Inside Macintosh: QuickTime Components
Progress functions provide the calling application an indication of how much of an operation is complete and a way for the user to cancel an operation. If the progressProcRecord field is set either in the compression parameters structure or the decompression parameters structure, then the image compressor component should call the progress function as it performs the operation. The progress function is typically called once for each scan line or row of pixel blocks processed, and it returns a completion value that is the percentage of the band that is complete, represented as a fixed-point number from 0 to 1.0.
If the result returned from a progress function is not 0, then the image compressor component should return as soon as possible (without completing the band that is being processed) with a return value of codecAbortErr .
For efficiency, many image compressor components have a streamlined path used for cases that do not require data-loading, data-unloading, or progress functions, and a slower path that supports any or all these application-defined functions when required.