Abstract:
A computer system with a monitor display, comprises a decoder unit configured to decode video data containing digital watermark information to derive decoded data, and configured to determine, based on information included in the video data, whether or not first certification for certifying that the video data is legitimately reproduced data succeeds, and a graphics controller which determines, using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, the graphics controller converting the decoded data into a protected decoded data from being copied and outputting the protected decoded data to the monitor display if the first certification and the second certification succeed, and the graphics controller inhibiting the decoded data from being output to the monitor display if the first certification fails and the second certification succeeds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2003-006182, filed Jan. 14, 2003; and No. 2003-165680, filed Jun. 10, 2003, the entire contents of both of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a computer system having a function for reproducing and outputting video data recorded on a medium, such as a DVD, using a monitor display. More particularly, it relates to a computer system capable of preventing, for example, copyright video programs from being illegally copied, and a method, employed in the computer system, for controlling the output of reproduced video data. 
   2. Description of the Related Art 
   In accordance with recent development of computer and multimedia techniques, so-called multimedia compatible personal computers have been widely put to practical use. Personal computers of this type have a function for reproducing video data (moving picture data) recorded on a medium such as a digital versatile disc (DVD), as well as text data and graphics data. 
   Video data recorded on a DVD is encoded based on the moving picture experts group (MPEG) phase 2 standard. To reproduce such MPEG-2 video data on a personal computer, an encoded stream of the video data is first read from a DVD drive into the main storage of the personal computer, and then transferred to an MPEG-2 decoder that is realized by software or hardware, under the control of a video reproduction application program. The MPEG-2 decoder performs decoding (decompress) of the coded stream. 
   The decoded data is once written into a video memory and then displayed on a monitor display, as in the case of standard graphics data provided by an operating system (OS) or an application program. In personal computers equipped with a GUI system, there is a case where a window (overlay surface) is assigned for each application program, and a case where video data is directly written to a video frame memory for display. 
   In response to strong requests from copyright owners who provide, for example, video software, a number of schemes for preventing illegal copying of video software have been proposed. For example, the Macrovision system developed by the US Macrovision company is the most popular copy protection technique employed in, for example, home VTRs (mainly, for VHS titles). However, no copy protection techniques have yet been established for protecting the video output to the monitor display of a personal computer from copying. 
   Jpn. Pat. Appln. KOKAI Publication No. 2001-16538, for example, has proposed a structure in which a copy protect function is imparted to a graphics controller, and an interface for executing the Macrovision function is provided in a graphics driver. This publication has also proposed a method in which the Macrovision function is activated for video data whose copying is to be prevented (i.e., where the data contains copy protection information) when the video data is decoded, thereby outputting, to a TV monitor, video data whose copying is to be prevented. This method, however, realizes copy protection for a TV monitor, but does not realize copy protection for a computer display. 
   In existing video applications, such as DVDs and digital broadcasting, copy protection information is represented by a flag belonging to the MPEG-2 stream. In the field of computers, programs for illegally altering the flag exist. To cope with this, a scheme for superimposing, on video data, copy protection information as digital watermark information is now being developed. 
   Since conventional personal computers do not have a function for protecting the output to the monitor display from being copied, video data recorded on a DVD and output to the monitor display of a computer is not protected from being copied, regardless of whether the video data contains copy protection information. Copy protection information contained in video data may be illegally copied by software. Further, in conventional personal computers, no consideration is made for the case where a plurality of video reproduction applications have different copy control information items. 
   BRIEF SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a computer system having a function for protecting the output to the computer monitor display from being copied if the output contains copy protection information, and also to provide a method for controlling the output of reproduced video data, employed in the computer system for realizing the function. 
   According to a first aspect of the invention, there is provided a computer system with a monitor display, comprising: 
   a decoder unit configured to decode video data containing digital watermark information to derive decoded data, and configured to determine, based on information included in the video data, whether or not first certification for certifying that the video data is legitimately reproduced data succeeds; and 
   a graphics controller which determines, using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds,
         the graphics controller converting the decoded data into a protected decoded data from being copied and outputting the protected decoded data to the monitor display if the first certification and the second certification succeed, and   the graphics controller inhibiting the decoded data from being output to the monitor display if the first certification fails and the second certification succeeds.       

   According to a second aspect of the invention, there is provided a method of controlling output of reproduced video data, comprising: decoding video data containing digital watermark information to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; forming a video display area in a video memory at a position corresponding to the video data, the video display area storing the decoded data; determining, using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, if the video display area is formed; converting the decoded data into a protected decoded data from being copied and writing the protected decoded data in the video display area if the first certification and the second certification succeed, and inhibiting the decoded data from being written in the video display area if the first certification fails and the second certification succeeds; and outputting, to a monitor display for reproduction, the protected decoded data stored in the video display area in the video memory. 
   According to a third aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data containing digital watermark information to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; dividing a screen of the monitor display into a plurality of areas; determining, for each of the areas using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds; and converting the decoded data into a protected decoded data from being copied and outputting the protected decoded data to the monitor display if the first certification and the second certification succeed, and inhibiting the decoded data from being output to the monitor display if the first certification fails and the second certification succeeds. 
   According to a fourth aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data containing digital watermark information to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; recognizing a plurality of windows on a screen of the monitor display; determining, for each of recognized windows using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds; and converting the decoded data into a protected decoded data from being copied and outputting the protected decoded data to the monitor display if the first certification and the second certification succeed, and inhibiting the decoded data from being output to the monitor display if the first certification fails and the second certification succeeds. 
   According to a fifth aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data containing digital watermark information to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; hooking an imaging instruction API (Application Programming Interface) to be transferred from an operating system to a graphics driver, the hooked imaging instruction API corresponding to the video data; determining whether or not second certification for certifying that the video data is to be protected from being copied succeeds, using the digital watermark information; and converting the decoded data into a protected decoded data from being copied and outputting the protected decoded data to the monitor display if the first certification and the second certification succeed, and inhibiting the decoded data from being output to the monitor display if the first certification fails and the second certification succeeds. 
   According to a sixth aspect of the invention, there is provided a computer system with a monitor display, comprising: 
   a decoder unit configured to decode video data to derive decoded data and configured to determine, based on information included in the video data, whether or not first certification for certifying that the video data is legitimately reproduced data succeeds; and 
   a graphics controller which determines whether or not the video data contains digital watermark information if the first certification succeeds, 
   the graphics controller determining, using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, 
   the graphics controller outputting decoded data to the monitor display in success of the first certification and failure of the second certification, and 
   the graphics controller limiting a manner of displaying the decoded data on the monitor display in one of failure of the first certification and success of the second certification. 
   According to a seventh aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; limiting a manner of displaying the decoded data on the monitor display if the first certification fails; determining whether or not the video data contains digital watermark information, if the first certification succeeds; forming a video display area on a video memory at a position corresponding to the video data, the video display area storing the decoded data; writing the decoded data in the video display area if the video data fails to contain the digital watermark information; determining, using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, if the video data contains the digital watermark information; writing the decoded data in the video display area if the second certification fails; limiting a manner of writing the decoded data in the video display area if the second certification succeeds; and outputting, to the monitor display for reproduction, the decoded data stored in the video display area. 
   According to an eighth aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; limiting a manner of displaying the decoded data on the monitor display if the first certification fails; dividing a screen of the monitor display into a plurality of areas if the first certification succeeds; determining, for each of the areas, whether or not the video data contains digital watermark information; outputting the decoded data to the monitor display if the video data fails to contain the digital watermark information; determining, for each of the areas using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, if the video data contains the digital watermark information; outputting the decoded data to the monitor display for reproduction if the second certification fails; and limiting a manner of displaying the decoded data on the monitor display if the second certification succeeds. 
