FLV 数据封装格式

https://www.cnblogs.com/chyingp/p/flv-getting-started.html

https://blog.csdn.net/ai2000ai/article/details/77530741

从整个文件上开看,FLV是由 header 和 Body 组成.

1、Header

Signature: FLV 文件的前3个字节为固定的‘F’‘L’‘V’,用来标识这个文件是flv格式的.在做格式探测的时候，

　　　　　　　　如果发现前3个字节为“FLV”，就认为它是flv文件.

Version: 第4个字节表示flv版本号.

Flags: 第5个字节中的第0位和第2位,分别表示 video 与 audio 存在的情况.(1表示存在,0表示不存在)

DataOffset : 最后4个字节表示FLV header 长度.

2 FLV body = TagSize|Tag | TagSize | Tag... ...

FLV header之后,就是 FLV File Body.

FLV File Body是由一连串的back-pointers + tags构成.back-pointers就是4个字节数据,表示前一个tag的size.

3.FLV Tag

FLV文件中的数据都是由一个个TAG组成,TAG里面的数据可能是video、audio、scripts.

onMetaData 包含在scripts 中，含有十分重要的信息，比如宽高、duration、filesize 等

下表是TAG的结构:

1.FLVTAG

Field	Type	Comment
Reserved	UB [2]	Reserved for FMS, should be 0
Filter	UB [1]	Indicates if packets are filtered. 0 = No pre-processing required. 1 = Pre-processing (such as decryption) of the packet is required before it can be rendered. Shall be 0 in unencrypted files, and 1 for encrypted tags. See Annex F. FLV Encryption for the use of filters.
TagType	UB [5]	Type of contents in this tag. The following types are defined: 8 = audio 9 = video 18 = script data
DataSize	UI24	Length of the message. Number of bytes after StreamID to end of tag (Equal to length of the tag – 11)
Timestamp	UI24	Time in milliseconds at which the data in this tag applies. This value is relative to the first tag in the FLV file, which always has a timestamp of 0.
TimestampExtended	UI8	Extension of the Timestamp field to form a SI32 value. This field represents the upper 8 bits, while the previous Timestamp field represents the lower 24 bits of the time in milliseconds.
StreamID	UI24	Always 0.
AudioTagHeader	IF TagType == 8 AudioTagHeader
VideoTagHeader	IF TagType == 9 VideoTagHeader
EncryptionHeader	IF Filter == 1 EncryptionTagHeader
FilterParams	IF Filter == 1 FilterParams
Data	IF TagType == 8 AUDIODATA IF TagType == 9 VIDEODATA IF TagType == 18 SCRIPTDATA	Data specific for each media type.

TagType: TAG中第1个字节中的前5位表示这个TAG中包含数据的类型,8 = audio,9 = video,18 = script data.

DataSize:StreamID之后的数据长度.

Timestamp和TimestampExtended组成了这个TAG包数据的PTS信息,记得刚开始做FVL demux的时候，并没有考虑TimestampExtended的值,直接就把Timestamp默认为是PTS，后来发生的现象就是画面有跳帧的现象,后来才仔细看了一下文档发现真正数据的PTS是PTS= Timestamp | TimestampExtended<<24.

StreamID之后的数据就是每种格式的情况不一样了，接下格式进行详细的介绍.

Audio Tags

如果TAG包中的TagType==8时，就表示这个TAG是audio。

StreamID之后的数据就表示是AudioTagHeader，AudioTagHeader结构如下：