上文中说到在read_thread线程中有个关键函数：avformat_open_input（utils.c），应当是读取视频文件的，这个函数属于ffmpeg层。这回进入到其中去看下：
int avformat_open_input(AVFormatContext **ps, const char *filename,
AVInputFormat *fmt, AVDictionary **options)
{
......
if ((ret = init_input(s, filename, &tmp)) < 0)
goto fail;
s->probe_score = ret;
if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
if (!s->protocol_whitelist) {
ret = AVERROR(ENOMEM);
goto fail;
}
}
if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
if (!s->protocol_blacklist) {
ret = AVERROR(ENOMEM);
goto fail;
}
}
if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {
av_log(s, AV_LOG_ERROR, "Format not on whitelist \'%s\'\n", s->format_whitelist);
ret = AVERROR(EINVAL);
goto fail;
}
......
} init_input这个函数的注释是：Open input file and probe the format if necessary.打开文件判断格式。进入到这个函数中观察，代码不多，关键点av_probe_input_buffer2，ffmpeg里的格式分析函数。里面会调用avio_read，读取文件。进入到avio_read函数内部，看到主要是循环读取packet,从AVIOContext的队列中读取，通过AVIOContext中的函数指针read_packet来进行。
下面退回到read_thread函数中，看看对h264的视频流如何处理的。
static int read_thread(void *arg)
{
......
for (i = 0; i < ic->nb_streams; i++) {
AVStream *st = ic->streams[i];
enum AVMediaType type = st->codecpar->codec_type;
st->discard = AVDISCARD_ALL;
if (type >= 0 && ffp->wanted_stream_spec[type] && st_index[type] == -1)
if (avformat_match_stream_specifier(ic, st, ffp->wanted_stream_spec[type]) > 0)
st_index[type] = i;
// choose first h264
if (type == AVMEDIA_TYPE_VIDEO) {
enum AVCodecID codec_id = st->codecpar->codec_id;
video_stream_count++;
if (codec_id == AV_CODEC_ID_H264) {
h264_stream_count++;
if (first_h264_stream < 0)
first_h264_stream = i;
}
}
}
......
}循环读取stream，然后判断是h264后记录下来。继续往下看又到了上文提到的stream_component_open函数，这回进入看看：
static int stream_component_open(FFPlayer *ffp, int stream_index)
{
......
codec = avcodec_find_decoder(avctx->codec_id);
......
switch (avctx->codec_type) {
case AVMEDIA_TYPE_AUDIO:
if ((ret = decoder_start(&is->auddec, audio_thread, ffp, "ff_audio_dec")) < 0)
goto out;
......
case AVMEDIA_TYPE_VIDEO:
if ((ret = decoder_start(&is->viddec, video_thread, ffp, "ff_video_dec")) < 0)
goto out;
......
case AVMEDIA_TYPE_SUBTITLE:
......
}这是个很长的函数，我看起来是分为2部分，前半部分是寻找解码器，后半部分的switch case是开始进行解码。分别为音频、视频和字母进行相关处理。decoder_start是个关键函数，开始进行解码处理。进入这个函数内部看：
static int decoder_start(Decoder *d, int (*fn)(void *), void *arg, const char *name)
{
packet_queue_start(d->queue);
d->decoder_tid = SDL_CreateThreadEx(&d->_decoder_tid, fn, arg, name);
if (!d->decoder_tid) {
av_log(NULL, AV_LOG_ERROR, "SDL_CreateThread(): %s\n", SDL_GetError());
return AVERROR(ENOMEM);
}
return 0;
}很短，不是吗，但是启动了线程。根据参数传递得知就是上面传递进来的video_thread和audio_thread是关键线程函数。看到这里可以了解，解码过程是完全异步的，那么再去看看关键的线程函数吧。
static int video_thread(void *arg)
{
FFPlayer *ffp = (FFPlayer *)arg;
int ret = 0;
if (ffp->node_vdec) {
ret = ffpipenode_run_sync(ffp->node_vdec);
}
return ret;
}很短，先看参数，从上面的可得知，是FFPlayer类型，从read_thread函数中就传递进来的一个结构，可以说是播放器的结构，播放所需要的所有内容这里几乎都有了。继续看关键函数ffpipenode_run_sync。向下跟踪两层，会发现，核心函数是ffplay_video_thread。这个函数内部看起来挺麻烦，进入看看吧：
static int ffplay_video_thread(void *arg)
{
......
for (;;) {
ret = get_video_frame(ffp, frame);
......
