多媒体文件格式：FLV 格式
Posted renhui
tags:
篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了多媒体文件格式：FLV 格式相关的知识，希望对你有一定的参考价值。
在网络的直播与点播场景中，FLV也是一种常见的格式，FLV是Adobe发布的一种可以作为直播也可以作为点播的封装格式，其封装格式非常简单，均以FLVTAG的形式存在，并且每一个TAG都是独立存在的，接下来就详细介绍一下FLV标准。
一、FLV 格式标准介绍
FLV包括文件头（File Header）和文件体（File Body）两部分，其中文件体由一系列的Tag组成。FLV文件的结构如下图：
1. 文件头 Header
Header 部分记录了FLV的类型、版本等信息，是FLV的开头。一般差不多占9bytes。具体格式如下：
1. 文件标识（3B）：总是为”FLV”, 0x46 0x4c 0x56
2. 版本（1B）：目前为0x01
3. 流信息（1B）：文件的标志位说明。前5位保留，必须为0；第6位为音频Tag：1表示有音频；第七位保留，为0； 第8位为视频Tag：1表示有视频
4. Header长度（4B）：整个Header的长度，一般为9（版本为0x01时）；大于9表示下面还有扩展信息。即0x00000009。
下图是使用工具FlvAnalyzer获取到的FLV的Header的详细信息：
2. 文件体 FLV Body
文件体由一系列的Tag组成。
其中，每个Tag前面还包含了Previous Tag Size字段，表示前面一个Tag的大小。Tag的类型可以是视频、音频和Script，每个Tag只能包含以上三种类型的数据中的一种。
下图是使用FlvAnalyzer获取到的Body信息：
3. Tag
每个Tag由也是由两部分组成的：Tag Header和Tag Data。Tag Header里存放的是当前Tag的类型、数据区（Tag Data）长度等信息，具体如下：
Tag类型（1）：0x08：音频; 0x09：视频; 0x12：脚本; 其他：保留
数据区长度（3）：数据区的长度
时间戳（3）：整数，单位是毫秒。对于脚本型的tag总是0 （CTS）
时间戳扩展（1）：将时间戳扩展为4bytes，代表高8位。很少用到
StreamsID（3）：总是0
数据区（由数据区长度决定）：数据实体
下面是三个Tag类型说明：
-
Audio Tag Data结构（音频类型） ：音频Tag Data区域开始的第一个字节包含了音频数据的参数信息，从第二个字节开始为音频流数据。
- video Tag Data结构(视频类型)：视频Tag Data开始的第一个字节包含视频数据的参数信息，从第二个字节开始为视频流数据。
-
Script Tag Data结构（脚本类型、帧类型）：该类型Tag又被称为MetaData Tag,存放一些关于FLV视频和音频的元信息，比如：duration、width、height等。通常该类型Tag会作为FLV文件的第一个tag，并且只有一个，跟在File Header后。
二、FLV 分析工具
在上节的内容中，我们介绍了FLV的格式信息，同时也提到了FlvAnalyzer工具，下面我们就介绍两个工具，帮助大家整理和学习FLV相关知识：
1. FlvAnalyzer
通过FlvAnalyzer可以很清晰的看到FLV文件的基本结构，这样能够结合上面了解的FLV的知识，更清晰的查看FLV的格式及结构。
工具地址：https://github.com/renhui/Thinking-in-AV/blob/master/多媒体格式/FLV/FlvAnalyzer.exe
工具使用如图：
左侧树状结构显示flv的信息，可以清楚了解flv文件的结构；
点击左侧节点，右侧显示对应hex与ascii信息，这样就不必打开二进制编辑器了;
通过此工具可以查看audio tag与video tag各个字节（精确到bit）的详细信息，了解每个tag是如何构造的，同时右下角黑色输出框显示某个值的意义;
2. FLV Format Analysis 工具
此工具是雷霄骅整理flvparse的开源代码，制作的flvformatanalysis工具，此工具可以用来帮助学习FLV封装格式结构。此外它还支持分离FLV中的视频流和音频流。
工具地址：https://github.com/renhui/Thinking-in-AV/blob/master/多媒体格式/FLV/SpecialFFLV.exe
工具使用如图：
三、FLV格式 与 FFmpeg 实战
1. 使用FFmpeg生成带关键索引信息的FLV
在网络视频点播文件为FLV格式文件时，人们经常用工具先对FLV文件进行一次转换，主要是将FLV文件中的关键帧建立一个索引，并将索引写到Metadata头中，这个步骤用FFmpeg同样也可以实现，使用参数add_keyframe_index即可：
ffmpeg -i 好汉歌.mp4 -c copy -f flv -flvflags add_keyframe_index out.flv
生成FLV包含了关键帧索引信息，这些关键帧索引信息并不是FLV的标准字段，但是由于其被广泛使用，已经成为常用的字段，所以FFmpeg也同样支持了这个功能。
2. 使用ffprobe查看FLV关键帧索引相关信息
除了在第二节介绍的两个工具，我们也可以使用ffprobe来解析FLV文件，并且还能将关键帧索引的相关信息打印出来，命令如下：
ffprobe -v trace -i out.flv
输出如下：
[NULL @ 0x7fc669002a00] Opening ‘out.flv‘ for reading [file @ 0x7fc667f00480] Setting default whitelist ‘file,crypto‘ Probing flv score:100 size:2048 Probing mp3 score:1 size:2048 [flv @ 0x7fc669002a00] Format flv probed with size=2048 and score=100 [flv @ 0x7fc669002a00] Before avformat_find_stream_info() pos: 13 bytes read:32768 seeks:0 nb_streams:0 [flv @ 0x7fc669002a00] type:18, size:1184, last:-1, dts:0 pos:21 [flv @ 0x7fc669002a00] keyframe stream hasn‘t been created [flv @ 0x7fc669002a00] type:9, size:45, last:-1, dts:0 pos:1220 [flv @ 0x7fc669002a00] keyframe filepositions = 1296 times = 0 [flv @ 0x7fc669002a00] keyframe filepositions = 159283 times = 3000 [flv @ 0x7fc669002a00] keyframe filepositions = 258004 times = 4000 [flv @ 0x7fc669002a00] keyframe filepositions = 272776 times = 4000 [flv @ 0x7fc669002a00] keyframe filepositions = 405340 times = 6000 [flv @ 0x7fc669002a00] keyframe filepositions = 1215104 times = 16000 [flv @ 0x7fc669002a00] keyframe filepositions = 2529035 times = 26000 [flv @ 0x7fc669002a00] keyframe filepositions = 3198814 times = 36000 [flv @ 0x7fc669002a00] keyframe filepositions = 3623757 times = 41000 [flv @ 