Patent Publication Number: US-8532195-B2

Title: Search algorithms for using related decode and display timelines

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/115,085, filed Apr. 25, 2005, entitled “Search Algorithms for Using Related Decode and Display Timelines”, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to video coding devices and systems, and to search algorithms employed in conjunction with such devices. 
     Digital-based electronic media formats have become widely accepted. Digital compact discs (CDs) and audio files, such as MP3s (MPEG Audio—layer 3), are now commonplace. Video media, however, has been slower to transition to digital storage and digital transmission formats than audio media. One reason for the slower integration of digital video media formats into the marketplace is the volume of information required to accurately produce video of an acceptable quality from a digital representation. Additionally, encoding and decoding video in a digital format consumes substantial system resources and requires systems capable of processing information at high speeds. Further, the large amounts of information used to represent digital video also necessitate high-bandwidth transmission systems and high-capacity storage systems. 
     The development of faster computer processors, high-density storage media, and efficient compression and encoding algorithms have led to more widespread implementation of digital video media formats in recent years. The Digital Versatile Disc (DVD) has rapidly replaced video cassettes as the primary storage media for video due to its high image quality, very high audio quality, convenience, and added functionality. Further, the digital Advanced Television Standards Committee video transmission system is in the process of replacing the analog National Television Standards Committee transmission system. 
     Computer systems have been using various digital video formats for a number of years. Specifically, computer systems have employed many different methods for compressing and encoding or decompressing and decoding digital video. A video compression/decompression method, implemented using hardware, software, or a combination of hardware and software, is commonly referred to as a CODEC. A number of popular digital video compression and encoding systems have been developed based on the standards propounded by the Moving Picture Experts Group (MPEG), including the MPEG-1, MPEG-2, and MPEG-4 standards. Video CDs and early consumer-grade digital video editing systems use the MPEG-1 digital video encoding format. DVDs, video games, and some direct broadcast satellite systems are encoded in accordance with the MPEG-2 standard. MPEG-4 is now being used to deliver DVD (MPEG-2) quality video at lower data rates and smaller file sizes, and thus enables digital video playback on products ranging from satellite television systems to wireless devices. 
     The MPEG standards set forth methods for compressing a series of images, such as frames or fields, and for encoding the compressed images into a digital bit stream. When a video image is encoded in an MPEG system, the video image is divided into multiple pixel arrays, such as 8×8 pixel blocks or 16×16 pixel macroblocks. Each pixel array can then be independently compressed and encoded. 
     When performing compression using an MPEG coder, such as a coder that is in compliance with the MPEG-2 or MPEG-4 standard, images may be encoded using three picture types. Specifically, images may be encoded using I-pictures, P-pictures, and B-pictures. I-pictures are encoded with reference only to the information within the picture, and thus may be decoded without reference to any other pictures. P-pictures are encoded with reference to preceding pictures, and thus permit the use of motion compensation to provide for a greater degree of compression. B-pictures are encoded with reference to succeeding pictures, and also permit the use of motion compensation. Because B-pictures are decoded using succeeding pictures, however, some reordering of the sequence of decoded pictures is required prior to display. 
     Digital video systems have also been developed based on standards other than those published by MPEG. For example, similar standards have been circulated by other organizations, such as the H.261-H.264 standards developed by the International Telecommunication Union. Additionally, proprietary codecs have been developed by other organizations and individuals. For example, Compression Technologies, inc. produces digital video compression tools based on the Cinepak codec and DivXNetworks produces a variety of applications based on the DivX codec. These standard and proprietary codecs represent only a few of the many different ways to compress and encode digital video information. 
       FIG. 1  presents a media sequence  10 , such as a portion of a movie. The media sequence  10  can be organized as a track  12  of digital video information that can include one or more image segments, such as the first image segment  14  and the second image segment  16 . In turn, each image segment is comprised of one or more samples, such as frames of image data. The track  12  also can include one or more empty segments  18 , which are not associated with any video information and during which no video information is presented. In addition to the track  12  of digital video information, the media sequence  10  also can include a track of audio information and a track of text information (not shown). 
     The track  12  of digital video information represents the movie timescale and the image segments included in the track  12  are sequentially ordered with respect to time. Therefore, the first image segment  14  in the track  12  is temporally ordered such that it precedes the second image segment  16 . During forward play, the first image segment  14  will thus be displayed prior to the display of the second image segment  16 . As with the image segments, each of the empty segments  18  included in the track  12  is temporally ordered with respect to the other segments. 
     The order of the image segments and the empty segments included in the track  12  is defined in an edit list  20 . For each image segment and empty segment included in the track  12 , there is a corresponding entry, or edit, in the edit list  20 . Each edit defines parameters associated with the image segment or the empty segment to which it corresponds. For example, an edit identifies the point in the movie timescale at which the corresponding image segment or empty segment is to begin. The edit also identifies the duration, expressed in terms of the movie timescale, over which the corresponding image segment or empty segment will be played. Further, with respect to image segments, an edit identifies the rate at which the corresponding image segment is to be played back. A playback rate of 1.0 can be associated with the first image segment  14  to indicate that playback should occur at a rate equal to the timing information associated with the first image segment  14 . Similarly, a playback rate of 2.0 can be associated with the second image segment  16  to indicate that playback should occur at a rate equal to twice the timing information associated with the second image segment  16 . 
