MP4文件quick start探究
最近在Flash上实现MP4文件点播时，发现部分MP4文件在原有的播放器上无法播放，经过定位发现，原来是MP4 BOX搞的鬼！
什么是MP4 BOX？
对于一般的MP4文件，BOX顺序为：ftyp-moov-free-mdat.
而对于部分经过视频软件处理后的MP4文件，BOX顺序发生了改变：ftyp-free-mdat-moov.
对于这种顺序的MP4文件，在完整下载文件之前，无法获取moov信息，因此是无法实现边下载边播放的。
解决方案
对源文件进行重新编码
既然BOX顺序有问题，可以从源头上进行处理：调整BOX顺序。工具qt-faststart即是实现该功能的利器。
qt-faststart主页下载了qt-faststart.exe binary for Windows之后，通过cmd对def.mp4文件进行转化： qt-faststart.exe def.mp4 def_new.mp4
转化后，box顺序果然发生了变化，播放正常。
使用AtomicParsley对MP4文件进行解析：
Atom ftyp @ 0 of size: 32, ends @ 32
Atom free @ 32 of size: 8, ends @ 40
Atom mdat @ 40 of size: 1025668, ends @ 1025708
Atom moov @ 1025708 of size: 13925, ends @ 1039633
Atom mvhd @ 1025716 of size: 108, ends @ 1025824
Atom trak @ 1025824 of size: 6608, ends @ 1032432
Atom tkhd @ 1025832 of size: 92, ends @ 1025924
Atom edts @ 1025924 of size: 36, ends @ 1025960
Atom elst @ 1025932 of size: 28, ends @ 1025960
Atom mdia @ 1025960 of size: 6472, ends @ 1032432
Atom mdhd @ 1025968 of size: 32, ends @ 1026000
Atom hdlr @ 1026000 of size: 45, ends @ 1026045
Atom minf @ 1026045 of size: 6387, ends @ 1032432
Atom vmhd @ 1026053 of size: 20, ends @ 1026073
Atom dinf @ 1026073 of size: 36, ends @ 1026109
Atom dref @ 1026081 of size: 28, ends @ 1026109
Atom stbl @ 1026109 of size: 6323, ends @ 1032432
Atom stsd @ 1026117 of size: 167, ends @ 1026284
Atom avc1 @ 1026133 of size: 151, ends @ 1026284
Atom avcC @ 1026219 of size: 49, ends @ 1026268
Atom pasp @ 1026268 of size: 16, ends @ 1026284
~
Atom stts @ 1026284 of size: 24, ends @ 1026308
Atom stss @ 1026308 of size: 24, ends @ 1026332
Atom ctts @ 1026332 of size: 3008, ends @ 1029340
Atom stsc @ 1029340 of size: 40, ends @ 1029380
Atom stsz @ 1029380 of size: 1532, ends @ 1030912
Atom stco @ 1030912 of size: 1520, ends @ 1032432
Atom trak @ 1032432 of size: 7104, ends @ 1039536
Atom tkhd @ 1032440 of size: 92, ends @ 1032532
Atom edts @ 1032532 of size: 36, ends @ 1032568
Atom elst @ 1032540 of size: 28, ends @ 1032568
Atom mdia @ 1032568 of size: 6968, ends @ 1039536
Atom mdhd @ 1032576 of size: 32, ends @ 1032608
Atom hdlr @ 1032608 of size: 45, ends @ 1032653
Atom minf @ 1032653 of size: 6883, ends @ 1039536
Atom smhd @ 1032661 of size: 16, ends @ 1032677
Atom dinf @ 1032677 of size: 36, ends @ 1032713
Atom dref @ 1032685 of size: 28, ends @ 1032713
Atom stbl @ 1032713 of size: 6823, ends @ 1039536
Atom stsd @ 1032721 of size: 103, ends @ 1032824
Atom mp4a @ 1032737 of size: 87, ends @ 1032824
Atom esds @ 1032773 of size: 51, ends @ 1032824
Atom stts @ 1032824 of size: 24, ends @ 1032848
Atom stsc @ 1032848 of size: 2524, ends @ 1035372
Atom stsz @ 1035372 of size: 2644, ends @ 1038016
Atom stco @ 1038016 of size: 1520, ends @ 1039536
Atom udta @ 1039536 of size: 97, ends @ 1039633
Atom meta @ 1039544 of size: 89, ends @ 1039633
Atom hdlr @ 1039556 of size: 33, ends @ 1039589
Atom ilst @ 1039589 of size: 44, ends @ 1039633
Atom ?too @ 1039597 of size: 36, ends @ 1039633
Atom data @ 1039605 of size: 28, ends @ 1039633
~ denotes an unknown atom
------------------------------------------------------
Total size: 1039633 bytes; 51 atoms total. AtomicParsley version: 0.9.0 (utf16)
Media data: 1025668 bytes; 13965 bytes all other atoms (1.343% atom overhead).
