如何计算DASH媒体段名称中的$Time$变量？结合MPD播放列表示例详解

Let's walk through exactly how to compute the $Time$ variable for each segment in your provided MPD, plus how to find the next segment's URL.

First, let's recap the key parts of your MPD snippet for context:

<SegmentTemplate timescale="10000000" initialization="Video-$RepresentationID$-init.mp4" media="Video-$RepresentationID$-$Time$.mp4">
  <SegmentTimeline>
    <S t="2083838574453329" d="100053337" />
    <S d="100053332" r="1" />
    <S d="99839999" />
    <S d="100053337" />
  </SegmentTimeline>
</SegmentTemplate>

Core Definitions You Need to Know #

Before diving into calculations, let's clarify what each attribute means:

• timescale="10000000": This is the time unit all timestamps (t) and durations (d) use. You don't need to convert this value for $Time$—it's directly used in the media URL.
• <S> elements: Each represents a segment (or a repeating pattern of segments)
  • t: The start timestamp of the segment (in timescale units). Only required for the first segment (or if there's a gap in the timeline).
  • d: The duration of the segment (in timescale units).
  • r: Number of times the segment is repeated. For example, r="1" means the segment pattern runs twice total (original + 1 repeat).

Step-by-Step $Time$ Calculation for Each Segment #

Let's compute the $Time$ value for every segment in your timeline:

1. Segment 1 (first <S> element)

   • The t value is explicitly given: 2083838574453329
   • $Time$ = 2083838574453329
   • Media URL: Video-[YourRepresentationID]-2083838574453329.mp4

2. Segment 2 (second <S> element, first instance)

   • No t provided, so we calculate it as the end time of the previous segment:
     • Previous segment's end time = Segment 1's t + Segment 1's d = 2083838574453329 + 100053337 = 2083838674506666
   • $Time$ = 2083838674506666

3. Segment 3 (second <S> element, repeated instance, r="1")

   • Since r="1", we repeat the segment once. Calculate using the end time of Segment 2:
     • Segment 2's end time = 2083838674506666 + 100053332 = 2083838774559998
   • $Time$ = 2083838774559998

4. Segment 4 (third <S> element)

   • Use the end time of Segment 3:
     • 2083838774559998 + 100053332 = 2083838874613330
   • $Time$ = 2083838874613330

5. Segment 5 (fourth <S> element)

   • Use the end time of Segment 4:
     • 2083838874613330 + 99839999 = 2083838974453329
   • $Time$ = 2083838974453329

Getting the Next Segment URL #

To get the next segment's URL:

1. Track which segment you're currently playing (e.g., if you're on Segment 3, next is Segment 4)
2. Calculate the $Time$ value for the next segment using the rules above
3. Replace $RepresentationID$ with your target quality's ID (e.g., 1080p, 720p) and $Time$ with the calculated timestamp in the media template string.

Quick Recap of Rules #

• For the first segment: Use the explicit t value if provided
• For all subsequent segments: $Time$ = Previous Segment's $Time$ + Previous Segment's $d$
• If a segment has r="N": Generate N+1 total segments for that <S> element, each with $Time$ incremented by the element's d each time

内容的提问来源于stack exchange，提问作者Boy