Abstract:
An image display system includes: an image data output device that multiplexes and outputs raster data and compressed moving picture data; and an image display device that is connected to the image data output device through a transmission line and displays an image on the basis of image data output from the image data output device. The image display system is capable of displaying a high-quality moving picture on the image display device by reducing the amount of image data flowing through the transmission line without imposing excessive load on the image data output device, the image display device, the image data output device, an image processing program, and a computer-readable recording medium having the image processing program recorded therein.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an image display system, an image display device, and an image data output device. 
     2. Related Art 
     In recent years, as an image processing function of a computer is highly improved, there is proposed a system in which a computer performs an image processing, which has been performed by an image display device in the related art, image processing data after the processing is transmitted to an image display device side in a digital format, and an image is displayed by the mage display device, such as a projector (for example, refer to JP-A-2004-69990). 
     In such an image display device, raster data captured by a frame memory of a video output unit of an image data output device, such as a computer, connected to the image display device, is encoded in a predetermined transmission format between the devices and is then transmitted and input to the image display device through a transmission line, such as a USB, and then the encoded data is decoded so as to realize image display. 
     In this case, if the raster data of a captured screen is transmitted under the uncompressed state, a large amount of data proportional to display resolution, color depth, and a frame rate flows through a transmission line, which causes a problem in that the load imposed on the transmission line or a driving circuit becomes significantly large. 
     In addition, even a main processor at the computer side should capture and transmit data corresponding to the entire screen, and accordingly, the load imposed on the main processor at the computer side becomes also large. 
     In order to solve the problems described above, there has been proposed a technique of reducing a required band in a transmission line by using a method in which only a part of the entire screen that has been updated as compared with a previous frame in a time manner, that is, only differential data, is transmitted. According to the technique, since only the differential part of a screen update is transmitted through a transmission line, the technique is very effective for transmitting screen data having a small amount of movement and a small update frequency or a narrow update range, which is typically displayed as a graphic screen by a computer. 
     However, in the case where an image data output device, such as a computer, reproduces a so-called moving picture based on a format specified by MPEG (moving picture experts group), for example, and then the moving picture is displayed on an image display device that is connected to the image data output device through a transmission line, it is general that a region where the moving picture is displayed occupies a relatively large area in the entire screen. In addition, if frame update is not made according to the frame rate of the moving picture, display based on the frame rate of the moving picture is not achieved. In this case, since a viewer can see dropping frames or jerky movement on the displayed moving picture, it is not possible to secure the quality of a display image. As a result, in order to display a moving picture, it is necessary to capture and transmit a large amount of data. 
     Furthermore, in the case where the moving picture is compressed moving picture data based on MPEG2 (ISO/IEC 13818-2) or the like, decoding of the moving picture, color conversion, a scaling, and a rasterizing process such as display processing need to be performed at the image data output device side, which also causes a problem in that the load imposed on the image data output device and the transmission line becomes even larger. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides an image display system capable of displaying a high-quality moving picture on an image display device by reducing the amount of image data flowing through a transmission line without imposing excessive load on an image data output device, an image display device, an image data output device, an image processing program, and a computer-readable recording medium having the image processing program recorded therein. 
     According to an aspect of the invention, an image display system includes: an image data output device that multiplexes and outputs raster data and compressed moving picture data; and an image display device that is connected to the image data output device through a transmission line and displays an image on the basis of image data output from the image data output device. The image data output device includes: a raster data acquisition unit that acquires the raster data; a compressed moving picture data acquisition unit that acquires the compressed moving picture data; a moving picture display region data generating unit that generates moving picture display region data indicating a display region, of a moving picture based on the compressed moving picture data, within the raster data; a data multiplexer that multiplexes the raster data acquired by the raster data acquisition unit, the compressed moving picture data acquired by the compressed moving picture data acquisition unit, and the moving picture display region data generated by the moving picture display region data generating unit; and a data transmitting unit that transmits multiplexed data through the transmission line. The image display device includes: a data receiving unit that receives data, which is transmitted from the image data output device, through the transmission line; a data demultiplexer that separates the data received by the data receiving unit into individual data; a moving picture rasterizing unit that performs a rasterizing process for separated compressed moving picture data so as to generate moving picture raster data; a display raster data synthesizing unit that synthesizes the moving picture raster data generated by the moving picture rasterizing unit and the raster data so as to generate display raster data; and an image forming unit that forms an image on the basis of the generated display raster data. 
