Patent Publication Number: US-2010117929-A1

Title: Multi-display system, information processor, and image data processing method in multi-display system

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a multi-display system, an information processor used in a multi-display system, and an image data processing method in a multi-display system. 
     2. Related Art 
     A multi-display system that arranges and displays images projected from a plurality of projection type image display devices (hereinafter referred to as projectors) on a screen and a multi-display system that displays a plurality of images simultaneously by arranging a plurality of direct-view-type image display devices have been known. In the multi-display systems, various kinds of image processing such as color correction and distortion correction have to be applied to an image to be displayed in each of the image display devices. 
     Especially in such a multi-display system that displays one large screen image by using a plurality of projectors, image processing has to be performed carefully on a region between adjacent images. For example, for making the seam between adjacent images inconspicuous, individual images displayed by the plurality of projectors have heretofore been displayed generally so as to have an overlapped region between adjacent images. When the display with the overlapped region is performed as described above, it is indispensable to apply a process making the overlapped region hardly visible (hereinafter referred to as edge-blending process). 
     In the multi-display system, various kinds of image processing including image correction are executed by a GPU (Graphic Processing Unit) mounted on a video card that functions as one hardware of a personal computer (hereinafter referred to as PC), thereby enabling higher-level image processing and the display of high quality images. 
     In general, image data corresponding to an. image displayed by a projector is generated by a PC, and the image data generated by the PC is transmitted to the projector. In the multi-display system including a plurality of projectors, images displayed by the individual projectors have to be generated and output by the corresponding PCs. Accordingly, in the case of using a plurality of projectors, a plurality of PCs are also required. 
     Video cards on which a GPU is mounted can generally provide output in two channels. Even when a special video card is used, output in eight channels is the limit at present. Accordingly, a multi-display system including over eight projectors has to use a plurality of PCs. Since the video card that can provide output in eight channels is extremely expensive, it cannot be said that the video card is generally usable. 
     Accordingly, when a typical video card that can provide image data output in two channels is used, one PC can simultaneously output image data to at least two projectors. However, when a multi-display system using more number of projectors than that is configured, a plurality of PCs are required. 
     In this manner, when a multi-display system is configured by using a number of projectors, as many PCs as corresponding to the number of projectors are required, leading to an increase in size of the multi-display system, the complexity of the configuration, and an increase in cost. For coping with this, a technique that can realize a multi-display system including a number of projectors with least number of PCs has heretofore been proposed (refer to JP-A-2006-349791, for example). 
     In the technique disclosed in JP-A-2006-349791 (hereinafter referred to as related-art multi-display system), for example, a PC, an image data distribution device that can distribute in parallel image data from the PC, and a plurality of image display devices (projectors) connected to the image data distribution device are provided, and individual ones of the plurality of projectors each cut out image data corresponding to an image to be displayed by itself from the image data distributed by the image data distribution device to display the same, thereby displaying one large screen image. 
     According to the related-art multi-display system described above, the image data distribution device and an image cut-out function provided to each of the projectors are used, whereby the multi-display system that displays a large screen image with the plurality of projectors can be realized with the least number of PCs. 
     In the related-art multi-display system, however, image processing that depends on the individual projectors, such as, for example, an edge-blending process on an overlapped region and correction of colors specific to the individual projectors has to be performed in the individual projectors. As described above, since the related-art multi-display system has to depend on an image data processing function provided to the individual projectors as to the various kinds of image processing that depend on the individual projectors, there arise a problem that high-level image processing can hardly be performed. 
     On the other hand, since higher quality is required for an image displayed in a multi-display system, higher-level image processing has to be performed. For achieving this, it is preferable that the image processing including the various kinds of correction processes be performed in a GPU on the PC side. As described above, however, a typical video card has a limitation of the number of pieces of image data to be output. As a result, even a multi-display system including a several projectors has to use a plurality of PCs at present. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a multi-display system that can display a high-quality image, an information processor, and an image data processing method in a multi-display system by enabling a multi-display system having a number of image display devices to be realized with a small number of information processors (personal computers) and enabling even image processing including correction regarding color and correction regarding shape to be performed on the information processor side. 
     A multi-display system according to a first aspect of the invention includes: a plurality of image display devices; and an information processor adapted to output image data to the plurality of image display devices. The information processor includes a composite image data generation unit adapted to combine partial image data corresponding to partial images to be displayed by individual ones of the plurality of image display devices to generate composite image data having a higher resolution than that of each of the image display devices, and an image data output unit adapted to output the composite image data. Each of the image display devices includes an image data input unit adapted to receive the composite image data generated in the information processor, a partial image data generation unit adapted to cut out partial image data corresponding to an partial image to be displayed by the image display device from the composite image data, and an image display unit adapted to display the partial image based on the partial image data cut out in the partial image data generation unit. 