   According to a ninth aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; limiting a manner of displaying the decoded data on the monitor display if the first certification fails; recognizing a plurality of windows on a screen of the monitor display if the first certification succeeds; determining, for each of the recognized windows, whether or not the video data contains digital watermark information; outputting the decoded data to the monitor display if the video data fails to contain the digital watermark information; determining, for each of the recognized windows using the digital watermark information, whether or not second certification for certifying that the video data is to be protected from being copied succeeds, if the video data contains the digital watermark information; outputting the decoded data to the monitor display for reproduction if the second certification fails; and limiting a manner of displaying the decoded data on the monitor display if the second certification succeeds. 
   According to a tenth aspect of the invention, there is provided a method of controlling output of reproduced video data to a monitor display, comprising: decoding video data to derive decoded data; determining, based on information included in the video data, whether or not first certification for certifying that the video data has been reproduced by a legitimate route succeeds; limiting a manner of displaying the decoded data on the monitor display if the first certification fails; hooking an imaging instruction API (Application Programming Interface) transferred from an operating system to a graphics driver, if the first certification succeeds; determining whether or not the video data contains digital watermark information; outputting the decoded data to the monitor display if the video data fails to contain the digital watermark information; determining, using the digital watermark information, whether or not second certification for certifying that the video data corresponding to the hooked imaging instruction API is to be protected from being copied succeeds, if the video data contains the digital watermark information; outputting the decoded data to the monitor display for reproduction if the second certification fails; and limiting a manner of displaying the decoded data on the monitor display if the second certification succeeds. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a block diagram illustrating the configuration of a computer system according to a first embodiment of the invention; 
       FIG. 2  is a block diagram useful in explaining a graphics controller incorporated in the first embodiment; 
       FIG. 3  is a flowchart illustrating the procedure of output control of reproduced video data in the first embodiment; 
       FIG. 4  is a block diagram illustrating the configuration of a computer system according to a second embodiment of the invention; 
       FIG. 5  illustrates a video data display form on a monitor display incorporated in the second embodiment; 
       FIG. 6  is a view useful in explaining copy protection in the second embodiment; 
       FIG. 7  is a flowchart illustrating the procedure of output control of reproduced video data in the second embodiment; 
       FIG. 8  is a block diagram illustrating the configuration of a computer system according to a third embodiment of the invention; 
       FIG. 9  is a view useful in explaining a window certification method using edge detection, employed in the third embodiment; 
       FIG. 10  is a view useful in explaining a window certification method using histograms, employed in the third embodiment; 
       FIG. 11  is a flowchart illustrating the procedure of output control of reproduced video data in the third embodiment; 
       FIG. 12  is a block diagram illustrating the configuration of a computer system according to a fourth embodiment of the invention; 
       FIG. 13  is a flowchart illustrating the procedure of output control of reproduced video data in the fourth embodiment; 
       FIG. 14  is a view useful in explaining the concept of copy protection employed in the fourth embodiment; 
       FIG. 15  is a block diagram illustrating the configuration of a computer system according to a fifth embodiment of the invention; 
       FIG. 16  is a flowchart illustrating the procedure of output control of reproduced video data in the fifth embodiment; 
       FIGS. 17A ,  17 B and  17 C are views useful in explaining display limitation in the fifth embodiment; 
       FIG. 18  is a view useful in explaining copy protection certification employed in the fifth embodiment; 
       FIG. 19  is a block diagram illustrating the configuration of a computer system according to a sixth embodiment of the invention; 
       FIG. 20  is a flowchart illustrating the procedure of output control of reproduced video data in the sixth embodiment; 
       FIG. 21  is a view useful in explaining operation areas of the computer system of the sixth embodiment; 
       FIG. 22  is a view useful in explaining operation areas of the computer system of the sixth embodiment; 
       FIGS. 23A ,  23 B,  23 C and  23 D illustrate changes with lapse of time in division processing performed in the sixth embodiment; 
       FIG. 24  is a block diagram illustrating the configuration of a computer system according to a seventh embodiment of the invention; 
       FIG. 25  is a flowchart illustrating the procedure of output control of reproduced video data in the seventh embodiment; 
       FIG. 26  is a block diagram illustrating the configuration of a computer system according to an eighth embodiment of the invention; 
       FIG. 27  is a flowchart illustrating the procedure of output control of reproduced video data in the eighth embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   FIRST EMBODIMENT 
     FIG. 1  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a first embodiment of the invention. This computer system comprises a decoder  11  for decoding video data  10 , graphics driver  12 , video memory  13 , and graphics controller  14  activated by the graphics driver  12 , and performs control for outputting reproduced video data to a monitor display  15 , using the graphics controller  14 . 
   The video data  10  is data compressed on the basis of, for example, the MPEG-2 standard, and is reproduced as an encoded stream from a DVD by a DVD drive under the control of a video reproduction application program. The video data  10  contains copy protection information that constitutes digital watermark information. The reproduced video data  10  is decoded by the decoder  11 . The decoder  11  is, for example, an MPEG-2 decoder, which is realized only by hardware, or using software such as video reproduction application program. In this embodiment, the decoder  11  also has a certification function for certifying whether or not the input video data  10  has been reproduced through a legitimate reproduction route (this certification will hereinafter be referred to as“decoder certification”). 
   The decoded data output from the decoder  11  is transferred to the video memory  13  via the operating system and graphics driver  12 . The data transferred to the video memory  13  is written to a video data display area generated in the video memory  13  (this area is generally called an overlay surface or video frame buffer, but will be referred to as the“overlay surface” in the description below). 
   The graphics controller  14  reads the data to be displayed and written to the overlay surface of the video memory  13 , and displays it on the monitor display  15 . The data written to the overlay surface is displayed on, for example, a video data reproduction window assigned by a window manager to the video data reproduction program, if the computer system of the embodiment is equipped with a window system in which a window is assigned to each application program. 
   In the embodiment, the graphics controller  14  has a copy protection function  14 A and certification function  14 B. When an overlay surface is generated in the video memory  13 , the graphics controller  14  performs certification (which is hereinafter referred to as“copy protection certification”) as to whether or not the video data  10  is copy-prevented data, i.e., whether or not the video data  10  contains digital watermark information. If the copy protection certification has succeeded, i.e., if it is determined that the video data  10  is copy-prevented data, the graphics controller  14  requests the decoder  11  of the above-mentioned decoder certification. 
   If the decoder certification has succeeded, the graphics controller  14  determines that the video data  10  has been reproduced through a legitimate reproduction route. The controller  14  then performs copy protection processing on the data decoded by the decoder  11  and supplied via the graphics driver  12 , writes the resultant data to the overlay surface of the video memory  13 , and outputs it to the monitor display  15 . On the other hand, if the decoder certification has failed, the graphics controller  14  determines that the video data  10  is reproduced by an illegitimate reproduction route, writes no data to the overlay surface, and inhibits the output of the decoded data to the monitor display  15 . 
   When the computer assigns a window to each application program for video data reproduction, the process of synthesizing an image using the overlay surface is included in the reproduction procedure. When an overlay surface is generated in the video memory  13 , the graphics controller  14  performs copy protection certification and refers to the result of decoder certification made by the decode  11 . After that, the controller  14  performs copy protection processing on the video data that contains copy protection information, and then outputs the resultant data to the monitor display  15 . Thus, copy protection can be performed effectively. 
   The operation of the graphics controller  14  will now be described roughly. 
   After the video data  10  is decoded by the decoder  11 , it is subjected to image synthesis. The to-be-displayed image data thus obtained is transferred to the video memory  13  and written to an overlay surface therein under the control of the graphics driver  12 . The image data written to the overlay surface is overlaid on the graphics data written in the video memory  13 , under the control of the graphics controller  14 .  FIG. 2  shows this state. 
   As seen from  FIG. 2 , the video memory  13  is divided into three areas—a primary surface  21 , secondary surface  22  and overlay surface  23 . The graphics controller  14  automatically switches the primary surface  21  and secondary surface  22  from one to the other to display graphics data. 
   The primary surface  21  and secondary surface  22  are used alternately. Specifically, while graphics data is read from the primary surface  21  and output to the monitor display  15 , new graphics data is written to the secondary surface  22 . As a result, data, which is now being written, is prevented from being displayed, thereby smoothing, for example, the movement of a window. 