     The one or more samples comprising each of the image segments included in the track  12  are contained in the media  22 . If the edit list  20  is modified to insert an additional image segment into the track  12  after the media  22  has been populated, the samples comprising the newly added image segment are also entered into the media  22 . It is also possible to delete an image segment from the track  12  during editing, but the samples comprising the deleted image segment are not removed from the media  22 . Once a sample has been added to the media  22 , its intrinsic properties cannot be modified and the sample cannot be removed from the media  22 . The media  22  thereby ensures that every sample associated with an image segment previously identified in the edit list  20  will be available if it is ever required in connection with the track  12 . 
     The media  22  also incorporates the concept of a media timescale, which is a positive integer. The media can be subdivided into X units of media time, where X is a positive integer. The duration of each unit of media time can then be determined to be X÷(media timescale). As described above, the media  22  contains every sample associated with an image segment that has previously been inserted into the track  12 . Therefore, the media  22  can be ordered as a sequential list of samples  24 , wherein each sample is assigned a unique sequence number. For example, the first sample  26  in the list of samples  24  can be assigned sequence number  1 . A sample duration  28  also is associated with each sample in the list of samples  24 , indicating how long, in media time, a given sample will be displayed. Sample durations, which are expressed as positive integers, may differ among different samples in the media  22 . The media duration  30 , in turn, is equal to the sum of the sample durations for all of the samples included in the media  22 . 
     The first sample  26  included in the media  22  has a sample time of zero, which also represents the zero point of the media timescale. The second sample  32  included in the media  22  has a sample time equal to the first sample duration  28 , because the sample time of the first sample  26  is zero. Each subsequent sample included in the list of samples  24  has a sample time equal to the sum of the sample time of the preceding sample and the duration of the preceding sample. Therefore, the samples included in the media  22  partition the media time for the entire media duration  30  without any gaps. The structure of the list of samples  24  cannot be adapted to permit the use of B-pictures, however, as it does not separately account for decode times and display times. 
     SUMMARY 
     The need to implement multiple-timeline strategies that will permit the use of pictures predicted or interpolated from future pictures, pictures that appear later in the display order, for video compression and decompression is recognized. Accordingly, the techniques and apparatus described here implement algorithms for searching separate decode and display timelines in order to identify one or more specific samples included in a media sequence. 
     In general, in one aspect, the techniques can be implemented to include identifying a first point on a first timeline associated with a sequence of video images, wherein the sequence of video images is comprised of one or more samples; determining a search range on a second timeline associated with the sequence of video images based on a positive offset and a negative offset associated with the one or more samples; and searching the second timeline based on the determined search range for a second point that corresponds to the identified first point. 
     The techniques also can be implemented to include associating a display offset with each of the samples comprising the sequence of video images, wherein one or more of the display offsets comprises a negative value. The techniques further can be implemented such that determining a search range further comprises determining a search range on the second timeline based on a maximum positive display offset and a maximum negative display offset associated with the one or more samples. Additionally, the techniques can be implemented to include adjusting the search range if the first point occurs within a decode duration or a display duration. 
     The techniques also can be implemented such that the search range is extended backward in time by a period equal to the decode duration or the display duration in which the first point occurs. The techniques further can be implemented to include storing the display offsets associated with the samples comprising the sequence of video images in a data structure, wherein the display offsets are run length encoded. Additionally, the techniques can be implemented to include identifying a sample number associated with the second point. 
     In general, in another aspect, the techniques can be implemented to include processor electronics configured to identify a first point on a first timeline associated with a sequence of video images, wherein the sequence of video images is comprised of one or more samples; determine a search range on a second timeline associated with the sequence of video images based on a positive offset and a negative offset associated with the one or more samples; and search the second timeline based on the determined search range for a second point that corresponds to the identified first point. 
     The techniques also can be implemented such that the processor electronics are further configured to associate a display offset with each of the samples comprising the sequence of video images and one or more of the display offsets can comprise a negative value. Further, the techniques can be implemented such that the processor electronics are further configured to determine a search range on the second timeline based on a maximum positive display offset and a maximum negative display offset associated with the one or more samples. Additionally, the techniques can be implemented such that the processor electronics are further configured to adjust the search range if the first point occurs within a decode duration or a display duration. 
     The techniques also can be implemented such that the processor electronics are further configured to extend the search range backward in time by a period equal to the decode duration or the display duration in which the first point occurs. Further, the techniques can be implemented such that the processor electronics are further configured to store the display offsets associated with the samples comprising the sequence of video images in a data structure, wherein the display offsets are run length encoded. Additionally, the techniques can be implemented such that the processor electronics are further configured to identify a sample number associated with the second point. 
