Programming Flash Communication Server

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Publishing Streams in Detail

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During publishing, bufferLength returns the amount of data, in seconds, currently in

the buffer. The

bufferTime property returns the total buffer time. In practice, the

buffer is always larger than the time set via setBufferTime( ); this allows the amount

of data in the buffer to start to exceed the buffer time without any frames being

immediately dropped. When the buffer time is exceeded, NetStream.onStatus( ) is

passed an information object with a

code value of “NetStream.Buffer.Full”. This pro-

vides an opportunity to increase the buffer time, reduce the amount of data being

sent into the stream, or stop recording altogether. If the buffer length reaches one

and a half times the buffer time, all video messages are dropped. If it reaches two

times the buffer time, all audio and video frames are dropped. When a buffer is

empty, a “NetStream.Buffer.Empty” message will be sent to the onStatus( ) handler.

See Example 5-2 and Example 5-5 for examples of responding to “NetStream.Buffer”

messages.

Snapshots, Stop Motion, and Time-Lapse Video

FlashCom makes it possible to capture a single video frame or snapshot from a cam-

era or video source, instead of capturing all the frames as they arrive from the source.

The NetStream.attachVideo( ) method has an optional second parameter named

snapShotMilliseconds. When the parameter is omitted, each frame from the source is

sent to the stream according to the frame rate set using Camera.setMode( ). If a value

of 0 is passed in, a single frame of video is placed in the stream. For example:

out_ns.attachVideo(cam, 0);

To be certain the frame is not dropped and has been sent to the server before the

stream is closed, two things must be done. The NetStream.setBufferTime( ) method

must be used to make sure the frame is buffered, and an onStatus( ) handler must be

used to make sure the buffer is empty before closing the stream. Example 5-2 lists the

essential code that sets up a stream to publish a snapshot and then takes one when

the Record button is clicked. The complete listing is available on the book’s web site.

Example 5-2. Taking buffered snapshots

// Called after a network connection is made.

function initProgram( ) {

if (out_ns) out_ns.close( );

out_ns = new NetStream(nc);

out_ns.setBufferTime(2);

out_ns.onStatus = function (info) {

writeln(info.code);

if (info.code == "NetStream.Buffer.Empty") {

this.close( );

in_ns.play("mugshot");

}

};

if (in_ns) in_ns.close( );

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164

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Chapter 5: Managing Streams

In Example 5-2, whenever a network connection is created, the code creates a Net-

Stream object, gives it a buffer time of 2 seconds, and creates an onStatus( ) event

handler:

out_ns = new NetStream(nc);

out_ns.setBufferTime(2);

out_ns.onStatus = function (info) {

writeln(info.code);

if (info.code == "NetStream.Buffer.Empty") {

this.close( );

in_ns.play("mugshot");

}

};

When the snapshot is taken, the buffer briefly holds some video information until it

is successfully sent. When all the video data has been sent, the onStatus( ) handler

receives an

info.code value of “NetStream.Buffer.Empty”. At that point, it can safely

close the stream without the snapshot frame being lost. To take the actual snapshot,

the code calls doRecord( ) to attach the video source to the stream and publish the

stream:

out_ns.attachVideo(cam, 0);

out_ns.publish("mugshot", "record");

Shortly afterward, the buffer empties and the stream closes.

The

snapshotMilliseconds parameter can also be used to create time-lapse videos in

which snapshots are taken at regular (usually long) intervals and then played back

quickly. This technique can be used to speed up an otherwise slow process, such as

watching a plant grow.

To make time-lapse video work, we cannot just publish a stream, leave it open, and

send snapshots at regular intervals. The stream time progresses as long as the stream

is being published. When such a stream is played back, the frames change at the

in_ns = new NetStream(nc);

snapshot_video.attachVideo(in_ns);

cam = Camera.get( );

preview_video.attachVideo(cam);

System.showSettings(0);

record_btn.setEnabled(true);

connect_btn.setLabel("Disconnect");

in_ns.play("mugshot");

}

// Called when the user clicks the Record button.

function doRecord( ) {

if (out_ns) out_ns.close( );

out_ns.attachVideo(cam, 0);

out_ns.publish("mugshot", "record");

}

Example 5-2. Taking buffered snapshots (continued)