Abstract:
A non-transitory computer readable storage medium and apparatus to reproduce from a storage medium are provided. The apparatus has a video decoder to decode audio-visual data and a subtitle decoder to receive text-based subtitle data having dialog presentation units and a dialog style unit defining a set of output styles to be applied to the dialog presentation units, converting the dialog presentation units into bitmap images based on the dialog style unit, and controlling an output of the converted dialog presentation units synchronized with decoded audio-visual data. Each dialog presentation unit has dialog text information, time information indicating a time for the dialog text information to be output, palette information defining colors to be applied to the dialog text information, and a color update flag indicating whether only the palette information has changed as compared with a graphical composition of a previous dialog presentation unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims all benefits of U.S. patent application Ser. No. 11/064,825, currently allowed, filed in the U.S. Patent Office on Feb. 25, 2005, now U.S. Pat. No. 7,529,467 which claims the benefit of Korean Patent Application Nos. 2004-13827 and 2004-32290, filed on Feb. 28, 2004 and May 7, 2004, respectively, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to reproduction of a multimedia image, and more particularly, to a storage medium for recording a multimedia image stream and a text-based subtitle stream, a reproducing apparatus and a reproducing method for reproducing the multimedia image stream and the text-based subtitle stream recorded on the storage medium. 
     2. Related Art 
     To provide a high-density (HD) multimedia image, video streams, audio streams, presentation graphics streams to provide subtitles, and interactive graphics streams to provide buttons or menus for interactive with a user, are multiplexed into a main stream, also known as an audio-visual “AV” data stream, and recorded on a storage medium. In particular, the presentation graphics stream for providing subtitles also provides a bitmap-based image in order to display subtitles or captions on an image. 
     In addition to its big size, the bitmap-based caption data has a problem in that production of subtitle or caption data and edition of the produced caption data are very difficult. This is because the caption data is multiplexed with other data streams such as video, audio, and interactive graphic streams. Furthermore, there is another problem in that an output style of the caption data cannot be changed in a variety of ways, that is, to change one output style of the caption into another output style of the caption. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention advantageously provide a storage medium in which a text-based subtitle stream is recorded, and a reproducing apparatus and method for reproducing text-based subtitle data recorded on such a storage medium. 
     According to an aspect of the present invention, an apparatus for reproducing image data and text-based subtitle data recorded on a storage medium to display a caption on an image based on the image data, comprises: a video decoder for decoding the image data; and a subtitle decoder for converting presentation information item into a bitmap image based on style information, and for controlling output of the converted presentation information item synchronized with the decoded image data. The text-based subtitle data includes the presentation information item which is a unit of displaying the caption and the style information which specifies an output style of the caption. 
     The subtitle decoder may decode the subtitle data recorded separated from the image data and output the subtitle data, overlaying the subtitle data on the decoded image data. The style information and the presentation information may be formed in units of packetized elementary streams (PESs), and the subtitle decoder may parse and process the style information and the presentation information in units of PESs. 
     The style information may be formed with one PES and recorded in the front part of the subtitle data, and a plurality of presentation information items may be recorded in units of PESs after the style information, and the subtitle decoder may apply one style information item to the plurality of presentation information items. 
     In addition, the presentation information may include text information indicating the contents of the caption and composition information controlling the output of a bitmap image obtained by converting the text information. The subtitle decoder may control the time when the converted text information is output by referring to the composition information. 
     The presentation information may specify one or more window regions in which a caption is to be output on a screen, and the subtitle decoder may output the converted text information in the one or more window regions at the same time. 
     An output start time and an output end time of the presentation information among the composition information may be defined as time information on a global time axis used in a playlist that is a reproducing unit of the image data, and the subtitle decoder may synchronize the output of the converted text information with the output of the decoded image data by referring to the output start time and the output end time. 
     If the output end time information of a presentation information item currently reproduced is the same as the output start time information of a next presentation information item, the subtitle decoder may continuously reproduce the two presentation information items. 
     If the next presentation information item does not have to be reproduced, the subtitle decoder may reset an internal buffer between the output start time and the output end time, and if the next presentation information item has to be reproduced continuously, the subtitle decoder may maintain the buffer without resetting the same. 
     The style information may be a set of output styles that are pre-defined by the producer of the storage medium and are to be applied to the presentation information, and the subtitle decoder may convert the plurality of presentation information items recorded thereafter into bitmap images based on the style information. 
     In addition, the text information among the presentation information may include text to be converted into a bitmap image and in-line style information to be applied to only part of the text, and by applying the in-line style information only part of the text to which the style information pre-defined by the producer is applied, the subtitle decoder may provide a function emphasizing part of the text. 
     As the in-line style information, the subtitle decoder may apply a relative value of predetermined font information or a predetermined absolute value included in the style information pre-defined by the producer to part of the text. 
     In addition, the style information may further include user changeable style information, and after receiving selection information from a user on one style among the user changeable style information items, the subtitle decoder may apply the style information pre-defined by the producer, then apply the in-line style information, and then finally apply the user changeable style information item corresponding to the selection information to the text. 
     As the user changeable style information, the subtitle decoder may apply a relative value of predetermined font information among the style information items pre-defined by the producer to the text. 
