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The QuickTime Architecture

The QuickTime architecture is made up of specific managers: the Movie Toolbox, the Image Compression Manager and Image Decompressor Manager, as well as the Component Manager, in addition to a set of predefined components. “Figure 1-3” shows the relationships of these managers and an application that is playing a movie.

In this section:

The Movie Toolbox
The Image Compression Manager
QuickTime Components
The Component Manager
Atoms

The Movie Toolbox

An application gains access to the capabilities of QuickTime by calling functions in the Movie Toolbox. The Movie Toolbox allows you to store, retrieve, and manipulate time-based data that is stored in QuickTime movies. A single movie may contain several types of data. For example, a movie that contains video information might include both video data and the sound data that accompanies the video.

The Movie Toolbox also provides functions for editing movies. For example, there are editing functions for shortening a movie by removing portions of the video and sound tracks, and there are functions for extending it with the addition of new data from other QuickTime movies.

The Image Compression Manager

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. Similarly, 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 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 provides a device-independent and driver-independent means of compressing and decompressing images and sequences of images. It also contains a simple interface for implementing software and hardware image-compression algorithms. It provides system integration functions for storing compressed images as part of PICT files, and it offers the ability to automatically decompress compressed PICT files on any QuickTime-capable Macintosh computer.

In most cases, applications use the Image Compression Manager indirectly, by calling Movie Toolbox functions or by displaying a compressed picture. However, if your application compresses images or makes movies with compressed images, you will call Image Compression Manager functions.

QuickTime Components

QuickTime provides components so that every application doesn’t need to know about all possible types of audio, visual, and storage devices. A component is a code resource that is registered by the Component Manager. The component’s code can be available as a systemwide resource or in a resource that is local to a particular application.

Each QuickTime component supports a defined set of features and presents a specified functional interface to its client applications. Applications are thereby isolated from the details of implementing and managing a given technology. For example, you could create a component that supports a certain data encryption algorithm. Applications could then use your algorithm by connecting to your component through the Component Manager, rather than by implementing the algorithm again.

QuickTime provides a number of useful components for application developers. These components provide essential services to your application and to the managers that make up the QuickTime architecture. The following Apple-defined components are among those used by QuickTime:

• movie controller components, which allow applications to play movies using a standard user interface

• standard image-compression dialog components, which allow the user to specify the parameters for a compression operation by supplying a dialog box or a similar mechanism

• image compressor components, which compress and decompress image data

• sequence grabber components, which allow applications to preview and record video and sound data as QuickTime movies

• video digitizer components, which allow applications to control video digitization by an external device

• media data-exchange components, which allow applications to move various types of data in and out of a QuickTime movie

• derived media handler components, which allow QuickTime to support new types of data in QuickTime movies

• clock components, which provide timing services defined for QuickTime applications

• preview components, which are used by the Movie Toolbox’s standard file preview functions to display and create visual previews for files

• sequence grabber components, which allow applications to obtain digitized data from sources that are external to a Macintosh computer

• sequence grabber channel components, which manipulate captured data for a sequence grabber component

• sequence grabber panel components, which allow sequence grabber components to obtain configuration information from the user for a particular sequence grabber channel component

The Component Manager

Applications gain access to components by calling the Component Manager. The Component Manager allows you to define and register types of components and communicate with components using a standard interface. A component is a code resource that is registered by the Component Manager. The component’s code can be stored in a systemwide resource or in a resource that is local to a particular application.

Once an application has connected to a component, it calls that component directly. If you create your own component class, you define the function-level interface for the component type that you have defined, and all components of that type must support the interface and adhere to those definitions. In this manner, an application can freely choose among components of a given type with absolute confidence that each will work.

Atoms

QuickTime stores most of its data using specialized memory structures called atoms. Movies and their tracks are organized as atoms. Media and data samples are also converted to atoms before being stored in a movie file.

There are two kinds of atoms: classic atoms, which your code accesses by offsets, and QT atoms, for which QuickTime provides a full set of access tools. Atoms that contain only data, and not other atoms, are called leaf atoms. QT atoms can nest indefinitely, forming hierarchies that are easy to pass from one process to another. Also, QuickTime provides a powerful set of tools by which you can search and manipulate QT atoms. You can use these tools to search through QT atom hierarchies until you get to leaf atoms, then read the leaf atom’s data from its various fields.

Each atom has a type code that determines the kind of data stored in it. By storing data in typed atoms, QuickTime minimizes the number and complexity of the data structures that you need to deal with. It also helps your code ignore data that’s not of current interested when it interprets a data structure.

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