     The techniques described in this specification can be implemented to realize one or more of the following advantages. For example, the techniques can be implemented to permit playback from random points in a media sequence. The techniques also can be implemented to permit scrubbing, or frame-by-frame playback, of a media sequence in forward and reverse directions. Additionally, the techniques can be implemented to permit the identification and queuing of samples required for decoding and display operations. Further, the techniques can be implemented to conserve resources by eliminating the need to store one or more additional tables describing display ordering. In addition, making intrinsic properties of samples non-modifiable once they have been added to the media facilitates search algorithms to cache information derived during one search that can help accelerate future searches while essentially eliminating a risk that the information will be later invalidated. 
     These general and specific techniques can be implemented using an apparatus, a method, a system, or any combination of an apparatus, methods, and systems. The details of one or more implementations are set forth in the accompanying drawings and the description below. Further features, aspects, and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a media sequence with an integrated list of samples. 
         FIGS. 2-3  are block diagrams of a media sequence with separate display and decode lists. 
         FIG. 4  is a timeline of a media sequence. 
         FIG. 5  presents a display table using run length encoding. 
         FIG. 6  presents a decode table using run length encoding. 
         FIG. 7  is a timeline of a media sequence. 
         FIGS. 8-9  are flowcharts of timeline searching algorithms. 
         FIG. 10  is a flowchart of a method of searching using related timelines. 
     
    
    
     Like reference symbols indicate like elements throughout the specification and drawings. 
     DETAILED DESCRIPTION 
       FIG. 2  presents a media sequence  50 , which is similar to the media sequence  10  presented with respect to  FIG. 1 . As described above, the media sequence  50  can be organized as a track  52  of digital video information that can include one or more image segments, such as the first image segment  54  and the second image segment  56 . In turn, each of the image segments can be comprised of one or more samples, such as frames of image data. Additionally, the one or more image segments can be encoded in a variety of formats, such as .mpg, .jpg, and .avi, and image segments encoded in different formats can be included in the same track. 
     The track  52  also can include one or more empty segments  58 , which are not associated with any video information and during which no video information is presented. In another implementation, the media sequence  50  can include two or more tracks of video information, and each of the tracks of video information can include one or more image segments. In such an implementation, an empty segment included in a first track can correspond temporally to an image segment included in a second track. In addition to the track  52  of digital video information, the media sequence  50  also can include one or more tracks of audio information and one or more tracks of text information (not shown). 
     Also as discussed above, the track  52  of digital video information represents the movie timescale and the image segments included in the track  52 , such as the first image segment  54  and the second image segment  56 , are sequentially ordered with respect to time. Further, the digital video information included in the track  52  can be encoded in a compressed format in order to reduce the amount of information that must be stored and to reduce the amount of bandwidth required to transmit the information. For example, the digital video information can be represented using only I-pictures, a combination of I-pictures and P-pictures, or a combination of I-pictures, P-pictures, and B-pictures. 
     The order of the image segments and the empty segments included in the track  52  is defined in the edit list  60 . For each image segment and empty segment included in the track  52 , there is a corresponding entry, or edit, in the edit list  60 . Each edit defines parameters associated with the image segment or empty segment to which it corresponds. For example, an edit identifies the point in the movie timescale at which the corresponding image segment or empty segment is to begin. The edit also identifies the duration, expressed in terms of the movie timescale, over which the corresponding image segment or empty segment will be played. Further, with respect to image segments, an edit identifies the rate at which the corresponding image segment is to be played back. A playback rate of 1.0 can be associated with the first image segment  54  to indicate that playback should occur at a rate equal to the timing information associated with the first image segment  54 . Similarly, a playback rate of 2.0 can be associated with the second image segment  56  to indicate that playback should occur at a rate equal to twice the timing information associated with the second image segment  56 . 
     The media  62  incorporates the concept of a media timescale, which is a positive integer and is independent of the movie timescale associated with the track  52 . The media can be subdivided into X units of media time, where X is a positive integer. The duration of each unit of media time can then be determined to be X÷(media timescale). 
     The one or more samples comprising each of the image segments presently or previously included in the track  52  are contained in the media  62 . Unlike the media sequence  10  described with reference to  FIG. 1 , however, the media sequence  50  includes two separate but related lists of samples. The display list of samples  64  and the decode list of samples  66  each represent a sequential ordering of the samples contained in the media  62 . In the display list of samples  64 , for each of the image segments included in the media  62 , all of the samples associated with a particular image segment are arranged in the order in which they are displayed during forward playback of that image segment. In the decode list of samples  66 , for each of the image segments included in the media  62 , all of the samples associated with a particular image segment are arranged in the order in which they are decoded during forward playback of that image segment. The display list of samples  64  also can be represented as a display timeline and the decode list of samples  66  can be represented as a decode timeline. Each sample identified in the decode list of samples  66  can be logically associated with a corresponding sample identified in the display list of samples  64  by a vector. For example, the first sample  70  identified in the decode list of samples  66  can be logically associated with the corresponding sample  68  identified in the display list of samples  64  by the first vector  74 . 