   Video data written to the overlay surface  23  is displayed in the form of an overlay window (video reproduction window)  25  on a present reproduction screen  24 . The graphics controller  14  computes the size of the overlay window  25 , and combines the video data in the overlay surface  23  with the video data in the primary or secondary surface  21  or  22  so that the size of the to-be-displayed video data is adjusted to that of the overlay window  25 . As a result, video data can be written to the overlay surface  23  regardless of the timing of display and the size of the overlay window. 
   The above-described copy protection certification by the graphics controller  14  and the reference by the controller  14  to the result of decoder certification must be made before particular image processing, such as attachment of an image to a sphere image. In other words, copy protection certification and reference to the result of decoder certification must be made concerning the data obtained before any image processing is performed thereon. 
   Referring to the flowchart of  FIG. 3 , a description will be given of the procedure of controlling the output of reproduced video data, employed in the embodiment, i.e., the procedure of decoding video data  10  and outputting the decoded data to the monitor display  15 . 
   Firstly, video data  10  reproduced by, for example, a DVD drive (not shown) is input and decoded by the decoder  11  (steps S 30  and S 31 ). Subsequently, a video display area for storing the decoded data, for example, overlay surface, is generated in the video memory  13  (step S 32 ). Copy protection certification as to whether or not the video data  10  is copy-prevented data, i.e., whether or not the video data  10  contains digital watermark information, is performed (step S 33 ). If the video data  10  contains no digital watermark information, the decoded data is written to the video data display area (step S 36 ). If, on the other hand, the video data  10  contains digital watermark information, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data (step S 34 ). 
   If the decoder certification concerning the video data has succeeded, i.e., if the video data is certified to have been reproduced by a legitimate reproduction route, copy protection certification as to whether or not the video data  10  should be protected from being copied is performed using the digital watermark information (step S 35 ). For example, at the step S 35 , it is determined from the digital watermark information whether or not the video data  10  has been copied so far less than a predetermined number of times. If the copy protection certification has failed (i.e., it is determined that the video data  10  has been copied so far less than the predetermined number of times), the decoded data is written to the video data display area (step S 36 ). On the other hand, if the copy protection certification has succeeded, the manner of writing the decoded data to the video data display area is limited (step S 38 ). Lastly, the decoded data stored in the video data display area of the video memory  13  is output to the monitor display  15  for reproduction (step S 37 ). 
   On the other hand, the video data whose decoder certification has failed, i.e., the video data reproduced by an illegitimate reproduction route, is prevented from being written to the overlay surface of the video memory  13  and output to the monitor display  15 . 
   SECOND EMBODIMENT 
     FIG. 4  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a second embodiment of the invention. In  FIG. 4 , elements similar to those of  FIG. 1  are denoted by corresponding reference numerals. In the second embodiment, the graphics controller  14  further comprises an area-dividing function  14 C. Using the area-dividing function  14 C, the graphics controller  14  divides the reproduction screen of the monitor display  15  into a plurality of areas when performing video reproduction on the display  15 . After that, the controller  14  performs copy protection certification as to whether or not video data  10  in each area is copy-prevented data, i.e., whether or not the video data  10  contains digital watermark information. 
   If the video data  10  is copy-prevented data, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data. Concerning the video data whose certification has succeeded, i.e., the video data certified to have been reproduced by a legitimate reproduction route, the graphics controller  14  performs copy protection processing on the data decoded by the decoder  11  and outputs the resultant data to the monitor display  15 . In contrast, concerning the video data whose decoder certification has failed, i.e., the video data reproduced by an illegitimate reproduction route, the controller  14  inhibits the output of this data to the monitor display  15 . 
   In a personal computer with the window system, if a window (overlay surface) is assigned to each application program, the process of synthesizing an image using the overlay surface is performed. Therefore, the same copy protection method as employed in the first embodiment can be utilized. 
   On the other hand, if video data is directly written into the video frame memory for display, the graphics controller  14  performs copy protection certification and refers to the result of decoder certification, concerning the decoded data in the display video frame memory that corresponds to the output to the monitor display  15 . As a result, the decoded data corresponding to copy-prevented video data is subjected to copy protection processing, and then output to the monitor display  15 . Thus, the video data is effectively protected from being copied. 
   In the embodiment, the reproduction screen may be divided, for example, in the following manners: 
   (1-1) The reproduction screen is divided into a predetermined number of equal-size columns and a predetermined number of equal-size rows. (1-2) The reproduction screen is divided such that the central area is larger than the peripheral areas. 
   In the above computer system, as shown in, for example,  FIG. 5 , it is possible to simultaneously reproduce DVD  1  data in a window on a reproduction screen  40 , DVD  2  data in another window on the screen  40 , spreadsheet data in a further window on the screen  40 , and DVD  3  data directly on the screen  40 . In this case, assume that the reproduction screen is divided into three equal columns and two equal rows, as in the reproduction screen  42  shown in  FIG. 6 , and it is necessary to protect DVD  1  data and DVD  2  data from being copied. If copy protection certification is performed as to whether or not the data in each of areas  1  to  6  is copy-prevented data, i.e., whether or not each area contains digital watermark information, copy protection information is detected from the areas  1 ,  3 ,  4  and  6 , since these areas contain the DVD  1  or  2  data. 
   Further, in the case of a reproduction screen  42  in  FIG. 6 , only the hatched area  1 , which contains the largest number of copy protection information items, is protected from being copied. In the case of a reproduction screen  43  in  FIG. 6 , the black areas  1  and  4 , signal values detected in which are higher than a predetermined threshold value set for the copy protection information, are protected from being copied. 
   The dividing manner of the reproduction screen of the monitor display  15  is not limited to that employed in the above reproduction screens  41  to  43 , in which the screen is divided into equal-size rows and equal-size columns. Alternatively, the reproduction screen may be divided like a reproduction screen  44  in  FIG. 6 , in which the central area is larger than the peripheral areas. Further, the reproduction screen may be divided into triangular areas like a reproduction screen  45  in  FIG. 6 , or circular areas. The reproduction screen may also be divided into triangular or rectangular areas like a reproduction screen  46 . 
   Referring then to the flowchart of  FIG. 7 , the procedure of controlling the output of reproduced video data will be described. 
   Firstly, video data  10  reproduced by, for example, a DVD drive (not shown) is input and decoded (steps S 50  and S 51 ). Subsequently, the decoded data is written into the video memory  13  (step S 52 ). At this time, a video display area, such as the overlay surface, may be generated as in the first embodiment. 
   Thereafter, the reproduction screen of the monitor display  15  is divided into a plurality of areas (step S 53 ), and performs copy protection certification for each area as to whether or not video data  10  is copy-prevented data, i.e., whether or not it contains digital watermark information (step S 54 ). If the video data  10  contains no digital watermark information, the decoded data is written to the video data display area (step S 36 ). If, on the other hand, the video data  10  contains digital watermark information, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning the video data  10  (step S 55 ). 
   Concerning the video data whose certification has succeeded, i.e., the video data certified to have been reproduced by a legitimate reproduction route, copy protection certification as to whether or not the video data  10  should be protected from being copied is performed using the digital watermark information (step S 56 ). For example, at the step S 56 , it is determined from the digital watermark information whether or not the video data  10  has been copied so far less than a predetermined number of times. If the copy protection certification has failed (i.e., it is determined that the video data  10  has been copied so far less than the predetermined number of times), the decoded data is written to the video data display area (step S 57 ). On the other hand, if the copy protection certification has succeeded, the manner of writing the decoded data to the video data display area is limited (step S 59 ). Lastly, the decoded data stored in the video data display area of the video memory  13  is output to the monitor display  15  for reproduction (step S 58 ). 
   On the other hand, the video data whose decoder certification has failed, i.e., the video data reproduced by an illegitimate reproduction route, is prevented from being written into the video memory  13  and output to the monitor display  15 . 