     If the storage medium permits predetermined style information defined in a reproducing apparatus in addition to the style information pre-defined by the producer, the subtitle decoder may apply the predetermined style information to the text. 
     In addition, the style information may include a set of color palettes to be applied to the presentation information and based on a color defined in the color palette, the subtitle decoder may convert all presentation information items after the style information into bitmap images. 
     Separately from the set of color palettes included in the style information, the presentation information may further include a set of color palettes and a color update flag, and if the color update flag is set to “1”, the subtitle decoder may apply the set of the color palettes included in the presentation information, and if the color update flag is set to “0”, apply the original set of color palettes included in the style information. 
     By setting the color update flag to “1” and gradually changing the transparency value of a color palette included in the plurality of continuous presentation information items, the subtitle decoder may implement a fade in/out effect, and if the fade in/out effect is finished, reset a color look-up table (CLUT) in the subtitle decoder, based on the original set of color palettes included in the style information. 
     In addition, the style information may include region information indicating the location of a window region for the converted presentation information to be output on the image, and font information required for converting the presentation information into a bitmap image, and by using the region information and the font information, the subtitle decoder may convert the converted presentation information into a bitmap image. 
     The font information may include at least one of an output start location of the converted presentation information, an output direction, sort, line space, a font identifier, a font style, a font size, or a color, and the subtitle decoder converts the presentation information into a bitmap image based on the font information. 
     As the font identifier, the subtitle decoder may refer to indication information on a font file included in a clip information file storing attribute information of a recording unit of the image data. 
     In addition, the subtitle decoder may buffer the subtitle data and a font file referred to by the subtitle data before the image data is reproduced. 
     In addition, if a plurality of the subtitle data items supporting a plurality of languages are recorded on the storage medium, the subtitle decoder may receive selection information on a desired language from a user, and reproduce a subtitle data item corresponding to the selection information among the plurality of subtitle data items. 
     According to another aspect of the present invention, a method of reproducing data from a storage medium storing image data and text-based subtitle data to display a caption on an image based on the image data, comprises: decoding the image data; reading style information and a presentation information item; converting the presentation information item into a bitmap image based on the style information; and controlling output of the converted presentation information item synchronized with the decoded image data. The text-based subtitle data includes the presentation information which is a unit of displaying the caption and the style information which specifies an output style of the caption. 
     According to still another aspect of the present invention, a storage medium is provided to store: image data; and text-based subtitle data to display a caption on an image based on the image data, wherein the subtitle data includes: one style information item specifying an output style of the caption; and a plurality of presentation information items that are displaying units of the caption, and the subtitle data is separated from the image data and recorded separately. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the following written and illustrated disclosure focuses on disclosing example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and that the invention is not limited thereto. The spirit and scope of the present invention are limited only by the terms of the appended claims. The following represents brief descriptions of the drawings, wherein: 
         FIG. 1  is a diagram for explaining a multimedia data structure recorded on a storage medium according to an embodiment of the present invention; 
         FIG. 2  illustrates an example data structure of a clip AV stream shown in  FIG. 1  and a text-based subtitle stream according to an embodiment of the present invention; 
         FIG. 3  is a diagram for explaining an example data structure of a text-based subtitle stream according to an embodiment of the present invention; 
         FIG. 4  illustrates a text-based subtitle stream having the data structure shown in  FIG. 3  according to an embodiment of the present invention; 
         FIG. 5  illustrates a dialog style unit shown in  FIG. 3  according to an embodiment of the present invention; 
         FIG. 6  is a diagram for explaining an example data structure of a dialog style unit according to an embodiment of the present invention; 
         FIG. 7  is a diagram for explaining an example data structure of a dialog style unit according to another embodiment of the present invention; 
         FIG. 8  illustrates an example dialog style unit shown in  FIG. 6  or  7  according to an embodiment of the present invention; 
         FIGS. 9A and 9B  illustrate an example clip information file including a plurality of font collections referred to by font information according to an embodiment of the present invention; 
         FIG. 10  is a diagram showing locations of a plurality of font files referred to by font file information, shown in  FIGS. 9A and 9B ; 
         FIG. 11  is a diagram for explaining an example data structure of a dialog presentation unit shown in  FIG. 3  according to another embodiment of the present invention; 
         FIGS. 12A and 12B  are diagrams for explaining an example data structure of a dialog presentation unit shown in  FIG. 3  according to other embodiments of the present invention; 
         FIG. 13  illustrates a dialog presentation unit shown in  FIGS. 11 through 12B  according to an embodiment of the present invention; 
         FIG. 14  is a diagram for explaining an example data structure of dialog text information shown in  FIG. 13 ; 
         FIG. 15  illustrates dialog text information of  FIG. 13  according to an embodiment of the present invention; 
         FIG. 16  is a diagram for explaining constraints in continuously reproducing continuous dialog presentation units (DPUs); 
         FIG. 17  is a diagram for explaining an example reproducing apparatus for reproducing a text-based subtitle stream according to an embodiment of the present invention; 
         FIG. 18  is a diagram for explaining a pre-loading process of a text-based subtitle stream in an example reproducing apparatus according to an embodiment of the present invention; 
         FIG. 19  is a diagram for explaining a reproduction process of a dialog presentation unit (DPU) in an example reproducing apparatus according to an embodiment of the present invention; 
         FIG. 20  is a diagram for explaining a process in which a text-based subtitle stream is synchronized with moving picture data and output in an example reproducing apparatus according to an embodiment of the present invention; 
         FIG. 21  is a diagram for explaining a process in which a text-based subtitle stream is output to a screen in an example reproducing apparatus according to an embodiment of the present invention; 
         FIG. 22  is a diagram for explaining a process of rendering a text-based subtitle stream in an example reproducing apparatus according to an embodiment of the present invention; 
         FIG. 23  illustrates an example status register disposed in an example reproducing apparatus for reproducing a text-based subtitle stream according to an embodiment of the present invention; and 
         FIG. 24  is a flowchart of a method of reproducing a text-based subtitle stream according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments of the invention are shown. 
