我在阅读有关如何解码视频的教程时遇到了这个片段：
private static long millisecondsUntilTimeToDisplay(IVideoPicture picture)
{
/**
* We could just display the images as quickly as we decode them, but it turns
* out we can decode a lot faster than you think.
*
* So instead, the following code does a poor-man's version of trying to
* match up the frame-rate requested for each IVideoPicture with the system
* clock time on your computer.
*
* Remember that all Xuggler IAudioSamples and IVideoPicture objects always
* give timestamps in Microseconds, relative to the first decoded item. If
* instead you used the packet timestamps, they can be in different units depending
* on your IContainer, and IStream and things can get hairy quickly.
*/
long millisecondsToSleep = 0;
if (mFirstVideoTimestampInStream == Global.NO_PTS)
{
// This is our first time through
mFirstVideoTimestampInStream = picture.getTimeStamp();
// get the starting clock time so we can hold up frames
// until the right time.
mSystemVideoClockStartTime = System.currentTimeMillis();
millisecondsToSleep = 0;
} else {
long systemClockCurrentTime = System.currentTimeMillis();
long millisecondsClockTimeSinceStartofVideo = systemClockCurrentTime - mSystemVideoClockStartTime;
// compute how long for this frame since the first frame in the stream.
// remember that IVideoPicture and IAudioSamples timestamps are always in MICROSECONDS,
// so we divide by 1000 to get milliseconds.
long millisecondsStreamTimeSinceStartOfVideo = (picture.getTimeStamp() - mFirstVideoTimestampInStream)/1000;
final long millisecondsTolerance = 50; // and we give ourselfs 50 ms of tolerance
millisecondsToSleep = (millisecondsStreamTimeSinceStartOfVideo -
(millisecondsClockTimeSinceStartofVideo+millisecondsTolerance));
}
return millisecondsToSleep;
}
我已经刮了很多，但不明白这种方法有什么作用？我们要返回什么？以及为什么我们在方法返回后让线程休眠 （该方法的目的是什么？）
这是链接中的完整代码：
import javax.sound.sampled.AudioFormat;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.DataLine;
import javax.sound.sampled.LineUnavailableException;
import javax.sound.sampled.SourceDataLine;
import com.xuggle.xuggler.demos.*;
import com.xuggle.xuggler.Global;
import com.xuggle.xuggler.IAudioSamples;
import com.xuggle.xuggler.IContainer;
import com.xuggle.xuggler.IPacket;
import com.xuggle.xuggler.IPixelFormat;
import com.xuggle.xuggler.IStream;
import com.xuggle.xuggler.IStreamCoder;
import com.xuggle.xuggler.ICodec;
import com.xuggle.xuggler.IVideoPicture;
import com.xuggle.xuggler.IVideoResampler;
import com.xuggle.xuggler.Utils;
public class DecodeAndPlayAudioAndVideo
{
/**
* The audio line we'll output sound to; it'll be the default audio device on your system if available
*/
private static SourceDataLine mLine;
/**
* The window we'll draw the video on.
*
*/
private static VideoImage mScreen = null;
private static long mSystemVideoClockStartTime;
private static long mFirstVideoTimestampInStream;
/**
* Takes a media container (file) as the first argument, opens it,
* plays audio as quickly as it can, and opens up a Swing window and displays
* video frames with <i>roughly</i> the right timing.
*
* @param args Must contain one string which represents a filename
*/
@SuppressWarnings("deprecation")
public static void main(String[] args)
{
if (args.length <= 0)
throw new IllegalArgumentException("must pass in a filename as the first argument");
String filename = args[0];
// Let's make sure that we can actually convert video pixel formats.
if (!IVideoResampler.isSupported(IVideoResampler.Feature.FEATURE_COLORSPACECONVERSION))
throw new RuntimeException("you must install the GPL version of Xuggler (with IVideoResampler support) for this demo to work");
// Create a Xuggler container object
IContainer container = IContainer.make();
// Open up the container
if (container.open(filename, IContainer.Type.READ, null) < 0)
throw new IllegalArgumentException("could not open file: " + filename);
// query how many streams the call to open found
int numStreams = container.getNumStreams();
// and iterate through the streams to find the first audio stream
int videoStreamId = -1;
IStreamCoder videoCoder = null;
int audioStreamId = -1;
IStreamCoder audioCoder = null;
for(int i = 0; i < numStreams; i++)
{
// Find the stream object
IStream stream = container.getStream(i);
// Get the pre-configured decoder that can decode this stream;
IStreamCoder coder = stream.getStreamCoder();
if (videoStreamId == -1 && coder.getCodecType() == ICodec.Type.CODEC_TYPE_VIDEO)
{
videoStreamId = i;
videoCoder = coder;
}
else if (audioStreamId == -1 && coder.getCodecType() == ICodec.Type.CODEC_TYPE_AUDIO)
{
audioStreamId = i;
audioCoder = coder;
}
}
if (videoStreamId == -1 && audioStreamId == -1)
throw new RuntimeException("could not find audio or video stream in container: "+filename);
/*
* Check if we have a video stream in this file. If so let's open up our decoder so it can
* do work.
