Android NDK开发之旅30 NDK FFmpeg音频解码与播放

###音频解码实现

音频解码也是直接使用FFmpeg的API来做。

public native void sound(String input,String output);
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其中，JNI实现如下：

//重采样
#include "libswresample/swresample.h"

#define MAX_AUDIO_FRME_SIZE 48000 * 4

JNIEXPORT void JNICALL
Java_com_nan_ffmpeg_utils_VideoPlayer_sound(JNIEnv *env, jobject instance, jstring input_,
                                            jstring output_) {
    const char *input = (*env)->GetStringUTFChars(env, input_, 0);
    const char *output = (*env)->GetStringUTFChars(env, output_, 0);

    //注册组件
    av_register_all();
    AVFormatContext *pFormatCtx = avformat_alloc_context();
    //打开音频文件
    if (avformat_open_input(&pFormatCtx, input, NULL, NULL) != 0) {
        LOGI("%s", "无法打开音频文件");
        return;
    }
    //获取输入文件信息
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {
        LOGI("%s", "无法获取输入文件信息");
        return;
    }
    //获取音频流索引位置
    int i = 0, audio_stream_idx = -1;
    for (; i < pFormatCtx->nb_streams; i++) {
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
            audio_stream_idx = i;
            break;
        }
    }

    //获取解码器
    AVCodecContext *codecCtx = pFormatCtx->streams[audio_stream_idx]->codec;
    AVCodec *codec = avcodec_find_decoder(codecCtx->codec_id);
    if (codec == NULL) {
        LOGI("%s", "无法获取解码器");
        return;
    }
    //打开解码器
    if (avcodec_open2(codecCtx, codec, NULL) < 0) {
        LOGI("%s", "无法打开解码器");
        return;
    }
    //压缩数据
    AVPacket *packet = (AVPacket *) av_malloc(sizeof(AVPacket));
    //解压缩数据
    AVFrame *frame = av_frame_alloc();
    //frame->16bit 44100 PCM 统一音频采样格式与采样率
    SwrContext *swrCtx = swr_alloc();

    //重采样设置参数-------------start
    //输入的采样格式
    enum AVSampleFormat in_sample_fmt = codecCtx->sample_fmt;
    //输出采样格式16bit PCM
    enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;
    //输入采样率
    int in_sample_rate = codecCtx->sample_rate;
    //输出采样率
    int out_sample_rate = 44100;
    //获取输入的声道布局
    //根据声道个数获取默认的声道布局（2个声道，默认立体声stereo）
    //av_get_default_channel_layout(codecCtx->channels);
    uint64_t in_ch_layout = codecCtx->channel_layout;
    //输出的声道布局（立体声）
    uint64_t out_ch_layout = AV_CH_LAYOUT_STEREO;

    swr_alloc_set_opts(swrCtx,
                       out_ch_layout, out_sample_fmt, out_sample_rate,
                       in_ch_layout, in_sample_fmt, in_sample_rate,
                       0, NULL);
    swr_init(swrCtx);

    //输出的声道个数
    int out_channel_nb = av_get_channel_layout_nb_channels(out_ch_layout);

    //重采样设置参数-------------end

    //16bit 44100 PCM 数据
    uint8_t *out_buffer = (uint8_t *) av_malloc(MAX_AUDIO_FRME_SIZE);

    FILE *fp_pcm = fopen(output, "wb");

    int got_frame = 0, index = 0, ret;
    //不断读取压缩数据
    while (av_read_frame(pFormatCtx, packet) >= 0) {
        //解码
        ret = avcodec_decode_audio4(codecCtx, frame, &got_frame, packet);

        if (ret < 0) {
            LOGI("%s", "解码完成");
        }
        //解码一帧成功
        if (got_frame > 0) {
            LOGI("解码：%d", index++);
            swr_convert(swrCtx, &out_buffer, MAX_AUDIO_FRME_SIZE, frame->data, frame->nb_samples);
            //获取sample的size
            int out_buffer_size = av_samples_get_buffer_size(NULL, out_channel_nb,
                                                             frame->nb_samples, out_sample_fmt, 1);
            fwrite(out_buffer, 1, out_buffer_size, fp_pcm);
        }

        av_free_packet(packet);
    }

    fclose(fp_pcm);
    av_frame_free(&frame);
    av_free(out_buffer);

    swr_free(&swrCtx);
    avcodec_close(codecCtx);
    avformat_close_input(&pFormatCtx);