     Referring to  FIG. 1 , a storage medium (such as the medium  230  shown, for example, in  FIG. 2 ) according to an example embodiment of the present invention is formed with multiple layers in order to manage a multimedia data structure  100  of multimedia image streams recorded thereon. The multimedia data structure  100  includes clips  110  that are recording units of a multimedia image, playlists  120  that are reproducing units of a multimedia image, movie objects  130  including navigation commands that are used to reproduce a multimedia image, and an index table  140  that is used to specify a movie object to be first reproduced and titles of movie objects  130 . 
     The clips  110  are implemented as one object which includes a clip AV stream  112  for an audio-visual (AV) data stream for a high picture quality movie and clip information  114  for the attribute corresponding to the AV data stream. For example, the AV data stream may be compressed according to a standard, such as the motion picture experts group (MPEG). However, such clips  110  need not require the AV data stream  112  to be compressed in all aspects of the present invention. In addition, the clip information  114  may include audio/video properties of the AV data stream  112 , an entry point map in which information regarding a location of a randomly accessible entry point is recorded in units of a predetermined section and the like. 
     The playlist  120  is a set of reproduction intervals of these clips  110 , and each reproduction interval is referred to as a play item  122 . The movie object  130  is formed with navigation command programs, and these navigation commands start reproduction of a playlist  120 , switch between movie objects  130 , or manage reproduction of a playlist  120  according to preference of a user. 
     The index table  140  is a table at the top layer of the storage medium to define a plurality of titles and menus, and includes start location information of all titles and menus such that a title or menu selected by a user operation, such as title search or menu call, can be reproduced. The index table  140  also includes start location information of a title or menu that is automatically reproduced first when a storage medium is placed on a reproducing apparatus. 
     Among these items, the structure of the clip AV stream in which a multimedia image is compression-coded will now be explained with reference to  FIG. 2 .  FIG. 2  illustrates an example data structure of an AV data stream  210  shown in  FIG. 1  and a text-based subtitle stream  220  according to an embodiment of the present invention. 
     Referring to  FIG. 2 , in order to solve problems relating to bitmap-based caption data as described above, a text-based subtitle data stream  220  according to an embodiment of the present invention is provided separately from the clip AV data stream  210  recorded on a storage medium  230 , such as a digital versatile disc (DVD). The AV data stream  210  includes a video stream  202 , an audio stream  204 , a presentation graphics stream  206  for providing subtitle data, and an interactive graphics stream  208  for providing buttons or menus to interact with a user, all of which are multiplexed in a moving picture main stream, known as an audio-visual “AV” data stream, and recorded on the storage medium  230 . 
     The text-based subtitle data  220  according to an embodiment of the present invention represents data for providing subtitles or captions of a multimedia image to be recorded on the storage medium  230 , and can be implemented by using a markup language, such as an extensible markup language (XML). However, such subtitles or captions of a multimedia image are provided using binary data. Hereinafter, the text-based subtitle data  220  providing the caption of a multimedia image using binary data will be referred to briefly as a “text-based subtitle stream”. The presentation graphics stream  206  for providing subtitle data or caption data also provides bitmap-based subtitle data in order to display subtitles (or captions) on a screen. 
     Since the text-based subtitle data stream  220  is recorded separately from the AV data stream  210 , and is not multiplexed with the AV data stream  210 , the size of the text-based subtitle data stream  220  is not limited thereto. As a result, subtitles or captions can be provided using a plurality of languages. Moreover, the text-based subtitle data stream  220  can be conveniently produced and effectively edited without any difficulty. 
     The text-based subtitle stream  220  is then converted into a bitmap graphic image, and is output on a screen, overlaid over a multimedia image. The process converting text-based data thus into a graphic-based bitmap image is referred to as rendering. The text-based subtitle stream  220  includes information required for rendering caption text. 
     The structure of the text-based subtitle stream  220  including rendering information will now be explained in detail with reference to  FIG. 3 .  FIG. 3  is a diagram for explaining an example data structure of the text-based subtitle stream  220  according to an embodiment of the present invention. 
     Referring to  FIG. 3 , the text-based subtitle stream  220  according to an embodiment of the present invention includes a dialog style unit (DSU)  310 , and a plurality of dialog presentation units (DPU)  320  through  340 . The DSU  310  and the DPUs  320 - 340  are also referred to as a dialog unit. Each of the dialog units  310 - 340  forming the text-based subtitle stream  220  is recorded in the form of a packetized elementary stream (PES), or simply known as a PES packet  350 . Also, the PES of the text-based subtitle stream  220  is recorded and transmitted in units of transport packets (TP)  362 . A series of the TPs is referred to as a transport stream (TS). 
