Using iChat Theater

iChat Theater allows applications to send additional audio and video tracks during an AV chat. You can use any NSView as a prebuilt video source or provide the auxiliary video through periodic callbacks for individual frames. Audio is provided through an audio device and channels.

Before implementing a video source, you should select the buffer type—a pixel buffer or an OpenGL buffer—that is most efficiently filled by your application during a callback. The pixel buffer is filled in the main memory—by the CPU rather than the GPU. If you are rendering content using OpenGL, then you typically use the OpenGL buffer.

There are several steps involved in using iChat Theater in your application:

  1. Set the video data source and any video options.

  2. If you are not using an NSView object as the video source, implement the callbacks that provide individual video frames.

    If you're using pixel buffers, implement the pixel buffer methods using Core Video. If you're using OpenGL, implement the OpenGL methods.

  3. Create audio channels and manage them using Core Audio.

  4. Use the start and stop methods to control the video playback.

  5. Register for state change notifications.

    You must register for notifications to establish a connection to iChat Theater.

The rest of this article explains how to do each of these steps. Read the Using Views as Video Data Sources article for details on how to use an NSView object as the video data source. Read the Using Pixel Buffers and Using OpenGL Buffers articles for details on implementing the IMVideoDataSource protocol.

Getting the Manager

The first step in using iChat Theater is to get the shared manager object that controls auxiliary audio and video playback. The sharedAVManager class method returns the shared IMAVManager object. This code fragment gets the state of the shared IMAVManager object:

IMAVManagerState state = [[IMAVManager sharedAVManager] state];

See IMAVManager Class Reference for descriptions of the different states returned by the state method.

Setting the Video Data Source

Your application provides the auxiliary video content that is sent over iChat Theater. This is accomplished using a delegation model. You set a video data source object that conforms to a defined protocol and the Instant Message framework sends a message to the data source object when it needs the next video frame. Hence, messages are sent periodically to your video data source object during playback.

For example, this code fragment sets the video data source for the shared IMAVManager object using the setVideoDataSource: method, then sets some optimization options using the setVideoOptimizationOptions: method, and starts the video playback using the start method:

Listing 1  Setting the video data source

IMAVManager *avManager = [IMAVManager sharedAVManager];
[avManager setVideoDataSource:videoDataSource];
[avManager setVideoOptimizationOptions:IMVideoOptimizationStills];
[avManager start];

Setting Video Options

Use the setVideoOptimizationOptions: method to give hints to the IMAVManager object so it can optimize the video playback based on the type of video source.

For example, use the IMVideoOptimizationStills option if you are sharing a slideshow as shown in Listing 1. This option is a hint to iChat Theater that the video doesn’t change for long periods of time. Consequently, iChat Theater assumes the video does not require much bandwidth to encode and send. However, if the video is full-motion, then setting this option has a negative impact on performance.

Use the IMVideoOptimizationReplacement option if you want to force iChat Theater to replace the outgoing local user’s video with your video data source instead of displaying both video sources side-by-side. If you set this option, iChat can devote full CPU and bandwidth resources to the iChat Theater video. However, if you do not set this option, there’s no guarantee that side-by-side video is used. iChat may replace the local video under certain circumstances—for example, it may replace the video if video chatting with a buddy on OS X v10.4 and earlier, with multiple buddies, or over a slow connection.

Implementing the Video Data Source

Your video data source needs to conform to the IMVideoDataSource informal protocol. You should select the type of buffer that is most efficient for your application.

If you’re using pixel buffers, then implement the getPixelBufferPixelFormat: and renderIntoPixelBuffer:forTime: methods. Read Using Pixel Buffers for tips on how to implement these methods.

If you’re using OpenGL, then implement the getOpenGLBufferContext:pixelFormat: and renderIntoOpenGLBuffer:onScreen:forTime: methods. Read Using OpenGL Buffers for tips on how to implement these methods.