     As described above, in the multi-display system according to the first aspect of the invention, on the information processor (for example, personal computer) side, composite image data obtained by combining partial image data corresponding to partial images to be displayed by the individual image display devices is generated, and the generated composite image data is given to the individual image display devices. On the image display device side, partial image data corresponding to a partial image to be displayed by itself is cut out, and an image based on the cut-out partial image data is displayed. This makes it possible to output a plurality of pieces of partial image data to be given to a plurality of image display devices from the information processor to the individual image display devices as one piece of image data (composite image data). Therefore, the multi-display system including a number of image display devices can be realized with a small number of information processors. 
     The multi-display system according to the first aspect of the invention can perform not only composite image data generation processing but also image processing including various kinds of correction processes on the information processor side. As described above, in the multi-display system according to the first aspect of the invention, the image processing including image correction that has heretofore been performed in each of the image display devices can be performed on the information processor side. Therefore, it is possible to perform high-level image processing and to thereby make an image displayed in the multi-display system of high quality. 
     In the first and other aspects of the invention, the “partial image(s)” means an individual image(s) displayed by an individual image display device(s). The “partial image” is not only an individual one of a plurality of images in the case where a plurality of images that are related to one another are arranged to form one large screen image but also an individual one of a plurality of images in the case where a plurality of images that are not related to one another are arranged to be displayed. 
     It is preferable that the information processor include a video card, and that the video card functions as the composite image data generation unit and the image data output unit. 
     This makes it possible to perform image processing including various kinds of image corrections with a GPU mounted on the video card, enabling high-level image processing. Since even a typical video card has an image data output function in two channels, image data corresponding to at least two image display devices can be output by using only one video card. By using two video cards, image data corresponding to at least four image display devices can be output. 
     Capability of processing of image data per output channel of the video card in terms of how many projectors the image data corresponds to depends on the resolution of the projector used in the multi-display system, processing ability for image data of the information processor side (video card side) and the like. Therefore, each output channel can output image data corresponding to, for example, four image display devices in some cases. This makes it possible to realize the multi-display system including a number of image display devices with a small number of information processors. The video card may be incorporated into the information processor or may be externally attached to the information processor. 
     It is preferable that the respective resolutions in horizontal and vertical directions of the composite image data be set to integral multiples of the resolutions in horizontal and vertical directions of each of the image display devices. 
     As described above, the respective resolutions in horizontal and vertical directions of the composite image data are set to integral multiples of the resolutions in horizontal and vertical directions of each of the image display devices. Therefore, when the multi-display system is configured with, for example, a plurality of image display devices disposed in each of the horizontal and vertical directions, image data having the resolution of each of the image display devices can be cut out as it is, eliminating the necessity of performing a scaling process and the like in each of the image display devices. 
     It is preferable that the multi-display system further include an image data distribution device adapted to distribute in parallel the composite image data output from the information processor to the individual image display devices. 
     This makes it possible to distribute in parallel composite image data generated on the information processor side to the plurality of image display devices. 
     It is preferable that the information processor apply image processing including image correction to the partial image data. 
     As described above, the image processing including various kinds of image corrections, which has heretofore been performed in each of the image display devices, is performed on the information processor side. Therefore, it is possible to perform high-level image processing. 
     it is preferable that the image correction applied to the partial image data include image correction based on individual difference among the plurality of image display devices. 
     As described above, the image correction based on individual difference among the image display devices, which has heretofore been performed in each of the image display devices, is performed on the information processor side. Therefore, it is possible to perform high-level correction and to thereby make an image displayed in the multi-display system of high quality. The correction can be realized by giving correction parameters necessary for performing the correction to an image data processing device of the information processor. 
     It is preferable that the image correction based on the individual difference be correction regarding color. 
     As the correction regarding color, a process for correcting color difference due to individual difference among the individual image display devices or a process for correcting color non-uniformities specific to the individual image display devices can be exemplified. The correction regarding color is performed on the information processor side, whereby it is possible to perform high-level correction. 
     It is preferable that the plurality of image display devices be a plurality of projectors, and that the plurality of projectors be disposed such that partial images displayed by the individual projectors are displayed in a state of being arranged on a screen. 
     According to the first aspect of the invention, it is possible to realize the multi-display system including a number of projectors with a small number of information processors. In addition, the composite image data generation processing and the image processing including a correction process can be performed on the information processor side. Therefore, it is possible to perform high-level image processing and to thereby make an image displayed in the multi-display system of high quality. 
     It is preferable that the information processor perform correction regarding shape of partial images to be displayed by the individual projectors on partial image data corresponding to the partial images. 
     As described above, the correction regarding shape that has heretofore been performed in each of the projectors is performed on the information processor side. Therefore, it is possible to perform high-level correction and to thereby make an image displayed in the multi-display system of high quality. The correction can be realized by giving correction parameters necessary for performing the correction to the image data processing device of the information processor. 
     It is preferable that the correction regarding shape of the partial images include at least one of correction of display size of partial images to be displayed by the individual projectors and distortion correction of the partial images to be displayed by the individual projectors. 