0x7fc669002a00] keyframe filepositions = 4882191 times = 51000 [flv @ 0x7fc669002a00] keyframe filepositions = 5951597 times = 61000 [flv @ 0x7fc669002a00] keyframe filepositions = 6256906 times = 63000 [flv @ 0x7fc669002a00] keyframe filepositions = 7235927 times = 73000 [flv @ 0x7fc669002a00] keyframe filepositions = 8175324 times = 83000 [flv @ 0x7fc669002a00] keyframe filepositions = 9203399 times = 93000 [flv @ 0x7fc669002a00] keyframe filepositions = 9936528 times = 103000 [flv @ 0x7fc669002a00] keyframe filepositions = 11056393 times = 113000 [flv @ 0x7fc669002a00] keyframe filepositions = 12183978 times = 123000 [flv @ 0x7fc669002a00] keyframe filepositions = 13014068 times = 133000 [flv @ 0x7fc669002a00] keyframe filepositions = 13610750 times = 143000 [flv @ 0x7fc669002a00] keyframe filepositions = 14628601 times = 153000 [flv @ 0x7fc669002a00] keyframe filepositions = 15873046 times = 163000 [flv @ 0x7fc669002a00] keyframe filepositions = 17112198 times = 173000 [flv @ 0x7fc669002a00] keyframe filepositions = 18301365 times = 182000 [flv @ 0x7fc669002a00] keyframe filepositions = 18604436 times = 186000 [flv @ 0x7fc669002a00] 0 17 0 [flv @ 0x7fc669002a00] type:8, size:9, last:-1, dts:0 pos:1280 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:2117, last:-1, dts:0 pos:1304 [flv @ 0x7fc669002a00] 0 17 0 [NULL @ 0x7fc668809e00] nal_unit_type: 7, nal_ref_idc: 3 [NULL @ 0x7fc668809e00] nal_unit_type: 8, nal_ref_idc: 3 [NULL @ 0x7fc668809e00] user data:"x264 - core 142 r2 dd79a61 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=8 deblock=1:-1:-1 analyse=0x1:0x131 me=umh subme=9 psy=1 psy_rd=1.00:0.15 mixed_ref=1 me_range=24 chroma_me=1 trellis=2 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=0 chroma_qp_offset=-3 threads=24 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 stitchable=1 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=2 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=60 rc=2pass mbtree=1 bitrate=680 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 cplxblur=20.0 qblur=0.5 ip_ratio=1.40 aq=1:1.00" [h264 @ 0x7fc668809e00] nal_unit_type: 7, nal_ref_idc: 3 [h264 @ 0x7fc668809e00] nal_unit_type: 8, nal_ref_idc: 3 [h264 @ 0x7fc668809e00] nal_unit_type: 6, nal_ref_idc: 0 [h264 @ 0x7fc668809e00] nal_unit_type: 5, nal_ref_idc: 3 [h264 @ 0x7fc668809e00] user data:"x264 - core 142 r2 dd79a61 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=8 deblock=1:-1:-1 analyse=0x1:0x131 me=umh subme=9 psy=1 psy_rd=1.00:0.15 mixed_ref=1 me_range=24 chroma_me=1 trellis=2 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=0 chroma_qp_offset=-3 threads=24 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 stitchable=1 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=2 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=60 rc=2pass mbtree=1 bitrate=680 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 cplxblur=20.0 qblur=0.5 ip_ratio=1.40 aq=1:1.00" [h264 @ 0x7fc668809e00] Reinit context to 576x432, pix_fmt: yuv420p [h264 @ 0x7fc668809e00] no picture [flv @ 0x7fc669002a00] type:9, size:1653, last:-1, dts:40 pos:3436 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:9, size:323, last:-1, dts:80 pos:5104 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:41, last:-1, dts:80 pos:5442 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:103, last:-1, dts:120 pos:5498 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:314, last:-1, dts:126 pos:5616 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:156, last:-1, dts:160 pos:5945 