ffmpeg源码跟踪笔记之av_read_frame
1、函数介绍
/**
* Return the next frame of a stream.
* This function returns what is stored in the file, and does not validate
* that what is there are valid frames for the decoder. It will split what is
* stored in the file into frames and return one for each call. It will not
* omit invalid data between valid frames so as to give the decoder the maximum
* information possible for decoding.
*
* If pkt->buf is NULL, then the packet is valid until the next
* av_read_frame() or until avformat_close_input(). Otherwise the packet
* is valid indefinitely. In both cases the packet must be freed with
* av_free_packet when it is no longer needed. For video, the packet contains
* exactly one frame. For audio, it contains an integer number of frames if each
* frame has a known fixed size (e.g. PCM or ADPCM data). If the audio frames
* have a variable size (e.g. MPEG audio), then it contains one frame.
*
* pkt->pts, pkt->dts and pkt->duration are always set to correct
* values in AVStream.time_base units (and guessed if the format cannot
* provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
* has B-frames, so it is better to rely on pkt->dts if you do not
* decompress the payload.
*
* @return 0 if OK, < 0 on error or end of file
*/
int av_read_frame(AVFormatContext *s, AVPacket *pkt);2、函数调用图
3、源码
int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
const int genpts = s->flags & AVFMT_FLAG_GENPTS;
int eof = 0;
int ret;
AVStream *st;
if (!genpts) {
ret = s->internal->packet_buffer
? read_from_packet_buffer(&s->internal->packet_buffer,//从缓存中读取
&s->internal->packet_buffer_end, pkt)
: read_frame_internal(s, pkt);//从文件，或探测缓存中读取
if (ret < 0)
return ret;
goto return_packet;
}
for (;;) {
AVPacketList *pktl = s->internal->packet_buffer;
if (pktl) {
AVPacket *next_pkt = &pktl->pkt;
if (next_pkt->dts != AV_NOPTS_VALUE) {
int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;
// last dts seen for this stream. if any of packets following
// current one had no dts, we will set this to AV_NOPTS_VALUE.
int64_t last_dts = next_pkt->dts;
while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {
if (pktl->pkt.stream_index == next_pkt->stream_index &&
(av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2LL << (wrap_bits - 1)) < 0)) {
if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (wrap_bits - 1))) {
// not B-frame
next_pkt->pts = pktl->pkt.dts;
}
if (last_dts != AV_NOPTS_VALUE) {
// Once last dts was set to AV_NOPTS_VALUE, we don't change it.
last_dts = pktl->pkt.dts;
}
}
pktl = pktl->next;
}
if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) {
// Fixing the last reference frame had none pts issue (For MXF etc).
// We only do this when
// 1. eof.
// 2. we are not able to resolve a pts value for current packet.
// 3. the packets for this stream at the end of the files had valid dts.