     This makes it possible to uniform the size of an image displayed by each of the projectors and correct non-linear distortion due to lens distortion or the like. 
     It is preferable that the information processor perform keystone correction on an entire image to be displayed on a screen. 
     This corrects trapezoidal distortion caused by a projection angle of each of the projectors with respect to a screen. A display image on a screen can be made in a rectangular shape by performing the keystone correction. 
     It is preferable that the keystone correction be performed by correcting respective trapezoidal distortions of partial images displayed by the individual projectors by using correction parameters corresponding to the partial images displayed by the individual projectors. 
     This performs the correction on partial image data corresponding to partial images to be displayed by the individual projectors by using correction parameters given to the respective partial images to be displayed by the individual projectors when performing the keystone correction. This makes it possible to perform the correction in accordance with the degree of distortion of partial images to be displayed by the individual projectors and to thereby properly make an image displayed on a screen rectangular. 
     It is preferable that the partial images displayed by the individual projectors be related to one another, and that the partial images that are related to one another form one large screen image. 
     This is the case of a multi-display system that forms one large screen image with partial images that are related to one another. In the case of such a multi-display system, image data corresponding to one large screen image is divided into partial image data corresponding to the individual projectors, predetermined image processing such as image correction is applied to the divided partial image data, and composite image data obtained by combining the image-processed partial image is given to the individual projectors. By performing the process, it is possible to configure the multi-display system that forms one large screen image with partial images that are related to one another. 
     It is preferable that the partial images displayed by the individual projectors be displayed so as to each have an overlapped region between adjacent images, and that the information processor generate partial image data while considering the overlapped region for each of the projectors and generate composite image data obtained by combining the partial image data. 
     This makes it possible to cut out partial image data including the overlapped region in each of the projectors when cutting out partial image data to be displayed by itself. 
     It is preferable that an edge-blending process is applied to the overlapped regions in the composite image data obtained by combining the partial image data. 
     The edge-blending process is a process for making the overlapped region hardly visible. In the multi-display system according to the first aspect of the invention, the edge-blending process is performed in the image data processing device of the information processor. Since the edge-blending process is performed on the information processor side, the edge-blending process does not have to be performed on each of the projector sides. In addition, the edge-blending process can be performed with high accuracy. 
     An information processor according to a second aspect of the invention is an information processor in a multi-display system including a plurality of image display devices and the information processor adapted to output image data to the plurality of image display devices. The information processor includes a composite image data generation unit adapted to combine partial image data corresponding to partial images to be displayed by individual ones of the plurality of image display devices to generate composite image data having a higher resolution than that of each of the image display devices and an image data output unit adapted to output the composite image data. 
     By using the information processor, the multi-display system according to the first aspect of the invention can be configured. It is preferable that also the information processor according to the second aspect of the invention have the features of the multi-display system according to the first aspect of the invention. 
     A method for processing image data in a multi-display system according to a third aspect of the invention is a method for processing image data in a multi-display system including a plurality of image display devices and an information processor adapted to output image data corresponding to images to be displayed by individual ones of the plurality of image display devices. The method includes: combining, by the information processor, partial image data corresponding to partial images to be displayed by the individual ones of the plurality of image display devices to generate composite image data having a higher resolution than that of each of the image display devices; outputting, by the information processor, the composite image data; receiving, by each of the image display devices, the composite image data generated by the information processor; cutting out, by each of the image display devices, partial image data corresponding to a partial image to be displayed by the image display device from the composite image data; and displaying, by each of the image display devices, the partial image based on the cut-out partial image data. 
     The above-described steps are executed in the information processor and each of the image display devices, whereby an effect similar to that of the multi-display system according to the first aspect of the invention can be provided. It is preferable that also the method for processing image data in the multi-display system according to the third aspect of the invention has the features of the multi-display system according to the first aspect of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  shows the configuration of a multi-display system according to an embodiment. 
         FIG. 2  is a functional block diagram of the multi-display system according to the embodiment. 
         FIG. 3  explains an image cut-out function provided to each of projectors. 
         FIG. 4  explains the case where four images that are not related to one another are arranged to be displayed on a screen. 
         FIG. 5  explains the case where four partial images that are related to one another are displayed on a screen as one large screen image. 
         FIGS. 6A to 6D  explain an example of correcting display size. 
         FIGS. 7A to 7C  explain an example of correcting non-linear distortion. 
         FIGS. 8A to 8C  explain an example of performing keystone correction. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENT 
     Hereinafter, an embodiment of the invention will be described. 
       FIG. 1  shows the configuration of a multi-display system according to the embodiment. As shown in  FIG. 1 , the multi-display system according to the embodiment includes a personal computer (hereinafter referred to as PC)  100  as an information processor having a video card  110  as an image data processing device and an image data acquisition unit  120  that acquires image data to be displayed, a plurality (two in each of horizontal and vertical directions, that is, four in total) of projectors PJ 1  to PJ 4 , and an image data distribution device  200  that distributes image data output from the PC  100  to each of the projectors PJ 1  to PJ 4 . 
