ffmpeg decoding

The previous section analyzes and decapsulates. The output of decapsulation is compressed data, which needs to be decoded. Today we will analyze the decoding process:

Basic process :

AVPacket: audio and video compression data structure; decoder input;

Decoder initialization:

avcodec_find_decoder(): find decoder;

AVCodec *avcodec_find_decoder(enum AVCodecID id);

avcodec_alloc_context3(): allocate AVCodecContext;

AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);

avcodec_open2(): Open the decoder:

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

avcodec_send_packet: Input data to decoder:

int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);

avcodec_receive_frame: Output data from decoder:

int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);

AVFrame: The decoded output data structure, the output of the decoder;

Official Demo :

decode_audio.c:

static void decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame,                   FILE *outfile){    int i, ch;    int ret, data_size;    //往decoder发送压缩数据    /* send the packet with the compressed data to the decoder */    ret = avcodec_send_packet(dec_ctx, pkt);    if (ret < 0) {        fprintf(stderr, "Error submitting the packet to the decoder\n");        exit(1);    }
    /* read all the output frames (in general there may be any number of them */    while (ret >= 0) {        //获取解码数据        ret = avcodec_receive_frame(dec_ctx, frame);        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)            return;        else if (ret < 0) {            fprintf(stderr, "Error during decoding\n");            exit(1);        }        data_size = av_get_bytes_per_sample(dec_ctx->sample_fmt);        if (data_size < 0) {            /* This should not occur, checking just for paranoia */            fprintf(stderr, "Failed to calculate data size\n");            exit(1);        }        //将解码的数据写文件；        for (i = 0; i < frame->nb_samples; i++)            for (ch = 0; ch < dec_ctx->channels; ch++)                fwrite(frame->data[ch] + data_size*i, 1, data_size, outfile);    }}
int main(int argc, char **argv){    const char *outfilename, *filename;    const AVCodec *codec;    AVCodecContext *c= NULL;    AVCodecParserContext *parser = NULL;    int len, ret;    FILE *f, *outfile;    uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];    uint8_t *data;    size_t   data_size;    AVPacket *pkt;    AVFrame *decoded_frame = NULL;    enum AVSampleFormat sfmt;    int n_channels = 0;    const char *fmt;
    if (argc <= 2) {        fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);        exit(0);    }    filename    = argv[1];    outfilename = argv[2];
    pkt = av_packet_alloc();    //查找到MP2 deocoder    /* find the MPEG audio decoder */    codec = avcodec_find_decoder(AV_CODEC_ID_MP2);    if (!codec) {        fprintf(stderr, "Codec not found\n");        exit(1);    }
    //这里输入文件是压缩音频码流，借且parser获取AVPacket数据     //AVParser初始化    parser = av_parser_init(codec->id);    if (!parser) {        fprintf(stderr, "Parser not found\n");        exit(1);    }    //分配AVCodecContext    c = avcodec_alloc_context3(codec);    if (!c) {        fprintf(stderr, "Could not allocate audio codec context\n");        exit(1);    }    //打开decoder    /* open it */    if (avcodec_open2(c, codec, NULL) < 0) {        fprintf(stderr, "Could not open codec\n");        exit(1);    }
    f = fopen(filename, "rb");    if (!f) {        fprintf(stderr, "Could not open %s\n", filename);        exit(1);    }    outfile = fopen(outfilename, "wb");    if (!outfile) {        av_free(c);        exit(1);    }
    /* decode until eof */    data      = inbuf;    data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, f);
    while (data_size > 0) {        if (!decoded_frame) {            if (!(decoded_frame = av_frame_alloc())) {                fprintf(stderr, "Could not allocate audio frame\n");                exit(1);            }        }        //通过parser获取AVPacket数据         ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size,                               data, data_size,                               AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);        if (ret < 0) {            fprintf(stderr, "Error while parsing\n");            exit(1);        }        data      += ret;        data_size -= ret;        //调用decode函数进行解码        if (pkt->size)            decode(c, pkt, decoded_frame, outfile);
        if (data_size < AUDIO_REFILL_THRESH) {            memmove(inbuf, data, data_size);            data = inbuf;            len = fread(data + data_size, 1,                        AUDIO_INBUF_SIZE - data_size, f);            if (len > 0)                data_size += len;        }    }    //最后刷新decoder内部缓冲数据     /* flush the decoder */    pkt->data = NULL;    pkt->size = 0;    decode(c, pkt, decoded_frame, outfile);
    /* print output pcm infomations, because there have no metadata of pcm */    sfmt = c->sample_fmt;
    if (av_sample_fmt_is_planar(sfmt)) {        const char *packed = av_get_sample_fmt_name(sfmt);        printf("Warning: the sample format the decoder produced is planar "               "(%s). This example will output the first channel only.\n",               packed ? packed : "?");        sfmt = av_get_packed_sample_fmt(sfmt);    }
    n_channels = c->channels;    if ((ret = get_format_from_sample_fmt(&fmt, sfmt)) < 0)        goto end;
    printf("Play the output audio file with the command:\n"           "ffplay -f %s -ac %d -ar %d %s\n",           fmt, n_channels, c->sample_rate,           outfilename);end:    fclose(outfile);    fclose(f);
    avcodec_free_context(&c);    av_parser_close(parser);    av_frame_free(&decoded_frame);    av_packet_free(&pkt);
    return 0;}

The above example is an officially provided example of decoding and reading MP2 audio bare streams from a file, and you need to use AVParser to obtain AVPacket. See decode_video.c for an example of decoding a bare video stream. The process is the same.

In most cases, we obtain AVPacket through av_read_frame. For specific examples, see demux_decoding.c. The decoding process is the same, except that before avcodec_open2 opens the decoder, we need to call avcodec_parameters_to_context() to copy the codec parameters from AVStream to AVCodecContext middle:

int avcodec_parameters_to_context(AVCodecContext *codec,                                  const AVCodecParameters *par);