Android音视频开发只硬件解码组件MediaCodec讲解

什么是MediaCodec

MediaCodec是Android提供的一种硬件解码组件，通过MediaCodec可以将硬件解码器(比如硬解码器)的硬件加速模块进行利用以提高视频进行解码的速度。

相比于软解码，硬解码可以有效的提高解码速度，使得更多的设备可以进行高清视频的播放。

使用MediaCodec进行视频解码

使用MediaCodec进行视频解码需要涉及到以下几个步骤：

1. 创建MediaFormat对象，并将需要进行解码的参数设置好。比如需要解码的视频的Mime类型、视频的分辨率、视频的码率等。

MediaFormat format = MediaFormat.createVideoFormat("video/avc", 1280, 720);
format.setInteger(MediaFormat.KEY_BIT_RATE, 10000000);
format.setInteger(MediaFormat.KEY_FRAME_RATE, 30);

1. 创建并初始化一个MediaCodec对象。

MediaCodec codec = MediaCodec.createDecoderByType("video/avc");
codec.configure(format, null, null, 0);
codec.start();

1. 将需要进行解码的视频数据传递给MediaCodec进行解码。

ByteBuffer[] inputBuffers = codec.getInputBuffers();
int inputBufferIndex = codec.dequeueInputBuffer(-1);
if (inputBufferIndex >= 0) {
ByteBuffer inputBuffer = inputBuffers[inputBufferIndex];
inputBuffer.clear();
inputBuffer.put(videoData);
codec.queueInputBuffer(inputBufferIndex, 0, videoData.length, 0, 0);
}

1. 从MediaCodec中获取到解码后的数据并进行处理。

```
ByteBuffer[] outputBuffers = codec.getOutputBuffers();
MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
int outputBufferIndex = codec.dequeueOutputBuffer(bufferInfo, 0);
if (outputBufferIndex >= 0) {
ByteBuffer outputBuffer = outputBuffers[outputBufferIndex];
byte[] outputData = new byte[bufferInfo.size];
outputBuffer.get(outputData);

   // 进行数据处理...

   codec.releaseOutputBuffer(outputBufferIndex, false);

}
```

示例1：视频播放器

下面是一个简单的视频播放器，使用MediaCodec进行硬解码：

public class VideoPlayer {
    private MediaCodec codec;
    private Surface surface;
    private boolean isPlaying;

    public void startPlay(String filePath, Surface surface) throws IOException {
        isPlaying = true;
        this.surface = surface;

        File file = new File(filePath);
        FileInputStream inputStream = new FileInputStream(file);
        int length = (int) file.length();
        byte[] data = new byte[length];
        inputStream.read(data);
        inputStream.close();

        MediaExtractor extractor = new MediaExtractor();
        extractor.setDataSource(filePath);

        int trackIndex = -1;
        for (int i = 0; i < extractor.getTrackCount(); i++) {
            MediaFormat format = extractor.getTrackFormat(i);
            String mime = format.getString(MediaFormat.KEY_MIME);
            if (mime.startsWith("video/")) {
                extractor.selectTrack(i);
                codec = MediaCodec.createDecoderByType(mime);
                codec.configure(format, surface, null, 0);
                break;
            }
        }
        codec.start();

        ByteBuffer[] inputBuffers = codec.getInputBuffers();
        ByteBuffer[] outputBuffers = codec.getOutputBuffers();
        MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();
        boolean isEOS = false;
        long startMs = System.currentTimeMillis();

        while (isPlaying) {
            if (!isEOS) {
                int inIndex = codec.dequeueInputBuffer(10000);
                if (inIndex >= 0) {
                    ByteBuffer buffer = inputBuffers[inIndex];
                    int sampleSize = extractor.readSampleData(buffer, 0);
                    if (sampleSize < 0) {
                        codec.queueInputBuffer(inIndex, 0, 0, 0, MediaCodec.BUFFER_FLAG_END_OF_STREAM);
                        isEOS = true;
                    } else {
                        codec.queueInputBuffer(inIndex, 0, sampleSize, extractor.getSampleTime(), 0);
                        extractor.advance();
                    }
                }
            }

            int outIndex = codec.dequeueOutputBuffer(info, 10000);
            switch (outIndex) {
                case MediaCodec.INFO_OUTPUT_BUFFERS_CHANGED:
                    outputBuffers = codec.getOutputBuffers();
                    break;
                case MediaCodec.INFO_OUTPUT_FORMAT_CHANGED:
                    break;
                case MediaCodec.INFO_TRY_AGAIN_LATER:
                    break;
                default:
                    ByteBuffer buffer = outputBuffers[outIndex];
                    codec.releaseOutputBuffer(outIndex, true);
                    break;
            }

            if ((info.flags & MediaCodec.BUFFER_FLAG_END_OF_STREAM) != 0) {
                break;
            }
        }

        codec.stop();
        codec.release();
        extractor.release();
    }

    public void stopPlay() {
        isPlaying = false;
    }
}

示例2：视频解码并保存

下面是一个使用MediaCodec进行视频解码并保存的示例：

public class VideoDecoder {
    public void decode(String inputPath, String outputPath) throws IOException {
        MediaExtractor extractor = new MediaExtractor();
        extractor.setDataSource(inputPath);

        int trackIndex = -1;
        for (int i = 0; i < extractor.getTrackCount(); i++) {
            MediaFormat format = extractor.getTrackFormat(i);
            String mime = format.getString(MediaFormat.KEY_MIME);
            if (mime.startsWith("video/")) {
                extractor.selectTrack(i);
                MediaCodec codec = MediaCodec.createDecoderByType(mime);
                codec.configure(format, null, null, 0);
                codec.start();

                ByteBuffer[] inputBuffers = codec.getInputBuffers();
                ByteBuffer[] outputBuffers = codec.getOutputBuffers();
                MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
                long timeOutUs = 10000;

                FileOutputStream outputStream = new FileOutputStream(outputPath);
                while (true) {
                    int inIndex = codec.dequeueInputBuffer(timeOutUs);
                    if (inIndex >= 0) {
                        ByteBuffer inputBuffer = inputBuffers[inIndex];
                        int size = extractor.readSampleData(inputBuffer, 0);
                        if (size < 0) {
                            codec.queueInputBuffer(inIndex, 0, 0, 0, MediaCodec.BUFFER_FLAG_END_OF_STREAM);
                            break;
                        } else {
                            codec.queueInputBuffer(inIndex, 0, size, extractor.getSampleTime(), 0);
                            extractor.advance();
                        }
                    }

                    int outIndex = codec.dequeueOutputBuffer(bufferInfo, timeOutUs);
                    while (outIndex >= 0) {
                        ByteBuffer outputBuffer = outputBuffers[outIndex];
                        byte[] outData = new byte[bufferInfo.size];
                        outputBuffer.get(outData);
                        outputStream.write(outData);

                        codec.releaseOutputBuffer(outIndex, false);
                        outIndex = codec.dequeueOutputBuffer(bufferInfo, timeOutUs);
                    }
                }

                codec.stop();
                codec.release();
                extractor.release();
                outputStream.close();
                break;
            }
        }
    }
}

总结

通过MediaCodec进行硬解码，可以极大的优化视频解码的性能，并且在优化过程中可以根据不同的视频格式，使用不同的硬解码器以获得更好的性能。