     Once a sample has been added to the media  62 , the sample cannot thereafter be removed from the media  62 . The media  62  thereby ensures that every sample previously identified in the edit list  60  will be available if it is ever required in connection with the track  52 . Further, once a sample has been added to the media, none of the intrinsic values associated with the sample can be modified. There is, however, one exception to this rule. If the sample is the last sample in the media  62 , the sample decode duration corresponding to that sample can be increased by the amount required to make the media decode end time  78  equal the media display end time  80 . 
     The multiple timeline schema is based on the decode order of the samples included in the media  62 , not the display order of the samples. As such, a sample decode duration is associated with each sample. The sample decode duration, which is always a positive value, indicates the difference between the decode time of the associated sample and the decode time of the next sample. For example, a first sample decode duration  72  is associated with the first sample  70  identified in the decode list of samples  66 . Therefore, the sample decode duration indicates the period of the first sample  70  expressed in media time. The sample decode duration  72  does not, however, identify the amount of time required to decode the first sample  70 . Sample decode durations associated with the samples identified in the decode list of samples  66  are not required to be identical. 
     The media decode duration  78 , or media decode end time  78 , is equal to the sum of the sample decode durations for every sample included in the media  62 . The first sample  70  included in the media  62  has a sample decode time of zero, which also represents the zero point of the media timescale. The second sample  76  included in the media  62  has a sample decode time equal to the first sample decode duration  72 , because the sample decode time of the first sample  70  is zero. Each subsequent sample identified in the decode list of samples  66  has a sample decode time equal to the sum of the sample decode time of the preceding sample and the sample decode duration of the preceding sample. Therefore, the samples included in the media  62  partition the media time for the entire media decode duration  78  without any gaps. 
     Each sample included in the media  62  also has a sample display offset, which can be a positive value, zero, or a negative value. The display time of a sample can therefore be derived by summing the sample decode time and the sample display offset associated with that sample. Additionally, the display time associated with a sample cannot be negative. Therefore, if the display offset associated with a sample is a negative value, the magnitude of the display offset can be no greater than the sample decode time. The media display end time  80  is derived by summing the display time associated with the last sample included in the media  62  with the decode duration associated with the last sample included in the media  62 . 
     The display duration of a sample is not an intrinsic property associated with that sample, but is instead determined with reference to other samples. If a sample with a subsequent display time exists in the media  62 , the display duration of the present sample can be determined as the difference between the display time of the present sample and the display time of the subsequent sample. If no sample with a subsequent display time exists in the media  62 , the display duration of the present sample can be set equal to a placeholder value, such as the sample decode duration associated with the sample. Further, because a fixed display duration is not associated with the samples included in the media  62 , the display duration associated with a sample is subject to change if one or more additional samples with subsequent display times are added to the media  62 . 
     As can be seen in  FIG. 2 , when the media sequence  50  contains only I-pictures, the display list of samples  64  and the decode list of samples  66  are identically ordered. For example, the first sample  70  identified in the decode list of samples  66  can be assigned the sample number  1 . Because no reordering is required, the sample number  1  also appears as the first sample  68  identified in the display list of samples  64 . Similarly, the display list of samples  64  and the decode list of samples  66  are identically ordered when the media sequence  50  contains I-pictures and P-pictures. In such circumstances, the zero point of the decode list of samples  66  can occur at the same point in time as the zero point of the display list of samples  64 . 
     Because B-pictures provide for a greater degree of compression and flexibility, it is advantageous to compress and encode a media sequence using a combination of I-pictures, P-pictures, and B-pictures. In the event the media sequence  50  includes one or more B-pictures, however, the ordering of the samples in the display list of samples  64  and the ordering of the samples in the decode list of samples  66  will differ as a result of the reordering that is required between decode and display. 
       FIG. 3  presents a media sequence  100  that includes multiple B-pictures. The media sequence  100  is similar to the media sequence  50  described with respect to  FIG. 2 , with the exception of the display list of samples  114  and the decode list of samples  116 . Because the one or more image segments included in the media  112 , such as the first image segment  104  and the second image segment  106 , contain one or more samples that have been compressed and encoded as B-pictures, the order of the samples identified in the decode list of samples  116  does not match the order of the samples identified in the display list of samples  114 . For example, the first sample  120  identified in the decode list of samples  116  can be assigned the sample number  1 . The corresponding sample  124  in the display list of samples  114 , which is also assigned the sample number  1 , is ordered as the fifth sample in the sequence. Similarly, the third sample  128  identified in the decode list of samples  116  can be assigned the sample number  3 . The corresponding sample  122  in the display list of samples  114 , which is also assigned the sample number  3 , appears as the first sample in the sequence. 
