16.FFmpeg学习笔记 - 解码音频

本篇文章用libavcodec API来解码一个音频文件(纯编码的音频文件)，将其解码成pcm格式，然后播放。

和之前的解复用解码不一样，由于没有解复用的步骤，需要手动选择解码器ID。读入文件的方式也不一样了，是先把文件读入内存，然后调用av_parser_parse2填充AVPacket的data和size等数据。

分离出来的pcm数据，可以用ffplay命令进行播放，示例如下：

ffplay -f f32le -ac 2 -ar 48000 ./sintel.pcm

参考15.FFmpeg学习笔记 - 解复用和解码(新的解码API)

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>
#include <libavutil/samplefmt.h>
#include <libavutil/timestamp.h>


#define AUDIO_INBUF_SIZE 20480
#define AUDIO_REFILL_THRESH 4096

static void decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame,
                   FILE *outfile)
{
    int i, ch;
    int ret, data_size;
    
    /* 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) {
        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);
        }
        if (av_sample_fmt_is_planar(dec_ctx->sample_fmt)) {
            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);
        }else {
            fwrite(frame->extended_data[0], 1, frame->linesize[0], outfile);
        }

    }
}

static void decode_audio(void)
{
    const char *infilename = "/Users/zhw/Desktop/sintel.aac";
    const char *outfilename = "/Users/zhw/Desktop/sintel.pcm";
    const AVCodec *codec;
    AVCodecContext *c= NULL;
    AVCodecParserContext *parser = NULL;
    size_t 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;
    
    pkt = av_packet_alloc();
    
    /* 此处应根据输入文件的实际编码格式选择正确的编码ID */
    codec = avcodec_find_decoder(AV_CODEC_ID_AAC);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }
    
    parser = av_parser_init(codec->id);
    if (!parser) {
        fprintf(stderr, "Parser not found\n");
        exit(1);
    }
    
    c = avcodec_alloc_context3(codec);
    if (!c) {
        fprintf(stderr, "Could not allocate audio codec context\n");
        exit(1);
    }
    
    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }
    
    f = fopen(infilename, "rb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", infilename);
        exit(1);
    }
    outfile = fopen(outfilename, "wb");
    if (!outfile) {
        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);
            }
        }
        
        ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size,
                               data, (int)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;
        
        if (pkt->size){
            decode(c, pkt, decoded_frame, outfile);
            av_packet_unref(pkt);
        }
        
        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;
        }
    }
    
    /* flush the decoder */
    pkt->data = NULL;
    pkt->size = 0;
    decode(c, pkt, decoded_frame, outfile);
    av_packet_unref(pkt);
    
    fclose(outfile);
    fclose(f);
    
    avcodec_free_context(&c);
    av_parser_close(parser);
    av_frame_free(&decoded_frame);
    av_packet_free(&pkt);
    
    printf("finish decode audio\n");
    
}