it-e-72 Introduction To the Image Compression Manager

The Image Compression Manager provides your application with an interface for compressing
and decompressing images and sequences of images that is independent of devices and algorithms.
Uncompressed image data requires a large amount of storage space. Storing a single
640-by-480 pixel image in 32-bit color can require as much as 1.2 MB. Sequences of images, like
those that might be contained in a QuickTime movie, demand substantially more storage than
single images. This is true even for sequences that consist of fairly small images, because the
movie consists of such a large number of those images. Consequently, minimizing the storage
requirements for image data is an important consideration for any application that works with
images or sequences of images.
The Image Compression Manager allows your application to
use a common interface for all image-compression and image-decompression operations;
take advantage of any compression software or hardware that may be present in a given
Macintosh configuration;
store compressed image data in pictures;
temporarily compress sequences of images, further reducing the storage requirements of
display compressed PICT files without the need to modify your application;
use an interface that is appropriate for your applicationa high-level interface if you do
not need to manipulate many compression parameters or a low-level interface that
provides you greater control over the compression operation.
The Image Compression Manager compresses images by invoking image compressor components
and decompresses images using image decompressor components. Compressor and decompressor
components are code resources that present a standard interface to the Image Compression Manager and
provide image-compression and image-decompression services, respectively. The Image Compression
Manager receives application requests and coordinates the actions of the appropriate components. The
components perform the actual compression and decompression. Compressor and decompressor
components are standard components and are managed by the Component Manager. For detailed
information about creating compressor and decompressor components, see Inside Macintosh:
QuickTime Components.
Because the Image Compression Manager is independent of specific compression algorithms
and drivers, it provides a number of advantages to developers of image- compression algorithms.
Specifically, compressor and decompressor components can
present a common application interface for software-based compressors and hardware-based

provide several different compressors and compression options, allowing the Image
Compression Manager or the application to choose the appropriate tool for a particular

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