       FIG. 1  shows an example in which individual images (hereinafter referred to as partial images) displayed by the individual projectors PJ 1  to PJ 4  are displayed so as to each have an overlapped region between adjacent partial images. However, the embodiment can be applied to the case where the individual images are displayed so as not to have the overlapped region. 
     Image data acquired by the image data acquisition unit  120  may be image data (moving image or still image) generated on the PC  100  or may be image data generated externally and loaded into the PC  100 . 
     Each of the projectors PJ 1  to PJ 4  has an image data cut-out function that cuts out partial image data corresponding to a partial image to be displayed by itself from image data that is distributed by the image data distribution device  200 . It is assumed that each of the projectors PJ 1  to PJ 4  has a resolution of XGA (1024×768 pixels). 
       FIG. 2  is a functional block diagram of the multi-display system according to the embodiment. As shown in  FIG. 2 , the video card  110  provided in the PC  100  includes a composite image data generation unit  111  that applies predetermined image processing including image correction to partial image data corresponding to partial images to be displayed individually by the projectors PJ 1  to PJ 4  and combines the partial image data to generate composite image data having a higher resolution than that of each of the projectors, an image data storage unit  112  that stores image data generated in the composite image data generation unit  111  (hereinafter referred to as composite image data), and an image data output unit  113  that outputs the composite image data. The composite image data generation unit  111  performs processing as one of functions of a GPU mounted on the video card  110 . The content of the processing will be described later. 
     The resolutions (resolution in the horizontal direction and resolution in the vertical direction) of composite image data generated in the composite image data generation unit  111  are preferably set to integral multiples of resolutions in the horizontal and vertical directions of each of the projectors. In the multi-display system according to the embodiment, integers used in the integral multiple are defined as the numbers of projectors in the horizontal and vertical directions. 
     That is, when the resolution of each of the projectors in the horizontal direction is M, and the number of projectors in the horizontal direction is a, the resolution of composite image data in the horizontal direction is “M×a”. While, when the resolution of each of the projectors in the vertical direction is N, and the number of projectors in the vertical direction is b, the resolution of composite image data in the vertical direction is “N×b”. Specifically, in the multi-display system according to the embodiment, the resolution of each of the projectors in the horizontal direction is 1024 pixels, the resolution in the vertical direction is 768 pixels, and two projectors are disposed in each of the horizontal and vertical directions. Therefore, the resolution of composite image data in the horizontal direction is 2048 pixels, and the resolution in the vertical direction is 1536 pixels. 
     The image data distribution device  200  distributes in parallel composite image data output from the image data output unit  113  of the PC  100  to the projectors PJ 1  to PJ 4 . The composite image data distributed from the image data distribution device  200  to the projectors PJ 1  to PJ 4  is image data corresponding to an entire image displayed on a screen SCR. 
     Each of the projectors PJ 1  to PJ 4  includes an image data input unit  310  that receives composite image data distributed from the image data distribution device  200 , a cut-out position setting unit  320  that sets cut-out positions of an image to be displayed by itself, a cut-out position storage unit  330  that holds the cut-out positions set in the cut-out position setting unit  320 , a partial image data generation unit  340  that cuts out the composite image data input to the image data input unit  310  based on the cut-out positions held in the cut-out position storage unit  330  and generates partial image data corresponding to a partial image to be displayed by itself, an image data storage unit  350  that holds the partial image data generated in the partial image data generation unit  340 , and an image projection unit  360  as an image display unit that generates a partial image based on the partial image data held in the image data storage unit  350  to project the generated partial image on the screen SCR. 
     The image projection unit  360  includes known constituent elements such as a light source, a light modulation device, an optical system for introducing light from the light source to the light modulation device, and a projection optical system for projecting an image generated by the light modulation device that are provided in a typical projector, and therefore the detailed description of the image projection unit  360  is omitted. 
     Although only the configuration of the projector PJ 1  is shown in  FIG. 2 , the projectors PJ 2  to PJ 4  each have the same configuration. 
       FIG. 3  explains an image cut-out function provided to each of the projectors. It is assumed that part (a) of  FIG. 3  is an image (composite image) corresponding to composite image data distributed by the image data distribution device  200 . Part (b) of  FIG. 3  shows an example of setting cut-out positions when an partial image is cut out in one projector (projector PJ 1 ). Part (c) of  FIG. 3  shows an example of displaying a cut-out partial image. 
     As shown in  FIG. 3 , in the projector PJ 1 , based on cut-out positions (cut-out positions held in the cut-out position storage unit  330 ) set by the cut-out position setting unit  320 , the partial image data generation unit  340  cuts out partial image data from composite image data. After performing a process (scaling process) for adjusting the resolution of the cut-out partial image data to its own resolution and the like on the cut-out partial image data, the projector PJ 1  projects a partial image based on the cut-out partial image data with the image projection unit  360 . The scaling process does not have to be performed when the resolution of the cut-out partial image data is the same as its own resolution. 
