DASH MPD中的段持续时间

Question

如果没有指定，但给定mp4视频的内容长度和索引范围，如何确定MPD文件中的片段持续时间？例如，如下所示的mpd文件（http://dash.edgesuite.net/akamai/streamroot/050714/Spring_4Ktest.mpd）：

<?xml version="1.0"?>
<!-- MPD file Generated with GPAC version 0.5.1-DEV-rev5208  on 2014-05-08T15:14:47Z-->
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" minBufferTime="PT1.500000S" type="static" mediaPresentationDuration="PT0H2M45.76S" profiles="urn:mpeg:dash:profile:isoff-on-demand:2011">
 <ProgramInformation moreInformationURL="http://gpac.sourceforge.net">
  <Title>Spring_3000k_dash.mpd generated by GPAC</Title>
 </ProgramInformation>

 <Period id="" duration="PT0H2M45.76S">
  <AdaptationSet segmentAlignment="true" maxWidth="3840" maxHeight="2160" maxFrameRate="24000/1001" par="16:9" subsegmentStartsWithSAP="1">
   <Representation id="1" mimeType="video/mp4" codecs="avc1.640033" width="1280" height="720" frameRate="24000/1001" sar="1:1" startWithSAP="1" bandwidth="2859078">
    <BaseURL>Spring_3000k_track1_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="896-1419"/>
   </Representation>
   <Representation id="2" mimeType="video/mp4" codecs="avc1.640033" width="1920" height="1080" frameRate="24000/1001" sar="1:1" startWithSAP="1" bandwidth="4872811">
    <BaseURL>Spring_5000k_track1_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="896-1419"/>
   </Representation>
   <Representation id="3" mimeType="video/mp4" codecs="avc1.640033" width="2880" height="1620" frameRate="24000/1001" sar="1:1" startWithSAP="1" bandwidth="7322004">
    <BaseURL>Spring_7500k_track1_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="897-1420"/>
   </Representation>
   <Representation id="4" mimeType="video/mp4" codecs="avc1.640033" width="3840" height="2160" frameRate="24000/1001" sar="1:1" startWithSAP="1" bandwidth="9604428">
    <BaseURL>Spring_10000k_track1_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="896-1419"/>
   </Representation>
   <Representation id="5" mimeType="video/mp4" codecs="avc1.640033" width="3840" height="2160" frameRate="24000/1001" sar="1:1" startWithSAP="1" bandwidth="14658452">
    <BaseURL>Spring_15000k_track1_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="896-1419"/>
   </Representation>
   <Representation id="6" mimeType="video/mp4" codecs="avc1.640033" width="3840" height="2160" frameRate="24000/1001" sar="1:1" startWithSAP="1" bandwidth="19683583">
    <BaseURL>Spring_20000k_track1_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="896-1419"/>
   </Representation>
  </AdaptationSet>
  <AdaptationSet segmentAlignment="true" subsegmentStartsWithSAP="1">
   <Representation id="7" mimeType="audio/mp4" codecs="mp4a.40.2" audioSamplingRate="48000" startWithSAP="1" bandwidth="193664">
    <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/>
    <BaseURL>Spring_3000k_track2_dashinit.mp4</BaseURL>
    <SegmentBase indexRangeExact="true" indexRange="823-1262"/>
   </Representation>
  </AdaptationSet>
 </Period>
</MPD>

非常感谢你！

闪耀

Answer 1

在这种情况下，似乎没有机会从MPD文件中检索段信息。你只需要分析mp4文件。以下是“Spring_3000k_track1_dashinit.mp4”的分析片段。

   0       24 ftyp
  24        8 free
  2c       3b free
  67      319 moov
 380      20c sidx
 58c      354 moof
 8e0   152f01 mdat
 ...

和“sidx”的数据（位于896-1914范围内，即“0x380”-“0x58b”）：

Box type    : sidx
Box version : 0
reference_ID    : 1
timescale   : 24000
earliest_presentation_time  : 0
first_offset    : 0
Entries : size = 41
 - [0]  
reference_type  : 0
referenced_size : 1389141
subsegment_duration : 96096
starts_with_SAP : 1
SAP_type    : 4
SAP_delta_time  : 0
 - [1]  
reference_type  : 0
referenced_size : 1422268
subsegment_duration : 96096
starts_with_SAP : 1
SAP_type    : 4
SAP_delta_time  : 0
...

所以视频中有41个片段，片段持续时间为4秒（subsegment_duration/时间尺度）。交叉检查：41 * 4 = 164~2M45.76S（内容长度）。