For performance reasons, all of these callbacks are not invoked on the main thread. If you are using OpenGL, which is not thread-safe, to render to both the screen and buffer, then you need to take some extra precautions. Read Using OpenGL Buffers to learn more about how to use OpenGL in a multithreaded application.

Creating Audio Channels

The audio tracks are not handled the same way as the video tracks. You set the number of audio channels before playing any AV using the setNumberOfAudioChannels: method. Currently, the audio can either be mono or stereo. You access the audio device and channels using the audioDeviceUID and audioDeviceChannels methods respectively. Use these methods when the shared IMAVManager is in the IMAVRunning state; otherwise, they return nil.

Use Core Audio to manage the channels and create audio content. For example, use the AudioHardwareGetProperty function in Core Audio by passing kAudioHardwarePropertyDeviceForUID and the value returned by audioDeviceUID to obtain the device. Read Core Audio Overview to get started with audio and Core Audio Framework Reference for details on Core Audio.

You can also play any NSSound over iChat Theater using the setPlaybackDeviceIdentifier: and setChannelMapping: methods of NSSound. Listing 2 shows how to use these method. See NSSound Class Reference for details on the setPlaybackDeviceIdentifier: and setChannelMapping: methods.

The playMonoForiChat: method in Listing 2 is intended to be a category method that you add to NSSound. If the sound has one channel, then use the playStereoForiChat: method instead of the play method of NSSound to play the sound over iChat Theater. There’s a similar category method in the sample code if the sound is stereo.

Listing 2  Playing sounds over iChat Theater

- (BOOL) playMonoForiChat:(BOOL)flag {
    if (flag) {
        // Set the audio output device.
        IMAVManager *avManager = [IMAVManager sharedAVManager];
        [self setPlaybackDeviceIdentifier:[avManager audioDeviceUID]];
        // Get the channel info for iChat Theater.
        NSArray *channels = [avManager audioDeviceChannels];
        NSUInteger channelCount = [channels count];
        // For a mono sound, map its single channel to those of the IMAVManager
        NSArray *mapping = (channelCount > 0) ? [NSArray arrayWithObject:channels] : nil;
        [self setChannelMapping:mapping];
    } else {
        // Use default playback device and channel mapping.
        [self setPlaybackDeviceIdentifier:nil];
        [self setChannelMapping:nil];
    return [self play];

Controlling Video Playback

After you set the video data source and create your audio channels, you are ready to start playing AV content in iChat. You simply send start to the shared IMAVManager object to play, and stop to stop the AV content. The IMAVManager object transitions through several states during playback.

When you send start to a stopped IMAVManager object, it changes state from IMAVRequested to IMAVStartingUp, then to IMAVPending, and finally to IMAVRunning. When you invoke the start method, the state changes immediately to IMAVStartingUp and the method returns. The IMAVManager object asynchronously transitions to the other states.

Conversely, when you send stop to a running IMAVManager object, it changes state from IMAVRunning, to IMAVShuttingDown, and then to IMAVRequested. When you invoke the stop method, the state changes immediately to IMAVShuttingDown and the method returns. The IMAVManager object asynchronously transitions to IMAVRequested. The stop method returns immediately if the IMAVManager object is not in the IMAVRunning state.

Registering for the State Change Notification

When using the iChat Theater API, the IMAVManager object can be in a number of different states at anytime—for example, depending on whether or not you invoke the start or stop method. Even after invoking these methods, the state of the IMAVManager object is not guaranteed because errors can occur while transitioning from a stopped to a running state or another application using the iChat Theater API can cause state transitions you might not expect. Invoking other methods while IMAVManager is not in an expected state can raise exceptions or do nothing.

Typically, you register for the IMAVManagerStateChangedNotification notification to be notified when the shared IMAVManager object changes state and then use the state method to get the new state. You should register for this notification early in your application, before sending state to the shared IMAVManager object, because registering for this notification establishes a connection to iChat Theater. Otherwise, state values returned by IMAVManagermay not be accurate.