     Here, the raster data means image data (data loaded on a video memory as information corresponding to each pixel, not moving picture data generally displayed by video overlay) based on graphic data generated by the computer or the like. The compressed moving picture data means image data subjected to a compression process according to a predetermined format, such as MotionJPEG and MPEG, and preferably, the invention is implemented by adopting a moving picture format, such as MPEG2 or MPEG4. 
     Moreover, the moving picture rasterizing unit included in the image display device decodes the compressed moving picture data and performs processes, such as a scaling and conversion of brightness or color of each pixel displayed by the image forming unit. 
     In the aspect of the invention, since the image display system includes the functional units described above, in advance, the compressed moving picture data is not decoded by the image data output device side, but the compressed moving picture data is transmitted to the image display device side through the transmission line and is then rasterized to be displayed. Accordingly, since the image data output device does not need to decode the compressed image data, a process load imposed on the image data output device is alleviated. As a result, excessive load is not imposed on the image data output device. In addition, since the compressed moving picture data flows through a transmission line under the compressed state, the amount of image data transmitted and received is reduced. In addition, since compressed moving picture data is decoded and rasterized on the image display device, a high-quality moving picture can also be obtained. 
     In the aspect of the invention, preferably, the image data output device further includes a differential data generating unit that generates differential data between data before and after a screen is updated, the raster data acquisition unit acquires, as raster data, the differential data generated by the differential data generating unit, and the image display device further includes a raster data reconstructing unit that reconstructs raster data on the basis of separated raster data and display raster data before a screen is updated. 
     In the aspect of the invention, when the screen update occurs in the raster data display region, only the differential data is transmitted to the image display device through the transmission line. Accordingly, since the amount of data flowing through the transmission line is reduced, it is possible to alleviate the load imposed on the transmission line. 
     Furthermore, in another aspect of the invention, preferably, the image display device further includes: a synchronization signal generating unit that generates an image synchronizing signal for updating a screen of the display raster data generated by the data synthesizing unit on the basis of one of frame rate information included in the compressed moving picture data, refresh rate information output from the image data output device, and update period information used as a data generation period by the differential data generating unit; and a scanning unit that performs a scanning process in the image forming unit on the basis of the generated image synchronizing signal. 
     Here, preferably, when a moving picture is displayed, a synchronizing signal for the screen update of the scanning unit in the image display device is generated on the basis of the frame rate information included as image attribute information in the compressed moving picture data. 
     In the aspect of the invention, since an image can be formed by using an optical synchronizing signal corresponding to the type of an image displayed on the image display device, a high-quality image can be displayed on the image display device. Moreover, in the case when a moving picture is displayed, the synchronizing signal is generated on the basis of the frame rate information included as image attribute information in the compressed moving picture data, such that the screen update based on the frame rate of the moving picture can be performed. As a result, it is possible to prevent, for example, dropping frames or jerky movement from occurring on the moving picture, and accordingly, a high-quality moving picture can be displayed. 
     Furthermore, in the invention, preferably, the image display device further includes: an image/audio separating unit that separates audio data included in the compressed moving picture data when the moving picture raster data is generated by the moving picture rasterizing unit; an audio data decoder that decodes the audio data separated by the image/audio separating unit; an audio output unit that outputs audio decoded by the audio data decoder; and an audio output synchronization adjusting unit that synchronizes timing of the audio output made by the audio output unit with generation time of the moving picture raster data generated by the moving picture rasterizing unit. 
     In the invention, in the case when it takes time to perform a rasterizing process for the compressed moving picture data, the audio output synchronization adjusting unit can cause audio to be output from the audio output unit in synchronization with a moving picture according to the generation time of the moving picture raster data. As a result, since there is no problem related to the timing between an image output and an audio output, that is, lip-sync, a viewer does not have any sense of incongruity. 
     The image display system according to the aspect of the invention includes an image display device and an image data output device, and each of the image display device and the image data output device has the following configuration. 
     According to another aspect of the invention, an image display device that is connected to an image data output device that multiplexes and outputs raster data and compressed moving picture data through a transmission line and displays an image on the basis of image data output from the image data output device, the image data output device multiplexing and outputting raster data and compressed moving picture data, includes: a data receiving unit that receives data, which is transmitted from the image data output device, through the transmission line; a data demultiplexer that separates the data received by the data receiving unit into individual data; a moving picture rasterizing unit that performs a rasterizing process for separated compressed moving picture data so as to generate moving picture raster data; a display raster data synthesizing unit that synthesizes the moving picture raster data generated by the compressed moving picture rasterizing unit and the raster data so as to generate display raster data; and an image forming unit that forms an image on the basis of the generated display raster data. The image data output device is configured to multiplex and output the raster data, the compressed moving picture data, and moving picture display region data indicating a display region, of a moving picture based on the compressed moving picture data, within the raster data. 