     The reordering necessitated by the use of B-pictures is further illustrated by the vectors used to logically associate the samples identified in the decode list of samples  116  with the corresponding samples identified in the display list of samples  114 , such as the first vector  118 . As illustrated by the first vector  118 , the first sample  120  identified in the decode list of samples  116  will be decoded prior to the decoding of any other samples included in the media  112 , but it will not be immediately displayed. Therefore, decoding of the samples included in the media  112  must begin at some point in time prior to the display of those samples. As such, an advance decode time is also associated with the media  112 . The advance decode time is set to equal the magnitude of the largest negative display offset associated with a sample identified in the display list of samples  114 . It is possible for the advance decode time to be zero under certain circumstances, such as when the samples included in the media  112  are encoded as I-pictures and P-pictures. 
     The advance decode time represents the period by which the sample decode time associated with the first sample  120  identified in the decode list of samples  116  precedes the display time associated with the sample  122  identified in the display list of samples  114  as the sample to be displayed first. Therefore, the advance decode time can be used to correlate the decode timeline with the display timeline. As described above, the display time associated with the sample  122  identified in the display list of samples  114  as the sample to be displayed first can be determined by summing the sample decode time and the sample display offset associated with the sample  122 . If a track is edited after the advance decode time has been determined, the advance decode time must be recalculated before the media sequence can be displayed. 
       FIG. 4  presents a display timeline  150  and a decode timeline  152  associated with the media sequence  100  presented in  FIG. 3 . Vectors, such as the vector  158  that logically associates the first sample  120  identified in the decode list of samples  116  with the corresponding sample  124  identified in the display list of samples  114 , graphically depict the display offsets associated with each of the samples included in the media  112 . When the zero point  154  of the display timeline  150  is aligned with the zero point  156  of the decode timeline  152 , it can be determined that the vectors associated with several samples, such as the vectors  158  and  160 , indicate a positive display offset between the sample decode time and the corresponding display time associated with a particular sample. Other vectors, such as vectors  162  and  164 , indicate a negative display offset between the sample decode time and the corresponding display time associated with a particular sample. The display time of a sample cannot occur prior to the sample decode time associated with that sample. Therefore, if any sample in the media is characterized by a negative display offset, the zero point  154  of the display timeline  150  cannot be aligned with the zero point  156  of the decode timeline  152 . 
     In order to properly display the media sequence  100 , the decode timeline  152  must be shifted backward in time with respect to the display timeline  150  by a period equal to the advance decode time. The vector  164  associated with the eleventh sample  126  identified in the display list of samples  114  in  FIG. 3 , represents the largest negative display offset of any sample included in the media  112 . Therefore, the advance decode time is set to equal the magnitude of the display offset associated with the eleventh sample  126  identified in the display list of samples  114  in  FIG. 3 . 
     The display offsets associated with each of the samples included in the media  112  can be stored in a table. As a display offset of the same sign and magnitude may be associated with two or more consecutive samples, the display offsets associated with the samples can be efficiently represented using run length encoding.  FIG. 5  presents an implementation of a display table  170  used to store the display offsets. The display table  170  includes a first column  172  indicating the count, which represents the number of consecutive frames that share a common display offset. The display table  170  also includes a second column  174  indicating the display offset measured in a common unit of time, such as media time. As one or more samples can be represented using B-pictures, the display offsets included in the display table  170  can include negative values. Each row of the display table  170  represents a number of consecutive frames included in the media that are associated with a common display offset. For example, the first row  176  of the display table  170  indicates that the first sample has a display offset of 20. The second row  178  of the display table  170  indicates that the two subsequent samples each have a display offset of −10. Further, the display table  170  can store the largest positive display offset  180  and the largest negative display offset  182 . In another implementation, the largest positive display offset  180  and the largest negative display offset  182  can be derived from the display table  170  as needed. 
     As discussed above, a sample decode duration is associated with each sample included in the media. Because the same sample decode duration may be associated with two or more consecutive samples, the sample decode durations also can be efficiently represented using run length encoding.  FIG. 6  presents an implementation of a decode table  190  used to store sample decode durations. The decode table  190  includes a first column  192  indicating the count, which represents the number of consecutive frames that share a common sample decode duration. The decode table  190  also includes a second column  194  indicating the sample decode duration measured in a common unit of time, such as media time. Each row of the decode table  190  represents a number of consecutive frames included in the media that are associated with a common sample decode duration. For example, the first row  196  of the decode table  190  indicates that four consecutive samples have a sample decode duration of 10. Additionally, the first row  196  corresponds to the first four samples included in the media. Further, the fourth row  198  of the decode table  190  indicates that sixteen consecutive samples, starting with the eighteenth sample included in the media, have a sample decode duration of 20. 