     In the example shown in  FIG. 3 , when an upper left end position, a right end position on the upper end side, and a lower end position on the left end side of the composite image are “0”, “1.0”, and “1.0”, respectively, cut-out positions set in the projector PJ 1  are set to “0.25” for a left side cut-out position (left), “0.75” for a right side cut-out position (right), “0.25” for an upper side cut-out position (top), and “0.75” for a lower side (bottom side) cut-out position (bottom). 
     Next, composite image data generation processing in the PC  100  will be described. In the multi-display system according to the embodiment, two of the four projectors PJ 1  to PJ 4  are disposed in the horizontal direction, the remaining two are disposed in the vertical direction, and the resolution of each of the projectors PJ 1  to PJ 4  is 1024×768 pixels. Therefore, composite image data in which image data each configured by 1024×768 pixels are combined for four screens is generated by the PC  100 . That is, the PC  100  generates composite image data having a resolution of 2048×1536 pixels. 
       FIG. 4  explains the case where four partial images that are not related to one another are arranged and displayed on a screen. In this case, the PC  100  combines partial image data respectively corresponding to the four partial images that are not related to one another to thereby generate one piece of composite image data and outputs the generated composite image data from one output channel of the video card  110  to the image data distribution device  200 . The composite image data is distributed in parallel from the image data distribution device  200  to each of the projectors PJ 1  to PJ 4 . 
     Parts (a) to (d) of  FIG. 4  are partial images to be displayed respectively by the projectors PJ 1  to PJ 4 . The PC  100  acquires image data corresponding to the partial images shown in the parts (a) to (d) of  FIG. 4  with the image data acquisition unit  120  and generates composite image data by combining the acquired image data (partial image data) (refer to part (e) of  FIG. 4 ). In this case, the resolution of the composite image data in the horizontal direction is set to be twice (1024×2=2048 pixels) that of each of the projectors in the horizontal direction, and the resolution of the composite image data in the vertical direction is also set to be twice (768×2=1536 pixels) that of each of the projectors in the vertical direction. 
     The combining of image data is performed by the composite image data generation unit  111  of the video card  110 . The generated composite image data is held in the image data storage unit  112 . Thereafter, the image data output unit  113  outputs the generated composite image data as one piece of image data from one output channel to the image data distribution device  200 . The image data distribution device  200  distributes the composite image data to each of the projectors PJ 1  to PJ 4 . 
     In the projectors PJ 1  to PJ 4 , on the other hand, the composite image data distributed from the image data distribution device  200  is input to the image data input unit  310 . Each of the projectors PJ 1  to PJ 4  cuts out partial image data to be displayed by itself from the composite image data based on the cut-out positions set in the cut-out position setting unit  320 . As described in  FIG. 3 , the cutting out of the partial image data is performed based on the cut-out positions set in the respective projectors PJ 1  to PJ 4 , that is, the left side cut-out position (left), the right side cut-out position (right), the upper side cut-out position (top), and the lower side (bottom side) cut-out position (bottom). 
     For example, in the projector PJ 1 , the cut-out positions (left, right, top, and bottom) are respectively set to (0, 0.5, 0, and 0.5). Therefore, the projector PJ 1  cuts out partial image data based on the cut-out positions so that partial image data corresponding to a partial image to be displayed by itself is cut out. In the same manner, the projectors PJ 2  to PJ 4  cut out partial image data based on cut-out positions respectively set therein. 
     The image projection unit  360  of each of the projectors PJ 1  to PJ 4  generates a partial image based on the partial image data cut out by itself to thereby display the partial image on the screen SCR. Therefore, the partial images shown in the parts (a) to (d) of  FIG. 4  are displayed (tiled) on the screen SCR in a state where two partial images are arranged in each of the horizontal and vertical directions. That is, the image shown in the part (e) of  FIG. 4  is displayed on the screen SCR. 
     Although  FIG. 4  shows the case of displaying the four partial images that are not related to one another, the invention can be applied to the case where four partial images that are related to one another are displayed on a screen as one large screen image. 
       FIG. 5  explains the case where four partial images that are related to one another are displayed on a screen as one large screen image. In  FIG. 5 , the individual partial images displayed by the projectors PJ 1  to PJ 4  are displayed so as to each have an overlapped region between adjacent partial images. 
     Parts (a) to (d) of  FIG. 5  are partial images to be displayed respectively by the projectors PJ 1  to PJ 4 . An edge blending process is applied to overlapped regions P. 
     Part (e) of  FIG. 5  is a large screen image to be displayed on the screen SCR. This is an image in which the four partial images shown in the parts (a) to (d) of  FIG. 5  are tiled so as to each have the overlapped region between adjacent partial images. 
     For tiling partial images as shown in  FIG. 5 , in the PC  100 , various kinds of image processing such as the edge-blending process on the overlapped regions p are applied to partial image data corresponding to the partial images to be displayed by the projectors PJ 1  to PJ 4 , and thereafter composite image data obtained by combining the partial image data is generated (refer to part (f) of  FIG. 5 ). 