     However, the text-based subtitle stream  220  according to an embodiment of the present invention is not multiplexed with the AV data stream  210  and is recorded as a separate TS on a storage medium  230 , as shown in  FIG. 2 . 
     Referring again to  FIG. 3 , in one PES packet  350  included in the text-based subtitle stream  220 , one dialog unit is recorded. The text-based subtitle stream  220  includes one DSU  310  positioned at the front and a plurality of DPUs  320  through  340  following the DSU  310 . The DSU  310  includes information specifying the output style of a dialog in a caption displayed on the screen on which a multimedia image is reproduced. Meanwhile, the plurality of DPUs  320  through  340  include text information items on the dialog contents to be displayed and information on respective output times. 
       FIG. 4  illustrates a text-based subtitle stream  220  having the data structure shown in  FIG. 3  according to an embodiment of the present invention. 
     Referring to  FIG. 4 , the text-based subtitle stream  220  includes one DSU  410  and a plurality of DPUs  420 . 
     In the example embodiment of the present invention, the number of DPUs is defined as num_of_dialog_presentation_units. However, the number of DPUs may not be specified separately. An example case is using a syntax such as while (processed_length&lt;end_of_file). 
     The data structures of a DSU and a DPU will now be explained in more detail with reference to  FIG. 5 .  FIG. 5  illustrates a dialog style unit shown in  FIG. 3  according to an embodiment of the present invention. 
     Referring to  FIG. 5 , in the DSU  310 , a set of dialog style information items, dialog_styleset( )  510 , in which output style information items of a dialog to be displayed as a caption are collected, is defined. The DSU  310  includes information on the location of a region in which a dialog is displayed in a caption, information required for rendering a dialog, information on styles that a user can control, and so on. The detailed contents of data will be explained later. 
       FIG. 6  is a diagram for explaining an example data structure of a dialog style unit (DSU) according to an embodiment of the present invention. 
     Referring to  FIG. 6 , the DSU  310  includes a palette collection  61  and a region style collection  620 . The palette collection  610  is a set of a plurality of color palettes defining colors to be used in a caption. A color combination or color information, such as a transparency degree, included in the palette collection  610  can be applied to all of the plurality of DPUs located after the DSU. 
     The region style collection  620  is a set of output style information items of respective dialogs forming a caption. Each region style includes region information  622  indicating a location in which a dialog is to be displayed on a screen; text style information  624  indicating an output style to be applied to the text of each dialog; and a user changeable style collection  626  indicating styles that a user can change arbitrarily to be applied to the text of each dialog. 
       FIG. 7  is a diagram for explaining an example data structure of a dialog style unit according to another embodiment of the present invention. 
     Referring to  FIG. 7 , unlike  FIG. 6 , the palette collection  610  is not included. That is, a color palette collection is not defined in the DSU  310 , but a palette collection  610  is defined in a DPU to be explained with reference to  FIG. 12A  and  FIG. 12B . The data structure of each region style  710  is the same as described above with reference to  FIG. 6 . 
       FIG. 8  illustrates a dialog style unit shown in  FIG. 6  or  7  according to an embodiment of the present invention. 
     Referring to  FIG. 8  and  FIG. 6 , the DSU  310  includes a palette collection  860  and  610  and a plurality of region styles  820  and  620 . As described above, the palette collection  610  is a set of a plurality of color palettes defining colors to be used in a caption. A color combination or color information, such as a transparency degree, included in the palette collection  610  can be applied to all of the plurality of DPUs located after the DSU. 
     Meanwhile, each region style  820  and  620  includes region information  830  and  622  indicating information on a window region in which a caption is to be displayed on a screen, and the region information  830  and  622  includes information on X, Y coordinates, the width, height, background color, and so on of the window region in which a caption is to be displayed on a screen. 
     Also, each region style  820  and  620  includes text style information  840  and  624  indicating an output style to be applied to the text of each dialog. That is, X, Y coordinates of the location in which the text of a dialog is to be displayed in the window region described above, the output direction of text such as from the left-hand side to the right-hand side, or from upside to downside, sort, line space, an identifier of a font to be referred to, a font style such as bold or Italic, a font size, and information on a font color, and so on can be included. 
     Furthermore, each region style  820  and  620  can also include a user changeable style collection  850  and  626  indicating styles that a user can change arbitrarily. However, the user changeable style collection  850  and  626  is optional. The user changeable style collection  850  and  626  can include change information on the location of a window region, the output location of text, the font size, and line space among text output style information items  840  and  624 . Each change information item can be expressed as a relatively increased or decreased value in relation to information on output styles  840  and  624  to be applied to the text of each dialog. 
     To summarize the above, there are three types of style relation information, style information (region_style)  620  defined in the region style  820  and  620 , inline style information (inline_style)  1510  to be explained later and to emphasize part of a caption, and user changeable style information (user_changeable_style)  850 , and the order of applying these information items is as follows: 
     1) Basically, region style information  620  defined in a region style is applied. 
     2) If there is in-line style information, the in-line style information  1510  is applied overlaid over a part in which region style information is applied, and emphasizing a part of the caption text. 
     3) If there is user changeable style information  850 , this information is applied finally. The presence of use changeable style information is optional. 