     In the image display device described above, in is preferable to further include: an image/audio separating unit that separates audio data included in the compressed moving picture data when the moving picture raster data is generated by the moving picture rasterizing unit; an audio data decoder that decodes the audio data separated by the image/audio separating unit; an audio output unit that outputs audio decoded by the audio data decoder; and an audio output synchronization adjusting unit that synchronizes timing of the audio output made by the audio output unit with generation time of the moving picture raster data generated by the moving picture rasterizing unit. 
     Further, according to still another aspect of the invention, an image data output device that multiplexes and outputs raster data and compressed moving picture data and displays an image on an image display device connected to the image data output device through a transmission line includes: a raster data acquisition unit that acquires the raster data; a compressed moving picture data acquisition unit that acquires the compressed moving picture data; a moving picture display region data generating unit that generates moving picture display region data indicating a display region, of a moving picture based on the compressed moving picture data, within the raster data; a data multiplexer that multiplexes the raster data acquired by the raster data acquisition unit, the compressed moving picture data acquired by the compressed moving picture data acquisition unit, and the moving picture display region data generated by the moving picture display region data generating unit; and a data transmitting unit that transmits multiplexed data through the transmission line. 
     Furthermore, according to still another aspect of the invention, there may be provided an image processing program serving as the functional units described above for an image data output device, such as a computer. In addition, there may be provided a recording medium having the image processing program recorded therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective view schematically illustrating the configuration of an image display system according to a first embodiment of the invention. 
         FIG. 2  is a view schematically illustrating the configuration of an image data output device according to the present embodiment. 
         FIG. 3  is a functional block diagram illustrating the image data output device according to the present embodiment. 
         FIG. 4  is a view schematically illustrating the configuration of an image display device according to the present embodiment. 
         FIG. 5  is a functional block diagram illustrating the image display device in the present embodiment. 
         FIG. 6  is a flow chart illustrating an operation of the image display system according to the present embodiment. 
         FIG. 7  is a flow chart illustrating an operation of the image display system according to the present embodiment. 
         FIG. 8  is a perspective view schematically illustrating the configuration of an image display system according to a second embodiment of the invention. 
         FIG. 9  is a functional block diagram illustrating the image display device in the present embodiment. 
         FIG. 10  is a flow chart illustrating an operation of the image display system according to the present embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings. 
     1. First Embodiment 
       FIG. 1  illustrates an image display system  1  according to a first embodiment of the invention. The image display system  1  includes a computer  2  serving as an image data output device, a projector  3  serving as an image display device, and a USB cable  4  serving as a transmission line by which the computer  2  and the projector  3  are connected to each other. Image data output from the computer  2  is input to the projector  3  through the USB cable  4 , and an image on a local display of the computer  2  can be displayed as a projection image on a screen. The USB cable  4  is a cable based on the specifications, such as USB 1.1 or USB 2.0, and two-way communication between connected devices can be performed through the USB cable  4 . 
     (1) Structure of Computer  2   
     As shown in  FIG. 2 , the computer  2  serving as an image data output device includes a chip set  21  provided on a mother board, a CPU  22 , a main memory  23 , a video card  24 , an optical disk drive  25 , a hard disk drive  26 , and a USB port  27 , and the chip set  21  is connected to the CPU  22 , the main memory  23 , the video card  24 , the optical disk drive  25 , the hard disk drive  26 , and the USB port  27  through bus lines. 
     The CPU  22  serves as an operation processing unit that executes various programs by using the main memory  23  as an operation region. For example, although not shown in  FIG. 2 , even graphic data generated by executing an operating system is generated by the CPU  22 . 
     The video card  24  performs an image processing for image data input to the computer  2  and performs image display on the local display belonging to the computer  2 . Although not shown, the video card  24  includes a processor, which performs an image processing for image to be output to the local display, a video memory that stores image data to be processed, or the like. In addition, in the present embodiment, even though the computer  2  is configured to include the local display, the computer  2  does not necessarily need to include the local display. Moreover, the video card  24  does not necessarily need to be a separate device. For example, the video card  24  may be a graphic function built in a chip set. 
