By PURPLELEC | 07 June 2024 | 0 Comments
DirectShow Technology of Streaming Media Video Capture Card
After the analog video signal output by the video signal source is digitized by the video capture card, an important concept involved in the transmission of digital video and sound is "stream media". Speaking of video capture/video acquisition processing, we must first look at the video capture card based on DirectShow. At present, the high-definition audio and video streaming acquisition card launched by Bodeyue supports DirectShow standard development and provides a complete secondary development package SDK. For the secondary development of multimedia video applications, it brings great convenience to video capture programming. DirectShow is a streaming media architecture on the Windows platform. DirectShow provides a high-quality capture and playback mechanism for multimedia streams. In particular, many software for streaming video (such as video conferencing, videophone, etc.) are developed and applied to video capture technology. Video capture and real-time processing are one of the most critical technologies in the current video processing system.
DirectShow simplifies media playback, format conversion and capture tasks. It is based on the control object model (COM). To write DirectShow applications, you must understand COM client programming. For most applications, there is no need to implement COM objects. DirectShow provides the controls you need. (If you want to extend the functionality of DirectShow by writing your own controls, you must implement them as COM objects.)
DirectShow uses a standardized structure. In the DirectShow standard structure, DirectShow filters interact with controls, various devices, local file systems, TV FM and video capture cards, VFW encoders, video display cards (through DirectDraw), and graphics cards. In order to complete the required audio and video stream processing, DirectShow applies DirectDraw and DirectSound technologies. These technologies can efficiently render data images and audio to the user's graphics card and sound card.
Streaming media DirectShow technology is mainly software application technology and development technology. COM technology is generally called COM component Filter in DirectShow technology. It is generally divided into three categories: source filter, transform filter, and rendering filter, which are used to process streaming media data. Filter has input and output pins, or both. Filter operates streaming media by reading and writing files, modifying data, and displaying data on output devices. In order to complete the entire task, all filters must be connected. These three filters form a filter graph manager. Source filters, conversion filters, and submission filters respectively complete data provision, data format conversion (compression encoding, etc.) and data rendering and playback functions. Therefore, in order to achieve video acquisition under the WIN32 system, we must first construct an appropriate filter graph, and then manage the filter graph through the application to complete the video acquisition function.
There are many software and programming languages for developing DirectShow technology. In general software development tools (such as VB, VC), you can directly call the MCI control interface) function or media playback control (WMP, etc.) under the windows system, and set the relevant file name and other properties to achieve video capture or playback. With the rapid development of multimedia technology and the application of various compression algorithms in this field, MCI technology is increasingly unable to cope with it. The most obvious thing is that it does not support variable bit rate compression algorithms, and it has become powerless to process DVD and other new media formats that have appeared in recent years.
Subsequently, Microsoft timely launched DirectShow technology based on DirectX. It is also an API (application programming interface). Each DirectX component is the sum of user-callable APIs, through which applications can directly access the computer's hardware. It supports decoding and playback of media files in various video and audio compression formats from local or network sources, can capture multimedia streams from devices, and can also process streaming media processed by various compression algorithms. These formats include: MPEG audio and video standards, audio and video interactive standards (AVI), WAVE, MIDI, and advanced streaming format ASF. The basic principle of DirectShow is that multimedia data flows in the filter graph, and functions are implemented through each filter in the filter graph, and finally the multimedia data is displayed and played back in the submitted filter.
At present, domestic research on such projects has been relatively mature, and they are mainly developed using C programming tools. Filters are generally used to complete various functions. Filters are a COM component within a process. DirectShow SDK provides a set of base class source codes, which can greatly reduce the difficulty of developing filters. DirectShow is located in the application layer. It uses the Filter Graph model to manage the entire data flow processing process. Each functional module involved in data processing is a Filter. Each Filter is connected in a certain order in the Filter Graph to form a "pipeline" to work together. The source filter is mainly responsible for obtaining data. The data source can be a file, the Internet, or a capture card or digital camera in the computer, and then transmit the data downward; the conversion filter is mainly responsible for data format conversion and transmission; the submission filter is mainly responsible for the final destination of the data. In the end, the data can be sent to the sound card or graphics card for multimedia presentation, or it can be output to a file for storage.
Like in China, many foreign software designers also use Filter to implement various functions. DirectShow was originally designed for C applications (C language can also be used). Using Microsoft's Visual Basic language, a large subset of the DirectShow API can be accessed, but support for VB is not complete. For example, using Microsoft's JScript, DirectShow also supports script-based DVD applications. It is also possible to write DirectShow applications using Borland Delphi, but Microsoft's software development kit (SDK) documentation does not have reference materials on this part.
The DirectShow streaming media architecture supports capture of digital and analog devices based on WDM (Windows Driver Model) and VFW (Video for Windows). DirectShow is integrated with other DirectX technologies. It supports multiple formats, including ASF (Advanced System Format), MPEG (Motion Picture Expert Group), AVI (Audio-Video Interleaved), MP3 (MPEG Audio Layer-3), and WAV sound files. Video capture in DirectShow describes any application that receives video from hardware. The captured video can be stored on disk or previewed in real time.
With the continuous development of multimedia technology, related software and hardware emerge in an endless stream. Now, software and hardware related to multimedia video conferencing (Video Conference) and video conversation are becoming hot applications that people pay attention to. And all of these applications involve the acquisition of video data. Traditional VFW video acquisition technology has many limitations and cannot be well applied to various current video systems. Although video image acquisition cards using VFW (Video for Windows) drivers are now outdated, DirectShow still supports them through VFW capture filters.
Nowadays, video capture cards generally use WDM drivers, so most new video capture devices use WDM (Windows Driver Model) drivers. In the WDM architecture, Microsoft provides a set of hardware-independent drivers called class drivers, and hardware vendors provide hardware-related mini drivers. The mini driver implements any device-related functions. By utilizing the good integration characteristics of DirectShow and WDM video capture cards and adopting the system architecture of the component object model, the shortcomings of traditional VFW video capture technology have been overcome, and satisfactory results have been achieved in practical applications.
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