精通Directshow编程：打造无乱码视频体验

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"微软的Directshow编程技术是用于处理数字视频和电视内容的关键工具，尤其在实现无乱码播放方面有显著优势。本书由Mark D. Pesce编写，详细介绍了如何利用Directshow进行高效且质量高的视频处理。" 在数字媒体领域，微软的Directshow是一个强大的多媒体框架，它为开发人员提供了构建高质量音频和视频应用的基础设施。Directshow主要设计用于处理流式媒体，如在线视频、电视广播和录制的媒体文件。这个框架包含了各种组件和服务，能够实现媒体的捕获、编辑、播放和过滤，确保了在多种编码格式下的流畅播放，有效地避免了乱码问题。无乱码播放是Directshow的一个核心特点，这得益于其对多种编码标准和容器格式的支持，包括MPEG、WMV、AVI等。通过使用Directshow，开发者可以创建应用程序，自动识别并正确解码不同的视频和音频流，从而提供无缝的用户体验。此外，Directshow的滤波器系统允许自定义处理链，以适应不同环境和用户需求，进一步优化性能和兼容性。《Programming Microsoft DirectShow for Digital Video and Television》这本书深入探讨了Directshow的各个方面，包括基本概念、滤波器图的构建、编程接口以及如何解决常见的播放问题。作者Mark Pesce是一位经验丰富的专家，他分享了如何利用Directshow的技术来开发高效、稳定的多媒体应用。书中包含的实例代码和实战技巧对于程序员来说是非常宝贵的资源。该书还强调了版权保护，指出未经出版商书面许可，不得复制或传播书中的内容。这表明Directshow的使用和开发也必须遵循相关的法律法规，尊重知识产权。了解和掌握Directshow编程对于在Windows平台上进行多媒体应用开发的IT专业人员来说至关重要。通过学习本书，读者不仅可以学会如何避免视频播放中的乱码问题，还能掌握构建复杂多媒体系统的技能，从而在数字视频和电视领域中取得竞争优势。

DirectShow filters make computations and decisions internally—for instance, they can change the

values of bits in a stream—but they cannot make decisions that affect the structure of the filter

graph. A filter simply passes its data along to the next filter in the filter graph. It can’t decide to pass

its data to filter A if some condition is true or filter B if the condition is false. This means that the

behavior of a filter graph is completely predictable; the way it behaves when it first begins to operate

on a data stream is the way it will always operate.

Although filter graphs are entirely deterministic, it is possible to modify the elements within a filter

graph programmatically. A C++ program could create filter graph A if some condition is true and

filter graph B if it’s false. Or both could be created during program initialization (so that the

program could swap between filter graph A and filter graph B on the fly) as the requirements of the

application change. Program code can also be used to modify the individual filters within a filter

graph, an operation that can change the behavior of a filter graph either substantially or subtly. So,

although filter graphs can’t make decisions on how to process their data, program code can be used

to simulate that capability.

For example, consider a DirectShow application that can capture video data from one of several

different sources, say from a digital camcorder and a webcam. Once the video has been captured, it

gets encoded into a compact Windows Media file, which could then be dropped into an e-mail

message for video e-mail. Very different source and transform filters are used to capture and process

a video stream from a digital camcorder than those used with a webcam, so the same filter graph

won’t work for both devices. In this case, program logic within the application could detect which

input device is being used—perhaps based on a menu selection—and could then build the

appropriate filter graph. If the user changes the selection from one device to another, program logic

could rebuild the filter graph to suit the needs of the selected device.

Modular Design

The basic power and flexibility of DirectShow derives directly from its modular design. DirectShow

defines a standard set of Component Object Model (COM) interfaces for filters and leaves it up to

the programmer to arrange these components in some meaningful way. Filters hide their internal

operations; the programmer doesn’t need to understand or appreciate the internal complexities of the

Audio Video Interleaved (AVI) file format, for example, to create an AVI file from a video stream.

All that’s required is the appropriate sequence of filters in a filter graph. Filters are atomic objects

within DirectShow, meaning they reveal only as much of themselves as required to perform their

functions.

Because they are atomic objects, filters can be thought of and treated just like puzzle pieces. The

qualities that each filter possesses determine the shape of its puzzle piece, and that, in turn,

determines which other filters it can be connected to. As long as the pieces match up, they can be

fitted together into a larger scheme, the filter graph.

All DirectShow filters have some basic properties that define the essence of their modularity. Each

filter can establish connections with other filters and can negotiate the types of connections it’s

willing to accept from other filters. A filter designed to process MP3 audio doesn’t have to accept a

connection from a filter that produces AVI video—and probably shouldn’t. Each filter can receive

some basic messages—run, stop, and pause—that control the execution of the filter graph. That’s

about it; there’s not much more a filter needs to be ready to go. As long as the filter defines these

properties publicly through COM, DirectShow will treat it as a valid element in a filter graph.