     The optical disk drive  25  is a device that reproduces optical disk media, such as a CD (compact disc) and a DVD (digital versatile disc). If an optical disk that stores compressed moving picture data is inserted into the optical disk drive  25 , the optical disk drive  25  reproduces the moving picture data stored in the optical disk and then outputs the moving picture data to the main memory  23  that is connected to the CPU  22  through the chip set  21   
     The hard disk drive  26  is a storage device that stores a program executed in the CPU  22  or data generated by operating the computer  2 . In addition, since the hard disk drive  26  can store image data, the hard disk drive  26  may output the stored image data in response to the request of a program operating on the CPU  22 . 
     The USB port  27  is a terminal located at the side of the computer  2  to which the above-described USB cable  4  is connected, and input and output of the USB port  27  are controlled by the chip set  21 , and thus a data transmission unit referred in the invention is implemented. 
     When an image processing with respect to graphic image data input by the CPU  22  and stream-output moving picture data is performed, an image processing program having functional units shown in  FIG. 3  is executed on the CPU  22 . Specifically, the CPU  22  is configured to include a graphic image data acquisition unit  221 , a moving picture display region generating unit  222 , a moving picture data acquisition unit  223 , a moving picture data analyzing unit  224 , and a data multiplexer  225 . 
     The graphic image data acquisition unit  221  serving as a raster data acquisition unit acquires graphic data generated by a graphic data generating unit  220  that operates on the operating system, and the graphic image data acquisition unit  221  includes a differential data generating section  221 A and a graphic data acquisition section  221 B. 
     The differential data generating section  221 A generates differential data of graphic images updated in a period of, for example, a refresh rate of the computer  2 . For example, the differential data generating section  221 A captures images before and after performing the updating process and then compares the images with each other so as to generate a changed part as differential data. 
     The graphic data acquisition section  221 B acquires only the differential data, which is generated by the differential data generating section  221 A, as graphic data, and outputs the acquired graphic data to the data multiplexer  225  to be described later. 
     The moving picture display region generating unit  222  serving as a moving picture display region data generating unit acquires, from input graphic data, on which part of the graphic screen based on the graphic data the moving picture data is to be displayed. For example, the moving picture display region generating unit  222  generates moving picture display region data indicating in which size and resolution and in which position of a graphic image the moving picture data should be displayed. 
     The moving picture data acquisition unit  223  serving as a compressed moving picture data acquisition unit acquires moving picture data that is compressed, without decoding moving picture data MPG sequentially input as a bit stream from the optical disk drive  25  or the hard disk drive  26 . Here, a compression format of the moving picture data includes MPEG2, MPEG4, MotionJPEG, and the like. 
     The moving picture data analyzing unit  224  acquires image attribution information, such as a frame rate, a bit rate, an aspect ratio, and resolution, which are included in the input moving picture data. The acquired image attribution information may be utilized as a motion control parameter in a multiplexing process or a transmission process for image data performed at a subsequent stage. In addition, for example, in the case when the compression format of moving picture data is MPEG2, the image attribution information is recorded on the sequence header (SH) of MPEG2 bit stream, and the moving picture data analyzing unit  224  can acquire the image attribution information of a moving picture by acquiring the information recorded on the header. 
     The data multiplexer  226  multiplexes the graphic data acquired by the graphic image data acquisition unit  221 , the moving picture display region data generated by the moving picture display region generating unit  222 , and the moving picture data acquired by the moving picture data acquisition unit  223 . As a multiplexing method includes an MPEG-TS (transport stream) method, an MPEG-PS (program stream) method, or the like, which are specified by ISO/IEC-13818-1. At this time, the data multiplexer  226  can also multiplex image correction data, such as a color and a gamma (γ) value, set on the computer  2 , in the same manner as described above. 
     Data multiplexed by the data multiplexer  226  is output and transmitted as serial data from the USB port  27  through the chip set  21 . 
     (2) Structure of Projector  3   
     As shown in  FIG. 4 , the projector  3  serving as an image display device is configured to include an optical engine  5  that forms an optical image and an image processing unit  6  that performs an image processing for the image data output and transmitted from the computer  2 . 
     The optical engine  5  includes a light source unit  51 , an integrator illumination optical system  52 , a liquid crystal panel  53  serving as an image forming unit, a color-combining optical system  54 , and a projection lens  55 . 
     Light emitted from the light source unit  51  is divided into a plurality of partial light components by means of the integrator illumination optical system  52  so as to be uniform illumination light, then the divided light components are modulated by the liquid crystal panel  53  on the basis of input image data, and then the modulated light components are projected onto a screen through the color-combining optical system  54  and the projection lens  55 . Moreover, although not shown in  FIG. 4 , in the projector  3  according to the present embodiment, the light modulation is performed by the liquid crystal panel  53  for each of the light components corresponding to R, G, and B colors. Thus, the color-combining optical system  54  synthesizes light components corresponding to R, G, and B colors, which have been subjected to an optical modulation process, and thus a color image is formed. 
