QuickTime 4 Reference

Asynchronous Decompression

In QuickTime 2.0 the Image Compression Manager (ICM) was enhanced to support scheduled asynchronous decompression operations. By calling the Image Compression Manager function DecompressSequenceFrameWhen (page 203) , applications can schedule decompression requests in advance. This allows decompressor components that support this functionality to provide reliable playback performance under a wider range of conditions.

The Apple Cinepak, Video, Animation, Component Video, and Graphics decompressors provided in QuickTime versions 2.0 and later support scheduled asynchronous decompression to 8-, 16-, and 32-bit destinations (the Cinepak decompressor also supports 4-bit grayscale destinations). QuickTime 2.5 added asynchronous decompression support to the JPEG and None decompressor components on PowerPC systems (with the QuickTime PowerPlug extension installed).

If you want to support this functionality, you must modify your decompressor component in the following ways:

Report your component's new capabilities in its compressor capability structure by setting the codecCanAsyncWhen and codecCanAsync flags.
Modify your component's ImageCodecFlush function to accept scheduled asynchronous decompression requests and process them correctly.
Implement the new function ImageCodecFlush ; this function allows the Image Compression Manager to instruct you to empty your input queue.
Optionally, implement logic to manage the shielding of the cursor during decompression operations.

All of these changes are discussed in detail in "Image Compressor Components Reference" .