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:634, last:-1, dts:173 pos:6116 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:751, last:-1, dts:200 pos:6765 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:636, last:-1, dts:219 pos:7531 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:208, last:-1, dts:240 pos:8182 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:266 pos:8405 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:134, last:-1, dts:280 pos:9045 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:607, last:-1, dts:312 pos:9194 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:73, last:-1, dts:320 pos:9816 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:599, last:-1, dts:359 pos:9904 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:358, last:-1, dts:360 pos:10518 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:9, size:108, last:-1, dts:400 pos:10891 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:628, last:-1, dts:405 pos:11014 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:61, last:-1, dts:440 pos:11657 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:614, last:-1, dts:452 pos:11733 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:53, last:-1, dts:480 pos:12362 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:630, last:-1, dts:498 pos:12430 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:287, last:-1, dts:520 pos:13075 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:544 pos:13377 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:47, last:-1, dts:560 pos:14017 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:622, last:-1, dts:591 pos:14079 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:79, last:-1, dts:600 pos:14716 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:637 pos:14810 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:324, last:-1, dts:640 pos:15450 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:9, size:55, last:-1, dts:680 pos:15789 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:605, last:-1, dts:684 pos:15859 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:67, last:-1, dts:720 pos:16479 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:613, last:-1, dts:730 pos:16561 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:350, last:-1, dts:760 pos:17189 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:604, last:-1, dts:777 pos:17554 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:89, last:-1, dts:800 pos:18173 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:620, last:-1, dts:823 pos:18277 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:68, last:-1, dts:840 pos:18912 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:601, last:-1, dts:869 pos:18995 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:880 pos:19611 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:603, last:-1, dts:916 pos:19684 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:307, last:-1, dts:920 pos:20302 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:9, size:89, last:-1, dts:960 pos:20624 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:596, last:-1, dts:962 pos:20728 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:64, last:-1, dts:1000 pos:21339 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:1009 pos:21418 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:1040 pos:22058 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:604, last:-1, dts:1055 pos:22131 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:333, last:-1, dts:1080 