     The image processing unit  6  performs a predetermined image processing on the basis of the image data input from the computer  2 , and performs a driving control of the liquid crystal panel  53  included in the optical engine  5  so as to form an optical image. The image processing unit  6  includes various circuit elements mounted on a circuit board provided within the projector  3 . 
     Specifically, the image processing unit  6  includes a USB port  61 , a USB controller  62 , a CPU  63 , a main memory  64 , a video rasterizer  65 , a demultiplexer  66 , a video frame memory  67 , an image processor  68 , and a panel controller  69 . 
     The USB port  61  is a terminal to which the USB cable  4  is connected and to which the image data output from the computer  2  is input. The USB controller  62  is a circuit element that controls input/output of the USB port  61 . In addition, the USB port  61  and the USB controller  62  are configured to serve as a data receiving unit referred in the invention. 
     The CPU  63  performs the entire control for the image processing unit  6  by executing various programs with the main memory  64  as an operation region. 
     The video rasterizer  65  serving as a moving picture rasterizing unit decodes moving picture data based on the format, such as MPEG2, and generates moving picture raster data. The moving picture raster data decoded by the video rasterizer  65  is input to the video frame memory  67  to be described later. 
     The demultiplexer  66  serving as a data demultiplexer separates the various data multiplexed by the data multiplexer  225  of the computer  2 , and the various data that has been separated is processed by other circuit elements, which will be described in detail later. In addition, the demultiplexer  66  in the present embodiment also serves as a differential image renderer, which will also be described in detail later. 
     The video frame memory  67  accumulates image raster data formed by various circuit elements and stores display raster data that is finally displayed on the liquid crystal panel  53 . The display raster data may be changed according to the data writing period of the liquid crystal panel  53  determined by the panel controller  69 . 
     The image processor  68  serving as a raster data synthesizing unit generates display raster data by finally synthesizing the data separated by the demultiplexer  66 , and in order to ensure the color reproducibility of the liquid crystal panel  53 , performs correction processes, such as brightness unevenness correction, color unevenness correction, or V-Tγ correction inherent in the liquid crystal panel  53 , in addition to the synthesis of graphic data and moving picture data. 
     The panel controller  69  serving as a scanning unit is a circuit element that performs a driving control of the liquid crystal panel  53 . By means of the panel controller  69 , the driving control of each pixel in the image display region of the liquid crystal panel  53  is performed. Further, in the present embodiment, the three-plate-type projector  3  described above is provided. Accordingly, although not shown in the drawing, the panel controller  69  is provided to the liquid crystal panel  53  for each of the light components corresponding to R, G, and B colors. 
     As described above, in the present embodiment, the image processing unit  6  has a configuration in which a plurality of circuit elements is combined. However, the hardware configuration of the image processing unit is not limited thereto. For example, one chip microprocessor may perform all the processes if it is an extremely high-performance processing unit. 
     The functional block configuration of the image processing unit  6  is shown in  FIG. 5 . 
     Of the circuit elements of the image processing unit  6 , the demultiplexer  66  includes a data demultiplexer  661  and a decoder  662  as functional units, and the video rasterizer  65  includes a decryptor  651 , a video decoder  652 , a scaler  653 , and a YUV-RGB converter  654  as functional units. Moreover, the CPU  63  includes a synchronization signal generating unit  631  as a functional unit for the purpose of synchronization of a screen update. 
     The data demultiplexer  661  separates multiplexed data, which has been input through the USB port  61 , into individual data, and specifically, the multiplexed data is divided into graphic data, moving picture data, moving picture display region data, and image correction data. 
     In addition, the data demultiplexer  661  outputs the separated graphic data to the decoder  662 , the separated moving picture data to the decryptor  651 , the separated moving picture display region data to the decoder  662  and the scaler  653 . Moreover, the data demultiplexer  661  outputs to the synchronization signal generating unit  631  information, such as a frame rate included in the image attribution information acquired at the time of a moving picture decoding process performed by the video decoder  652  to be described later, a refresh rate set by the computer  2 , and a screen update period of graphic data input as differential data. 