ret = av_buffersrc_add_frame(filt_in, frame);
if (ret < 0)
goto the_end;
while (ret >= 0) {
is->frame_last_returned_time = av_gettime_relative() / 1000000.0;
ret = av_buffersink_get_frame_flags(filt_out, frame, 0);
if (ret < 0) {
if (ret == AVERROR_EOF)
is->viddec.finished = is->viddec.pkt_serial;
ret = 0;
break;
}
is->frame_last_filter_delay = av_gettime_relative() / 1000000.0 - is->frame_last_returned_time;
if (fabs(is->frame_last_filter_delay) > AV_NOSYNC_THRESHOLD / 10.0)
is->frame_last_filter_delay = 0;
tb = filt_out->inputs[0]->time_base;
#endif
duration = (frame_rate.num && frame_rate.den ? av_q2d((AVRational){frame_rate.den, frame_rate.num}) : 0);
pts = (frame->pts == AV_NOPTS_VALUE) ? NAN : frame->pts * av_q2d(tb);
ret = queue_picture(ffp, frame, pts, duration, av_frame_get_pkt_pos(frame), is->viddec.pkt_serial);
av_frame_unref(frame);
#if CONFIG_AVFILTER
}
#endif
}
}关键点首先是get_video_frame，然后是av_buffersrc_add_frame和后面的while循环部分里的queue_picture。那么不得不进入到get_video_frame中看下：
static int get_video_frame(FFPlayer *ffp, AVFrame *frame)
{
VideoState *is = ffp->is;
int got_picture;
ffp_video_statistic_l(ffp);
if ((got_picture = decoder_decode_frame(ffp, &is->viddec, frame, NULL)) < 0)
return -1;
if (got_picture) {
double dpts = NAN;
if (frame->pts != AV_NOPTS_VALUE)
dpts = av_q2d(is->video_st->time_base) * frame->pts;
frame->sample_aspect_ratio = av_guess_sample_aspect_ratio(is->ic, is->video_st, frame);
if (ffp->framedrop>0 || (ffp->framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) {
if (frame->pts != AV_NOPTS_VALUE) {
double diff = dpts - get_master_clock(is);
if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD &&
diff - is->frame_last_filter_delay < 0 &&
is->viddec.pkt_serial == is->vidclk.serial &&
is->videoq.nb_packets) {
is->frame_drops_early++;
is->continuous_frame_drops_early++;
if (is->continuous_frame_drops_early > ffp->framedrop) {
is->continuous_frame_drops_early = 0;
} else {
av_frame_unref(frame);
got_picture = 0;
}
}
}
}
}
return got_picture;
}关键点只有一个就是decoder_decode_frame，好吧，继续往下看，层级有点多了哈：
static int decoder_decode_frame(FFPlayer *ffp, Decoder *d, AVFrame *frame, AVSubtitle *sub) {
int got_frame = 0;
do {
int ret = -1;
if (d->queue->abort_request)
return -1;
if (!d->packet_pending || d->queue->serial != d->pkt_serial) {
AVPacket pkt;
do {
if (d->queue->nb_packets == 0)
SDL_CondSignal(d->empty_queue_cond);
if (packet_queue_get_or_buffering(ffp, d->queue, &pkt, &d->pkt_serial, &d->finished) < 0)
return -1;
if (pkt.data == flush_pkt.data) {
avcodec_flush_buffers(d->avctx);
d->finished = 0;
d->next_pts = d->start_pts;
d->next_pts_tb = d->start_pts_tb;
}
} while (pkt.data == flush_pkt.data || d->queue->serial != d->pkt_serial);
av_packet_unref(&d->pkt);
d->pkt_temp = d->pkt = pkt;
d->packet_pending = 1;
}
switch (d->avctx->codec_type) {
case AVMEDIA_TYPE_VIDEO: {
ret = avcodec_decode_video2(d->avctx, frame, &got_frame, &d->pkt_temp);
if (got_frame) {
ffp->stat.vdps = SDL_SpeedSamplerAdd(&ffp->vdps_sampler, FFP_SHOW_VDPS_AVCODEC, "vdps[avcodec]");
if (ffp->decoder_reorder_pts == -1) {
frame->pts = av_frame_get_best_effort_timestamp(frame);
} else if (!ffp->decoder_reorder_pts) {
frame->pts = frame->pkt_dts;
}
}
}
break;
case AVMEDIA_TYPE_AUDIO:
ret = avcodec_decode_audio4(d->avctx, frame, &got_frame, &d->pkt_temp);
if (got_frame) {
AVRational tb = (AVRational){1, frame->sample_rate};
if (frame->pts != AV_NOPTS_VALUE)
frame->pts = av_rescale_q(frame->pts, av_codec_get_pkt_timebase(d->avctx), tb);
else if (d->next_pts != AV_NOPTS_VALUE)
frame->pts = av_rescale_q(d->next_pts, d->next_pts_tb, tb);
if (frame->pts != AV_NOPTS_VALUE) {
d->next_pts = frame->pts + frame->nb_samples;
d->next_pts_tb = tb;
}
}
break;
case AVMEDIA_TYPE_SUBTITLE:
ret = avcodec_decode_subtitle2(d->avctx, sub, &got_frame, &d->pkt_temp);
break;
default:
break;
}
if (ret < 0) {
d->packet_pending = 0;
} else {
d->pkt_temp.dts =
d->pkt_temp.pts = AV_NOPTS_VALUE;
if (d->pkt_temp.data) {
if (d->avctx->codec_type != AVMEDIA_TYPE_AUDIO)
ret = d->pkt_temp.size;
d->pkt_temp.data += ret;
d->pkt_temp.size -= ret;
if (d->pkt_temp.size <= 0)
d->packet_pending = 0;
} else {
if (!got_frame) {
d->packet_pending = 0;
d->finished = d->pkt_serial;
}
}
}
} while (!got_frame && !d->finished);
return got_frame;
}packet_queue_get_or_buffering从解码前的队列中读取一帧的数据，然后调用avcodec_decode_video2。不能进去看了，没完了，总之是读取一帧。往回倒，回到ffplay_video_thread里，下面就是queue_picture，将一帧解码后的图像放入解码后队列。
至此解码算完了。整个过程真是粗略分析啊，对自己也很抱歉，暂时先这样吧。后面有空继续就某个点深入进行。
java
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