Program Flow

The following figure illustrates the interaction between the captioning components and the media control interface layer. Source code for the Caption Sample Application and Caption DLL are located respectively in the \TOOLKIT\SAMPLES\MM\CAPSAMP and \TOOLKIT\SAMPLES\MM\CAPDLL subdirectories.

┌───────────────────┐
│      Caption      │
│      Sample       ├───(2D)──────┐
│    Application    │             │
└┬──┬──┬──┬──┬──┬──┬┘             │
(2)(3)(4)(1)(2)(4)(5)             │
 │  │  │                      │
 │  │  │ ┌───────────────┐     ┌──┴────┐
 │  │  │ │  Caption DLL  │(2D) │Caption│
 │  │  │ │               │     │ File  │
 │  │  │ └┬──────────────┘     └───────┘
 │  │  │ (2B) 
 │  │  │  │  (2C)
----------│-----------------------------------
┌───────────────────────────┐     OS/2 Multimedia
│   Media Control Interface │
└───────────────────────────┘

• As part of its initialization and termination routines, the Caption Sample Application issues ccInitialize and ccTerminate, respectively, to notify the Caption DLL to begin and end captioning. As part of the termination process, the Caption DLL releases any resources previously allocated for captioning.

• When you select Play, the Caption Sample Application opens the audio file, obtains a device ID, and plays the audio file. The Caption Sample Application queries the captioning flag. (You can set this flag by selecting the Captioning check box on the System page of the Multimedia Setup.) If this flag is set, the application issues ccSendCommand with a CC_START message to the Caption DLL. The Caption DLL then begins to provide captioning for the application.

• 2B)
  When the Caption DLL receives the request to begin captioning, it issues a set position advise message to the multimedia system for every 1500 time units.
  (2C)
  When the device moves 1500 time units, the Caption DLL receives an MM_MCIPOSITIONCHANGE message from the multimedia system.
  (2D)
  Whenever the Caption DLL receives the MM_MCIPOSITIONCHANGE message, it checks the caption file for the appropriate line to display, based on the current position of the audio file. The Caption DLL then scrolls the caption window in the application, to display the appropriate line.
  (3)
  If you pause the audio file, change the volume, or move the audio slider position, the Caption Sample Application does not have to perform any additional processing to manage the caption window. This processing is managed by the Caption DLL.
  (4)
  When you select Stop, the Caption Sample Application sends an MCI_STOP message to the audio device. The application then issues ccSendCommand with a CC_STOP message, informing the Caption DLL to stop displaying the caption window in the application.
  (5)
  You can change several properties of the caption window by selecting Settings from the Options menu of the Caption Sample Application. The application issues ccSendCommand with a CC_STATUS message to query the current properties of the caption window. When you select OK to save the desired properties, the Caption Sample Application issues the ccSendCommand with a CC_SET message to the Caption DLL. The Caption DLL handles changing and displaying the new properties of the caption window.

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