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Using Image Transcoder Components

QuickTime’s image transcoding support is part of the Image Compression Manager API. The Image Compression Manager uses an image sequence when compressing or decompressing data. An image sequence allows QuickTime to make certain optimizations because it knows that a similar operation will be repeated multiple times (that is, images will be repeatedly compressed to the same image data format).

A transcoder translates image data from one compressed format to another. This is done automatically, if an appropriate transcoder component is available, when QuickTime plays a movie that has been compressed in a format for which there is no decompressor on the playback machine. Transcoder components can also be used when an application provides an export function that saves data in a compressed format different from the compressed format of the source data.

Transcoding has two distinct advantages over the decompress-then-recompress approach to converting the format of compressed data. The first advantage is that the operation is usually substantially faster, since much of the data can be copied directly from the source image data format to the destination image data format. The second advantage is that the operation is usually more accurate because decompressing and recompressing provides two steps for introducing rounding and quantization errors. By directly transcoding, opportunities for small errors are substantially reduced.

QuickTime’s image transcoding support is part of the Image Compression Manager API. Image transcoding can be invoked either explicitly, using transcoder-specific Image Compression Manager functions such as the ImageTranscoderBeginSequence function, or implicitly, using standard routines for decompressing images that in turn may use a transcoder when needed.

As with most other services in QuickTime, the details of image transcoding are handled by components. The Image Compression Manager uses image transcoder components to perform both implicit and explicit image transcoding. Application developers that perform image transcoding interact with the Image Compression Manager, not directly with the image transcoder components themselves. The Image Compression Manager takes care of the details of working with image transcoder components.

If you want to add new image transcoding capabilities to QuickTime, you can write an image transcoder component, as described in “Creating Image Transcoder Components.”

Contents:

Invoking an Image Transcoding Process
Transcoding Paths

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