     Meanwhile, among text style information items  840  and  624  to be applied to text of each dialog, font file information to be referred to by the identifier of a font (font_id)  842  can be defined as follows. 
       FIG. 9A  illustrates an example clip information file  910  including a plurality of font collections referred to by font information  842  shown in  FIG. 8  according to an embodiment of the present invention. 
     Referring to  FIG. 9A ,  FIG. 8 ,  FIG. 2  and  FIG. 1 , in StreamCodingInfo( )  930  that is a stream coding information structure included in the clip information file  910  and  110 , information on a variety of streams recorded on a storage medium according to the present invention is included. That is, information on a video stream  202 , an audio stream, a presentation graphics stream, an interactive graphics stream, a text-based subtitle stream and so on is included. In particular, in relation to the text-based subtitle stream  220 , information (textST_language_code)  932  on a language to display a caption can be included. Also, a font name  936  and a file name  938  of a file storing font information, corresponding to font_id  842  and  934  indicating the identifier of a font to be referred to and shown in  FIG. 8  can be defined. A method for finding a font file corresponding to the identifier of a font to be referred to and defined here will be explained later with reference to  FIG. 10 . 
       FIG. 9B  illustrates an example clip information file  940  including a plurality of font collections referred to by font information  842  shown in  FIG. 8  according to another embodiment of the present invention. 
     Referring to  FIG. 9B , a structure, ClipInfo( ), can be defined in the clip information file  910  and  110 . In this structure, a plurality of font collections referred to by the font information  842  shown in  FIG. 8  can be defined. That is, a font file name  952  corresponding to font_id  842  indicating the identifier of a font to be referred and shown in  FIG. 8  is specified. The method for finding a font file corresponding to the identifier of a font defined here and to be referred to will now be explained. 
       FIG. 10  is a diagram showing locations of a plurality of font files referred to by the font file name  938  and  952  in  FIG. 9A  and  FIG. 9B . 
     Referring to  FIG. 10 , a directory structure of files to be related to a multimedia image recorded on a storage medium according to an embodiment of the present invention is shown. In particular, by using the directory structure, the location of a font file such as 11111.font  1010  or 99999.font  1020  stored in an auxiliary data (AUXDATA) directory can be easily found. 
     Meanwhile, the structure of a DPU forming a dialog unit will now be explained in more detail with reference to  FIG. 11 . 
       FIG. 11  is a diagram for explaining an example data structure of the DPU  320  shown in  FIG. 3  according to other embodiments of the present invention. 
     Referring to  FIG. 11  and  FIG. 3 , the DPU  320  including text information on dialog contents to be output and information on a display time includes time information  1110  indicating a time for a dialog to be output on a screen, palette reference information  1120  specifying a color palette to be referred to, and dialog region information  1130  for a dialog to be output on a screen. In particular, the dialog region information  1130  for a dialog to be output on a screen includes style reference information  1132  specifying an output style to be applied to the dialog and dialog text information  1134  indicating the text of a dialog to be actually output on a screen. In this case, it is assumed that the color palette collection indicated by the palette reference information  1120  is defined in a DSU (Refer to  610  of  FIG. 6 ). 
     Meanwhile,  FIG. 12A  is a diagram for explaining an example data structure of the DPU  320  shown in  FIG. 3  according to an embodiment of the present invention. 
     Referring to  FIG. 12A  and  FIG. 3 , the DPU  320  includes time information  1210  indicating a time for a dialog to be output on a screen, a palette collection  1220  defining a color palette collection, and dialog region information  1230  for a dialog to be output on a screen. In this case, the palette collection  1220  is not defined in a DSU as shown in  FIG. 11 , but is defined directly in the DPU  320 . 
     Meanwhile,  FIG. 12B  is a diagram for explaining an example data structure of the DPU  320  shown in  FIG. 3  according to an embodiment of the present invention. 
     Referring to  FIG. 12B , the DPU  320  includes time information  1250  indicating a time for a dialog to be output on a screen, a color update flag  1260 , a color palette collection  1270  to be used when the color update flag is set to 1, and a dialog region information  1280  for a dialog to be output on a screen. In this case, the palette collection  1270  is also defined in a DSU as in  FIG. 11 , and is also stored in the DPU  320 . In particular, in order to express fade-in/out using continuous reproduction, in addition to a basic palette collection defined in a DSU, a color palette collection  1270  to be used to express fade-in/out is defined in the DPU  320  and the color update flag  1260  can be set to 1. This will be explained in more detail with reference to  FIG. 19 . 
       FIG. 13  illustrates the DPU  320  shown in  FIGS. 11 through 12B  according to an embodiment of the present invention. 
     Referring to  FIG. 13 ,  FIG. 11 ,  FIG. 12A  and  FIG. 12B , the DPU includes dialog start time information (dialog_start_PTS) and dialog end time information (dialog_end_PTS)  1310  as time information  1110  indicating a time for a dialog to be output on a screen. Also, dialog palette identifier (dialog_palette_id) is included as palette reference information  1120 . In case of  FIG. 12A , the color palette collection  1220  can be included instead of the palette reference information  1120 . Dialog text information (region_subtitle)  1334  is included as dialog region information  1230  for a dialog to be output, and in order to specify an output style to be applied to it, a region style identifier (region_style_id)  1332  can be included. The example shown in  FIG. 13  is just an embodiment of a DPU and the DPU having data structures shown in  FIGS. 11 through 12B  can be implemented with modification in a variety of ways. 