    (*env)->ReleaseStringUTFChars(env, input_, input);
    (*env)->ReleaseStringUTFChars(env, output_, output);
}
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最终会输出pcm格式的文件。

###音频播放

PCM音频播放的常用方法：

下面我们通过Audio Track来进行播放。

主要的实现思路就是：

1. 在C层调用Java层的createAudioTrack方法，创建AudioTrack对象。
2. 然后在C层调用AudioTrack的pkay、write、stop等方法进行播放控制。

例子：

package com.nan.ffmpeg.utils;

public class VideoPlayer {

    static {
        System.loadLibrary("avutil-54");
        System.loadLibrary("swresample-1");
        System.loadLibrary("avcodec-56");
        System.loadLibrary("avformat-56");
        System.loadLibrary("swscale-3");
        System.loadLibrary("postproc-53");
        System.loadLibrary("avfilter-5");
        System.loadLibrary("avdevice-56");
        System.loadLibrary("ffmpeg-lib");
    }

    public native void render(String input, Surface surface);

    public native void play(String input);

    /**
     * 创建一个AudioTrac对象，用于播放
     *
     * @return
     */
    public AudioTrack createAudioTrack(int sampleRateInHz, int nb_channels) {
        //固定格式的音频码流
        int audioFormat = AudioFormat.ENCODING_PCM_16BIT;
        Log.i("jason", "nb_channels:" + nb_channels);
        //声道布局
        int channelConfig;
        if (nb_channels == 1) {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_MONO;
        } else if (nb_channels == 2) {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_STEREO;
        } else {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_STEREO;
        }

        int bufferSizeInBytes = AudioTrack.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);

        AudioTrack audioTrack = new AudioTrack(
                AudioManager.STREAM_MUSIC,
                sampleRateInHz, channelConfig,
                audioFormat,
                bufferSizeInBytes, AudioTrack.MODE_STREAM);
        //播放
        //audioTrack.play();
        //写入PCM
        //audioTrack.write(audioData, offsetInBytes, sizeInBytes);
        //播放完调用stop即可

        audioTrack.stop();
        return audioTrack;
    }

}
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JNI实现如下：

//音频播放
JNIEXPORT void JNICALL
Java_com_nan_ffmpeg_utils_VideoPlayer_play(JNIEnv *env, jobject instance, jstring input_) {
    const char *input = (*env)->GetStringUTFChars(env, input_, 0);

    LOGI("%s", "sound");
    //注册组件
    av_register_all();
    AVFormatContext *pFormatCtx = avformat_alloc_context();
    //打开音频文件
    if (avformat_open_input(&pFormatCtx, input, NULL, NULL) != 0) {
        LOGI("%s", "无法打开音频文件");
        return;
    }
    //获取输入文件信息
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {
        LOGI("%s", "无法获取输入文件信息");
        return;
    }
    //获取音频流索引位置
    int i = 0, audio_stream_idx = -1;
    for (; i < pFormatCtx->nb_streams; i++) {
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
            audio_stream_idx = i;
            break;
        }
    }

    //获取解码器
    AVCodecContext *codecCtx = pFormatCtx->streams[audio_stream_idx]->codec;
    AVCodec *codec = avcodec_find_decoder(codecCtx->codec_id);
    if (codec == NULL) {
        LOGI("%s", "无法获取解码器");
        return;
    }
    //打开解码器
    if (avcodec_open2(codecCtx, codec, NULL) < 0) {
        LOGI("%s", "无法打开解码器");
        return;
    }
    //压缩数据
    AVPacket *packet = (AVPacket *) av_malloc(sizeof(AVPacket));
    //解压缩数据
    AVFrame *frame = av_frame_alloc();
    //frame->16bit 44100 PCM 统一音频采样格式与采样率
    SwrContext *swrCtx = swr_alloc();