     The decoder  662  functions as the differential image renderer described above and performs a conversion such that graphic data output from the data demultiplexer  661  serves as a graphic image forming one screen. In the present embodiment, as described above, since the differential data is transmitted as graphic data, the decoder  662  holds graphic data before updating in the main memory  64 , and if the graphic data as new differential data is input, the decoder  662  rewrites only a part of data corresponding to the differential data so as to generate new graphic data. The graphic data converted by the decoder  662  is output as RGB data to the image processor  68 . That is, in the present embodiment, the decoder  662  functions as a raster data reconstructing unit referred in the invention. 
     Moreover, the decoder  662  sets a region, in which a moving picture should be displayed, within a graphic image on the basis of the moving picture display region data output from the data demultiplexer  661 , and generates graphic data corresponding to a region other than the region where the moving picture is displayed. 
     The decryptor  651  decodes parts, which are encoded for copy protection related to, for example, copyright, before decoding the input moving picture data, and performs a processing that causes the input moving picture data to be decoded by the video decoder  652  located at a subsequent stage. In addition, the decryptor  651  is not necessarily required, but the decryptor  651  is required in the case of moving picture data including the copy protection. 
     The video decoder  652  decodes the moving picture data transmitted under the compressed state. For example, in the case when the compression format is MPEG2, the video decoder  652  decodes the compressed moving picture data to moving picture data based on a raster data format in a unit of GOP (group of pictures) by applying inverse DCT (discrete cosine transform) to the compressed moving picture data. 
     The scaler  653  performs an expansion and contraction adjustment for the decoded moving picture data, and performs the adjustment depending on the capacity of a display device, such as resolution or an aspect ratio. The scaler  653  sets a region where moving picture data is to be displayed and resolution within the region on the basis of the resolution information included in the moving picture, the specification of the liquid crystal panel  53 , the moving picture display region data output from the data demultiplexer  661 , or the like. 
     The YUV-RGB converter  654  converts data, based on the YUV format, of the moving picture data to the RGB format for a computer, and the conversion is performed on the basis of a predetermined relational expression. 
     As described above, the image processor  68  synthesizes the graphic data decoded by the decoder  662  and the moving picture data decoded by the video decoder  652  so as to generate display raster data. Furthermore, the image processor  68  performs a correction process on the basis of the image correction data, which is set by the computer  2 , output from the data multiplexer  661  and picture correction data set to be inherent in the liquid crystal panel  53 , and stores display raster data after the correction in the video frame memory  67 . 
     The synchronization signal generating unit  631  generates a synchronizing signal for screen update of the panel controller  69  on the basis of information on the screen update output from the data demultiplexer  661 . In the present embodiment, the synchronizing signal is generated on the basis of the frame rate obtained from the serial header of the moving picture data. Moreover, the synchronizing signal generated by the synchronization signal generating unit  631  is also output to the image processor  68 , and the image processor  68  sets the timing at which the display raster data is stored in the video frame memory  67  on the basis of the synchronizing signal. 
     (3) Operation of Image Display System  1   
     Next, an operation of the image display system  1  having the above-described configuration will be described with reference to flow charts shown in  FIGS. 6 and 7 . 
     (3-1) Process at the Side of Computer  2   
     First, the graphic image data acquisition unit  221  captures a graphic image that is currently displayed and then acquires the captured graphic image as graphic data (process S 1 ). 
     Then, the differential data generating section  221 A of the graphic image data acquisition unit  221  generates differential data corresponding to an updated part on the basis of a graphic image, which has been previously captured, and the graphic image that has been captured this time (process S 2 ). In addition, the moving picture display region generating unit  222  generates moving picture display region data, such as the X-Y position on a screen, the size, and the resolution, with respect to a moving picture display region on the basis of the captured graphic image (process S 3 ). 
     On the other hand, in parallel with the acquisition of the graphic data, the moving picture data acquisition unit  223  acquires moving picture data input from the optical disk drive  25  or the like without performing a rasterizing process (process S 4 ). Then, the moving picture data analyzing unit  224  acquires, as image attribution information, various information, such as a frame rate, a bit rate, an aspect ratio, and resolution of the moving picture data, from a part of the sequence header (in the case of MPEG2) of the acquired moving picture data processing S 5 ). 
     If the above variety of information is acquired, the data multiplexer  225  multiplexes the moving picture data in the format based on the MPEG-TS, the MPEG-PS, or the like (process S 6 ). The multiplexed data is encoded by means of a function of the chip set  21  serving as a USB controller, and the encoded data is output from the USB port  27  to the projector  3  through the USB cable  4  (process S 7 ). 
