Video Capture under Windows 2000 Professional is based on the WDM Stream class driver. Windows 2000 Professional provides minidrivers for USB and IEEE 1394 cameras, as well as PCI and videoport analog video devices. Support includes DirectShow filters for WDM video capture interfaces, and for compatibility with previous interface versions, a Video for Windows (VFW)-to-WDM mapper. The mapper, also called the VFWWDM mapper, allows WDM video capture devices to take advantage of existing 32-bit VFW applications, using the AVICap interfaces.
Capturing video with WDM has the following advantages:
Compatibility with Windows 98
Synergy with Microsoft DirectShow and Connection and Streaming Architecture
Single class driver architecture for hardware (such as video ports and chip sets) that is shared between video capture devices and DVD or MPEG devices
Television tuner, input selection, and support for fields, vertical blanking interval (VBI), and video port extensions
Capture applications have been developed using both DirectShow and VFW. A sample DirectShow capture application (Amcap.exe) is included in the DirectShow Software Development Kit.
Vidcap32.exe is a sample Video for Windows (VFW) capture application included in the Win32 SDK. It allows you to capture video sequences and images from a VCR, videodisc player, or video camera. Video Capture provides two modes for capturing video sequences:
Using Real-Time Capture
Real-time capture processes a video sequence and audio as the events occur naturally or as the video source plays without interruption. A video source for real-time capture (such as a video camera or videodisc) provides an uninterrupted stream of information to the capture hardware. The capture hardware copies each frame of the video sequence (and each portion of audio) and transfers it to the hard disk before the next frame of data enters the capture hardware. A video frame contains one image of the video sequence.
Real-time capture demands a fast computer and hard disk. The computer must process and store each incoming video frame before the next frame is received in the capture board. If the system lags during capture, frames of video data are lost.
Using Step-Frame Capture
Step-frame capture pauses the video source as it collects each frame (image) of data. If audio is also selected, this capture mode rewinds the media in the video source and collects audio data as the video source plays a second time. Step-frame capture collects video frames from a video sequence in a series of steps. Frames are captured one at a time, generally from a paused video device. You can perform step-frame capture manually, advancing the video source using the controls on the video device. Video Capture also provides automatic step-frame capture for video devices that support the Media Control Interface. With this method, Video Capture issues frame-advance commands to the source device and captures the sequence frame-by-frame. When Video Capture finishes capturing the current frame, it advances the video source to the next capture point.
Step-frame capture provides an alternative for systems that cannot process a video sequence in real time. Because the system can fully process a video frame before contending with the next frame, you can use larger frame sizes and color formats, and you can compress the video sequence during capture. When a step-frame capture is complete, you can capture the audio segment associated with the video frames by playing the source video a second time.