156   b  of the train  15  pass over the optical path and intercept the light. The signal processor  422  detects pulse interval τ (FIG. 15) of the detection signals  425   a  and  425   b  resulting from the passing of the pair of wheels. The velocity computation section  423  finds out the velocity of the train  15  based on pulse interval τ. The pulse generator  424  generates the velocity detection pulses  52  based on the obtained velocity and inputs the pulses to the switch  125  (FIG. 4). In FIG. 14 the switch  125  and the switch controller  126  are omitted. The velocity sensor unit of the outgoing velocity detector may have a similar configuration. In such a configuration no expensive television camera and complicated image processing are required so that the costs are reduced. In this case, as in the foregoing second embodiment (FIG. 11), the pulse generator  424  may be removed from the velocity detector  420  and the timing pulses  24  may be generated after switching the velocity data.  
     [0085] In the image display apparatus of the foregoing embodiments, if the train length is shorter than the overall system length, a period in which no velocity detection pulse (or velocity data) is outputted from either the incoming velocity detector  12  (or  32 ) or the outgoing velocity detector  13  (or  33 ) may result. However, if the difference between the train length and the overall system length is slight, such a period is short and the possibility that a change in velocity occurs during the period is reduced. Therefore, in such a non-detection period, there is no serious problem in controlling display timing by using the velocity detection pulse (or velocity data) last outputted from the velocity detector  12  (or  32 ).  
     [0086] For showing moving images or still-frame images the passengers for a long period of time as in the case of the train passing through a long tunnel, the image display apparatus shown in FIG. 1 may be provided as a unit and a plurality of the units may be connected to one another.  
     [0087] Although the image display apparatus is placed in a tunnel in the foregoing embodiments, the apparatus may be provided in any other place. For example, the apparatus may be installed in any place along the railroad and used at night. Furthermore, the apparatus may be used for not only the train or underground (subway) but also any other type of vehicle that carries passengers. For example, the apparatus may be applied to vehicles as attractions in an amusement park and the like. The system that gives the passengers a fantastic or thrilling feeling of virtual reality may be thereby implemented.  
     [0088] According to the image display apparatus or method of the invention thus described, timing of display of the still-frame images by the image display means is controlled, based on the velocity obtained by the first velocity detection means placed near the foremost part of the row of the image display means and the velocity obtained by the second velocity detection means placed near the rearmost part of the row of the image display means. As a result, timing control of still-frame image display is achieved in accordance with the velocity of the vehicle nearly throughout the period in which the vehicle passes through the section where the image display means are placed. Therefore, even if the velocity of the vehicle changes while the vehicle passes through the section, image display by the image display means is performed with timing corresponding to the change in velocity. The image viewed by the passenger on the vehicle is thereby stabilized.  
     [0089] In the image display apparatus, the distance between the position where the first velocity detection means is placed and the position where the second velocity detection means is placed may be shorter than the length of the vehicle. As a result, the period in which the velocity of the vehicle is not detected is eliminated when the vehicle passes through the section where the image display means are placed. The image viewed by the passenger on the vehicle is thereby stabilized throughout the period in which the vehicle passes through the section.  
     [0090] The image display means may be each capable of instantaneously displaying still-frame images forming a moving image. As a result, the passenger on the vehicle is allowed to view the moving image with stability.  
     [0091] The image display means may be each capable of instantaneously displaying still-frame images based on image information given as electrical information. As a result, images to display may be easily changed.  
     [0092] According to the other image display apparatus or method of the invention, information to output is switched from first velocity information obtained by the first velocity detection means placed near the foremost part of the row of the image display means to second information obtained by the second velocity detection means placed near the rearmost part of the row of the image display means when the difference between the first velocity information and the second velocity information falls within a specific range, and the second information is outputted. A signal for controlling timing of displaying the still-frame images by the image display means is generated, based on the first velocity information or the second velocity information outputted. As a result, the interval of display timing instructed by the signal remains constant before and after the switching of information and smooth switching is achieved. Therefore sudden horizontal shifts of the image position viewed by the passenger on the vehicle are prevented. Stable image display is thus provided for the passenger nearly throughout the period in which the vehicle passes through the section where the image display means are placed.  
     [0093] The image display apparatus may further comprise a means for determining whether the velocity information outputted from the first and second velocity detection means is valid or not so that the velocity information that is determined as valid by the means for determining is only inputted to the switch means to effect switching. A malfunction due to noise and so on is thereby prevented. As a result, reliability of the operation is further improved.  
     [0094] Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.