     The edge-blending process and the combining process for combining the partial image data can be performed by the composite image data generation unit  111 . The composite image data (composite image data applied with the edge-blending process) generated in the composite image data generation unit  111  is written to the image data storage unit  112 . Since pixels present in the overlapped region p are common between adjacent projectors, image data of the same pixels is redundantly written to the image data storage unit  112 . 
     The image data output unit  113  outputs the composite image data written to the image data storage unit  112  from one output channel to the image data distribution device  200 . The image data distribution device  200  distributes the composite image data corresponding to the composite image shown in the part (f) of  FIG. 5  to the projectors PJ 1  to PJ 4 . 
     In the projectors PJ 1  to PJ 4 , on the other hand, the composite image data distributed from the image data distribution device  200  is input to the image data input unit  310 , and partial image data corresponding to a partial image to be displayed by itself is cut out based on the respective cut-out positions set in the projectors PJ 1  to PJ 4 . 
     In the projector PJ 1  in this case, since the cut-out positions (left, right, top, and bottom) are respectively set to (0, 0.5, 0, and 0.5), partial image data is cut out based on the set cut-out positions. In the same manner, in each of the projectors PJ 2  to PJ 4 , partial image data is cut out based on the set cut-out positions. Based on the cut-out partial image data, the image projection units  360  of the projectors PJ 1  to PJ 4  respectively project the partial images shown in the parts (a) to (d) of  FIG. 5 . The edge-blending process is applied to the overlapped regions P in the partial images shown in the parts (a) to (d) of  FIG. 5 . Therefore, the overlapped regions P between adjacent partial images are overlapped with each other to thereby form an image whose overlapped regions are hardly visible on the screen SCR (refer to the part (e) of  FIG. 5 ). 
     In addition, a color correction process for correcting color difference among partial images due to individual difference in color among the projectors PJ 1  to PJ 4  and correction of color non-uniformities specific to the projectors PJ 1  to PJ 4  can be applied to the partial image data in the composite image data generation unit  111  of the video card  110  by giving correction parameters for performing the corrections to the PC  100 . 
     Composite image data obtained by combining the correction-processed partial image data is given to the projectors PJ 1  to PJ 4 , whereby the projectors PJ 1  to PJ 4  can cut out the corrected partial image data from the composite image data. In the embodiment of the invention, the correction for correcting the color difference among the partial images and the correction of the color non-uniformities are collectively hereinafter referred to as “correction regarding color”. 
     The correction regarding color can be performed not only in the case where four partial images that are related to one another are displayed on a screen as one large screen image (refer to  FIG. 5 ) but also in the case where four partial images that are not related to one another are arranged and displayed on a screen in the same manner (refer to  FIG. 4 ). 
     In the composite image data generation unit  111 , correction of the relative size (hereinafter referred to as display size) of a partial image displayed by each of the projectors and distortion correction of the partial image are possible in addition to the correction regarding color. Especially when the partial images displayed by the projectors are different in display size, or when the image is distorted, the overlapped region between adjacent images cannot be set accurately. As a result, the edge-blending process cannot be performed properly. Therefore, it is important to properly perform the display size correction or the distortion correction of image on the partial image displayed by each of the projectors. In the embodiment of the invention, the display size correction of partial image and the distortion correction of image are collectively hereinafter referred to as “correction regarding shape”. 
     In the same manner as in the correction regarding color, the composite image data generation unit  111  of the video card  110  in the PC  100  can apply the correction regarding shape to the partial image data by giving correction parameters for performing the correction regarding shape to the PC  100  side. 
       FIGS. 6A to 6D  explain an example of correcting the display size.  FIG. 6A  schematically shows a state where partial images from the projectors PJ 1  to PJ 4  are tiled on the screen SCR so as to each have an overlapped region between adjacent partial images. 
     For tiling the partial images so as to have the overlapped region between adjacent partial images, the composite image data generation unit  111  of the PC  100  applies the various kinds of image processing such as the edge-blending process on the overlapped regions to the partial image data corresponding to the partial images to be displayed by the projectors PJ 1  to PJ 4 , and thereafter generates composite image data obtained by combining the partial image data as described in  FIG. 5 . Based on the thus generated composite image data, each of the projectors PJ 1  to PJ 4  cuts out the partial image data to be displayed by itself to display the same. 
     In this case, as shown in  FIG. 6A , it is assumed that the partial image displayed by the projector PJ 2  has a relatively large display size compared with the partial images of the projectors PJ 1  and PJ 2 . As shown in  FIG. 6A , when the images displayed by the projectors are different in display size, the overlapped regions are not uniform between adjacent images. Therefore, when the edge-blending process is performed, the edge-blending process cannot be performed properly. 