   THIRD EMBODIMENT 
     FIG. 8  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a third embodiment of the invention. In  FIG. 8 , elements similar to those of  FIG. 1  are denoted by corresponding reference numerals. In the third embodiment, the graphics controller  14  further comprises a window recognition function  14 D. Using the window recognition function  14 D, the graphics controller  14  recognizes a window or windows on the reproduction screen of the monitor display  15  before reproducing video data on the display  15 , and performs copy protection certification on video data  10  in each recognized window, thereby determining whether or not the data contains digital watermark information. 
   If it is determined that the video data  10  contains digital watermark information, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data. Concerning the video data whose certification has succeeded, i.e., the video data certified to have been reproduced by a legitimate reproduction route, the graphics controller  14  performs copy protection processing on decoded data and outputs the resultant data to the monitor display  15 . 
   In contrast, concerning the video data whose certification has failed, i.e., the video data reproduced by an illegitimate reproduction route, the controller  14  inhibits the output of this data to the monitor display  15 . 
   If video data is directly written into the display video frame memory, the graphics controller  14  performs copy protection certification and refers to the result of decoder certification, concerning the decoded data in the display video frame memory that corresponds to the output to the monitor display  15 . As a result, the decoded data corresponding to video data that is copy-prevented data is subjected to copy protection processing, and then output to the monitor display  15 . Thus, the video data is effectively protected from being copied. 
   The following methods, for example, may be used as the window recognition method in the embodiment: 
   (2-1) Method using edge detection 
   (2-2) Method using the frequency of access to the memory 
   (2-3) Method using a pixel value distribution 
   Firstly, a method for recognizing a window based on edge detection using primary differential will be described. In the boundary of areas, the pixels have significantly different brightness values. Therefore, if differential is performed concerning changes in pixel value, edge detection can be performed. If standard difference computation is executed, noise components contained in video data may also be involved in the computation process. To avoid this, some filters having a function for reducing the level of noise and removing noise itself have been proposed. An example using a Sobel filter will now be described. 
   As is shown in  FIG. 9 , the Sobel filter multiplies, by respective predetermined coefficients, the values of nine pixels that include a target pixel and its peripheral eighth pixels, and sums up the multiplication results. In the example of  FIG. 9 , this process is performed, using a coefficient matrix  61  in the horizontal direction and a coefficient matrix  62  in the vertical direction. If the horizontal and vertical sums obtained in the process are represented by Gh and Gv, respectively, the value of the target pixel is given by (Gh 2 +Gv 2 ) 1/2 . From this, a window edge for recognizing a window is detected. 
   A method for recognizing a window based on the frequency of access will be described. Assume, for example, that as shown in  FIG. 10 , the computer system simultaneously reproduces DVD  1  data, DVD  2  data and spreadsheet data in respective windows on a reproduction screen  71 , and DVD  3  data directly on the screen  71  (i.e., the screen  71  exhibits the same state as the screen  40  in  FIG. 5 ). In this case, the video memory  13  defines therein memory areas corresponding to the DVD  1 , DVD  2  and DVD  3 . During reproduction of a DVD, the frequency of access to the memory area corresponding to the DVD is considered very high. In other words, it is strongly possible that the memory area now accessed at a high frequency corresponds to the window that displays DVD data that is now being reproduced. 
   Further, assume that pixel value distributions  72  and  73  have been acquired in the horizontal line H 1  and vertical line V 1  of the reproduction screen  71  of  FIG. 10 , respectively. In this case, the position of a window can be determined from change points Ha and Hb in the pixel value distribution  72  and change points Va and Vb in the pixel value distribution  73 . Thus, the window is recognized. 
   Referring to the flowchart of  FIG. 11 , a description will be given of the procedure of controlling the output of reproduced video data, employed in the embodiment. 
   Firstly, video data  10  reproduced by, for example, a DVD drive (not shown) is input and decoded (steps S 70  and S 71 ). The decoded data is written into the video memory  13  (step S 72 ), After that, window recognition is performed on the reproduction screen of the monitor display  15  (step S 73 ), and copy protection certification as to whether the video data  10  is copy-prevented data, i.e., whether or not the data contains digital watermark information, is performed (step S 74 ). If it is determined at the step S 74  that the video data  10  contains digital watermark information, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data (step S 75 ). 
   Concerning the video data whose certification has succeeded, i.e., the video data certified to have been reproduced by a legitimate reproduction route, copy protection certification as to whether or not the video data  10  should be protected from being copied is performed using the digital watermark information (step S 76 ). For example, at the step S 76 , it is determined from the digital watermark information whether or not the video data  10  has been copied so far less than a predetermined number of times. If the copy protection certification has failed (i.e., it is determined that the video data  10  has been copied so far less than the predetermined number of times), the decoded data is written to the video data display area (step S 77 ). On the other hand, if the copy protection certification has succeeded, the manner of writing the decoded data to the video data display area is limited (step S 79 ). Lastly, the decoded data stored in the video data display area of the video memory  13  is output to the monitor display  15  for reproduction (step S 78 ). 
   On the other hand, concerning the video data whose decoder certification has failed, i.e., the video data reproduced by an illegitimate reproduction route, the graphics controller  14  does not write the data decoded by the decoder  11  into the video memory  13 , thereby preventing the data from being output to the monitor display  15 . 
   FOURTH EMBODIMENT 
     FIG. 12  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a fourth embodiment of the invention. An operating system (OS)  16  and API hook unit  17  are shown. 
   In the fourth embodiment, when the OS  16  executes an imaging instruction API (Application Programming Interface), the API hook unit  17  hooks an imaging instruction API call. The graphics controller  14  performs copy protection certification as to whether or not video data  10 , which is to be output using the hooked API call, contains digital watermark information. 
   If the video data  10  is copy-prevented data, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data. Concerning the video data certified to have been reproduced by a legitimate reproduction route, the graphics controller  14  performs copy protection processing on the data decoded by the decoder  11 , and outputs the resultant data to the monitor display  15 . 
   On the other hand, concerning the video data reproduced by an illegitimate reproduction route, the graphics controller  14  prevents the decoded data from being output to the monitor display  15 . 
   A detailed description will be given of the operations related to the imaging instruction API call. 
   When the OS  16  executes an imaging instruction API, the API hook unit  17  hooks an imaging instruction API call, transfers control to the copy protection function  14 A of the graphics controller  14 , and generates an API call after copy protection information is detected. More specifically, when an application has issued an imaging instruction, a certain imaging routine is called and an interruption occurs, therefore a pointer in a jump table corresponding to the interruption is fetched. 
   The type of interruption can be determined by reading the contents of the memory indicated by the pointer. If it is determined that the interruption relates to an imaging routine, the graphics controller  14  performs copy protection certification as to whether or not video data  10  is copy-prevented data, i.e., whether or not the data contains digital watermark information. 
   If the video data  10  is copy-prevented data, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data. If it is determined from the decoder certification that the video data  10  has been reproduced by a legitimate reproduction route, the graphics controller  14  causes an API call to be generated, then performs copy protection processing on the decoded data and outputs the resultant data to the monitor display  15 . 
   On the other hand, if it is determined that the video data has been reproduced by an illegitimate reproduction route, the graphics controller  14  prevents the generation of an API call to thereby prevent the decoded data from being output to the monitor display  15 . 
   The copy protection method employed in the first to third embodiments expects no supports from the OS  16 , while the copy protection method employed in the fourth embodiment expects supports from the OS  16 . When video data is reproduced, an imaging instruction API for flushing the screen and an imaging instruction API for window operation, etc. are utilized. When such an imaging instruction API is transferred to the graphics driver  12 , the graphics controller  14  performs copy protection certification and refers to the result of decoder certification, thereby performing copy protection processing on the copy-prevented data and outputting the resultant data to the monitor display  15 . Thus, copy protection can be performed effectively. 
   Referring now to the flowchart of  FIG. 13 , a description will be given of the procedure of controlling the output of reproduced video data, employed in the embodiment. 
   Firstly, video data  10  reproduced by, for example, a DVD drive (not shown) is input and decoded by the decoder  11  (steps S 90  and S 91 ). Subsequently, the decoded data is written into the video memory  13  (step S 92 ). 