     Because P-pictures and B-pictures are ultimately decoded with reference to an I-picture, or key picture, a record identifying the samples that are encoded as I-pictures also can be maintained (not shown). During an operation in which a sample is selected other than through sequential play, such as scrubbing, the closest preceding I-picture can be accessed and used in decoding the selected sample. In order to identify which sample is associated with a particular point on the display timeline, however, it can be necessary to determine the corresponding sample decode time on the decode timeline. Additionally, in order to identify the next display time that will occur following a selected point on the display timeline, it can first be necessary to identify the sample that corresponds to the selected point. Therefore, it can be necessary to search the display timeline for a point corresponding to a known location on the decode timeline and also to search the decode timeline for a point corresponding to a known location on the display timeline. Conducting an end-to-end search of either timeline, however, is unnecessary. 
       FIG. 7  illustrates that a search of the display timeline  200  or the decode timeline  202  can be limited to an identifiable search range using the largest positive display offset  180  and the largest negative display offset  182  included in the display table  170 . For example, given a sample decode time t a    204  associated with a sample identified in the decode timeline  202 , it can be understood that the corresponding display time can be no earlier than that given by the largest negative display offset  182 , or Δ min . Therefore, the earliest point in the display timeline search range  210  can be represented as t a +Δ min    206 . Further, given the sample decode time t a    204 , it can be understood that the corresponding display time can be no later than that given by the largest positive display offset  180 , or Δ max . Therefore, the latest point in the display timeline search range  210  can be represented as t a +Δ max    208 . 
     A similar decode timeline search range  218  can be determined given a display time t b    212  in the display timeline  200 . Given the display time t b    212 , it can be understood that the corresponding sample decode time can be no earlier than that given by the largest positive display offset  180 , or Δ max . Therefore, the earliest point in the decode timeline search range  218  can be represented as t b -Δ max    214 . Further, given the display time t b    212 , it can be understood that the corresponding decode time can be no later than that given by the largest negative display offset  182 , or Δ min . Therefore, the latest point in the decode timeline search range  218  can be represented as t b −Δ min    216 . 
     In another implementation, the display timeline search range  210  and the decode timeline search range  218  can be defined using different display offsets. For example, the largest positive and negative display offsets occurring within a predetermined period of the display time or decode time of interest can be selected, where the predetermined period is less than the full media duration. 
     Additionally, if the point t a    204  on the decode timeline  202  represents a point in time during a sample decode duration, rather than the sample decode time, the display timeline search range  210  can be shifted backward in time by an adjustment period, such as a period equal to the sample decode duration. Similarly, if the point t b    212  on the display timeline  200  represents a point in time during a display duration, rather than the display time, the decode timeline search range  218  can be shifted backward in time by an adjustment period, such as a period equal to the display duration. 
       FIG. 8  describes a technique  250  for searching a decode timeline to identify a sample corresponding to a given point t t  in a corresponding display timeline. The target display time corresponding to the point t t  in the display timeline is identified ( 252 ). The target display time is compared with the display start time of the media and the display end time of the media. The target display time is valid if it occurs on or after the display start time of the media and before the display end time of the media ( 254 ). If the target display time is not valid, the search is terminated ( 256 ). If the target display time is valid, the point t s  in the decode timeline characterized by the same time as the point t t  in the display timeline is identified. BaseSampleNumber is set equal to the number of the sample associated with the sample decode time or sample decode duration occurring at the point t s  in the decode timeline. Further, BaseDecodeTime is set equal to the sample decode time of the sample number corresponding to BaseSampleNumber ( 258 ). 
     As described above with respect to  FIG. 7 , the decode timeline search range is determined using the largest positive display offset, Δ max , and the largest negative display offset, Δ min , associated with the samples included in the media. GreatestDecodeTime is determined by subtracting the largest negative display offset, Δ min , from the point t s  in the decode timeline. Further, LeastDecodeTime is determined by subtracting the largest positive display offset, Δ max , from the point t s  in the decode timeline ( 260 ). Additionally, the flag SawLaterDisplayTime is set to equal false ( 262 ). 
     The forward walk loop is initiated by setting the value BaseSampleNumber+j, where j=1 for the first iteration of the forward walk loop. For each subsequent iteration of the forward walk loop, j is incremented such that j=j+1 ( 264 ). BaseSampleNumber+j is then compared with the number of samples included in the media. BaseSampleNumber+j is valid if it does not exceed the number of samples included in the media ( 266 ). If BaseSampleNumber+j is invalid, BaseSampleNumber+j is decremented such that BaseSampleNumber+j=BaseSampleNumber+j−1 and the forward walk loop is terminated ( 268 ). If BaseSampleNumber+j is valid, ProbeDecodeTime is set equal to the sample decode time associated with the sample number corresponding to BaseSampleNumber+j. Additionally, ProbeDisplayTime is set equal to the display time associated with the sample number corresponding to BaseSampleNumber+j ( 270 ). 
