你真的懂 MP4 格式吗？ (3)

遍历累加下表所示 stts 所有项目，计算得到 3640000 位于第 110 个 Sample。

type stts size 224 flags 0 version 0 sample_counts 1,10,1,1,11,1,1,2,1,25,1,1,1,17,1,10,1,1,1,7,1,1,1,1,10,1 sample_deltas 33362,33363,33362,33364,33363,33362,33364,33363,33362,33363,33362,33364,33362,33363,33362,33363,33362,33364,33362,33363,33362,33364,33363,33362,33363,0

查询下表所示 stsc 所有项目，计算得到第 110 个 Sample 位于第 109 个 Chunk，并且在该 Chunk 中位于第 2 个 Sample。

type stsc size 40 flags 0 version 0 first_chunk 1,109 samples_per_chunk 1,2 sample_description_index 1,1

查询下表所示 stco 所有项目，得到第 109 个 Chunk 在文件中偏移位置为 1710064。

Property name Property value type stco size 452 flags 0 version 0 chunk_offsets 4750,47007,54865,61967,75519,88424,105222,117892,133730,149529,165568,182034,194595,210776,225470,240756,255358,270711,285459,300135,315217,330899,347372,363196,376409,394509,407767,424615,438037,455603,469784,487287,505197,519638,536714,553893,567187,584744,599907,615298,630669,645918,662605,678655,693510,708980,724061,738946,754170,771520,787233,800847,816997,832490,847814,862559,877929,898379,911054,925810,943883,956497,974403,991527,1009478,1025198,1041806,1062609,1078401,1091360,1105142,1118748,1132815,1145281,1156966,1171871,1186742,1202760,1218235,1236688,1249330,1263163,1280880,1297903,1313162,1332885,1345726,1359017,1376283,1391401,1405512,1419550,1433644,1452103,1475241,1492689,1511291,1522606,1535368,1559413,1575331,1588853,1609829,1626623,1642798,1658640,1674160,1693972,1710064

查询下表所示 stsz 所有项目，得到第 109 个 Sample 的 size 为 14808。计算得到 3.64 秒视频 Sample 数据在文件中：

offset：1710064 + 14808 = 1724872
size：17930

type stsz size 460 flags 0 version 0 sample_sizes 42072,7354,6858,13110,12684,16416,12490,15497,15630,15865,16116,12387,15775,14519,14929,14433,15181,14390,14496,14717,15507,16101,15643,12843,17911,13070,16455,13221,17186,14002,17139,17737,14251,16708,16999,12911,17356,14801,15213,15016,15062,16505,15689,14657,15053,14907,14527,15048,17161,15308,13432,15777,15307,14971,14568,14987,20264,12494,14382,17873,12235,17718,16770,17766,15366,16420,20623,15403,12761,13394,13390,13714,12295,11505,14541,14689,15635,15291,18091,12458,13645,17346,16847,14902,19530,12446,13105,16872,14937,13944,13657,13908,18092,22959,17080,18421,11129,12400,23844,15564,13340,20603,16609,15984,15474,15339,19451,15719,14808,17930 sample_size 0 sample_count 110

验证：用编辑器打开 MP4 文件，定位到文件偏移 offset = 1724872 的位置，前 4 字节值为 0x00004606。在 avcc 中一个 Sample 的前 4 个字节代表这个包的大小，转换为十进制是 17926，该值正好等于 size = 17930 减去表示长度的四个字节。

参考资料

在线 MP4 解析工具

「视频云技术」你最值得关注的音视频技术公众号，每周推送来自阿里云一线的实践技术文章，在这里与音视频领域一流工程师交流切磋。