    //重采样设置参数-------------start
    //输入的采样格式
    enum AVSampleFormat in_sample_fmt = codecCtx->sample_fmt;
    //输出采样格式16bit PCM
    enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;
    //输入采样率
    int in_sample_rate = codecCtx->sample_rate;
    //输出采样率
    int out_sample_rate = in_sample_rate;
    //获取输入的声道布局
    //根据声道个数获取默认的声道布局（2个声道，默认立体声stereo）
    //av_get_default_channel_layout(codecCtx->channels);
    uint64_t in_ch_layout = codecCtx->channel_layout;
    //输出的声道布局（立体声）
    uint64_t out_ch_layout = AV_CH_LAYOUT_STEREO;

    swr_alloc_set_opts(swrCtx,
                       out_ch_layout, out_sample_fmt, out_sample_rate,
                       in_ch_layout, in_sample_fmt, in_sample_rate,
                       0, NULL);
    swr_init(swrCtx);

    //输出的声道个数
    int out_channel_nb = av_get_channel_layout_nb_channels(out_ch_layout);

    //重采样设置参数-------------end

    //JNI begin------------------
    //JasonPlayer
    jclass player_class = (*env)->GetObjectClass(env, instance);

    //AudioTrack对象
    jmethodID create_audio_track_mid = (*env)->GetMethodID(env, player_class, "createAudioTrack",
                                                           "(II)Landroid/media/AudioTrack;");
    jobject audio_track = (*env)->CallObjectMethod(env, instance, create_audio_track_mid,
                                                   out_sample_rate, out_channel_nb);

    //调用AudioTrack.play方法
    jclass audio_track_class = (*env)->GetObjectClass(env, audio_track);
    jmethodID audio_track_play_mid = (*env)->GetMethodID(env, audio_track_class, "play", "()V");
    jmethodID audio_track_stop_mid = (*env)->GetMethodID(env, audio_track_class, "stop", "()V");
    (*env)->CallVoidMethod(env, audio_track, audio_track_play_mid);

    //AudioTrack.write
    jmethodID audio_track_write_mid = (*env)->GetMethodID(env, audio_track_class, "write",
                                                          "([BII)I");
    //JNI end------------------

    //16bit 44100 PCM 数据
    uint8_t *out_buffer = (uint8_t *) av_malloc(MAX_AUDIO_FRME_SIZE);

    int got_frame = 0, index = 0, ret;
    //不断读取压缩数据
    while (av_read_frame(pFormatCtx, packet) >= 0) {
        //解码音频类型的Packet
        if (packet->stream_index == audio_stream_idx) {
            //解码
            ret = avcodec_decode_audio4(codecCtx, frame, &got_frame, packet);

            if (ret < 0) {
                LOGI("%s", "解码完成");
            }
            //解码一帧成功
            if (got_frame > 0) {
                LOGI("解码：%d", index++);
                swr_convert(swrCtx, &out_buffer, MAX_AUDIO_FRME_SIZE,
                            (const uint8_t **) frame->data, frame->nb_samples);
                //获取sample的size
                int out_buffer_size = av_samples_get_buffer_size(NULL, out_channel_nb,
                                                                 frame->nb_samples, out_sample_fmt,
                                                                 1);

                //out_buffer缓冲区数据，转成byte数组
                jbyteArray audio_sample_array = (*env)->NewByteArray(env, out_buffer_size);
                jbyte *sample_bytep = (*env)->GetByteArrayElements(env, audio_sample_array, NULL);
                //out_buffer的数据复制到sampe_bytep
                memcpy(sample_bytep, out_buffer, out_buffer_size);
                //同步
                (*env)->ReleaseByteArrayElements(env, audio_sample_array, sample_bytep, 0);

                //AudioTrack.write PCM数据
                (*env)->CallIntMethod(env, audio_track, audio_track_write_mid,
                                      audio_sample_array, 0, out_buffer_size);
                //释放局部引用
                (*env)->DeleteLocalRef(env, audio_sample_array);
            }
        }

        av_free_packet(packet);
    }

    (*env)->CallVoidMethod(env, audio_track, audio_track_stop_mid);

    av_frame_free(&frame);
    av_free(out_buffer);

    swr_free(&swrCtx);
    avcodec_close(codecCtx);
    avformat_close_input(&pFormatCtx);

    (*env)->ReleaseStringUTFChars(env, input_, input);
}
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测试：

@Override
public void onClick(View v) {
    String inputVideo = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separatorChar
            + "input.mp4";

    String inputAudio = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separatorChar
            + "love.mp3";

    switch (v.getId()) {

        case R.id.btn_play_audio:

            mPlayer.play(inputVideo);
            break;

    }
}
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