This modularity makes designing custom DirectShow filters a straightforward process. The

programmer’s job is to design a COM object with the common interfaces for a DirectShow filter,

plus whatever custom processing the filter requires. A custom DirectShow filter might sound like a

complex affair, but it’s really a routine job, one that will be covered extensively in the examples in

Part III.

The modularity of DirectShow extends to the filter graph. Just as the internals of a filter can be

hidden from the programmer, the internals of a filter graph can be hidden from view. When the filter

graph is treated as a module, it can assume responsibility for connecting filters together in a

meaningful way. It’s possible to create a complete, complex filter graph by adding a source filter and

a renderer filter to the filter graph. These filters are then connected with a technique known as

Intelligent Connect. Intelligent Connect examines the filters in the filter graph, determines the right

way to connect them, adds any necessary conversion filters, and makes the connections—all without

any intervention from the programmer. Intelligent Connect can save you an enormous amount of

programming time because DirectShow does the tedious work of filter connection for you.

There is a price to be paid for this level of automation: the programmer won’t know exactly which

filters have been placed into the filter graph or how they’re connected. Some users will have installed

Filters

Filters are the basic units of DirectShow programs, the essential components of the filter graph. A

filter is an entity complete unto itself. Although a filter can have many different functions, it must

have some method to receive or transmit a stream of data. Each filter has at least one pin, which

provides a connection point from that filter to other filters in the filter graph. Pins come in two varie-

ties: input pins can receive a stream, while output pins produce a stream that can be sent along to

another filter.

Filter Types

There are three basic classes of DirectShow filters, which span the path from input, through

processing, to output (or, as it’s often referred to, rendering). All DirectShow filters fall into one of

these broad categories. A filter produces a stream of data, operates on that stream, or renders it to

some output device.

Source Filters

Any DirectShow filter that produces a stream is known as a source filter. The stream might

originate in a file on the hard disk, or it might come from a live device, such as a microphone,

webcam, or digital camcorder. If the stream comes from disk, it could be a pre-recorded WAV

(sound), AVI (movie), or Windows Media file. Alternately, if the source is a live device, it could be

any of the many thousands of Windows-compatible peripherals. DirectShow is closely tied in to the

Windows Driver Model (WDM), and all WDM drivers for installed multimedia devices are

automatically available to DirectShow as source filters. So, for example, webcams with properly

installed Windows drivers become immediately available for use as DirectShow source filters.

Source filters that translate live devices into DirectShow streams are known as capture source

filters. Chapter 12 covers the software design of a source filter in detail.

Transform Filters

Transform filters are where the interesting work gets done in DirectShow. A transform filter

receives an input stream from some other filter (possibly a source filter), performs some operation

on the stream, and then passes the stream along to another filter. Nearly any imaginable operation on

an audio or video stream is possible within a transform filter. A transform filter can parse (interpret)

a stream of data, encode it (perhaps converting WAV data to MP3 format) or decode it, or add a

text overlay to a video sequence. DirectShow includes a broad set of transform filters, such as filters

for encoding and decoding various types of video and audio formats.

Transform filters can also create a tee in the stream, which means that the input stream is duplicated

and placed on two (or more) output pins. Other transform filters take multiple streams as input and

multiplex them into a single stream. Using a transform filter multiplexer, separate audio and video

streams can be combined into a video stream with a soundtrack.

Renderer Filters

A renderer filter translates a DirectShow stream into some form of output. One basic renderer filter

can write a stream to a file on the disk. Other renderer filters can send audio streams to the speakers

or video streams to a window on the desktop. The Direct in DirectShow reflects the fact that

DirectShow renderer filters use DirectDraw and DirectSound, supporting technologies that allow

DirectShow to efficiently pass its renderer filter streams along to graphics and sound cards. This

ability means that DirectShow’s renderer filters are very fast and don’t get tied up in a lot of user-to-

kernel mode transitions. (In operating system parlance, this process means moving the data from an

unprivileged level in an operating system to a privileged one where it has access to the various

output devices.)

A filter graph can have multiple renderer filters. It is possible to put a video stream through a tee,

sending half of it to a renderer filter that writes it to a file, and sending the other half to another

renderer filter that puts it up on the display. Therefore, it is possible to monitor video operations

while they’re happening, even if they’re being recorded to disk—an important feature we’ll be using

later on.

The Complete Picture

All DirectShow filter graphs consist of combinations of these three types of filters, and every

DirectShow filter graph will have at least one source filter, one renderer filter, and (possibly) several

transform filters. In each filter graph, a source filter creates a stream that is then operated on by any

number of transform filters and is finally output through a renderer filter. These filters are connected

together through their pins, which provide a well-defined interface point for transfer of stream data

between filters.

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精通Directshow编程：打造无乱码视频体验

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