Flash Video (FLV) 文件格式详解

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"FLV（Flash Video）是一种流行的网络视频格式，由Adobe Systems开发，主要用于在网页上播放视频内容。此格式被广泛应用于在线流媒体服务，如YouTube早期曾大量使用FLV格式。本文档详细介绍了FLV文件格式，包括其数据包结构和版本8的相关规范。" FLV（Flash Video）格式是Adobe Macromedia推出的一种流媒体容器格式，旨在为Web上的动态内容提供高效、低延迟的视频播放体验。自2007年起，随着Flash Player的广泛普及，FLV成为了互联网视频的主流格式之一。以下是对FLV格式的详细解说： 1. 文件头：FLV文件以一个5字节的文件头开始，包含文件类型标识符（'F', 'L', 'V', '1', '\x9'或'2'，分别代表版本1和2）、文件数据长度以及文件头长度。 2. 文件标签：FLV文件由一系列的数据标签组成，每个标签包含一个标签头和一个或多个数据块。标签头包含了标签类型（音频、视频或脚本数据）、数据长度和时间戳，这些信息用于播放和同步。 3. 视频数据：FLV支持两种主要的视频编码格式——Sorenson Spark（H.263变种）和VP6。视频数据块包含编码后的帧数据，以及可能的帧类型（关键帧、插帧或丢弃帧）信息。 4. 音频数据：音频数据支持MP3和ADPCM编码，每帧音频数据可以是单声道或立体声，采样率和位深也各不相同。FLV格式还允许在文件中嵌入AAC音频，通过扩展支持更多编码格式。 5. 脚本数据：脚本数据通常用于元信息、事件处理和交互控制，如视频的播放、暂停、进度条等。ActionScript 2.0和3.0可以通过这种数据类型在FLV中实现。 6. 文件尾：FLV文件结束时有一个文件尾，包含一个结束标记（'E'）和一个填充字节，用于确保文件长度的准确性。 7. 版本8的改进：FLV版本8引入了对H.264视频编码的支持，增强了视频质量，同时也增加了对AAC音频编码的原生支持。此外，还改进了时间戳的表示方式，以更好地处理不同类型的帧。 8. 编解码器配置：FLV文件中可以包含编解码器配置信息，允许客户端正确地解析和播放不同编码格式的视频和音频。 9. 性能优化：FLV格式的设计考虑了网络传输效率，如使用固定大小的标签头，减少了数据包的解析开销，提升了播放性能。 10. 应用场景：除了网页播放，FLV也被用作服务器端流媒体服务，例如RTMP协议就是基于FLV的实时传输协议，适用于直播和点播服务。尽管FLV格式在现代已经逐渐被其他格式如MP4和HLS取代，但它在互联网视频历史中的地位不容忽视，尤其是对于那些需要向后兼容旧版Flash Player的项目。了解FLV的内部结构和工作原理，对于网络视频开发者来说仍然具有重要的参考价值。

16 Introduction

Scriptability The format includes tags that provide sequences of byte codes to be

interpreted by a stack machine. The byte codes support the ActionScript language. Flash

Player provides a runtime ActionScript object model that allows interaction with drawing

primitives, servers, and features of Flash Player.

SWF files have the extension .swf and a MIME type of application/x-shockwave-flash.

The SWF format has evolved through several versions. Through SWF 5, substantial additions

were made to the SWF tag set. Starting with SWF 6 and later, the SWF format changes less,

as more new Flash features are implemented partly or entirely at the ActionScript level. For

this reason, anyone planning to generate SWF file content that uses newer features should

become familiar with the ActionScript object model that Flash Player exposes. The best

references for this information are Learning ActionScript 2.0 in Flash (see http://

livedocs.macromedia.com/flash/8/main/00001303.html) and Essential ActionScript 2.0 by

Colin Moock.

Macromedia seriously considers all feedback to the SWF format specification. In addition,

any known errors from previously released specifications are corrected in this document. If

you encounter unclear or potentially erroneous information within the specification, please

e-mail Macromedia at Flash_Format@macromedia.com, and Macromedia will investigate an

improvement to the document.

