ffmpeg解码

2022年8月3日 415点热度 0人点赞 0条评论

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上一节分析了解封装,解封装输出的是压缩数据,需要进行解码。今天我们来分析解码的过程:

基本流程

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AVPacket: 音视频压缩数据结构体;decoder的输入;

Decoder的初始化:

  • avcodec_find_decoder(): 查找decoder;

AVCodec *avcodec_find_decoder(enum AVCodecID id);
  • avcodec_alloc_context3(): 分配AVCodecContext;

AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);
  • avcodec_open2(): 打开解码器:

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);
  • avcodec_send_packet: 往decoder输入数据:

int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);
  • avcodec_receive_frame: 从decoder输出数据:

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

AVFrame: 解码后的输出数据结构体,解码器的输出;

官方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;}

上面的例子是官方提供的从文件写读取MP2 音频裸流解码的例子,需要利用AVParser获取AVPacket。解码视频裸流例子参见decode_video.c 过程是一样的。

而大多数情况下, 我们都是通过av_read_frame获取AVPacket,具体例子可以参见demux_decoding.c,解码流程也是一样的,只不过在avcodec_open2打开解码器前,需要调用avcodec_parameters_to_context()从AVStream中拷贝codec参数到AVCodecContext中:

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

79350ffmpeg解码

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