Image Transcoder
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 contained within the Image Compression Manager. Image transcoding can be invoked either explicitly, using new APIs in the Image Compression Manager, or implicitly, by using existing routines for decompressing images.
QuickTime's support for decompressing images has been enhanced so that if a request is issued to decompress an image, but no image decompressor component is installed for that image format, QuickTime will attempt to locate an image transcoder to convert the image data into a supported format. This transcoding is performed transparently to the calling application. This automatic image transcoding is supported for both QuickTime movies and compressed image data stored in QuickDraw pictures.
QuickTime also provides an API for applications to transcode images. These APIs make it possible for any application to take compressed image data and transcode it into another format. This capability is useful for applications that create QuickTime movies by combining segments of other QuickTime movies. These applications often convert the format of the compressed image data by decompressing the image and then recompressing it to the new format. If no other processing is to be performed on the compressed data, you can use an image transcoder to increase the speed and fidelity of the operation.
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 operations to QuickTime, you can write an image transcoder component.