*/
IVideoResampler resampler = null;
if (videoCoder != null)
{
if(videoCoder.open() < 0)
throw new RuntimeException("could not open audio decoder for container: "+filename);
if (videoCoder.getPixelType() != IPixelFormat.Type.BGR24)
{
// if this stream is not in BGR24, we're going to need to
// convert it. The VideoResampler does that for us.
resampler = IVideoResampler.make(videoCoder.getWidth(), videoCoder.getHeight(), IPixelFormat.Type.BGR24,
videoCoder.getWidth(), videoCoder.getHeight(), videoCoder.getPixelType());
if (resampler == null)
throw new RuntimeException("could not create color space resampler for: " + filename);
}
/*
* And once we have that, we draw a window on screen
*/
openJavaVideo();
}
if (audioCoder != null)
{
if (audioCoder.open() < 0)
throw new RuntimeException("could not open audio decoder for container: "+filename);
/*
* And once we have that, we ask the Java Sound System to get itself ready.
*/
try
{
openJavaSound(audioCoder);
}
catch (LineUnavailableException ex)
{
throw new RuntimeException("unable to open sound device on your system when playing back container: "+filename);
}
}
/*
* Now, we start walking through the container looking at each packet.
*/
IPacket packet = IPacket.make();
mFirstVideoTimestampInStream = Global.NO_PTS;
mSystemVideoClockStartTime = 0;
while(container.readNextPacket(packet) >= 0)
{
/*
* Now we have a packet, let's see if it belongs to our video stream
*/
if (packet.getStreamIndex() == videoStreamId)
{
/*
* We allocate a new picture to get the data out of Xuggler
*/
IVideoPicture picture = IVideoPicture.make(videoCoder.getPixelType(),
videoCoder.getWidth(), videoCoder.getHeight());
/*
* Now, we decode the video, checking for any errors.
*
*/
int bytesDecoded = videoCoder.decodeVideo(picture, packet, 0);
if (bytesDecoded < 0)
throw new RuntimeException("got error decoding audio in: " + filename);
/*
* Some decoders will consume data in a packet, but will not be able to construct
* a full video picture yet. Therefore you should always check if you
* got a complete picture from the decoder
*/
if (picture.isComplete())
{
IVideoPicture newPic = picture;
/*
* If the resampler is not null, that means we didn't get the video in BGR24 format and
* need to convert it into BGR24 format.
*/
if (resampler != null)
{
// we must resample
newPic = IVideoPicture.make(resampler.getOutputPixelFormat(), picture.getWidth(), picture.getHeight());
if (resampler.resample(newPic, picture) < 0)
throw new RuntimeException("could not resample video from: " + filename);
}
if (newPic.getPixelType() != IPixelFormat.Type.BGR24)
throw new RuntimeException("could not decode video as BGR 24 bit data in: " + filename);
long delay = millisecondsUntilTimeToDisplay(newPic);
// if there is no audio stream; go ahead and hold up the main thread. We'll end
// up caching fewer video pictures in memory that way.
try
{
if (delay > 0)
Thread.sleep(delay);
}
catch (InterruptedException e)
{
return;
}
// And finally, convert the picture to an image and display it
mScreen.setImage(Utils.videoPictureToImage(newPic));
}
}
else if (packet.getStreamIndex() == audioStreamId)
{
/*
* We allocate a set of samples with the same number of channels as the
* coder tells us is in this buffer.
*
* We also pass in a buffer size (1024 in our example), although Xuggler
* will probably allocate more space than just the 1024 (it's not important why).
*/
IAudioSamples samples = IAudioSamples.make(1024, audioCoder.getChannels());
/*
* A packet can actually contain multiple sets of samples (or frames of samples
* in audio-decoding speak). So, we may need to call decode audio multiple
* times at different offsets in the packet's data. We capture that here.