       FIG. 14  is a diagram for explaining the data structure of dialog text information (region_subtitle) shown in  FIG. 13 . 
     Referring to  FIG. 14 , dialog text information  1134  shown in  FIG. 11 ,  1234  shown in  FIG. 12A ,  1284  shown in  FIG. 12B , and  1334  shown in  FIG. 13 , includes inline style information  1410  and dialog text  1420  as an output style to emphasize part of a dialog. 
       FIG. 15  illustrates dialog text information  1334  shown in  FIG. 13  according to an embodiment of the present invention. As shown in  FIG. 15 , the dialog text information  1334  is implemented by inline style information (inline_style)  1510  and dialog text (text_string)  1520 . Also, it is preferable that information indicating the end of an inline style is included in the embodiment shown in  FIG. 15 . Unless the end part of an in-line style is defined, an in-line style once specified may be applied to continuously thereafter contrary to the intention of a producer. 
     Meanwhile,  FIG. 16  is a diagram for explaining constraints in continuously reproducing continuous DPUs. 
     Referring to  FIG. 16  and  FIG. 13 , when the plurality of DPUs as described above are needed to be reproduced continuously, the following constraints are needed. 
     1) Dialog start time information (dialog_start_PTS)  1310  defined in a DPU indicates a time when a dialog object begins to be output on a graphic plane (GP) to be explained later with reference to  FIG. 17 . 
     2) Dialog end time information (dialog_end_PTS)  1310  defined in a DPU indicates a time to reset a text-based subtitle decoder processing a text-based subtitle to be explained later with reference to  FIG. 17 . 
     3) When a plurality of DPUs described above are needed to be reproduced continuously, the dialog end time information (dialog_end_PTS) of a current DPU should be the same as the dialog start time information (dialog_start_PTS) of a DPU to be reproduced continuously next. That is, in  FIG. 16 , in order to continuously reproduce DPU # 2  and DPU # 3 , the dialog end time information included in DPU # 2  should be the same as the dialog start time information included in DPU # 3 . 
     Meanwhile, it is preferable that a DSU according to the present invention satisfies the following constraints. 
     1) A text-based subtitle stream  220  includes one DSU. 
     2) The number of user changeable style information items (user_control_style) included in all region styles (region_style) should be identical. 
     Meanwhile, it is preferable that a DPU according to the present invention satisfies the following constraints. 
     1) A window region for at least two captions should be defined. 
     The structure of an example reproducing apparatus based on the data structure of a text-based subtitle stream  220  recorded on a storage medium according to an embodiment of the present invention will now be explained with reference to  FIG. 17  herein below. 
       FIG. 17  is a diagram for explaining the structure of an example reproducing apparatus for reproducing a text-based subtitle stream according to an embodiment of the present invention. 
     Referring to  FIG. 17 , the reproducing apparatus  1700 , also known as a playback device, includes a buffer unit which comprises a font preloading buffer (FPB)  1710  for storing font files, and a subtitle preloading buffer (SPB)  1712  for storing text-based subtitle files, and a text-based subtitle decoder  1730  which decodes and reproduces the text-based subtitle stream previously recorded on a storage medium, as an output, via a graphics plane (GP)  1750  and a color look-up table (CLUT)  1760 . 
     Specifically, the buffer unit includes a subtitle pre-loading buffer (SPB)  1710  in which a text-based subtitle data stream  220  is pre-loaded, and a font pre-loading buffer (FPB)  1712  in which font information is pre-loaded. 
     The subtitle decoder  1730  includes a text subtitle processor  1732 , a dialog composition buffer (DCB)  1734 , a dialog buffer (DB)  1736 , a text subtitle renderer  1738 , a dialog presentation controller  1740 , and a bitmap object buffer (BOB)  1742 . 
     The text subtitle processor  1732  receives the text-based subtitle data stream  220  from the text subtitle pre-loading buffer (SPB)  1710 , transfers style related information included in the DSU and dialog output time information included in the DPU described above, to the dialog composition buffer (DCB)  1734 , and transfers dialog text information included in the DPU, to the dialog buffer (DB)  1736 . 
     The presentation controller  1740  controls the text renderer  1738  by using the style related information included in the dialog composition buffer (DCB)  1734 , and by using the dialog output time information, controls a time for a bitmap image rendered in the bitmap object buffer (BOB)  1742  to be output to the graphics plane (GP)  1750 . 
     According to control of the presentation controller  1740 , the text subtitle renderer  1738  converts dialog text information into a bitmap image, that is, performs rendering, by applying a font information item corresponding to dialog text information stored in the dialog buffer (DB)  1736  among font information items pre-loaded in the font preloading buffer (FPB)  1712 , to the dialog text information. The rendered bitmap image is stored in the bitmap object buffer (BOB)  1742  and according to control of the presentation controller  1740 , is output to the graphics plane (GP)  1750 . At this time, a color specified in the DSU is applied by referring to the color look-up table (CLUT)  1760 . 
