Inside Macintosh: QuickTime Components
If available memory is insufficient to hold the entire image that is being compressed or decompressed, the image compressor component must call data-loading or data-unloading functions to spool--that is, read or write the data from storage in stages. The calling application indicates this in the data-loading or data-unloading structure, as described in the following sections.
Decompressor components use data loading. The data buffer still exists when the calling application supplies a data-loading function; however, the data buffer holds only part of the data and you must use the data-loading function to load the remaining data into this buffer. The bufferSize parameter of the decompression parameters structure (described on The Decompression Parameters Structure ) indicates the size of the data buffer.
To use the data-loading function, the decompressor component calls it with the pointer to the current position in the data buffer as a parameter. The decompressor specifies the number of bytes it needs (this number must be less than or equal to the size of the data buffer). The data-loading function fills in the data buffer with the number of bytes requested and may adjust the pointer as necessary to remove some of the used data and make room for new data.
If the decompressor component needs to skip data in the compressed stream or go back to data earlier in the stream, the decompressor should call the data-loading function with a nil pointer (instead of the pointer to the data buffer of the data-loading function) and with the size parameter set to the number of bytes that the decompressor wants to skip relative to the current position in the stream. A positive number seeks forward and a negative one seeks backward. To ensure that the position in the stream is known by the data-loading function, the decompressor should call the function before specifying a seek operation with an actual pointer to the current position in the data buffer and a 0 byte count. After the seek operation, the decompressor component should call the data-loading function again with the number of bytes needed from the new position to make sure the needed bytes are read into the buffer.
A decompressor component should not depend on the ability to skip backward in the data stream since not all applications are able to take advantage of this feature. The decompressor should check the error from the data-loading function during a seek operation and should not use the seek feature if an error code is returned. Seeking forward works in most situations; however, it may entail reading the data and throwing it out. Hence, seeking forward may not always be faster than reading the data.
Figure 1 shows several image bands and their measurements.
Figure 1Image bands and their measurements
Data-unloading functions are used by compressor components when there is insufficient memory to hold the buffer for the compressed data produced by the compressor component. The compressor component needs to use a data-unloading function if the flushProcRecord field in the compression parameters structure is not nil . (For details on the compression parameters structure, see The Compression Parameters Structure ). A data buffer is provided even if the data-unloading function is present, and it should be used to hold the data to be unloaded by the data-unloading function. The size of the data buffer is indicated by the bufferSize field in the parameters.
To use the data-unloading function, the compressor fills the data buffer with as much data as possible (within the size limitations of the data buffer). The compressor component then calls the data-unloading function with a pointer to the start of the data buffer and the number of bytes written. The data-unloading function then unloads the data from the buffer. The compressor should then use the entire buffer for the next piece of data--and continue in this manner until all the data is unloaded.
If the compressor component needs to skip forward or backward in the data stream, it should call the data-unloading function with a nil data pointer, and the compressor should specify the number of bytes to seek relative to the current position in the size parameter. A positive number seeks forward and a negative one seeks backward. The compressor component should make sure that all data is unloaded from the buffer before commencing the seek operation. After the seek operation, the next data unloaded from the buffer with the data-unloading function is written starting at the new location. The new data overwrites any data previously written at that location in the data stream.
Not all applications support the ability to seek forward or backward with a data-unloading function. The compressor component should check the error result when performing such an operation.