next_pkt->pts = last_dts + next_pkt->duration;
}
pktl = s->internal->packet_buffer;
}
/* read packet from packet buffer, if there is data */
st = s->streams[next_pkt->stream_index];
if (!(next_pkt->pts == AV_NOPTS_VALUE && st->discard < AVDISCARD_ALL &&
next_pkt->dts != AV_NOPTS_VALUE && !eof)) {
ret = read_from_packet_buffer(&s->internal->packet_buffer,
&s->internal->packet_buffer_end, pkt);
goto return_packet;
}
}
ret = read_frame_internal(s, pkt);
if (ret < 0) {
if (pktl && ret != AVERROR(EAGAIN)) {
eof = 1;
continue;
} else
return ret;
}
if (av_dup_packet(add_to_pktbuf(&s->internal->packet_buffer, pkt,
&s->internal->packet_buffer_end)) < 0)
return AVERROR(ENOMEM);
}
return_packet:
st = s->streams[pkt->stream_index];
if ((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY) {
ff_reduce_index(s, st->index);
av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME);
}
if (is_relative(pkt->dts))
pkt->dts -= RELATIVE_TS_BASE;
if (is_relative(pkt->pts))
pkt->pts -= RELATIVE_TS_BASE;
return ret;
}4、解释
av_read_frame函数会判断在未解码缓存中是否有数据，如果有则调用read_from_packet_buffer，看看其代码：
static int read_from_packet_buffer(AVPacketList **pkt_buffer,
AVPacketList **pkt_buffer_end,
AVPacket *pkt)
{
AVPacketList *pktl;
av_assert0(*pkt_buffer);
pktl = *pkt_buffer;
*pkt = pktl->pkt;
*pkt_buffer = pktl->next;
if (!pktl->next)
*pkt_buffer_end = NULL;
av_freep(&pktl);
return 0;
}
若缓存中没有数据，av_read_frame函数则会调用read_frame_internal函数来获取数据，看下代码：
static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
int ret = 0, i, got_packet = 0;
AVDictionary *metadata = NULL;
av_init_packet(pkt);
//如果不满足条件就一直循环
while (!got_packet && !s->internal->parse_queue) {
AVStream *st;
AVPacket cur_pkt;
/* read next packet */
ret = ff_read_packet(s, &cur_pkt);
if (ret < 0) {
if (ret == AVERROR(EAGAIN))
return ret;
/* flush the parsers */
for (i = 0; i < s->nb_streams; i++) {
st = s->streams[i];
if (st->parser && st->need_parsing)
parse_packet(s, NULL, st->index);
}
/* all remaining packets are now in parse_queue =>
* really terminate parsing */
break;
}
ret = 0;
st = s->streams[cur_pkt.stream_index];
if (cur_pkt.pts != AV_NOPTS_VALUE &&
cur_pkt.dts != AV_NOPTS_VALUE &&
cur_pkt.pts < cur_pkt.dts) {
av_log(s, AV_LOG_WARNING,
"Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n",
cur_pkt.stream_index,
av_ts2str(cur_pkt.pts),
av_ts2str(cur_pkt.dts),
cur_pkt.size);
}
if (s->debug & FF_FDEBUG_TS)
av_log(s, AV_LOG_DEBUG,
"ff_read_packet stream=%d, pts=%s, dts=%s, size=%d, duration=%d, flags=%d\n",
cur_pkt.stream_index,
av_ts2str(cur_pkt.pts),
av_ts2str(cur_pkt.dts),
cur_pkt.size, cur_pkt.duration, cur_pkt.flags);
if (st->need_parsing && !st->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {
st->parser = av_parser_init(st->codec->codec_id);
if (!st->parser) {
av_log(s, AV_LOG_VERBOSE, "parser not found for codec "
"%s, packets or times may be invalid.\n",
avcodec_get_name(st->codec->codec_id));
/* no parser available: just output the raw packets */
st->need_parsing = AVSTREAM_PARSE_NONE;
} else if (st->need_parsing == AVSTREAM_PARSE_HEADERS)
st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
else if (st->need_parsing == AVSTREAM_PARSE_FULL_ONCE)
st->parser->flags |= PARSER_FLAG_ONCE;
else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW)
st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
}
if (!st->need_parsing || !st->parser) {
/* no parsing needed: we just output the packet as is */
*pkt = cur_pkt;