     Accordingly, the composite image data generation unit  111  generates partial image data for correcting the display size (hereinafter referred to as partial image data for correcting display size) for the partial image data corresponding to the projector PJ 2  (refer to  FIG. 6B ). The gray region in  FIG. 6B  is a black image region. 
     That is, considering that the partial image of the projector PJ 2  is larger in display size than the partial images of the other projectors, the composite image data generation unit  111  generates partial image data with a black image region (partial image data for correcting display size) is generated as partial image data corresponding to the projector PJ 2 . 
     The partial image data for correcting display size is obtained by reducing the partial image data before correction so that the partial image displayed on the screen SCR has a size to be displayed and making a portion not to be displayed a black image region. Correction parameters (such as positional data for setting the black image region) for generating the partial image data for correcting display size shown in  FIG. 6B  can be set based on picked-up image data obtained by, for example, picking-up an image displayed on the screen SCR with image pick-up means (not shown). 
       FIG. 6C  schematically shows composite image data obtained by combining partial image data corresponding to the projectors PJ 1  to PJ 4 , in which the partial image data corresponding to the projector PJ 2  is partial image data with a black image region (partial image data for correcting display size). It is assumed that the edge-blending process is applied to the partial image data forming the composite image data shown in  FIG. 6C . 
     When the composite image data shown in  FIG. 6C  is given to each of the projectors PJ 1  to PJ 4 , each of the projectors PJ 1  to PJ 4  cuts out partial image data to be displayed by itself to display the same. In this case, in the projector PJ 2 , since a partial image is displayed by using the partial image data for correcting display size, the partial image displayed by the projector PJ 2  has the same display size as those of the other projectors. Therefore, the respective partial images displayed by the projectors PJ 1  to PJ 4  are displayed in the same display size (refer to  FIG. 6D ). 
     When the display shown in  FIG. 6D  is performed, the black image region appears as black floating. For eliminating the black floating, a light shielding plate is disposed corresponding to the black image region, whereby it is possible to make the black floating inconspicuous. 
     Non-linear distortion due to lens distortion or the like can also be corrected in addition to the correction of the simple linear shape shown in  FIGS. 6A  to  6 D. 
       FIGS. 7A to 7C  explain an example of correcting non-linear distortion.  FIGS. 7A to 7C  show partial images displayed by one projector.  FIG. 7A  shows a partial image including non-linear distortion (hereinafter referred to as non-linearly distorted partial image) due to lens distortion or the like.  FIG. 7B  schematically shows partial image data for correcting the non-linearly distorted partial image (hereinafter referred to as partial image data for correcting non-linear distortion) shown in  FIG. 7A . 
     The partial image data for correcting non-linear distortion is obtained by correcting partial image data before correction so that a partial image to be displayed on the screen SCR becomes a rectangular partial image from which the distortion is removed. The partial image data for correcting non-linear distortion shown in  FIG. 7B  is given to the projector, whereby the non-linearly distorted partial image shown in  FIG. 7A  is made into a rectangular partial image from which the distortion is removed as shown by the bold line in  FIG. 7C . 
     In this manner, non-linear distortion can be removed by performing the correction shown in  FIGS. 7A to 7C  on a non-linearly distorted partial image. When the partial images in the projectors PJ 1  to PJ 4  are different in display size, after performing the non-linear distortion correction shown in  FIGS. 7A to 7C , the display size correction described in  FIGS. 6A to 6D  may be performed. 
     When trapezoidal distortion based on the projection angle of each of the projectors PJ 1  to PJ 4  with respect to a screen occurs in an entire image (for example, refer to the part (e) of  FIG. 5 ) displayed by the projectors PJ 1  to PJ 4 , it is also possible to perform correction for correcting the trapezoidal distortion (keystone correction). 
       FIGS. 8A to 8C  explain an example of performing the keystone correction. It is assumed in  FIGS. 8A to 8C  that partial image data corresponding to the partial image to be displayed by the projector PJ 2  is the partial image data for correcting display size. 
       FIG. 8A  shows a state where trapezoidal distortion occurs in an entire image displayed by the projectors PJ 1  to PJ 4  on the screen SCR, in which the gray region is a black image region in the partial image of the projector PJ 2 . 
     When the trapezoidal distortion shown in  FIG. 8A  occurs in the entire image displayed by the projectors PJ 1  to PJ 4  on the screen SCR, the composite image data generation unit  111  generates correction parameters different for each of partial images to be displayed by the individual projectors. The composite image data generation unit  111  generates partial image data for correcting the trapezoidal distortions (hereinafter referred to as partial image data for correcting trapezoidal distortion) of the respective partial images by using the correction parameters for the respective partial images and generates composite image data obtained by combining the partial image data for correcting trapezoidal distortion generated for the respective partial images. The partial image data for correcting trapezoidal distortion is obtained by correcting partial image data before correction so that the partial image to be displayed on the screen SCR is made into a keystone-corrected image. 
