Inside Macintosh: QuickTime Components
Sequence grabber components provide callback functions that allow application developers to customize some aspects of capturing video data. It is your channel component's responsibility to call these callback functions at specified points in the data capture process. The application's function can then perform any special processing that is appropriate for the application. For example, an application can overlay text, such as a frame number, on each frame of video data as it is captured. These functions are discussed in detail in the next section.
Sound channel components do not support any callback functions.
Sequence grabber components allow application developers to define a number of callback functions in their applications. Your channel component calls these functions at specific points in the process of collecting, compressing, and displaying the source visual data. By defining callback functions, a developer can control the process more precisely or customize the operation of the sequence grabber component and its channel components.
For example, a developer could use a callback function to draw a frame number on each video frame as it is collected. In this case, the developer could use either a compress callback function or a grab-complete callback function. You call the compress function after each frame is collected, in order to compress the frame. You call the grab-complete function just before the compress function or as soon as the frame has been captured.
Note that your channel component need not call each and every callback function. If some functions are inappropriate to the operation of your channel, do not call them. However, if your component calls one function of a pair, be sure to call the other. For example, if your component calls an application's grab function, you must also call its grab-complete function.
The sequence grabber component uses the SGSetVideoBottlenecks function to assign callback functions to your video channel. The SGGetVideoBottlenecks function allows the sequence grabber to determine the callback functions that have been assigned to your video channel. See the chapter "Sequence Grabber Components" in this book for details on SGSetVideoBottlenecks and SGGetVideoBottlenecks .
The following application-defined functions are supported by video channels and are described in the chapter "Sequence Grabber Components" in this book.
Sequence grabber components provide a number of functions that application-defined functions can use. Several channel functions support those sequence grabber component functions.
The sequence grabber component uses the SGGetBufferInfo function to obtain information about a buffer that contains data to be manipulated by a callback function. Application callback functions can use the SGGetBufferInfo function to obtain information about a buffer that you have passed. This information is valid only during record operations, or after your channel has been prepared to record. The SGGetBufferInfo function is described in detail in the chapter "Sequence Grabber Components" in this book.
The following functions provide default behavior for application-defined grab, grab-complete, display, compress, compress-complete, add-frame, transfer-frame, display-compress, and grab-compress-complete functions: