ffmpeg实战教程（十二）为视频添加/更换背景音乐

2017年，各种关于视频处理的功能层出不穷，现在基本上大多APP都有视频上传播放功能。
然而上传，播放可用的优秀框架有很多，我们在此就不说了。
本篇我们就做一个用ffmpeg给视频添加/更换背景音乐的功能

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UI界面有两个输入框，第一个填入要加背景音乐的视频文件，第一个要添加音乐的文件。
然后点击按钮。
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可见在手机根目录生成了一个合成的视频文件。试播一下，发现已经有了背景音乐。

其实现原理主要是剥开视频文件拿到视频裸流，然后拿到音频文件根据时间戳一帧一帧的封装成一个新的视频文件

下面是具体实现的代码逻辑。

首先项目中导入编译好的ffmpeg库，然后书写CMakeLists.txt链接这些库

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然后编写界面，书写界面的逻辑。
主要代码如下：

 @Override
    public void onClick(View view) {
        switch (view.getId()) {

            case R.id.button:
                startDecode();
                break;

            default:
                break;
        }
    }

  private void startDecode() {

        String folderurl= Environment.getExternalStorageDirectory().getPath();

        String videoUrl=folderurl+"/"+editText1.getText().toString();

        String musicUrl=folderurl+"/"+editText2.getText().toString();

        Log.e("ws-----------videoUrl",videoUrl);
        Log.e("ws------------musicUrl",musicUrl);

        addBgMusci(videoUrl,musicUrl);

    }

    public native int addBgMusci(String videoUrl, String musicUrl);

最后调用ffmpeg API实现native 方法addBgMusci。
JNI源码如下：

#include <jni.h>
#include <string>

extern "C"
{
#include <libavutil/log.h>
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>

#include <android/log.h>
#define LOGE(format, ...)  __android_log_print(ANDROID_LOG_ERROR, "(>_<)", format, ##__VA_ARGS__)
#define LOGI(format, ...)  __android_log_print(ANDROID_LOG_INFO,  "(^_^)", format, ##__VA_ARGS__)

//Output FFmpeg's av_log()
void custom_log(void *ptr, int level, const char* fmt, va_list vl){
    FILE *fp=fopen("/storage/emulated/0/av_log.txt","a+");
    if(fp){
        vfprintf(fp,fmt,vl);
        fflush(fp);
        fclose(fp);
    }
}

JNIEXPORT jint JNICALL
Java_com_ws_ffmpegandroidaddmusic_MainActivity_addBgMusci(
        JNIEnv *env,
        jobject , jstring input_video, jstring input_music) {

    AVOutputFormat *ofmt = NULL;
    //Input AVFormatContext and Output AVFormatContext
    AVFormatContext *ifmt_ctx_v = NULL, *ifmt_ctx_a = NULL,*ofmt_ctx = NULL;
    AVPacket pkt;
    int ret, i;
    int videoindex_v=-1,videoindex_out=-1;
    int audioindex_a=-1,audioindex_out=-1;
    int frame_index=0;
    int64_t cur_pts_v=0,cur_pts_a=0;


    const char *in_filename_v =  env->GetStringUTFChars(input_video,NULL);
    const char *in_filename_a =  env->GetStringUTFChars(input_music,NULL);

    const char *out_filename = "/storage/emulated/0/wsAddMusic.flv";//Output file URL
    av_register_all();
    //Input
    if ((ret = avformat_open_input(&ifmt_ctx_v, in_filename_v, 0, 0)) < 0) {//打开输入的视频文件
        LOGE( "Could not open input file.");
        goto end;
    }
    if ((ret = avformat_find_stream_info(ifmt_ctx_v, 0)) < 0) {//获取视频文件信息
        LOGE( "Failed to retrieve input stream information");
        goto end;
    }

    if ((ret = avformat_open_input(&ifmt_ctx_a, in_filename_a, 0, 0)) < 0) {//打开输入的音频文件
        LOGE( "Could not open input file.");
        goto end;
    }
    if ((ret = avformat_find_stream_info(ifmt_ctx_a, 0)) < 0) {//获取音频文件信息
        LOGE( "Failed to retrieve input stream information");
        goto end;
    }
    LOGE("===========Input Information==========\n");
    av_dump_format(ifmt_ctx_v, 0, in_filename_v, 0);
    av_dump_format(ifmt_ctx_a, 0, in_filename_a, 0);
    LOGE("======================================\n");
    //Output
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);//初始化输出码流的AVFormatContext。
    if (!ofmt_ctx) {
        LOGE( "Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        return -1;
    }
    ofmt = ofmt_ctx->oformat;

    //从输入的AVStream中获取一个输出的out_stream
    for (i = 0; i < ifmt_ctx_v->nb_streams; i++) {
        //Create output AVStream according to input AVStream
        if(ifmt_ctx_v->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO){
            AVStream *in_stream = ifmt_ctx_v->streams[i];
            AVStream *out_stream = avformat_new_stream(ofmt_ctx, in_stream->codec->codec);//创建流通道AVStream
            videoindex_v=i;
            if (!out_stream) {
                LOGE( "Failed allocating output stream\n");
                ret = AVERROR_UNKNOWN;
                break;
            }
            videoindex_out=out_stream->index;
            //Copy the settings of AVCodecContext
            if (avcodec_copy_context(out_stream->codec, in_stream->codec) < 0) {
                LOGE( "Failed to copy context from input to output stream codec context\n");
                break;
            }
            out_stream->codec->codec_tag = 0;
            if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
                out_stream->codec->flags |= CODEC_FLAG_GLOBAL_HEADER;
            break;
        }
    }

    for (i = 0; i < ifmt_ctx_a->nb_streams; i++) {
        //Create output AVStream according to input AVStream
        if(ifmt_ctx_a->streams[i]->codec->codec_type==AVMEDIA_TYPE_AUDIO){
            AVStream *in_stream = ifmt_ctx_a->streams[i];
            AVStream *out_stream = avformat_new_stream(ofmt_ctx, in_stream->codec->codec);
            audioindex_a=i;
            if (!out_stream) {
                LOGE( "Failed allocating output stream\n");
                ret = AVERROR_UNKNOWN;
                goto end;
            }
            audioindex_out=out_stream->index;
            //Copy the settings of AVCodecContext
            if (avcodec_copy_context(out_stream->codec, in_stream->codec) < 0) {
                LOGE( "Failed to copy context from input to output stream codec context\n");
                goto end;
            }
            out_stream->codec->codec_tag = 0;
            if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
                out_stream->codec->flags |= CODEC_FLAG_GLOBAL_HEADER;

            break;
        }
    }

    LOGE("==========Output Information==========\n");
    av_dump_format(ofmt_ctx, 0, out_filename, 1);
    LOGE("======================================\n");
    //Open output file
    if (!(ofmt->flags & AVFMT_NOFILE)) {
        if (avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE) < 0) {//打开输出文件。
            LOGE( "Could not open output file '%s'", out_filename);
            return -1;
        }
    }
    //Write file header
    if (avformat_write_header(ofmt_ctx, NULL) < 0) {
        LOGE( "Error occurred when opening output file\n");
        return -1;
    }


    //FIX
#if USE_H264BSF
    AVBitStreamFilterContext* h264bsfc =  av_bitstream_filter_init("h264_mp4toannexb");
#endif
#if USE_AACBSF
    AVBitStreamFilterContext* aacbsfc =  av_bitstream_filter_init("aac_adtstoasc");
#endif

    while (1) {
        AVFormatContext *ifmt_ctx;
        int stream_index=0;
        AVStream *in_stream, *out_stream;

        //Get an AVPacket .   av_compare_ts是比较时间戳用的。通过该函数可以决定该写入视频还是音频。
        if(av_compare_ts(cur_pts_v,ifmt_ctx_v->streams[videoindex_v]->time_base,cur_pts_a,ifmt_ctx_a->streams[audioindex_a]->time_base) <= 0){
            ifmt_ctx=ifmt_ctx_v;
            stream_index=videoindex_out;

            if(av_read_frame(ifmt_ctx, &pkt) >= 0){
                do{
                    in_stream  = ifmt_ctx->streams[pkt.stream_index];
                    out_stream = ofmt_ctx->streams[stream_index];