     As style relation information to be applied to dialog text, information defined in the DSU by a producer can be used, and style related information pre-defined by a user can also be applied. The reproducing apparatus  1700 , as shown in  FIG. 17 , applies the style information defined by a user prior to the style related information defined by a producer. 
     As described with reference to  FIG. 8 , as style related information to be applied to dialog text, region style information (region_style) defined in a DSU by a producer is basically applied, and if an in-line style is included in a DPU including dialog text to which the region style information is applied, in-line style information (inline_style) is applied to the corresponding part. Also, if a producer defines user changeable styles additionally in the DSU and one of the user changeable styles defined by a user is selected, the region style and/or in-line style are applied and then, the user changeable style is finally applied. Also, as described with reference to  FIG. 15 , it is preferable that information indicating the end of applying an inline style is included in the contents of the in-line style. 
     Furthermore, a producer can specify whether to permit or prohibit using style related information defined in the reproducing apparatus itself separately from the style related information defined by the producer and recorded on a storage medium. 
       FIG. 18  is a diagram explaining a pre-loading process of a text-based subtitle data stream  220  in a reproducing apparatus  1700 , as shown, for example, in  FIG. 17 , according to an embodiment of the present invention. 
     Referring to  FIG. 18 , a text-based subtitle data stream  220 , shown in  FIG. 2 , is defined in a subpath of the playlist described above. In the subpath, a plurality of text-based subtitle data streams  220  supporting a plurality of languages can be defined. Also, a font file to be applied to a text-based subtitle can be defined in a clip information file  910  or  940  as described above with reference to  FIG. 9A  and  FIG. 9B . Up to 255 text-based subtitle data streams  220  that can be included in one storage medium can be defined in each playlist. Also, up to 255 font files that can be included in one storage medium can be defined. However, in order to guarantee seamless presentation, the size of a text-based subtitle data stream  220  should be smaller than or equal to that of the pre-loading buffer  1710  of a reproducing apparatus  1700 , shown, for example, in  FIG. 17 . 
       FIG. 19  is a diagram explaining a reproduction process of a DPU in a reproducing apparatus according to the present invention. 
     Referring to  FIG. 19 ,  FIG. 13 , and  FIG. 17 , the process of reproducing a DPU is shown. The presentation controller  1740  controls a time for a rendered dialog to be output on a graphics plane (GP)  1750 , shown in  FIG. 17 , by using dialog start time information (dialog_start_PTS) and dialog end time information (dialog_end_PTS) specifying the output time  1310  of a dialog included in a DPU. At this time, the dialog start time information specifies a time when transferring a rendered dialog bitmap image stored in the bitmap object buffer (BOB)  1742  included in the text-based subtitle decoder  1730  to the graphics plane (GP)  1750  is completed. That is, if it is the dialog start time defined in the DPU, bitmap information required for constructing a dialog should be ready to be used after transferring the information to the graphics plane (GP)  1750  is completed. Also, the dialog end time information specifies a time when reproducing a DPU is finished. At this time, both the subtitle decoder  1730  and the graphics plane (GP)  1750  are reset. It is preferable that a buffer in the subtitle decoder  1730  such as the bitmap object buffer (BOB)  1742  is reset between a start time and an end time of a DPU regardless of continuous reproduction. 
     However, when continuous reproduction of a plurality of DPUs is needed, the subtitle decoder  1730  and the graphics plane  1750  are not reset and the contents stored in each buffer, such as the dialog composition buffer (DCB)  1734 , the dialog buffer (DB)  1736 , and the bitmap object buffer (BOB)  1742 , should be maintained. That is, when the dialog end time information of a DPU currently being reproduced and the dialog start time information of a DPU to be continuously reproduced thereafter is the same, the contents of each buffer are not reset but maintained. 
     In particular, as an example of applying continuous reproduction of a plurality of DPUs, there is a fade in/out effect. The fade in/out effect can be implemented by changing a color look-up table (CLUT)  1760  of a bitmap object which is transferred to the graphics plane (GP)  1750 . That is, a first DPU includes composition information such as a color, a style, and an output time, and a plurality of DPUs continuing thereafter have composition information identical to that of the first DPU, but update only color palette information. In this case, by gradually changing transparency among color information items, from 0% to 100%, the fade in/out effect can be implemented. 
     In particular, when the data structure of the DPU as shown in  FIG. 12B  is used, the fade in/out effect can be effectively implemented by using the color update flag  1260 . That is, if the dialog presentation controller  1740  checks and confirms that the color update flag  1260  included in the DPU is set to “0”, that is, if it is an ordinary case not requiring the fade in/out effect, color information included in the DSU shown in  FIG. 6  is basically used. However, if the presentation controller  1740  confirms that the color update flag  1260  is set to “1”, that is, if the fade in/out effect is needed, by using color information  1270  included in the DPU, instead of using the color information  610  included in the DSU shown in  FIG. 6 , the fade in/out effect can be implemented. At this time, by adjusting the transparency of the color information  1270  included in the DPU, the fade in/out effect can be implemented simply. 
     After thus showing the fade in/out effect, it is preferable to update the color look-up table (CLUT)  1760  to the original color information included in the DSU. This is because unless it is updated, the color information once specified can be applied continuously thereafter contrary to the intention of the producer. 
