版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/nonmarking/article/details/53439481
在DASH（自适应流媒体传输）中推荐使用的是fragmented Mp4(fMp4)格式，那么这种格式和传统的mp4格式有什么区别呢？
先说结论：在fMp4格式中包含一系列的segments(moof+mdat的组合)，这些segments可以被独立的request（利用byte-range request），这有利于在不同质量级别的码流之间做码率切换操作
介绍一个可以输出mp4 box信息的工具：MP4dump 下面我们分别dump出一个regular mp4和一个fmp4的box信息，化简如下
regular mp4
[ftyp] size=8+16
[moov] size=8+9149
[mdat] size=8+17923439```如果我们要在两个码流之间做码率切换，就需要找到两个码流中对应时间点的byte position，然而这时候我们只有一个巨大的mdat box，要在这里面找到一个具体的byte position无疑是复杂的。而且，在regular mp4中，有时moov会在巨大的mdat box之后，这也会影响起播的速度。
fmp4
[ftyp] size=8+28
[moov] size=8+790
[sidx] size=12+368
[moof] size=8+1304
[mdat] size=8+2447381
[moof] size=8+132
[mdat] size=8+164418
[moof] size=8+1304
[mdat] size=8+2612620
[moof] size=8+132
[mdat] size=8+124621```但是在fmp4中就简单多了，首先我们注意到有一个sidx(segment index) box，它记录了各个moof+mdat组成的segment的精确byte position，所以我们只需要Load一个很小的sidx box就能方便的实现码率切换了。sidx box的具体内容如下，size对应各个segment的大小，duration对应时长。
entry 0000 = reference_type=0, referenced_size=388331, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0001 = reference_type=0, referenced_size=1135797, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0002 = reference_type=0, referenced_size=1266343, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0003 = reference_type=0, referenced_size=1160677, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0004 = reference_type=0, referenced_size=1180917, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0005 = reference_type=0, referenced_size=842092, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0006 = reference_type=0, referenced_size=1160963, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0007 = reference_type=0, referenced_size=1203216, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0008 = reference_type=0, referenced_size=1234885, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0009 = reference_type=0, referenced_size=1166036, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0010 = reference_type=0, referenced_size=1237774, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0011 = reference_type=0, referenced_size=1162960, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0012 = reference_type=0, referenced_size=1235166, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
......
entry 0051 = reference_type=0, referenced_size=1195136, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0052 = reference_type=0, referenced_size=1178021, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0053 = reference_type=0, referenced_size=1222620, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0054 = reference_type=0, referenced_size=1197275, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0055 = reference_type=0, referenced_size=1161201, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0056 = reference_type=0, referenced_size=1239388, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0057 = reference_type=0, referenced_size=1186850, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0058 = reference_type=0, referenced_size=1012514, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0059 = reference_type=0, referenced_size=1110407, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0060 = reference_type=0, referenced_size=1055466, subsegment_duration=147456, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0
entry 0061 = reference_type=0, referenced_size=7954, subsegment_duration=24576, starts_with_SAP=1, SAP_type=1, SAP_delta_time=0不仅如此，在fmp4中各个segment的duration是可以由我们自己指定的，从而可以保证不同码流的各个segment是time aligned且一定start with关键帧。
最后介绍一个工具Mp4Fragment，可以将regular MP4转换为fmp4.
各位看官，如果您觉得本人的博客对您有所帮助，可以扫描如下二维码进行打赏，打赏多少您随意