                    if(pkt.stream_index==videoindex_v){
                        //FIX：No PTS (Example: Raw H.264) H.264裸流没有PTS，因此必须手动写入PTS
                        //Simple Write PTS
                        if(pkt.pts==AV_NOPTS_VALUE){
                            //Write PTS
                            AVRational time_base1=in_stream->time_base;
                            //Duration between 2 frames (us)
                            int64_t calc_duration=(double)AV_TIME_BASE/av_q2d(in_stream->r_frame_rate);
                            //Parameters
                            pkt.pts=(double)(frame_index*calc_duration)/(double)(av_q2d(time_base1)*AV_TIME_BASE);
                            pkt.dts=pkt.pts;
                            pkt.duration=(double)calc_duration/(double)(av_q2d(time_base1)*AV_TIME_BASE);
                            frame_index++;
                        }

                        cur_pts_v=pkt.pts;
                        break;
                    }
                }while(av_read_frame(ifmt_ctx, &pkt) >= 0);
            }else{
                break;
            }
        }else{
            ifmt_ctx=ifmt_ctx_a;
            stream_index=audioindex_out;
            if(av_read_frame(ifmt_ctx, &pkt) >= 0){
                do{
                    in_stream  = ifmt_ctx->streams[pkt.stream_index];
                    out_stream = ofmt_ctx->streams[stream_index];

                    if(pkt.stream_index==audioindex_a){

                        //FIX：No PTS
                        //Simple Write PTS
                        if(pkt.pts==AV_NOPTS_VALUE){
                            //Write PTS
                            AVRational time_base1=in_stream->time_base;
                            //Duration between 2 frames (us)
                            int64_t calc_duration=(double)AV_TIME_BASE/av_q2d(in_stream->r_frame_rate);
                            //Parameters
                            pkt.pts=(double)(frame_index*calc_duration)/(double)(av_q2d(time_base1)*AV_TIME_BASE);
                            pkt.dts=pkt.pts;
                            pkt.duration=(double)calc_duration/(double)(av_q2d(time_base1)*AV_TIME_BASE);
                            frame_index++;
                        }
                        cur_pts_a=pkt.pts;

                        break;
                    }
                }while(av_read_frame(ifmt_ctx, &pkt) >= 0);
            }else{
                break;
            }

        }

        //FIX:Bitstream Filter
#if USE_H264BSF
        av_bitstream_filter_filter(h264bsfc, in_stream->codec, NULL, &pkt.data, &pkt.size, pkt.data, pkt.size, 0);
#endif
#if USE_AACBSF
        av_bitstream_filter_filter(aacbsfc, out_stream->codec, NULL, &pkt.data, &pkt.size, pkt.data, pkt.size, 0);
#endif


        //Convert PTS/DTS
        pkt.pts = av_rescale_q_rnd(pkt.pts, in_stream->time_base, out_stream->time_base, (AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
        pkt.dts = av_rescale_q_rnd(pkt.dts, in_stream->time_base, out_stream->time_base, (AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
        pkt.duration = av_rescale_q(pkt.duration, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;
        pkt.stream_index=stream_index;

        LOGE("Write 1 Packet. size:%5d\tpts:%lld\n",pkt.size,pkt.pts);
        //Write AVPacket 音频或视频裸流
        if (av_interleaved_write_frame(ofmt_ctx, &pkt) < 0) {
            LOGE( "Error muxing packet\n");
            break;
        }
        av_free_packet(&pkt);

    }
    //Write file trailer
    av_write_trailer(ofmt_ctx);

#if USE_H264BSF
    av_bitstream_filter_close(h264bsfc);
#endif
#if USE_AACBSF
    av_bitstream_filter_close(aacbsfc);
#endif

    end:
    avformat_close_input(&ifmt_ctx_v);
    avformat_close_input(&ifmt_ctx_a);
    /* close output */
    if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))
        avio_close(ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);
    if (ret < 0 && ret != AVERROR_EOF) {
        LOGE( "Error occurred.\n");
        return -1;
    }
    return 0;
}
}

如果不了解ffmpeg API的客官，可先看这里的系列文章：
http://blog.csdn.net/king1425/article/details/70598827