     (3-2) Process at the Side of Projector  3   
     The multiplexed data output from the computer  2  is received by the USB port  61  and the USB controller  62  (process S 8 ), is then decoded by the USB controller  62  in the format that can be processed in the image processing unit  6 , and is then output to the data demultiplexer  661 . 
     The data demultiplexer  661  separates the multiplexed data into individual graphic data, moving picture display region data, and moving picture data (process S 9 ). 
     The decoder  662  acquires the separated graphic data as differential data (process S 10 ), and updates graphic data, in which only a part corresponding to the differential data is changed, on the basis of a graphic image before updating (process S 11 ). 
     In addition, the video rasterizer  65  acquires moving picture data and moving picture display region data (processes S 12  and S 13 ), and performs a rasterizing process on the moving picture on the basis of image attribution information included in the data and the moving picture data (processing S 14 ). 
     If rasterization of the graphic data performed by the decoder  662  and rasterization performed by the video rasterizer  65  are completed, the image processor  68  synthesizes the rasterized data so as to generate display raster data (process S 15 ). Furthermore, the image processor  68  performs a picture correction process inherent in the liquid crystal panel  53 , forms an optical image on the liquid crystal panel  53  by the scanning process performed by the panel controller  69 , and displays a projection image on a screen through the projection lens  55  (process S 16 ). 
     In the present embodiment described above, a process, such as the screen update, with respect to the graphic data as raster data is performed by only the computer  2 , and the moving picture data as compressed moving picture data is directly transmitted to the projector  3  so as to be rasterized by the video rasterizer  65  of the projector  3 , without being decoded in the computer  2 . Accordingly, it is advantageous in that the image processing load at the side of the computer  2  is reduced. In addition, since the moving picture data is output to the USB cable  4 , which serves as a transmission line, under the compressed state, it is possible to reduce the communication load imposed on the transmission line. In particular, since the process of rasterizing the moving picture data is performed by the projector  3  side, the communication speed of the transmission line is controlled, which makes it possible to prevent, for example, dropping frames or jerky movement from occurring on the displayed moving picture. 
     2. Second Embodiment 
     Next, a second embodiment of the invention will be described. In addition, in the following description, the same components as those described above have the same reference numerals, and a detailed description thereof will be omitted. 
     In the first embodiment described above, the projector  3  serving as an image display device performs only image display on the basis of the compressed moving picture data output from the computer  2 . 
     On the other hand, as shown in  FIG. 8 , an image display system according to the second embodiment is different from that according to the first embodiment in that a projector  7  is provided with a speaker  73  and audio data recorded in synchronization with image data is output from the speaker  73  when compressed moving picture data is decoded. 
     (1) Configuration of Image/Audio Processing Unit  8   
     Specifically, as shown in  FIG. 8 , the projector  7  according to the second embodiment includes circuit elements for audio output, and an image/audio processing unit  8  of the projector  7  includes a video rasterizer/audio decoder  71  and an audio output unit  72 , in addition to a USB port  61 , a USB controller  62 , a CPU  63 , a main memory  64 , a demultiplexer  66 , an image processor  68 , and a panel controller  69 , which are the same components as in the first embodiment. 
     The video rasterizer/audio decoder  71  has a function of rasterizing compressed moving picture data and a function of separating moving picture data and audio data from each other, which are included in the compressed moving picture data. Furthermore, the video rasterizer/audio decoder  71  also serves as an audio decoder that decodes the audio data. The audio data decoded by the video rasterizer/audio decoder  71  is stored in the main memory  64 . 
     The audio output unit  72  sequentially converts the decoded audio data stored in the main memory  64  into analog data, amplifies the converted audio data, and then outputs the amplified audio data to the speaker  73 . 
     The functional block configuration of the image/audio processing unit  8  is shown in  FIG. 9 . 
     The video rasterizer/audio decoder  71  of the image/audio processing unit  8  includes an image/audio separating unit  711 , an audio decoder  712 , and an audio output synchronization adjusting unit  713  as functional units, in addition to a decryptor  651 , a video decoder  652 , a scaler  653 , and a YUV-RGB converter  654  that also serve as functional units. 
     The image/audio separating unit  711  separates the moving picture data, which is separated by the data demultiplexer  661  and is then processed by the decryptor  651 , into image data and audio data, and the separated image data is output to the video decoder  652  and the separated audio data is output to the audio decoder  712 . In addition, in the present embodiment, the image/audio separating unit  711  is described as a unit different from the data demultiplexer  661  for the convenience of explanation. However, since the image/audio separating unit  711  and the data demultiplexer  661  are the same from the view point in which each of the image/audio separating unit  711  and the data demultiplexer  661  performs a separating process, a process using the same demultiplexer is possible. That is, the image/audio separating unit  711  and the data demultiplexer  661  may be configured as the same functional unit. 