*/
int offset = 0;
/*
* Keep going until we've processed all data
*/
while(offset < packet.getSize())
{
int bytesDecoded = audioCoder.decodeAudio(samples, packet, offset);
if (bytesDecoded < 0)
throw new RuntimeException("got error decoding audio in: " + filename);
offset += bytesDecoded;
/*
* Some decoder will consume data in a packet, but will not be able to construct
* a full set of samples yet. Therefore you should always check if you
* got a complete set of samples from the decoder
*/
if (samples.isComplete())
{
// note: this call will block if Java's sound buffers fill up, and we're
// okay with that. That's why we have the video "sleeping" occur
// on another thread.
playJavaSound(samples);
}
}
}
else
{
/*
* This packet isn't part of our video stream, so we just silently drop it.
*/
do {} while(false);
}
}
/*
* Technically since we're exiting anyway, these will be cleaned up by
* the garbage collector... but because we're nice people and want
* to be invited places for Christmas, we're going to show how to clean up.
*/
if (videoCoder != null)
{
videoCoder.close();
videoCoder = null;
}
if (audioCoder != null)
{
audioCoder.close();
audioCoder = null;
}
if (container !=null)
{
container.close();
container = null;
}
closeJavaSound();
closeJavaVideo();
下面 的方法是做什么的？
private static long millisecondsUntilTimeToDisplay(IVideoPicture picture)
{
/**
* We could just display the images as quickly as we decode them, but it turns
* out we can decode a lot faster than you think.
*
* So instead, the following code does a poor-man's version of trying to
* match up the frame-rate requested for each IVideoPicture with the system
* clock time on your computer.
*
* Remember that all Xuggler IAudioSamples and IVideoPicture objects always
* give timestamps in Microseconds, relative to the first decoded item. If
* instead you used the packet timestamps, they can be in different units depending
* on your IContainer, and IStream and things can get hairy quickly.
*/
long millisecondsToSleep = 0;
if (mFirstVideoTimestampInStream == Global.NO_PTS)
{
// This is our first time through
mFirstVideoTimestampInStream = picture.getTimeStamp();
// get the starting clock time so we can hold up frames
// until the right time.
mSystemVideoClockStartTime = System.currentTimeMillis();
millisecondsToSleep = 0;
} else {
long systemClockCurrentTime = System.currentTimeMillis();
long millisecondsClockTimeSinceStartofVideo = systemClockCurrentTime - mSystemVideoClockStartTime;
// compute how long for this frame since the first frame in the stream.
// remember that IVideoPicture and IAudioSamples timestamps are always in MICROSECONDS,
// so we divide by 1000 to get milliseconds.
long millisecondsStreamTimeSinceStartOfVideo = (picture.getTimeStamp() - mFirstVideoTimestampInStream)/1000;
final long millisecondsTolerance = 50; // and we give ourselfs 50 ms of tolerance
millisecondsToSleep = (millisecondsStreamTimeSinceStartOfVideo -
(millisecondsClockTimeSinceStartofVideo+millisecondsTolerance));
}
return millisecondsToSleep;
}
/**
* Opens a Swing window on screen.
*/
private static void openJavaVideo()
{
mScreen = new VideoImage();
}
/**
* Forces the swing thread to terminate; I'm sure there is a right
* way to do this in swing, but this works too.
*/
private static void closeJavaVideo()
{
System.exit(0);
}
private static void openJavaSound(IStreamCoder aAudioCoder) throws LineUnavailableException
{
AudioFormat audioFormat = new AudioFormat(aAudioCoder.getSampleRate(),
(int)IAudioSamples.findSampleBitDepth(aAudioCoder.getSampleFormat()),
aAudioCoder.getChannels(),
true, /* xuggler defaults to signed 16 bit samples */
false);
DataLine.Info info = new DataLine.Info(SourceDataLine.class, audioFormat);
mLine = (SourceDataLine) AudioSystem.getLine(info);
/**
* if that succeeded, try opening the line.
*/
mLine.open(audioFormat);
/**
* And if that succeed, start the line.
*/
mLine.start();
}
private static void playJavaSound(IAudioSamples aSamples)
{
/**
* We're just going to dump all the samples into the line.
*/
byte[] rawBytes = aSamples.getData().getByteArray(0, aSamples.getSize());
mLine.write(rawBytes, 0, aSamples.getSize());
}
private static void closeJavaSound()
{
if (mLine != null)
{
/*
* Wait for the line to finish playing
*/
mLine.drain();
/*
* Close the line.
*/
mLine.close();
mLine=null;
}
}
}