       FIG. 8B  schematically shows composite image data generated by the composite image data generation unit  111 . In  FIG. 8B , the gray region is a black image region. When the composite image data is given to each of the projectors PJ 1  to PJ 4 , each of the projectors PJ 1  to PJ 4  cuts out a partial image to be displayed by itself from the composite image data given by the PC  100  to display the same. Therefore, the keystone-corrected image is displayed on the screen SCR as shown in  FIG. 8C . 
     The correction parameters for performing the display size correction and the distortion correction described in  FIGS. 6A to 6D  and  FIGS. 7A to 7C  and the correction parameters for performing the keystone correction described in  FIGS. 8A to 8C  can be automatically generated based on picked-up image data obtained by picking up an image on a screen using image pick-up means. However, a user may manually generate the correction parameters. 
     When performing both the display size correction and/or distortion correction and the keystone correction shown in  FIGS. 8A to 8C , the order of the corrections may be set in advance such that, for example, after performing the display size correction and/or distortion correction, the keystone correction is performed. Alternatively, these corrections may be combined into one computation. 
     As described above, in the multi-display system according to the embodiment, the composite image data generation unit  111  of the video card  110  provided in the PC  100  applies various kinds of image processing such as the edge-blending process, the correction regarding color, and the correction regarding shape to partial image data corresponding to the partial images displayed by the projectors PJ 1  to PJ 4  and combines them, thereby generating composite image data having a higher resolution than that of each of the projectors. Then, the image data output unit  113  of the video card  110  outputs the generated composite image data to the image data distribution device  200 . 
     In this manner, in the multi-display system according to the embodiment, four pieces of image data (four pieces of partial image data) to be given to the four projectors PJ 1  to PJ 4  are combined together to be output as one piece of image data (one piece of composite image data). This makes it possible to output image data to be given to the four projectors PJ 1  to PJ 4  with one PC by using only one output channel of a typical video card having two output channels. 
     Accordingly, when using the two output channels of the typical video card, image data corresponding to eight projectors can be generated and output with one PC. Further, if it is possible to use an expensive video card that can provide eight output channels, one PC can generate and output image data corresponding to 32 projectors when each output channel can output four projectors worth of image data. 
     Capability of processing of image data per output channel of the video card  110  in terms of how many projectors the image data corresponds to depends on the resolution of the projector used in the multi-display system, processing ability for image data of the PC side (video card side) and the like. A current typical PC (video card) can sufficiently perform image data generation and output processing corresponding to four projectors for each output channel when the resolution of a projector is XGA (1024×768 pixels). Further, it is conceivable that the PC can sufficiently cope with the resolution of the so-called full high vision standard (1920×1080 pixels) frequently employed for home-use projectors or the like in recent years. 
     In the multi-display system according to the embodiment, a GPU mounted on a video card applies image processing including image corrections such as the correction regarding color and the correction regarding shape. Therefore, high-level image processing is possible compared with the case where the image processing is applied in a projector, providing an effect that an image to be displayed can be of high quality. 
     Many of typical PCs have a video card that can realize the multi-display system according to the embodiment. In addition, projectors generally have an image cut-out function. Therefore, when realizing the multi-display system according to the embodiment, there is no need to provide a special hardware to the PC and projector, and therefore the multi-display system can be configured inexpensively. 
     The invention is not limited to the above-described embodiment but can be implemented in various modifications within a range not departing from the spirit of the invention. For example, the following modifications (1) to (3) are possible. 
     (1) In the above-described multi-display system according to the embodiment, a multi-display system in which projectors are used as a plurality of image display devices has been exemplified. However, a multi-display system in which direct-view-type image display devices are arranged may be employed. In the case of employing the direct-view-type image display devices, each of the direct-view-type image display devices has to have the image cut-out function. In the case of employing the direct-view-type image display devices, an overlapped region is not present between images displayed by the image display devices. Therefore, a multi-display system that performs display as shown in, for example,  FIG. 4  can be configured. 
     (2) In the above-described multi-display system according to the embodiment, the plurality (four) of projectors PJ 1  to PJ 4  are connected in parallel to the image data distribution device  200 , and composite image data from the PC  100  is distributed to the projectors PJ 1  to PJ 4 . However, the projectors PJ 1  to PJ 4  may be multistage-connected (cascade-connected) without using the image data distribution device  200 . In this case, although each of the projectors PJ 1  to PJ 4  has to have an external output terminal for outputting composite image data as it is from the former stage projector to the later stage projector, there is a projector originally including the external output terminal. Therefore, this modification can be easily implemented by using the projector. 
     (3) When a plurality of images displayed by a plurality of projectors are displayed so as to each have an overlapped region between adjacent images (refer to  FIG. 5 ), the edge-blending process is performed through image processing in the embodiment. However, the edge-blending process through image processing and optical edge-blending means due to a light shielding plate that can adjust the brightness of the overlapped region can be used in combination. 
     The entire of Disclosure of Japanese Patent Application Nos: 2008-287427, filed Nov. 10, 2008 and 2009-022295, filed Feb. 3, 2009 are expressly incorporated by reference herein.