QuickTime 3 Reference
The functionality of the ImageFieldSequenceExtractCombine function described in "Image Compression Manager," is performed by individual image codecs. This is because the way in which fields are stored is different for every compression format. A new codec component function, ImageCodecExtractAndCombineFields , has been defined for this purpose. Apple encourages developers of codecs to incorporate this function, if their compressed data format is capable of separately storing both fields of a video frame.