Total free atom space: 8 bytes; 0.001% waste.
------------------------------------------------------
修改后：
Atom ftyp @ 0 of size: 32, ends @ 32
Atom moov @ 32 of size: 13925, ends @ 13957
Atom mvhd @ 40 of size: 108, ends @ 148
Atom trak @ 148 of size: 6608, ends @ 6756
Atom tkhd @ 156 of size: 92, ends @ 248
Atom edts @ 248 of size: 36, ends @ 284
Atom elst @ 256 of size: 28, ends @ 284
Atom mdia @ 284 of size: 6472, ends @ 6756
Atom mdhd @ 292 of size: 32, ends @ 324
Atom hdlr @ 324 of size: 45, ends @ 369
Atom minf @ 369 of size: 6387, ends @ 6756
Atom vmhd @ 377 of size: 20, ends @ 397
Atom dinf @ 397 of size: 36, ends @ 433
Atom dref @ 405 of size: 28, ends @ 433
Atom stbl @ 433 of size: 6323, ends @ 6756
Atom stsd @ 441 of size: 167, ends @ 608
Atom avc1 @ 457 of size: 151, ends @ 608
Atom avcC @ 543 of size: 49, ends @ 592
Atom pasp @ 592 of size: 16, ends @ 608
~
Atom stts @ 608 of size: 24, ends @ 632
Atom stss @ 632 of size: 24, ends @ 656
Atom ctts @ 656 of size: 3008, ends @ 3664
Atom stsc @ 3664 of size: 40, ends @ 3704
Atom stsz @ 3704 of size: 1532, ends @ 5236
Atom stco @ 5236 of size: 1520, ends @ 6756
Atom trak @ 6756 of size: 7104, ends @ 13860
Atom tkhd @ 6764 of size: 92, ends @ 6856
Atom edts @ 6856 of size: 36, ends @ 6892
Atom elst @ 6864 of size: 28, ends @ 6892
Atom mdia @ 6892 of size: 6968, ends @ 13860
Atom mdhd @ 6900 of size: 32, ends @ 6932
Atom hdlr @ 6932 of size: 45, ends @ 6977
Atom minf @ 6977 of size: 6883, ends @ 13860
Atom smhd @ 6985 of size: 16, ends @ 7001
Atom dinf @ 7001 of size: 36, ends @ 7037
Atom dref @ 7009 of size: 28, ends @ 7037
Atom stbl @ 7037 of size: 6823, ends @ 13860
Atom stsd @ 7045 of size: 103, ends @ 7148
Atom mp4a @ 7061 of size: 87, ends @ 7148
Atom esds @ 7097 of size: 51, ends @ 7148
Atom stts @ 7148 of size: 24, ends @ 7172
Atom stsc @ 7172 of size: 2524, ends @ 9696
Atom stsz @ 9696 of size: 2644, ends @ 12340
Atom stco @ 12340 of size: 1520, ends @ 13860
Atom udta @ 13860 of size: 97, ends @ 13957
Atom meta @ 13868 of size: 89, ends @ 13957
Atom hdlr @ 13880 of size: 33, ends @ 13913
Atom ilst @ 13913 of size: 44, ends @ 13957
Atom ?too @ 13921 of size: 36, ends @ 13957
Atom data @ 13929 of size: 28, ends @ 13957
Atom free @ 13957 of size: 8, ends @ 13965
Atom mdat @ 13965 of size: 1025668, ends @ 1039633
~ denotes an unknown atom
------------------------------------------------------
Total size: 1039633 bytes; 51 atoms total. AtomicParsley version: 0.9.0 (utf16)
Media data: 1025668 bytes; 13965 bytes all other atoms (1.343% atom overhead).
Total free atom space: 8 bytes; 0.001% waste. Padding available: 8 bytes.
------------------------------------------------------
- 客户端动态加载BOX
对于自有内容网站来说，对自身视频库进行转码并不麻烦。但是对于第三方公司来说，可能无法去对甲方庞大的视频库进行处理，更理想的方式，是从客户端及时获取moov信息并实现播放。
//TODO
参考文章：
- http://blog.zhourunsheng.com/2012/05/android-视频播放之流媒体格式处理/
- http://stackoverflow.com/questions/19803458/how-to-download-only-the-moov-atom-from-mp4-file