   After that, the OS  16  hooks an imaging instruction API to be transferred to the graphics driver  12  (step S 93 ), thereby performing copy protection certification as to whether or not the video data  10  is copy-prevented data, i.e., whether or not the video data  10  contains digital watermark information (step S 94 ). If the video data  10  contains no digital watermark information, the decoded data is written to the video data display area (step S 97 ). If, on the other hand, the video data  10  contains digital watermark information, the graphics controller  14  refers to the result of certification made by the decoder  11  concerning this data (step S 95 ). 
   If the certification of the video data has succeeded, the graphics controller  14  determines that the video data has been reproduced by a legitimate reproduction route, copy protection certification as to whether or not the video data  10  should be protected from being copied is performed using the digital watermark information (step S 96 ). For example, at the step S 96 , it is determined from the digital watermark information whether or not the video data  10  has been copied so far less than a predetermined number of times. If the copy protection certification has failed (i.e., it is determined that the video data  10  has been copied so far less than the predetermined number of times), the decoded data is written to the video data display area (step S 97 ). On the other hand, if the copy protection certification has succeeded, the manner of writing the decoded data to the video data display area is limited (step S 99 ). Lastly, the decoded data stored in the video data display area of the video memory  13  is output to the monitor display  15  for reproduction (step S 98 ). 
   On the other hand, the video data whose decoder certification has failed, i.e., the video data reproduced by an illegitimate reproduction route, is prevented from being written into the video memory  13  and output to the monitor display  15 . 
   (Copy Protection) 
   The following methods, for example, may be used as the copy protection method in the first to fourth embodiments: 
   (3-1) The level of the brightness or color difference component of the target portion is changed. 
   (3-2) The target portion is hatched by a noise line, such as a red line. 
   (3-3) The target portion is hatched by thin noise lines. 
   (3-4) The target portion is solidly shaded with a single color. 
   (3-5) The target portion is covered with a mosaic pattern. 
   (3-6) The brightness or color difference component of the target portion is reversed. 
   Assume, for example, that in the example of  FIG. 5  where DVD  1  data, DVD  2  data and spreadsheet data are reproduced in respective windows displayed on the reproduction screen  40  of the monitor display  15 , and DVD  3  data is reproduced directly on the screen  40 , the reproduction screen is divided, by the area-dividing method employed in the second embodiment, into two rows and three columns like the reproduction screen  41  in  FIG. 6 . 
   In  FIG. 6 , the reproduction screen  42  is an example where data in the area  1  should be protected from being copied, and is copy-prevented by hatching. Similarly, the reproduction screen  43  is an example where data in the areas  1  and  4  should be protected from being copied, and is copy-prevented by solidly shading them. 
   Where a plurality of copy protection manners are employed on a single reproduction screen (for example, in  FIG. 5 , DVD  1  data should not be copied, DVD  2  data may be copied a predetermined number of times or less, and DVD  3  data may be copied), the following methods, for example, may be used: 
   (4-1) Method for protecting data in areas in accordance with respective conditions for the areas; 
   (4-2) Method for performing copy protection based on the data to be protected in the most reliable manner; 
   (4-3) Method for protecting the entire monitor display; 
   (4-4) Method for protecting only a data-reproducing area with high priority; and 
   (4-5) Method for protecting Macrovision and CGMS-A data in accordance with the data to be protected in the most reliable manner 
   Assume, for example, that in the example of  FIG. 5  where DVD  1  data, DVD  2  data and spreadsheet data are reproduced in respective windows displayed on the reproduction screen  40  of the monitor display  15 , and DVD  3  data is reproduced directly on the screen  40 , the reproduction screen is divided, by the area-dividing method employed in the second embodiment, into two rows and three columns like the reproduction screen  41  in  FIG. 6 . 
   In  FIG. 6 , the reproduction screen  42  is an example where data in the area  1  should be protected from being copied, and is copy-prevented by hatching. In this case, data in each area is protected in accordance with the condition for each area. Similarly, the reproduction screen  43  is an example where data in the areas  1  and  4  should be protected from being copied. In this case, it is requested that data in the area  1  be copy-prevented by hatching, and data in the area  4  be copy-prevented based on the Macrovision and CGMS-A schemes. Therefore, data in the areas  1  and  4  is copy-prevented in the most limited display manner (i.e., by solid black shading). 
   (Copy Protection Certification and Decoder Certification) 
   Assume here that copy protection information has been illegally altered using software. In each of the above-described embodiments, two-stage certification, i.e., copy protection certification by the graphics controller  14  and decoder certification by the decoder  11 , is performed. In this structure, even if an error occurs in copy protection certification because of illegal alteration of copy protection information, illegal copying is prevented by decoder certification by the decoder  11 . 
   Assume, for example, that video data  80  to  83  recorded on a rewritable disk, such as a DVD-RAM, is reproduced and output to the monitor display  15  as shown in  FIG. 14 . Further, assume that the decoder  11  performs certification of disks from which the video data is reproduced, and only a disk of number  5  is a legitimate one. In other words, assume that the decoder  11  determines that only the video data reproduced from the disk of number  5  is the one reproduced through a legitimate reproduction route. 
   The process shown in  FIG. 14  comprises the step of inputting the video data  80  to  83  reproduced from the disks, a copy protection certification step S 84  by the graphics controller  14 , and a disk certification step S 85  by the decoder  11 . Specifically, disk certification is performed in the following manner. For example, the decoder  11  reads a serial number recorded on a disk, inquires at a certification system via the Internet whether or not the read serial number is a legitimate one, and certifies that the disk is legitimate if the certification system certifies so. 
   When video data  80  is input, copy protection certification as to whether or not the data is copy-limited data is performed using digital watermark information detected from the data (step S 84 ). The copy protection information contained in copy-limited video data includes data for managing the number of copy-permitted occasions (including zero). From the certification, the video data  80  is detected to be copy-limited data (in this case, the number of copy-permitted occasions is not zero). Subsequently, the decoder  11  performs disk certification (step S 85 ). In this case, since the number of the disk from which the video data  80  is reproduced is “5”, the decoder  11  determines that disk certification has succeeded, whereby the data decoded by the decoder  11  is directly output to and reproduced on the monitor display  15 . 
   Further, when video data  81  is input, copy protection certification as to whether or not the data is copy-prevented data is performed at the step S 84  using digital watermark information detected from the data. From the certification, the video data  81  is detected to be copy-prevented data (in this case, the number of copy-permitted occasions is zero). Subsequently, the decoder  11  performs disk certification (step S 85 ). In this case, since the number of the disk from which the video data  81  is reproduced is “5”, the decoder  11  determines that disk certification has succeeded. Therefore, the data decoded by the decoder  11  is subjected to copy protection (at the time, the data is covered with a mosaic pattern), and then output to and reproduced on the monitor display  15 . 
   When video data  82  is input, copy protection certification as to whether or not the data is copy-limited data is performed using digital watermark information detected from the data (step S 84 ). From the certification, the video data  82  is detected to be copy-limited data (in this case, the number of copy-permitted occasions is not zero). Subsequently, the decoder  11  performs disk certification (step S 85 ). In this case, since the number of the disk from which the video data  82  is reproduced is “4”, the decoder  11  determines that disk certification has failed, whereby the data decoded by the decoder  11  is not output to the monitor display  15 , i.e., is not reproduced. It can be assumed that the video data  82  is illegally altered from “copy prevention” to “copy permission” by certain software. 
   When video data  83  is input, copy protection certification as to whether or not the data is copy-prevented data is performed at the step S 84  using digital watermark information detected from the data. From the certification, the video data  83  is detected to be copy-prevented data (in this case, the number of copy-permitted occasions is zero). Subsequently, the decoder  11  performs disk certification (step S 85 ). In this case, since the number of the disk from which the video data  83  is reproduced is “4”, the decoder  11  determines that disk certification has failed. Therefore, the data decoded by the decoder  11  is not output to the monitor display  15 , i.e., is not reproduced. 
