Inside Macintosh: QuickTime

Compressing Sequences

The Image Compression Manager also provides functions that allow your application to compress and decompress sequences of images, such as might constitute a QuickTime movie. The tools provided by the Image Compression Manager focus on image compression and decompression and on the ordering of the images in a sequence, not on timing considerations. Use the Movie Toolbox to handle all the issues relating to the amount of time each image should be shown on the screen. For information on decompressing image sequences, see the next section, "Decompressing Sequences."

A series of images can be compressed as a sequence if those images share an image description. That is, each image in the sequence must have the same compressor type, pixel depth, color lookup table, and boundary dimensions. To take best advantage of temporal compression, the images should also be related to each other (like frames in a movie), but this relationship is not necessary for them to be grouped as a sequence. If you create a sequence from completely unrelated images, you may not be able to achieve significant temporal compression.

When compressing image sequences, your application must perform several steps in addition to those required for single-frame image compression. This section describes a typical function for compressing an image sequence. Note that much of the setup processing is the same as that performed for single-frame images.

First, determine the parameters for the compression operation. As with single-image compression, the user may specify these parameters in a dialog box you can supply via the standard image-compression dialog component (see the chapter "Standard Image-Compression Dialog Components" in Inside Macintosh: QuickTime Components for details). Your application may choose to give the user the ability to specify such parameters as the compression algorithm, image quality, and so on. Note that image sequences require additional parameters, such as temporal quality.

Your application may give the user the option of specifying a compression algorithm based on an important performance characteristic. For example, the user may be most concerned with size, speed, or accuracy. The Image Compression Manager allows your application to choose the compressor component that meets the specified criterion.

Your application signals its intention to compress an image sequence by issuing the Image Compression Manager's CompressSequenceBegin function (see Working With Sequences for more information about this function). At this time your application specifies many of the parameters that govern the sequence-compression operation. When you set the compression parameters and the temporalQuality parameter is not 0, then be sure to set the value of either the codecFlagUpdatePrevious or codecFlagUpdatePreviousComp flag to 1 in the flags parameter of the CompressSequenceBegin function.

Once you have started the sequence, you then compress each image in the sequence by performing the following steps:

Your application must call the Image Compression Manager's GetMaxCompressionSize function to determine the maximum size of the compressed data that will result from the current image (see "Getting Information About Compressed Data" for more information about this function). You provide the specified compression parameters to this function. In response, the Image Compression Manager invokes the appropriate compressor component to determine the number of bytes required to store the largest compressed image in the sequence. Your application should then reserve sufficient memory to accommodate that compressed image. You can use this returned value until you change the settings of the compression parameters.
Your application must call the CompressSequenceFrame function to compress the image (see "Working With Sequences" for more information about this function). It may be necessary or desirable for your application to change one or more of the compression parameters while processing a sequence. The Image Compression Manager provides several functions that allow your application to modify such parameters as the spatial or temporal quality or the data-unloading function. See "Changing Sequence-Compression Parameters" for more information about these functions.
The Image Compression Manager manages the compression operation and invokes the appropriate compressor. The manager returns the compressed image and its associated image description to your application.
Your application is then free to store the compressed image with the others in the sequence.

After the entire sequence is compressed, you end the process by calling the CDSequenceEnd function (see CDSequenceEnd for more information about this function).

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