     ProbeDisplayTime is compared with the target display time ( 272 ). If ProbeDisplayTime equals the target display time, BaseSampleNumber+j is returned as the target sample number and the search is terminated ( 274 ). If ProbeDisplayTime does not equal the target display time, it is determined whether ProbeDisplayTime exceeds the target display time ( 276 ). If ProbeDisplayTime exceeds the target display time, the flag SawLaterDisplayTime is set to true ( 278 ). Additionally, the sample corresponding to BaseSampleNumber+j is examined to determine whether the flag SampleEarlierDisplayTimesAllowed is set to true ( 280 ). If the flag SampleEarlierDisplayTimesAllowed is set to true, BaseSampleNumber+j is incremented and an additional iteration of the forward walk loop is executed ( 264 ). If the flag SampleEarlierDisplayTimesAllowed is set to false, the forward walk loop is terminated. 
     If ProbeDisplayTime does not exceed the target display time, it is determined whether ProbeDisplayTime exceeds GreatestDecodeTime ( 282 ). If ProbeDisplayTime does not exceed GreatestDecodeTime, BaseSampleNumber+j is incremented and an additional iteration of the forward walk loop is executed ( 264 ). If ProbeDisplayTime exceeds GreatestDecodeTime, the forward walk loop is terminated. 
     CeilingSampleNumber is set equal to BaseSampleNumber+j ( 284 ). CeilingSampleNumber+1 is then compared with the number of samples in the media ( 286 ). If CeilingSampleNumber+1 is less than or equal to the number of samples in the media, the flag SawLaterDisplayTime is set to true ( 288 ). CandidateSampleNumber is set equal to 0 and CandidateDisplaySustain is assigned the maximum positive value of TimeValue 64 , which is a signed 64-bit integer ( 290 ). 
     The backward walk loop is initiated by setting the value CeilingSampleNumber−k, where k=0 for the first iteration of the backward walk loop. For each subsequent iteration of the backward walk loop, k is incremented such that k=k+1 ( 292 ). CeilingSampleNumber−k is then validated by determining whether CeilingSampleNumber−k is less than 1 ( 294 ). If CeilingSampleNumber−k is less than 1, it is invalid and the backward walk loop is terminated ( 296 ). If CeilingSampleNumber−k is valid, ProbeDecodeTime is set equal to the sample decode time associated with the sample number corresponding to CeilingSampleNumber−k. Additionally, ProbeDisplayTime is set equal to the display time associated with the sample number corresponding to CeilingSampleNumber−k ( 298 ). 
     ProbeDisplayTime is compared with the target display time to determine whether they are equal ( 300 ). If ProbeDisplayTime equals the target display time, CeilingSampleNumber−k is returned as the target sample number and the search is terminated ( 302 ). If ProbeDisplayTime is not equal to the target display time, it is determined whether ProbeDisplayTime is less than the target display time ( 304 ). If ProbeDisplayTime is less than the target display time, it is determined whether the target display time−ProbeDisplayTime is less than CandidateDisplaySustain ( 306 ). If the target display time−ProbeDisplayTime is less than CandidateDisplaySustain, CandidateDisplaySustain is set equal to the target display time−ProbeDisplayTime, CandidateSampleNumber is set equal to CeilingSampleNumber−k, and CandidateDisplayTime is set equal to the display time associated with the sample corresponding to CandidateSampleNumber ( 308 ). If the target display time−ProbeDisplayTime is greater than or equal to CandidateDisplaySustain, the backward walk loop is terminated. 
     If ProbeDisplayTime is greater than or equal to the target display time, it is determined whether CandidateSampleNumber is equal to 0 or whether ProbeDecodeTime+GreatestDisplayOffset is greater than CandidateDisplayTime ( 310 ). If CandidateSampleNumber is equal to 0 or ProbeDecodeTime+GreatestDisplayOffset is greater than CandidateDisplayTime, CeilingSampleNumber−k is decremented and an additional iteration of the backward walk loop is executed ( 292 ). Otherwise, the backward walk loop is terminated. 
     CandidateSampleNumber is evaluated to determine whether it equals 0 ( 314 ). If CandidateSampleNumber equals 0, no sample corresponding to the target display time was located during the search and an invalid time indicator is returned ( 316 ). Alternatively, if CandidateSampleNumber does not equal 0, it is determined whether the flag SawLaterDisplayTime is set to false ( 318 ). If the flag SawLaterDisplayTime is set to false, it is determined whether the display time associated with CandidateSampleNumber+the sample decode duration of the sample corresponding to CandidateSampleNumber is less than or equal to the target display time ( 320 ). If the display time associated with CandidateSampleNumber+the sample decode duration of the sample corresponding to CandidateSampleNumber is less than or equal to the target display time, an invalid time indicator is returned ( 322 ). If the flag SawLaterDisplayTime is set to true, or if the flag SawLaterDisplayTime is set to false and the display time associated with CandidateSampleNumber+the sample decode duration of the sample corresponding to CandidateSampleNumber is greater than the target display time, CandidateSampleNumber is returned as the sample number corresponding to the target display time ( 324 ). 
