Inside Macintosh: QuickTime Components
Image compressor components are registered by the Component Manager, and they present a standard interface to the Image Compression Manager (see "Functions," for a detailed description of the functions that image compressor components must provide). An image compressor component can be a systemwide resource, or it can be local to a particular application.
Applications never communicate directly with these components. Applications request compression and decompression services by issuing the appropriate Image Compression Manager functions. The Image Compression Manager then performs its necessary processing before invoking the component. Of course, an application could install its own image compressor component. However, any interaction between the application and the component is still managed by the Image Compression Manager.
The Image Compression Manager knows about two types of image compressor components. Components that can compress image data carry a component type of 'imco' and are called image compressors . Components that can decompress images have a component type of 'imdc' and are called image decompressors .
#define compressorComponentType 'imco' /* compressor component
type */
#define decompressorComponentType 'imdc' /* decompressor
component type */
The value of the component subtype indicates the compression algorithm supported by the component. For example, the graphics compressor has the component subtype 'cvid' . (A component subtype is an element in the classification hierarchy used by the Component Manager to define the services provided by a 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. While compressing an image, a compressor must not produce data that decompressors of the same subtype cannot handle during decompression.
The Image Compression Manager provides a set of utility functions for compressor components. These functions allow compressors and decompressors to create custom color lookup tables, among other things. For a complete description of these utility functions, along with the functions that must be supported by compressor components, see "Image Compression Manager Utility Functions," which begins on Image Compression Manager Utility Functions .
The Image Compression Manager defines four callback functions that may be provided to compressors and decompressors by applications. 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 components to report that asynchronous operations have completed. Progress functions provide a mechanism for components to report their progress toward completing an operation. For more information about these callback functions, see the chapter "Image Compression Manager" in Inside Macintosh: QuickTime .