The Sequence Grabber

Sequence Grabber Components

Sequence Grabber components allow applications to obtain digitized data from external sources, such as video capture boards. The digitized data can be previewed, saved as a QuickTime movie, or both. Sequence grabber components allow applications to capture audio and video easily, without concern for the details of how the data is acquired.

These components rely on the services of lower-level components, called channel components, to obtain digitized data from different sources. For example, a single sequence grabber component could provide an application with video and audio data, using the services of different channel components for audio and video. The channel components, in turn, may rely on the services of still lower-level components, such as video digitizer components.

While the entire process of capturing digital data from different sources is quite complex a QuickTime developer is shielded from these complexities by using the higher-level Sequence Grabber API.

The diagram below shows the relationships among a QuickTime application, a sequence grabber component, and channel components.

Sequence grabber components are standard components that are managed by the Component Manager.

You can use sequence grabber components in two ways: to play digitized data for the user or to save captured data in a QuickTime movie. The process of displaying data that is to be captured is called previewing; saving captured data in a movie is called recording. You can use previewing to allow the user to prepare to make a recording. If you do so, your application can move directly from the preview operation to a record operation, without stopping the process.

Previewing

Previewing captured data involves playing the data for the user as it is captured. For video data, this means displaying the video images on the computer screen. For audio data, this means playing the sound through the computer's sound system.

Recording

During a record operation, a sequence grabber component collects the data it captures and formats that data into a QuickTime movie. During a record operation, the sequence grabber can also play the captured data for the user. However, the sequence grabber tries to prevent the playback from interfering with the quality of the recording process.

Channel Components

Sequence grabber channel components, also known simply as channel components are used by higher-level sequence grabber components, and act to isolate the sequence grabber from the details of working with actual data types. Channel components may, in turn, depend on the services of still lower-level components, such as video digitizer components. QuickTime application programmers use the services of a sequence grabber channel component but usually do not have to interact with them directly.

Panel Components

A sequence grabber panel component creates a settings dialog box, which has settings that affect the behavior of a channel component. For example, a dialog might let the user control the frame capture rate of a video digitizer and the image quality of an image compressor.

Sequence grabber panel components are never called directly by an application. QuickTime application developers indirectly use panel components by calling the sequence grabber component. The sequence grabber component uses a panel component to obtain user preferences for configuration of channel components.

References

Sequence Grabber

Sequence Grabber Channel Components

Sequence Grabber Panel Components

Video Digitizers