Inside Macintosh: QuickTime
This section discusses key attributes of compressor components and the functional interfaces these components must support. ( Compressor components here refers to both image compressor components and image decompressor components.) This information is intended for developers of compressor components. Application developers do not need to be familiar with this material to use the Image Compression Manager.
A compressor component is a code resource that provides image compression or decompression services for image data. These components may also utilize additional hardware to provide their services. Compressor components are registered by the Component Manager, and they present a standard set of function interfaces to the Image Compression Manager (see Inside Macintosh: QuickTime Components for a detailed description of the functions that compressors must provide). A compressor can be a systemwide resource, or it can be local to a particular application.
Applications never communicate directly with compressors. Applications request compressor services by issuing the appropriate Image Compression Manager functions. The Image Compression Manager then performs its necessary processing before invoking the compressor. Of course, an application could install its own compressor component. However, any interaction between the application and the compressor is still managed by the Image Compression Manager.
The Image Compression Manager knows about two types of compressor components. Components that can compress image data carry a component type (described by the compressorComponentType data type) of 'imco' and are referred to as compressors . Components that can decompress images have a component type (described by the decompressorComponentType data type) of 'imdc' and are called decompressors . The value of the component subtype indicates the compression algorithm supported by the component. All compressor components with the same subtype must be able to handle the same format of compressed data. During decompression a component should handle all variations of the data specified for a subtype. Conversely, while compressing an image a compressor must not produce data that decompressors of the same subtype cannot handle during decompression.
The Image Compression Manager defines four callback functions that may be provided to compressors or decompressors by applications. A callback function is an application-defined function that is invoked at a specified time or based on specified criteria. These callback functions are data-loading functions, data-unloading functions, completion functions, and progress functions. Data-loading functions and data-unloading functions support spooling of compressed data. Completion functions allow compressors and decompressors to report that asynchronous operations have completed. Progress functions provide a mechanism for compressors and decompressors to report their progress toward completing an operation. For more information about these callback functions, see "Application-Defined Functions," .