       FIG. 20  is a diagram explaining a process in which a text-based subtitle stream is synchronized with moving picture data and output in a reproducing apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 20 , the dialog start time information and dialog end time information included in a DPU of the text-based subtitle data stream  220  should be defined as time points on a global time axis used in a playlist so as to be synchronized with the output time of an AV data stream  210  of a multimedia image. Accordingly, discontinuity between the system time clock (STC) of an AV data stream and the dialog output time (PTS) of the text-based subtitle data stream  220  can be prevented. 
       FIG. 21  is a diagram explaining a process in which a text-based subtitle data stream is output to a screen in a reproducing apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 21 , shown is a process in which by applying rendering information  2102  including style related information, dialog text information  2104  is converted into a bitmap image  2106 , and the converted bitmap image is output at a corresponding location on a graphics plane (GP)  1750 , based on output location information (such as region_horizontal_position, and region_vertical_position) included in composition information  2108 . 
     Rendering information  2102  represents style information such as the width of a region, the height, a foreground color, a background color, text sort, a font name, a font style, and a font size. As described above, the rendering information  2102  is defined in the region style collection in a DSU. Meanwhile, the composition information  2108  indicates the start time and end time of presentation, horizontal and vertical location information of a window region in which a caption is output on a graphics plane (GP)  1750  and so on. This is defined in a DPU. 
       FIG. 22  is a diagram explaining a process of rendering a text-based subtitle data stream  220  in a reproducing apparatus  1700 , shown, for example, in  FIG. 17  according to an embodiment of the present invention. 
     Referring to  FIG. 22 ,  FIG. 21 , and  FIG. 8 , a window region specified by using region_horizontal_position, region_vertical_position, region_width, and region_height that are location information  830  of a window region for a caption defined in a DSU is specified as a region in which a caption is displayed on a graphics plane (GB)  1750 . The bitmap image of a rendered dialog is displayed from a start point location specified by text_horizontal_position and text_vertical_position that are the output location  840  of the dialog in the window region. 
     Meanwhile, a reproducing apparatus according to the present invention stores style information (style_id) selected by a use in a system register area.  FIG. 23  illustrates an example status register disposed in a reproducing apparatus for reproducing a text-based subtitle data stream according to an embodiment of the present invention. 
     Referring to  FIG. 23 , the status register (player status registers, hereinafter referred to as PSRs) stores style information (Selected Style  2310 ) selected by a user in the 12th register. Accordingly, for example, even after the reproducing apparatus  1700 , shown, for example, in  FIG. 17 , performs a menu call or another operation, if the user pushes a style information change button, the style information selected previously by the user can be made to be applied first by referring to PSR  12 . The register storing the information can be changed. 
     A method of reproducing a text-based subtitle data stream  220  based on a storage medium recording the text-based subtitle stream  220  and the structure of a reproducing apparatus reproducing the subtitle data stream  220  described above will now be explained with reference to  FIG. 24 .  FIG. 24  is a flowchart of the operations of a method of reproducing a text-based subtitle data stream  220  according to an embodiment of the present invention. 
     Text-based subtitle data stream  220  including DSU information and DPU information is read from a storage medium  230 , shown, for example, in  FIG. 2 , in operation  2410 , and based on rendering information included in the DSU information, caption text included in the DPU information is converted into a bitmap image in operation  2420 . The converted bitmap image is output on a screen according to time information and location information that are composition information included in the DPU information, in operation  2430 . 
     As described from the foregoing, the present invention advantageously provides a storage medium storing a text-based subtitle data stream separately from image data, a reproducing apparatus and a reproducing method for reproducing such text-based subtitle data stream such that production of subtitle data and edition of produced subtitle data can be made easier. Also, regardless of the number of subtitle data items, a caption can be provided in a plurality of languages. 
     In addition, since subtitle data is formed with one style information item (DSU) and a plurality of presentation information items (DPUs), an output style to be applied to the entire presentation data can be defined in advance, and can be changed in a variety of ways, and an in-line style to emphasize part of a caption and a user changeable style can also be defined. 
     Furthermore, by using a plurality of neighboring presentation information items, continuous reproduction of a caption is enabled and by applying this, fade in/out and other effects can be easily implemented. 
     Example embodiments of the present invention can also be written as computer programs and can be implemented in general-use digital computers that execute the programs using a computer readable medium. Examples of the computer readable medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, DVDs, etc.), and storage media such as carrier waves (e.g., transmission through the Internet). The computer readable medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     While there have been illustrated and described what are considered to be example embodiments of the present invention, it will be understood by those skilled in the art and as technology develops that various changes and modification may be made, and equivalents may be substituted for elements thereof without departing from the spirit and scope of the present invention. Many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from the scope thereof. For example, any computer readable media or data storage devices may be utilized, as long as text-based subtitle data and AV data are recorded separately thereon. In addition, text-based subtitle data can also be configured differently as shown in  FIG. 3  or  FIG. 4 . Moreover, a reproducing apparatus as shown in  FIG. 17  can be implemented as part of a recording apparatus, or alternatively a single apparatus for performing recording and/or reproducing functions with respect to a storage medium. Similarly, the CPU can be implemented as a chipset having firmware, or alternatively, a general or special purposed computer programmed to perform the methods as described, for example, with reference to  FIG. 24 . Accordingly, it is intended, therefore, that the present invention not be limited to the various example embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims.