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Integrating Markers

Introduction

Markers are identifiers that are inserted in a WAV file and used to tag a position in the waveform. These markers are usually created in a WAV editor application, such as SoundForge®, Adobe® Audition®, or CueTool.

An application can use these markers to be notified when a certain position is reached during playback. For example, you could use this information to synchronize content drawing with the audio being played, or to know which file is being played by a random container so that the correct subtitles can be displayed in your game.

Note.gif
Note: Using Wwise marker notifications is usually the most efficient way to integrate a lip-sync or subtitle solution.

Working with the.wav File Format

Chunks are data storage units used to store cue points, or data about cue points. Cue points are points of interest flagged in .wav format files.

Cue Chunks

The cue chunk format is used to store marker positions that indicate points of interest in a .wav file.

Offset  Bytes   Description         Value
0x00    4       Chunk ID            "cue " (0x63756520)
0x04    4       Chunk Data Size     Depends on the number of cue points
0x08    4       Num Cue Points      Number of cue points in list
0x0c    Cue Points

#define CueID 'cue '  /* chunk ID for cue chunk */
typedef struct {
  ID        chunkID;
  long      chunkSize;
  long      dwCuePoints;
  CuePoint  points[];
} CueChunk;

typedef struct {
  long    dwIdentifier;
  long    dwPosition;
  ID      fccChunk;
  long    dwChunkStart;
  long    dwBlockStart;
  long    dwSampleOffset;
} CuePoint;

List Chunks

A list chunk is a container inside the .wav file which contains subchunks. The associated data list chunk (adtl) format is used to store labels, notes, and text associated to a cue point.

Offset  Bytes   Description         Value
0x00    4       Chunk ID            "list" (0x6C696E74)
0x04    4       Chunk Data Size     Depends on contained sub-chunks
0x08    4       Type ID             "adtl" (0x6164746C)
0x0c    SubChunks start here

Label Sub-Chunks

The label sub-chunk format is used to store a string associated with a cue point. It should be inside the associated data list chunk.

Offset  Bytes   Description         Value
0x00    4       Chunk ID            "labl" (0x6C61626C)
0x04    4       Chunk Data Size     Depends on size of text
0x08    4       Cue Point ID        Refers to cue chunk dwIdentifier
0x0c    Label (variable-size text)

How to Use Marker Notifications

Here are instructions on how to design your application so that it receives the marker notifications:

  • When posting a play event, you should add the AK_Marker flag to in_uiFlags. If you also want to have the end of event notification, you should use AK_EndOfEvent | AK_Marker since the flags are bitwise exclusive.
AkPlayingID AK::SoundEngine::PostEvent(
    AkUniqueID in_eventID,                  // Unique ID of the event
    AkGameObjectID in_gameObjectID,         // Associated game object ID
    AkUInt32 in_uFlags = 0,                 // Bitmask: see AkCallbackType
    AkCallbackFunc in_pfnCallback = NULL,   // Callback function
    void * in_pCookie = NULL                // Callback cookie that will be sent to the callback function 
                                            // along with additional information
);
  • Your callback function must have the following form:
static void MarkersCallback(
    AkCallbackType in_eType,            // Type of callback reason, in this case AK_Marker on 
                                        // reception of a marker event.
    AkCallbackInfo* in_pCallbackInfo    // Pointer to callback information structure, in this case
                                        // AkMarkerCallbackInfo*.
    )
  • When your callback function is called, you first need to check what type of notification is passed. For example, if you only want to process AK_Marker notifications, you should return when any other event type is received.
  • Based on the type of notification, you can typecast in_pCallbackInfo into the appropriate information structure type. For AK_Marker notification, it is AkMarkerCallbackInfo.
// Callback information structure corresponding to AK_Marker.
struct AkMarkerCallbackInfo : public AkEventCallbackInfo
{
    AkUInt32    uIdentifier;        // Cue point identifier
    AkUInt32    uPosition;          // Position in the cue point (unit: sample frames)
    const char * strLabel;          // Label of the marker, read from the file
};
  • Make sure you copy the contents of the strLabel string member if you plan to reference it later, as the pointer can be invalidated after the callback returns.
See also:
Quick Start Sample Integration - Events

Notification Latency

Currently, notification is sent when the buffer is passed down to the hardware. This means that there is a certain constant delay between when the notification is sent and the marker is encountered. This gives your application enough time to gather the information on the marker and process it before the sound associated with that marker is really played.

Note that this delay is platform-dependent.

Callback Thread

The callbacks for markers, as well as for the end of event notification, are done from the sound engine's main thread. This means that your application should gather all the information it needs from the notification and return immediately. If any processing needs to be done, it should be performed in a separate thread once the relevant information has been copied from the notification.

If the application holds the thread for too long, the sound engine might fall into an underrun state and the output might stop playing.

Note:
The label string from the callback function should be copied because the referenced data will be released by the sound engine after the callback function returns.

Wwise Capture Log and Markers

The Wwise Profiler can display marker notifications from the sound engine. In order to do so, you must ensure that the Markers Notification Data option in the Profiler Settings dialog is enabled. When a marker is reached and a notification has been requested, a new line will appear in the Capture Log. Note that these notifications can be filtered out if needed by clearing the Markers Notification Data check box in the Capture Log Filter dialog.

See also:
Integration Details - Events

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