   Fifth to eighth embodiments obtained by modifying the first to fourth embodiments will now be described. In the fifth to eighth embodiments, the points that differ from the first to fourth embodiments will be mainly explained. 
   FIFTH EMBODIMENT 
     FIG. 15  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a fifth embodiment of the invention. This computer system has a basic structure similar to that of the computer system of  FIG. 1 , and differs therefrom only in that in the former, the graphics controller  14  further comprises a copy protection certification function  14 E, display limiting function  14 F and display area data holding unit  14 G. 
   In the fifth embodiment, the graphics controller  14  detects whether or not video data  10  contains digital watermark information. If the data does not contain digital watermark information, the controller  14  outputs decoded data to the monitor display  15 . If, on the other hand, the data contains digital watermark information, the controller  14  performs, when an overlay surface is generated in the video memory  13 , copy protection certification as to whether or not video data  10  should be protected from being copied, using the digital watermark information detected from the video data  10 . 
   If the graphics controller  14  determines from the copy protection certification that the video data  10  should be protected from being copied, it refers to the result of certification made by the decoder  11 . If the decoder certification has succeeded and the copy protection certification has failed, the graphics controller  14  writes the data decoded by the decoder  11  to the overlay surface of the video memory  13 , and outputs it to the monitor display  15 . On the other hand, if the decoder certification has failed, the graphics controller  14  prevents the decoded data from being written to the overlay surface, thereby preventing the output of the decoded data to the monitor display  15 . 
   When the computer reproduces video data with a window assigned to each application program, it performs the process of synthesizing an image using an overlay surface. In this case, the graphics controller  14  performs copy protection certification and refers to the result of decoder certification. If the copy protection certification concerning video data has succeeded or the decoder certification concerning video data has failed, the controller  14  limits the manner of displaying the video data on the monitor display  15  (this will be also referred to as “display limitation”). Thus, copy protection can be made effectively. Since the operation of the graphics controller  14  is substantially the same as that of the corresponding controller in the first embodiment, no detailed description is given thereof. 
   Referring to the flowchart of  FIG. 16 , a description will be given of the procedure of controlling the output of reproduced video data, according to the embodiment, i.e., the process of decoding video data  10  and outputting the decoded data to the monitor display  15 . 
   Firstly, video data  10  reproduced by, for example, a DVD drive (not shown) is input and decoded by the decoder  11  (steps S 100  and S 101 ). Subsequently, the decoder  11  performs decoder certification as to whether or not the video data  10  has been reproduced by a legitimate reproduction route (step S 102 ). If the decoder certification has failed, i.e., if the video data  10  has been reproduced by an illegitimate reproduction route, the manner of displaying the decoded data on the monitor display  15  is limited (step S 108 ). 
   If the decoder certification has failed, i.e., if the video data  10  has been reproduced by a legitimate reproduction route, a video data display area for storing the decoded data, such as an overlay surface, is generated in the video memory  13 , thereby detecting whether or not the video data  10  contains digital watermark information (steps S 103  and S 104 ). 
   If the video data  10  contains no digital watermark information, the decoded data is written to the video data display area (step S 106 ). If, on the other hand, the video data  10  contains digital watermark information, copy protection certification as to whether or not the video data  10  should be protected from being copied is performed using the digital watermark information (step S 105 ). For example, at the step S 105 , it is determined from the digital watermark information whether or not the video data  10  has been copied so far less than a predetermined number of times. If the copy protection certification has failed (i.e., it is determined that the video data  10  has been copied so far less than the predetermined number of times), the decoded data is written to the video data display area (step S 106 ). On the other hand, if the copy protection certification has succeeded, the manner of writing the decoded data to the video data display area is limited (step S 108 ). Lastly, the decoded data stored in the video data display area of the video memory  13  is output to the monitor display  15  for reproduction (step S 107 ). 
   If, on the reproduction screen of the monitor display  15 , the display of video data whose decoder certification has succeeded and copy protection certification has failed is superimposed upon that of video data whose decoder certification has failed or copy protection certification has succeeded, the manner of writing data to the overlay surface is limited. Thus, the manner of displaying decoded data on the monitor display  15  is limited. A description will be given of specific methods for limiting the manner of displaying data on the monitor display  15 , with reference to the reproduction screens  91 ,  92  and  93  shown in  FIGS. 17A ,  17 B and  17 C. 
   In the reproduction screen  91  of  FIG. 17A , assume that decoder certification has failed or copy protection certification concerning DVD  1  data has succeeded, and decoder certification has succeeded and copy protection certification concerning DVD  2  data has failed (i.e., the DVD  2  data has been reproduced by a legitimate route). In this case, it is necessary to limit the display manner of the DVD  1  data. Therefore, the area for the DVD  1  data in the overlay surface (except for the area for the DVD  2  data) is masked as indicated by hatching in  FIG. 17A . 
   In contrast, in the reproduction screen  92  of  FIG. 17B , assume that decoder certification concerning DVD  2  data has failed or copy protection certification concerning DVD  2  data has succeeded, and decoder certification concerning DVD  1  data has succeeded and copy protection certification concerning DVD  1  data has failed (i.e., the DVD  1  data has been reproduced by a legitimate route). In this case, it is necessary to limit the display manner of the DVD  2  data. Therefore, only the area for the DVD  2  data in the overlay surface is masked as indicated by hatching in  FIG. 17B . 
   Further, in reproduction screen  93  of  FIG. 17C , assume that decoder certification concerning DVD  1  data has failed or copy protection certification concerning DVD  1  data has succeeded, and decoder certification concerning DVD  2  data has succeeded and copy protection certification concerning DVD  2  data has failed (i.e., the DVD  2  data has been reproduced by a legitimate route). In this case, to limit the display manner of the DVD  1  data, the area for the DVD  1  data (including the area for the DVD  2  data) is masked as indicated by hatching in  FIG. 17C . 
   The method for limiting the manner of displaying data on the area in which the area for data whose decoder certification concerning DVD  1  data has failed or copy protection certification concerning DVD  1  data has succeeded is superimposed upon the area for data whose decoder certification concerning DVD  2  data has succeeded and copy protection certification concerning DVD  2  data has failed may differ from that for limiting the manner of displaying data on the area in which no such superimposition exists. 
   The limitation or inhibition of the writing of decoded data may be realized by changing a value on the overlay surface. For example, in the reproduction screen  92  of  FIG. 17B , to limit the display manner of the DVD  2  data whose decoder certification has failed and copy protection certification has succeeded the area of the overlay surface corresponding to the DVD  2  data may be masked as indicated by hatching in  FIG. 17B . 
   When the display area for video data whose decoder certification has succeeded and copy protection certification has failed is changed on the reproduction screen of the monitor display  15 , the changed display area can be subjected to copy protection if the contents of change is reported to the graphics controller  14 . The display area change is, for example, a change in position or size, clipping, etc. 
   Specifically, when writing video data  10  into the video memory  13  in the system of  FIG. 15 , the graphics controller  14  holds, in the display area data holding unit  14 G, information as to the video data writing position in the video memory  13 . When the display area for decoded data has been changed on the monitor display  15 , the contents of the display area data holding unit  14 G is updated, and the graphics controller  14  performs copy protection based on the updated display area information. 
   Although in the embodiment, a description has been given of copy protection certification performed in the overlay surface, copy protection certification may be performed in other display areas as stated below. 
   (5-1) Copy protection certification in overlay surface 
   (5-2) Copy protection certification in primary surface 
   (5-3) Copy protection certification in overlay surface and that in primary surface 
   (5-4) Copy protection certification in overlay surface and that in primary surface, and copy protection certification in a video display area formed of overlay surface and primary surface 
     FIG. 18  is a view useful in explaining copy protection certification performed in a display area and the output of data to the monitor display  15 . As shown, copy protection certification may be performed in various areas, such as a primary surface  101 , overlay surface  102  or a synthesis section  103  of the primary and overlay surfaces  101  and  102 . 