     The audio decoder  712  decodes the audio data separated by the image/audio separating unit  711 , and the decoded audio data is stored in the main memory  64 . 
     The audio output synchronization adjusting unit  713  adjusts the timing of an audio output according to the output time stamp of moving picture and audio, which are encoded within the multiplexed stream of the moving picture and the audio, such that synchronization between the moving picture and the audio can be properly realized. 
     Specifically, the audio output synchronization adjusting unit  713  adjusts the timing of audio data output to a D/A converter  721 , which is stored in the main memory  64 , such that output timings based on PTS (presentation time stamp) of a moving picture and audio, which have been encoded at the time of multiplexing by MPEG-TS or MPEG-PS, match each other. 
     As shown in  FIG. 9 , the audio output unit  72  of  FIG. 8  includes the D/A converter  721  and an amplifier  722 , and the speaker  73  is connected to the amplifier  722 . 
     The D/A converter  721  converts the audio data based on a digital format, which is stored in the main memory  64 , to audio signals based on an analog format and then outputs the converted audio signals. 
     The amplifier  722  amplifies the audio signals, which have been converted to analog signals by the D/A converter  721 , and then outputs the amplified audio signals to the speaker  73 , and thus the audio data included in the moving picture data is output as audio from the speaker  73 . 
     (2) Operation 
     Next, an operation of the image/audio processing unit  8  having the above-described configuration will be described with reference to a flow chart shown in  FIG. 10 . In addition, since the image data processing is the same as in the first embodiment, the same reference numerals are applied and a detailed description thereof will be omitted. Here, an audio data processing will be mainly described. 
     Moving picture data input from a computer is received by the USB port  61  and the USB controller (process S 8 ), is then decoded in the format that can be processed by the image/audio processing unit  8 , and is then output to the data demultiplexer  661 . 
     The data demultiplexer  661  separates multiplexed data into individual data and outputs moving picture data to the image/audio separating unit  711  (process S 9 ). 
     The image/audio separating unit  711  separates image data and audio data, which are included in the moving picture data separated by the data demultiplexer  661 , from each other, and outputs output time information on a moving picture and audio to the audio output synchronization adjusting unit  713  (process S 21 ). 
     The audio decoder  712  decodes compressed audio data and stores the decoded data in the main memory  64  (process S 22 ). 
     When the audio decoder  712  performs the decoding process, the audio output synchronization adjusting unit  713  acquires output time information input from the image/audio separating unit  712 , sets the delay time of an audio output on the basis of the output time information, and performs a synchronization adjustment of the audio output with respect to the moving picture output (process S 23 ). 
     Data decoded by the audio decoder  712  is output to the D/A converter  721  on the basis of the delay time set by the audio output synchronization adjusting unit  713  and is then D/A converted by the D/A converter  721 . Then, the audio signal is amplified by the amplifier  722  and then an audio output is obtained through the speaker  73  (process S 24 ). 
     3. Modifications 
     In addition, the invention is not limited to the above-described embodiments, but various modifications and improvements may be made within the range where objects of the invention can be achieved. 
     For example, in the embodiments described above, the computer  2  has been used as an image data output device; however, the invention is not limited thereto. That is, it is possible to use a device, such as a DVD player or a game machine, as the image data output device. 
     Moreover, in the embodiments described above, the liquid crystal projector  3  has been adopted as an image display device; however, the invention is not limited thereto. For example, the invention may be applied to a projector having an optical modulation unit other than liquid crystal, such as a DLP, a backlight-type liquid crystal display, or a fixed-pixel-type display such as a plasma display or an organic EL display. 
     Furthermore, in the embodiments described above, the USB cable  4  has been used as a transmission line that connects the computer  2  and the projector  3  with each other; however, the invention is not limited thereto. That is, the invention may be applied to a system in which the image data output device and the image display device are connected to each other by means of a transmission line using LAN, such as TCP/IP, or a transmission line through which image can be transmitted, such as IEEE 1394. 
     In addition, the specific structure and shape in the embodiments of the invention may be substituted with different structure and shape within a range where the objects of the invention can be achieved. 
     The entire disclosure of Japanese Patent Application No. 2005-258407, filed Sep. 6, 2005 is expressly incorporated by reference herein.