ffmpeg源码分析--12.mov的mov_read_packet

1.mov_read_packet的调用过程
av_read_frame
 -->read_frame_internal
   --> ff_read_packet
      --> mov_read_packet

"stsz"定义了每个video_sample与每个sound_sample的sample_sizes大小
"stco"定义了每个video_chunk与每个sound_chunk在san.mp4这个文件内的offset
所以从san.mp4这个文件的offset处读取sample_sizes就是音视频的数据.
2. mov_find_next_sample说明
mov_read_packet
-->mov_find_next_sample

static AVIndexEntry *mov_find_next_sample(AVFormatContext *s, AVStream **st)
{
    AVIndexEntry *sample = NULL;
    int64_t best_dts = INT64_MAX;
    int i;
    for (i = 0; i < s->nb_streams; i++) {
        AVStream *avst = s->streams[i];
        MOVStreamContext *msc = avst->priv_data;
        if (msc->pb && msc->current_sample < avst->nb_index_entries) {
            AVIndexEntry *current_sample = &avst->index_entries[msc->current_sample];
            int64_t dts = av_rescale(current_sample->timestamp, AV_TIME_BASE, msc->time_scale);
            if (!sample || (!s->pb->seekable && current_sample->pos < sample->pos) ||
                (s->pb->seekable &&
                 ((msc->pb != s->pb && dts < best_dts) || (msc->pb == s->pb &&
                 ((FFABS(best_dts - dts) <= AV_TIME_BASE && current_sample->pos < sample->pos) ||
                  (FFABS(best_dts - dts) > AV_TIME_BASE && dts < best_dts)))))) {
                sample = current_sample;
                best_dts = dts;
                *st = avst;
            }
        }
    }
    return sample;
}

在"stco"中有一系列的chunk_offsets,既包含video的也包含sound的chunk_offsets
上面函数的作用就是从avst->index_entries中分离出所有的chunk_offsets
AV_TIME_BASE=1000000
mov_read_mdhd[1156]: sc->time_scale=0x5dc2=24002  -->视频的time_scale
mov_read_mdhd[1156]: sc->time_scale=0xac44=44100  -->音频的time_scale

从stco中读取出来如下的video与audio的chunk:
video:
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[0]=0x30
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[1]=0x137d
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[2]=0x253c
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[3]=0x3143
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[4]=0x3ca7
audio:
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[0]=0x1363
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[1]=0x22ec
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[2]=0x22f5
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[3]=0x22fe
cong:libavformat/mov.c:mov_read_stco[1693]: chunk_offsets[4]=0x239f

next_sample的顺序就是如下:
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x30,sample->timestamp=0xfffffffffffffc17,size=0x1333
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x1363,sample->timestamp=0x0,size=0x1a
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x137d,sample->timestamp=0x0,size=0xf6f
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x22ec,sample->timestamp=0x400,size=0x9
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x22f5,sample->timestamp=0x800,size=0x9
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x22fe,sample->timestamp=0xc00,size=0xa1
cong:libavformat/mov.c:mov_read_packet[5026]: sample->pos=0x239f,sample->timestamp=0x1000,size=0xc9

mov_find_next_sample按照chunk_offset的值从小到大依次排列
这个函数中计算的dts，只是一个中间的结果，不是真正的dts

3.mov_read_packet

static int mov_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    MOVContext *mov = s->priv_data;
    MOVStreamContext *sc;
    AVIndexEntry *sample;
    AVStream *st = NULL;
    int ret;
    mov->fc = s;
 retry:
    sample = mov_find_next_sample(s, &st);   //查找下一个sample
    sc = st->priv_data;
    sc->current_sample++;

    if (st->discard != AVDISCARD_ALL) {
        //通过sample中的pos找到媒体文件中的sample起始点与大小，然后根据这个去读取媒体文件的内容
        int64_t ret64 = avio_seek(sc->pb, sample->pos, SEEK_SET);
        ret = av_get_packet(sc->pb, pkt, sample->size);  //读取媒体文件的内容，大小是sample->size
    }
    pkt->stream_index = sc->ffindex;
    pkt->dts = sample->timestamp;                            //dts是计算出来的"start_time+samplduration"
    if (sc->ctts_data && sc->ctts_index < sc->ctts_count) {
        pkt->pts = pkt->dts + sc->dts_shift + sc->ctts_data[sc->ctts_index].duration;
        sc->ctts_sample++;
        if (sc->ctts_index < sc->ctts_count &&
            sc->ctts_data[sc->ctts_index].count == sc->ctts_sample) {
            sc->ctts_index++;
            sc->ctts_sample = 0;
        }
    } else {
        int64_t next_dts = (sc->current_sample < st->nb_index_entries) ?
            st->index_entries[sc->current_sample].timestamp : st->duration;
        pkt->duration = next_dts - pkt->dts;
        pkt->pts = pkt->dts;
    }
    pkt->flags |= sample->flags & AVINDEX_KEYFRAME ? AV_PKT_FLAG_KEY : 0;
    pkt->pos = sample->pos;

    return 0;
}

到此，mov_read_packet己经很明确了
a. mov_find_next_sample找到要读取的文件的偏移 + size
b. avio_seek(sc->pb, sample->pos, SEEK_SET); 将文件的指针定位到文件的偏移处
c. av_get_packet(sc->pb, pkt, sample->size); 从文件的偏移读取size的数据