   SIXTH EMBODIMENT 
     FIG. 19  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a sixth embodiment of the invention. In  FIG. 19 , elements similar to those of  FIG. 1  are denoted by corresponding reference numerals. In the sixth embodiment, the graphics controller  14  comprises an area-dividing function  14 C, as well as the copy protection certification function  14 E and display limiting function  14 F. Using the area-dividing function  14 C, the graphics controller  14  divides the reproduction screen of the monitor display  15  into a plurality of areas when performing video reproduction on the display  15 . After that, the controller  14  performs copy protection certification as to whether or not video data  10  in each area should be protected from being copied, using digital watermark information detected from the video data  10 . 
   In a personal computer with the window system, if a window (overlay surface) is assigned to each application program, the process of synthesizing an image using the overlay surface is performed. Therefore, the same copy protection method as employed in the first embodiment can be utilized. 
   On the other hand, if video data is directly written into the video frame memory for display, copy protection certification and decoder certification are performed on the decoded data in the display video frame memory that corresponds to the output to the monitor display  15 . The decoded data corresponding to video data whose decoder certification has failed or copy protection certification has succeeded is output to the monitor display  15  with the manner of display limited. Thus, the decoded data is effectively protected from being copied. 
   In the sixth embodiment, the reproduction screen may be divided in the following manners, as in the second embodiment: 
   (1-1) The reproduction screen is divided into a predetermined number of equal-size columns and a predetermined number of equal-size rows. 
   (1-2) The reproduction screen is divided such that the central area is larger than the peripheral areas. 
   In addition to the above, (1-3) the division method and area may be changed with lapse of time. 
   In the sixth embodiment, it is possible, like the second embodiment, to simultaneously reproduce DVD  1  data, DVD  2  data and spreadsheet data in respective windows on the screen  40 , and DVD  3  data directly on the screen  40 , as shown in  FIG. 5 . 
   In this case, a reproduction screen  41  is divided into three equal columns and two equal rows as shown in  FIG. 6 , and assume that DVD  1  data has been reproduced by an illegitimate route, copy protection certification concerning DVD  1  and DVD  2  data has succeeded, and copy protection certification concerning DVD  3  data has failed. Firstly, it is determined whether or not data in each area  1  to  6  contains digital watermark information. As a result, it is determined that data in the areas  1  and  4  contains digital watermark information. After that, copy protection certification is performed on the data in the areas  1  and  4 , and it is determined that only the certification in the area  1  has succeeded, therefore the display manner of the data in the area  1  is limited. The reproduction screen  42  of  FIG. 6  is the case where the display manner of data is limited in the area  1  as indicated by hatching. Further, the reproduction screen  43  is the case where it is determined that the DVD  2  data has been produced by an illegitimate route, and the display manner of data is limited in the areas  1  and  4  by solid shading. In the sixth embodiment, the reproduction screen of the monitor display  15  can be divided by various manners as in the second embodiment. 
   Referring to the flowchart of  FIG. 20 , the procedure of controlling the output of reproduced video data employed in the sixth embodiment will be described. This procedure is similar to that of the fifth embodiment except that in the former, the overlay surface-generating step S 103  in  FIG. 16  is replaced with an area-dividing step S 113 . 
   Specifically, video data  10  is input and decoded, and decoder certification is performed on the video data  10  (step S 110  to S 112 ). If the decoder certification result indicates that the video data  10  has been reproduced by an illegitimate route, the manner of displaying the decoded data on the monitor display  15  is limited (step S 118 ). If the decoder certification result indicates that the video data  10  has been reproduced by a legitimate route, the reproduction screen of the monitor display  15  is divided into a plurality of areas (step S 113 ). At this time, a video display area, such as an overlay surface, may be generated as in the fifth embodiment. 
   Each of the following steps S 114  to S 117  is executed for each area obtained by division at the step S 113 . Specifically, it is determined whether or not video data  10  contains digital watermark information. If the video data  10  contains no digital watermark information, decoded data is written into the video memory  13 , while if the video data  10  contains digital watermark information, it is subjected to copy protection certification using the digital watermark information. If copy protection certification has failed, the decoded data is written into the video memory  13 , while if copy protection certification has succeeded, the manner of writing the decoded data into the video memory  13  is limited. The decoded data written into the video memory  13  is output to the monitor display  15 . 
   When the display manner of the decoded data is limited, this limitation may be performed on the data in the area of the reproduction screen of the monitor display  15  other than the operation areas for the computer system. For example, in reproduction screens  111  to  116  shown in  FIG. 21 , a task bar and system menu as operation areas are displayed on the lower portion of the screen. These operation areas can be used even after copy protection is performed. On the other hand, if the operation areas, such as the task bar, system menu, etc., are protected, the computer system cannot be operated. In light of this, the operation areas are not subjected to copy protection processing so that they can be always used. 
   When the display manner of decoded data is limited, this limitation may be performed on the data in the area of the screen of the monitor display  15  other than the peripheral frame on the screen. For example,  FIG. 22  shows a reproduction screen  121  similar to the reproduction screen  40  of  FIG. 5 , and a reproduction screen  122  obtained after display limitation is performed. In the reproduction screen  122  after the display limitation, a certain pattern of display limitation is performed inside the frame, while the system menu on the frame is displayed. Thus, it is considered that system operation areas, such as the system menu, are located on the peripheral frame of the screen. Therefore, even if the positions of the system operation areas are unknown, when copy protection processing is performed on the area except for the area corresponding to the peripheral frame as shown in the screen  122  of  FIG. 22 , and display limitation is performed on the area other than the operation areas, the operation areas can be always displayed. 
   In the sixth embodiment, the manners shown in, for example,  FIGS. 23A ,  23 B,  23 C and  23 D may be employed as those for changing the division areas with lapse of time. Firstly, as shown in  FIG. 23A , it is determined whether or not the whole screen of the monitor display  15  contains digital watermark information. If no digital watermark information is detected, the screen is divided into four areas as shown in  FIG. 23B , and it is determined whether or not each area contains digital watermark information. If no digital watermark information is detected in each area, the screen is divided into six areas as shown in  FIG. 23C , thereby determining whether or not each area contains digital watermark information. These operations are repeated until each area thus obtained has a size smaller than a predetermined one. 
   In the examples shown in  FIGS. 23A ,  23 B,  23 C and  23 D, DVD  1  video data that contains digital watermark information is reproduced. Digital watermark information is not found in each area of the examples of  FIGS. 23A ,  23 B and  23 C, but found in the area of the example of  FIG. 23D  indicated by hatching. Therefore, based on this digital watermark information, display limitation indicated by hatching is performed on the screen of the monitor display  15 . 
   SEVENTH EMBODIMENT 
     FIG. 24  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to a seventh embodiment of the invention. In  FIG. 24 , elements similar to those of  FIG. 15  are denoted by corresponding reference numerals. In the seventh embodiment, the graphics controller  14  comprises a window recognition function  14 D, as well as the copy protection certification function  14 E and display limiting function  14 F. Using the window recognition function  14 D, the graphics controller  14  recognizes a window or windows on the reproduction screen of the monitor display  15  before reproducing video data on the display  15 . After that, the controller  14  performs copy protection certification as to whether or not video data  10  in each recognized window should be protected from being copied, using digital watermark information detected from the video data  10 . To recognize a window on the reproduction screen, various methods as employed in the third embodiment can be utilized. 
   If video data is directly written into the display video frame memory, decoder certification and copy protection certification are performed on the decoded data in the display video frame memory that corresponds to the output to the monitor display  15 . If decoder certification has failed or copy protection certification has succeeded, the decoded data is output to the monitor display  15 , with the manner of display limited. As a result, effective copy protection can be made. 
   Referring to the flowchart of  FIG. 25 , a description will be given of the procedure of controlling the output of reproduced video data, employed in the seventh embodiment. The procedure of the seventh embodiment is similar to that of the fifth embodiment except that in the former, the overlay surface-generating step S 103  in  FIG. 16  is replaced with a window recognition step S 123 . 