The SWF header

All SWF files begin with the following header. The types are defined in Basic Data Types:

SWF File Header

Field Type Comment

Signature UI8 Signature byte:

“F” indicates uncompressed

“C” indicates compressed (SWF 6 and later only)

Signature UI8 Signature byte always “W”

Signature UI8 Signature byte always “S”

Version UI8 Single byte file version (for example, 0x06 for SWF 6)

FileLength UI32 Length of entire file in bytes

FrameSize RECT Frame size in twips

FrameRate UI16 Frame delay in 8.8 fixed number of frames per second

FrameCount UI16 Total number of frames in file

Macromedia Flash (SWF) and Flash Video

(FLV) File Format Specification Version 8

To view the contents of this document, you must agree to the

License Agreement. Requires Adobe® Reader® or Adobe® Acrobat®

version 6.0 or above. 

SWF file structure 17

The header begins with a three-byte signature of either 0x46, 0x57, 0x53 (“FWS”); or 0x43,

0x57, 0x53 (“CWS”). An FWS signature indicates an uncompressed SWF file; CWS

indicates that the entire file after the first 8 bytes (that is, after the FileLength field) was

compressed by using the ZLIB open standard. The data format that the ZLIB library uses is

described by Request for Comments (RFCs) documents 1950 to 1952. CWS file compression

is permitted in SWF 6 or later only.

A one-byte version number follows the signature. The version number is not an ASCII

character, but an 8-bit number. For example, for SWF 4, the version byte is 0x04, not the

ASCII character “4” (0x35).

The FileLength field is the total length of the SWF file, including the header. If this is an

uncompressed SWF file (FWS signature), the FileLength field should exactly match the file

size. If this is a compressed SWF file (CWS signature), the FileLength field indicates the total

length of the file after decompression, and thus generally does not match the file size. Having

the uncompressed size available can make the decompression process more efficient.

The FrameSize field defines the width and height of the on-screen display. This field is stored

as a RECT structure, meaning that its size may vary according to the number of bits needed

to encode the coordinates. The FrameSize RECT always has Xmin and Ymin of 0; the Xmax

and Ymax members define the width and height (see Using bit values).

The FrameRate is the desired playback rate in frames per second. This rate is not guaranteed

if, for example, Flash Player is running on a slow or busy CPU.

The FrameCount is the total number of frames in this SWF file.

SWF file structure

Following the header is a series of tagged data blocks. All tags share a common format, so any

program parsing a SWF file can skip over blocks it does not understand. Data inside the block

can point to offsets within the block, but can never point to an offset in another block. This

ability enables tags to be removed, inserted, or modified by tools that process a SWF file.

The FileAttributes tag is only required for SWF 8 and later.

SWF File Structure

Header Tag Tag Tag End tag

...

FileAttributes

tag

Macromedia Flash (SWF) and Flash Video

(FLV) File Format Specification Version 8

To view the contents of this document, you must agree to the

License Agreement. Requires Adobe® Reader® or Adobe® Acrobat®

version 6.0 or above. 

The dictionary 19

Tag ordering in SWF files

Generally speaking, tags in a SWF can occur in any order. However, you must observe the

following rules:

■ The FileAttributes tag must be the first tag in the SWF file for SWF 8 and later.

■ A tag should only depend on tags that come before it. A tag should never depend on a tag

that comes later in the file.

■ A definition tag that defines a character must occur before any control tag that refers to

that character.

■ Streaming sound tags must be in order. Out-of-order streaming sound tags result in the

sound being played out of order.

■ The End tag is always the last tag in the SWF file.

The dictionary

The dictionary is a repository of characters that are defined, and are available for control tags

to use. The process of building and using the dictionary is as follows:

1. The definition tag defines some content, such as a shape, font, bitmap, or sound.

2. The definition tag assigns a unique CharacterId to the content.

3. The content is saved in the dictionary under the CharacterId.

4. A control tag uses the CharacterId to retrieve the content from the dictionary, and performs

some action on the content, such as displaying a shape, or playing a sound.

Every definition tag must specify a unique ID. Duplicate IDs are not allowed. Typically, the

first CharacterId is 1, the second CharacterId is 2, and so on. The number zero (0) is special

and is considered a null character.

Control tags are not the only tags that reference the dictionary. Definition tags can use

characters from the dictionary to define more complex characters. For example, the

DefineButton and DefineSprite tags refer to other characters to define their contents. The

DefineText tag can refer to font characters to select different fonts for the text.

Macromedia Flash (SWF) and Flash Video

(FLV) File Format Specification Version 8

To view the contents of this document, you must agree to the

License Agreement. Requires Adobe® Reader® or Adobe® Acrobat®

version 6.0 or above. 

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