       FIG. 9  describes a technique  350  for identifying the next display time in the display timeline based on a present sample. DisplayTime is set equal to the display time corresponding to the present sample ( 352 ). BaseDecodeTime is set equal to DisplayTime−the largest positive display offset, Δ max . If DisplayTime−the largest positive display offset, Δ max , is less than 0, BaseDecodeTime is set equal to 0 ( 354 ). BaseSampleNumber is set equal to the sample number corresponding to BaseDecodeTime ( 356 ). The sample number corresponding to BaseDecodeTime can be determined by searching the corresponding decode table, such as the decode table  190  presented with respect to  FIG. 6 . A time variable T can be initialized to a decode time zero and a sample variable S can be initialized to 1. While T is less than the BaseDecodeTime, each row of the decode table can be successively referenced. For each row of the decode table that is referenced, T can be incremented by the product of the count and the sample decode duration. Additionally, S can be incremented by count. Once T is greater than or equal to the BaseDecodeTime, T can be divided by the duration corresponding to the current row of the decode table, and the result can be subtracted from S to arrive at the sample number corresponding to BaseDecodeTime. Additionally, the values associated with the variables S and T can be saved for use as the starting point of a subsequent search. If T is greater than the BaseDecodeTime in a subsequent search, the decode table can be traversed in reverse by successively subtracting the product of the count and the sample decode duration from T and the count from the S. Once determined, BaseSampleNumber represents the lower bound on the sample number that could correspond to the next display time. Further, the flag SawLaterDisplayTime is set to false ( 358 ). 
     Before beginning the forward walk loop, CandidateSampleNumber and CandidateDisplayTime are each set to equal 0. Additionally, BaseSampleNumber+j, is initialized such that j=0 for the first iteration of the forward walk loop ( 360 ). Before executing an iteration, it is first determined whether the forward walk loop is in its first iteration ( 361 ). For all but the first iteration of the forward walk loop, j is incremented such that j=j+1 ( 362 ). BaseSampleNumber+j is compared with the number of samples included in the media. BaseSampleNumber+j is valid unless it exceeds the number of samples included in the media ( 364 ). If BaseSampleNumber+j is invalid, the search is terminated ( 366 ). If BaseSampleNumber+j is valid, ProbeDecodeTime is set equal to the sample decode time associated with the sample corresponding to BaseSampleNumber+j. Additionally, ProbeDisplayTime is set equal to the display time associated with the sample corresponding to BaseSampleNumber+j ( 368 ). 
     ProbeDisplayTime is then compared with DisplayTime ( 370 ). If ProbeDisplayTime is less than or equal to DisplayTime, BaseSampleNumber+j is incremented and an additional iteration of the forward walk loop is executed ( 362 ). If ProbeDisplayTime is greater than DisplayTime, it is determined whether the flag SawLaterDisplayTime is set to false or ProbeDisplayTime is less than CandidateDisplayTime ( 372 ). If the flag SawLaterDisplayTime is set to true and ProbeDisplayTime is greater than or equal to CandidateDisplayTime, then ProbeDecodeTime is evaluated to determine whether it is greater than GreatestDecodeTime ( 378 ). 
     If either the flag SawLaterDisplayTime is set to false or ProbeDisplayTime is less than CandidateDisplayTime, then CandidateDisplayTime is set equal to ProbeDisplayTime, CandidateSampleNumber is set equal to BaseSampleNumber+j, and GreatestDecodeTime is set equal to CandidateDisplayTime−the largest negative display offset, or Δ min  ( 374 ). Additionally, the flag SawLaterDisplayTime is set to true ( 376 ). ProbeDecodeTime is then evaluated to determine whether it is greater than GreatestDecodeTime ( 378 ). 
     If ProbeDecodeTime is less than or equal to GreatestDecodeTime, BaseSampleNumber+j is incremented and an additional iteration of the forward walk loop is executed ( 362 ). If ProbeDecodeTime is greater than GreatestDecodeTime, the forward walk loop is terminated ( 380 ). The flag SawLaterDisplayTime is evaluated to determine whether it is set to true ( 382 ). If the flag SawLaterDisplayTime is set to true, CandidateSampleNumber is returned as the sample to be displayed next, the next display time is set equal to CandidateDisplayTime, and the search is terminated ( 384 ). If the flag SawLaterDisplayTime is set to false, there is no next display time and the search is terminated ( 386 ). 
     In another implementation, the technique  350 , described with respect to  FIG. 9 , can be used to execute a reverse search. In order to identify the previous display time based on a present sample, the technique  350  can be initiated by setting DisplayTime=DisplayTime−1. 
       FIG. 10  describes a method of searching multiple related timelines representing a sequence of video images. In a first step  400 , a first point on a first timeline associated with a sequence of video images is identified, wherein the sequence of video images is comprised of one or more samples. In a second step  402  a search range on a second timeline associated with the sequence of video images is determined based on a positive offset and a negative offset associated with the one or more samples. Once the search range has been determined, the third step  404  is to search the second timeline based on the determined search range for a second point that corresponds to the identified first point. 
     A number of implementations have been disclosed herein. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the claims. Accordingly, other implementations are within the scope of the following claims.