   Firstly, video data  10  is input, decoded and subjected to decoder certification (steps S 120  to S 122 ). If the result of decoder certification indicates that the video data  10  has been reproduced by an illegitimate route, the manner of displaying the decoded data on the monitor display  15  is limited (step S 128 ). If the decoder certification result indicates that the video data  10  has been reproduced by a legitimate route, the reproduction screen of the monitor display  15  is divided into a plurality of areas (step S 123 ). At this time, a video display area, such as an overlay surface, may be generated as in the fifth embodiment. 
   Each of the following steps S 124  to S 127  is executed for each area obtained by division at the step S 123 . Specifically, it is determined whether or not video data  10  contains digital watermark information. If the video data  10  contains no digital watermark information, decoded data is written into the video memory  13 , while if the video data  10  contains digital watermark information, it is subjected to copy protection certification using the digital watermark information. If copy protection certification has failed, the decoded data is written into the video memory  13 , while if copy protection certification has succeeded, the manner of writing the decoded data into the video memory  13  is limited. The decoded data written into the video memory  13  is output to the monitor display  15 . 
   EIGHTH EMBODIMENT 
     FIG. 26  is a block diagram illustrating the part of a computer system that relates to video reproduction and accords to an eighth embodiment of the invention. As seen from  FIG. 26 , the eighth embodiment is obtained by adding an operating system (OS)  16  and API hook unit  17  to the configuration of  FIG. 15 , as in the fourth embodiment. The basic operation of the eighth embodiment is the same as that of the fifth embodiment, and the operations of the OS  16  and API hook unit  17  are the same as those of the corresponding elements in the fourth embodiment. Therefore, no detailed description is given thereof. 
   The copy protection method employed in the fifth to seventh embodiments expects no supports from the OS  16 , while the copy protection method employed in the eighth embodiment expects supports from the OS  16 . When video data is reproduced, an imaging instruction API for flushing the screen and an imaging instruction API for window operation, etc. are utilized. When such an imaging instruction API is transferred to the graphics driver  12 , the graphics controller  14  performs copy protection certification, and refers to the result of decoder certification. If decoder certification has failed or copy protection certification has succeeded, the video data is output to the monitor display  15  with the manner of display limited. Thus, copy protection can be performed effectively. 
   On the other hand, when the OS  16  is an open source, even if copy protection processing is performed on video data using a function (e.g., API function) of the OS, the video data may well be used illegally by analyzing the source. In light of this, an application for, for example, the decoder  11  may use an imaging instruction API that directly accesses the video memory  13 , thereby directly connecting this application to the graphics controller  14  to enable the controller  14  to perform direct certification. 
   Referring to the flowchart of  FIG. 27 , the procedure of controlling the output of reproduced video data employed in the eighth embodiment will be described. This procedure is similar to that of the fifth embodiment except that in the former, the overlay surface-generating step S 103  in  FIG. 16  is replaced with an API hook step S 133 . Specifically, video data  10  is input and decoded by the decoder  11 , and decoder certification is performed as to whether or not the video data  10  has been reproduced by a legitimate route (step S 130  to S 132 ). If the decoder certification has succeeded, the OS  16  hooks the imaging instruction API transferred to the graphics driver  12  (step S 133 ). At this time, a video display area, such as an overlay surface, may be generated, as in the fifth embodiment. 
   After that, it is determined whether or not the video data  10  corresponding to the hooked imaging instruction API contains digital watermark information (step S 134 ). If the video data  10  contains no digital watermark information, the decoded data is written into the video memory  13 , whereas if the data  10  contains digital watermark information, copy protection certification is performed on the video data  10  using the digital watermark information (step S 135 ). If the copy protection certification on the video data  10  has failed, the decoded data is written into the video memory  13 , while if the copy protection certification on the video data  10  has succeeded, the manner of writing the decoded data into the video memory  13  is limited (step S 136 ). Lastly, the decoded data stored in the video memory  13  is output to the monitor display  15  for reproduction (step S 137 ). 
   The following manners, for example, may be employed for display limitation in the fifth to eighth embodiments: 
   (6-1) The brightness or color difference component in a target portion is changed; 
   (6-2) A noise line, such as a red line, is attached in a target portion; 
   (6-3) A plurality of thin noise lines are attached in a target portion; 
   (6-4) A target portion is solidly shaded in a single color; 
   (6-5) A target portion is covered with a mosaic pattern; 
   (6-6) The brightness or color difference component in a target portion is reversed; and 
   (6-7) Output to a target portion is inhibited. 
   Assume, for example, that DVD  1  data, DVD  2  data and spreadsheet data are reproduced in respective windows on the reproduction screen of the monitor display  15 , and DVD  3  data is directly reproduced on the screen, as is shown in  FIG. 5 . Further, assume that the reproduction screen is divided into three equal columns and two equal rows as shown in  FIG. 6 , using area-dividing method employed in the sixth embodiment. 
   The reproduction screen  42  of  FIG. 6  is an example where it is necessary to perform display limitation on the data in area  1 , and display limitation is performed by hatching. Further, the reproduction screen  43  of  FIG. 6  is an example where it is necessary to perform display limitation on the data in areas  1  and  4 , and display limitation is performed by solid black shading. 
   The following methods, for example, may be used for realizing different types of display limitation for a plurality of simultaneously reproduced video data items: 
   (1) Method for limiting the manner of displaying data items in respective areas in accordance with the states of the data items; 
   (2) Method for limiting the display manner based on the data whose display manner is to be limited in the most reliable manner; 
   (3) Method for limiting the display manner of the entire monitor display; 
   (4) Method for limiting the display manner of a data-reproducing area with high priority; and 
   (5) Method for limiting the display manner of Macrovision and CGMS-A data in accordance with the data whose display manner is to be limited in the most reliable manner 
   Assume, for example, that in the example of  FIG. 5  where DVD  1  data, DVD  2  data and spreadsheet data are reproduced in respective windows displayed on the reproduction screen  40  of the monitor display  15 , and DVD  3  data is reproduced directly on the screen  40 , the reproduction screen is divided, by the area-dividing method employed in the sixth embodiment, into two rows and three columns like the reproduction screen  41  in  FIG. 6 . In  FIG. 6 , the reproduction screen  42  is an example where data in the area  1  should be protected from being copied, and is copy-prevented by hatching. In this case, data in each area is protected in accordance with the condition for each area. Similarly, the reproduction screen  43  is an example where data in the areas  1  and  4  should be protected from being copied. In this case, it is requested that data in the area  1  be copy-prevented by hatching, and data in the area  4  be copy-prevented based on Macrovision and CGMS-A. Therefore, data in the areas  1  and  4  is display-limited in the most reliable manner (i.e., by solid black shading). Further, like display limitation for simultaneous reproduction of a plurality of video data items, it is not always necessary to use the same type of display limitation between the case where the decoder  11  determines that data has been reproduced by an illegitimate route, and the case where copy protection certification performed by the graphics controller  14  has failed. 
   In each of the above-described embodiments, two-stage certification is performed—copy protection certification by the graphics controller  14 , and certification performed by the decoder  11  as to whether data has been reproduced by a legitimate route. As a result, illegal copying can be effectively prevented. 
   Macrovision, color-stripe, CGMS-A, DTCP and CPRM schemes are used as copy protection schemes for images, while CSS and CPRM scheme are used as copy protection schemes for DVD files. 
   In the above-described embodiments, video data reproduced from a DVD is used as input video data  10 . However, the recording medium for recording video data is not limited to DVDs, but may be magnetic disks such as flexible or hard disks, optical disks other than DVDs, such as CD-ROMs, or semiconductor memories. Further, the input video data  10  may be the data that can be transmitted through a communication medium, such as a network. However, when the monitor display is connected to a network to display an image transmitted via the network, if the image contains digital watermark information, the display of the image on the monitor display may be inhibited. Alternatively, the display manner may be limited. This is because the existing IP communication system cannot protect image information from being copied. 
   Furthermore, the copy protection method of the invention can be utilized even when the monitor display is used, rotated. In this case, the rotation of the monitor display is monitored. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.