pos:22750 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:598, last:-1, dts:1102 pos:23098 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:94, last:-1, dts:1120 pos:23711 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:605, last:-1, dts:1148 pos:23820 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:1160 pos:24440 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:613, last:-1, dts:1195 pos:24513 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:1200 pos:25141 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:9, size:352, last:-1, dts:1240 pos:25214 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:607, last:-1, dts:1241 pos:25581 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:129, last:-1, dts:1280 pos:26203 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:620, last:-1, dts:1287 pos:26347 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:81, last:-1, dts:1320 pos:26982 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:604, last:-1, dts:1334 pos:27078 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:57, last:-1, dts:1360 pos:27697 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:618, last:-1, dts:1380 pos:27769 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:350, last:-1, dts:1400 pos:28402 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:609, last:-1, dts:1427 pos:28767 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:83, last:-1, dts:1440 pos:29391 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:626, last:-1, dts:1473 pos:29489 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:44, last:-1, dts:1480 pos:30130 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:9, size:86, last:-1, dts:1520 pos:30189 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:608, last:-1, dts:1520 pos:30290 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:298, last:-1, dts:1560 pos:30913 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] type:8, size:629, last:-1, dts:1566 pos:31226 [flv @ 0x7fc669002a00] 1 AF 0 [flv @ 0x7fc669002a00] type:9, size:88, last:-1, dts:1600 pos:31870 [flv @ 0x7fc669002a00] 0 27 0 [flv @ 0x7fc669002a00] All info found [flv @ 0x7fc669002a00] stream 0: start_time: 0.080 duration: -9223372036854776.000 [flv @ 0x7fc669002a00] stream 1: start_time: 0.080 duration: -9223372036854776.000 [flv @ 0x7fc669002a00] format: start_time: 0.080 duration: 189.440 bitrate=787 kb/s [flv @ 0x7fc669002a00] After avformat_find_stream_info() pos: 31965 bytes read:32768 seeks:0 frames:74 Input #0, flv, from ‘out.flv‘: Metadata: major_brand : isom minor_version : 512 compatible_brands: isomiso2avc1mp41 artist : yinyuetai.com album : Yinyuetai date : 04/01/15 15:51:32 comment : Yinyuetai-1TR1026 encoder : Lavf57.83.100 hasVideo : true hasKeyframes : true hasAudio : true hasMetadata : true canSeekToEnd : true datasize : 18639072 videosize : 16303552 audiosize : 2335015 lasttimestamp : 189 lastkeyframetimestamp: 187 lastkeyframelocation: 18603951 Duration: 00:03:09.44, start: 0.080000, bitrate: 787 kb/s Stream #0:0, 41, 1/1000: Video: h264 (Main), 1 reference frame, yuv420p(progressive, left), 576x432, 0/1, 684 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc Stream #0:1, 33, 1/1000: Audio: aac (HE-AAC), 44100 Hz, stereo, fltp, 95 kb/s [h264 @ 0x7fc668824400] nal_unit_type: 7, nal_ref_idc: 3 [h264 @ 0x7fc668824400] nal_unit_type: 8, nal_ref_idc: 3 [AVIOContext @ 0x7fc667f005c0] Statistics: 32768 bytes read, 0 seeks
从以上内容可以看到，输出信息包含了keyframe关键帧存储在文件中的偏移位置及时间戳。
以上是关于多媒体文件格式：FLV 格式的主要内容，如果未能解决你的问题，请参考以下文章