QuickTime 3 Reference
QuickTime's ability to include any compressed image data in a QuickDraw picture is a helpful feature from a compatibility perspective. However, it presents several technical challenges for applications that need to work with compressed image data contained within pictures. Determining if compressed data is present, and extracting it, requires special code installed in QuickDraw bottlenecks to detect and copy compressed data as it processes. Additional problems are posed by special cases such as multiple compressed images in a single file. The QuickTime image file (QTIF) format solves this and other issues.
QuickTime image files are intended to provide the most useful container for QuickTime compressed still images. The format uses the same atom-based structure as a QuickTime movie. (See "Movie Toolbox: Fundamentals" for information about atoms.) There are two defined atom types: 'idsc' , which contains an image description, and 'idat' , which contains the image data. For a JPEG image, the image description atom contains a QuickTime image description describing the JPEG image's size, resolution, depth, and so on, and the image data atom contains the actual JPEG compressed data.
A QuickTime image file can also contain other atoms. For example, it can contain single-fork preview atoms. Because the QuickTime image file is a single fork format, it works well in cross-platform applications. On Mac OS systems, QuickTime image files are identified by the file type 'qtif' . Apple recommends using the filename extension .QIF to identify QuickTime image files on other platforms.