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Data Flow in the DVD Navigator

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Microsoft DirectShow 9.0

Data Flow in the DVD Navigator

The DVD Navigator has methods to stop and pause playback. These methods are similar — but not identical — to the Stop and Pause methods in IMediaControl. Here is the difference between them:

  • The IDvdControl2 methods change what the DVD Navigator reads from the disk. They do not change the state of the graph.
  • The IMediaControl methods change the state of the graph. They do not change what the DVD Navigator reads from the disk. (There is one important exception, explained in the next section, related to the Stop method.)

For example, IDvdControl2::Pause method issues the Annex J "Pause_On" command, but does not pause the filter graph. The IMediaControl::Pause method, on the other hand, pauses the graph but does not issue any DVD command.

In general, use the IMediaControl::Pause and Stop methods instead of the corresponding IDvdControl2 methods. The IMediaControl methods have very small latencies, whereas the IDvdControl2 methods can have up to two seconds of latency.

Stopping Playback

The behavior of IMediaControl::Stop depends on a flag that you can set with the IDvdControl2::SetOption method.

  • If the DVD_ResetOnStop flag is FALSE, IMediaControl::Stop stops the graph, but does not change the DVD Navigator's domain. When you call run again, playback resumes from the current position.
  • If DVD_ResetOnStop is TRUE, IMediaControl::Stop causes the DVD Navigator to reset. When you call IMediaControl::Run again, the DVD Navigator plays from the First Play domain, as if you were inserting the DVD for the first time.

The DVD_ResetOnStop flag is true by default, for compatibility with older applications. Generally, however, you should override the default and set the flag to FALSE. The reason is that certain events can cause the graph to stop during playback. For example, if the display resolution changes, the filter graph stops, reconnects the video renderer, and restarts. If DVD_ResetOnStop is TRUE, playback will restart from the beginning of the disc. That is probably not what the user expects.

At the beginning of your application, therefore, call SetOption with DVD_ResetOnStop set to FALSE. If you want to stop playback and have it resume from the same location, call IMediaControl::Stop or IMediaControl::Pause. If you want to stop playback and reset the disk, call SetOption with DVD_ResetOnStop equal to TRUE; then call IMediaControl::Stop; finally, call SetOption again and reset DVD_ResetOnStop to FALSE.

Pausing Playback

If you give the DVD Navigator a command while the graph is paused, the command may not complete until the graph runs again. In some situations, this can cause a deadlock in your application. There are two rules you should follow to avoid deadlocks:

  • While paused, do not issue more than one asynchronous DVD command.
  • While paused, do not block the application's UI thread or the thread that changes the state of the graph.

The second rule is worth examining in more detail. Here are some specific scenarios that may cause a deadlock:

  • Scenario: While paused, the application issues a DVD command with the blocking flag. This can cause a deadlock if the thread that issues the DVD command is the same thread that issues the run command. The DVD command blocks until the graph runs, but the graph cannot run until the command completes.

    Recommendation: Issue the DVD command on a separate worker thread, or don't use the blocking flag.

  • Scenario: While paused, the application issues a DVD command, then calls IDvdCmd::WaitForEnd on the command object. This situation is equivalent to the previous example. If you call Wait from the UI thread, the UI thread cannot run the graph until the Wait method unblocks, but the Wait method will not unblock until the graph runs.

    Recommendation: Call Wait on a worker thread.

  • Scenario: While the graph is running, the application issues a DVD command with the blocking flag, and then calls pause from another thread. This is a possible race condition because the graph may pause before the command is issued. If one of the two threads is the UI thread, you may cause a deadlock similar to the previous two examples. This example illustrates the importance of writing thread-safe code if your application uses multiple threads.

    Recommendation: If you use worker threads, make sure your code is thread-safe.

  • Scenario: While paused, the application disables the run command from the UI, and then issues an asynchronous DVD command. This case is not strictly a deadlock, because the application thread is still running. However, the user is now prevented from running the graph, and therefore the command will never complete.

    Recommendation: When pausing, always leave the run command enabled.

Seeking a DVD to a Specified Time

To accurately seek to a specified time on a disc, call IMediaControl::Run. Then call IDvdControl2::PlayAtTime, specifying the time and setting dwFlags to DVD_CMD_FLAG_Flush.

See Also