compute_pkt_fields(s, st, NULL, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);//检查和计算pts、dts、duration
if ((s->iformat->flags & AVFMT_GENERIC_INDEX) &&
(pkt->flags & AV_PKT_FLAG_KEY) && pkt->dts != AV_NOPTS_VALUE) {
ff_reduce_index(s, st->index);
av_add_index_entry(st, pkt->pos, pkt->dts,
0, 0, AVINDEX_KEYFRAME);
}
got_packet = 1;
} else if (st->discard < AVDISCARD_ALL) {
if ((ret = parse_packet(s, &cur_pkt, cur_pkt.stream_index)) < 0)
return ret;
} else {
/* free packet */
av_free_packet(&cur_pkt);
}
if (pkt->flags & AV_PKT_FLAG_KEY)
st->skip_to_keyframe = 0;
if (st->skip_to_keyframe) {
av_free_packet(&cur_pkt);
if (got_packet) {
*pkt = cur_pkt;
}
got_packet = 0;
}
}
if (!got_packet && s->internal->parse_queue)
ret = read_from_packet_buffer(&s->internal->parse_queue, &s->internal->parse_queue_end, pkt);
if (ret >= 0) {
AVStream *st = s->streams[pkt->stream_index];
int discard_padding = 0;
if (st->first_discard_sample && pkt->pts != AV_NOPTS_VALUE) {
int64_t pts = pkt->pts - (is_relative(pkt->pts) ? RELATIVE_TS_BASE : 0);
int64_t sample = ts_to_samples(st, pts);
int duration = ts_to_samples(st, pkt->duration);
int64_t end_sample = sample + duration;
if (duration > 0 && end_sample >= st->first_discard_sample &&
sample < st->last_discard_sample)
discard_padding = FFMIN(end_sample - st->first_discard_sample, duration);
}
if (st->skip_samples || discard_padding) {
uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);
if (p) {
AV_WL32(p, st->skip_samples);
AV_WL32(p + 4, discard_padding);
av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %d\n", st->skip_samples);
}
st->skip_samples = 0;
}
if (st->inject_global_side_data) {
for (i = 0; i < st->nb_side_data; i++) {
AVPacketSideData *src_sd = &st->side_data[i];
uint8_t *dst_data;
if (av_packet_get_side_data(pkt, src_sd->type, NULL))
continue;
dst_data = av_packet_new_side_data(pkt, src_sd->type, src_sd->size);
if (!dst_data) {
av_log(s, AV_LOG_WARNING, "Could not inject global side data\n");
continue;
}
memcpy(dst_data, src_sd->data, src_sd->size);
}
st->inject_global_side_data = 0;
}
if (!(s->flags & AVFMT_FLAG_KEEP_SIDE_DATA))
av_packet_merge_side_data(pkt);
}
av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);
if (metadata) {
s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
av_dict_copy(&s->metadata, metadata, 0);
av_dict_free(&metadata);
av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);
}
if (s->debug & FF_FDEBUG_TS)
av_log(s, AV_LOG_DEBUG,
"read_frame_internal stream=%d, pts=%s, dts=%s, "
"size=%d, duration=%d, flags=%d\n",
pkt->stream_index,
av_ts2str(pkt->pts),
av_ts2str(pkt->dts),
pkt->size, pkt->duration, pkt->flags);
return ret;
}int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
int ret, i, err;
AVStream *st;
for (;;) {
AVPacketList *pktl = s->internal->raw_packet_buffer;
//如果缓存中有数据，则到缓存中取
if (pktl) {
*pkt = pktl->pkt;
st = s->streams[pkt->stream_index];
if (s->internal->raw_packet_buffer_remaining_size <= 0)//raw_packet_buffer剩下的有效buf大小小于0
if ((err = probe_codec(s, st, NULL)) < 0)
return err;
if (st->request_probe <= 0) {//如果 探测结束 ，则更新信息，并返回
s->internal->raw_packet_buffer = pktl->next;
s->internal->raw_packet_buffer_remaining_size += pkt->size;
av_free(pktl);
return 0;
}
}
pkt->data = NULL;
pkt->size = 0;
av_init_packet(pkt);//用InputFormat的read_packet函数指针从文件读取数据
ret = s->iformat->read_packet(s, pkt);
if (ret < 0) {
if (!pktl || ret == AVERROR(EAGAIN))
return ret;
for (i = 0; i < s->nb_streams; i++) {
st = s->streams[i];
if (st->probe_packets)
if ((err = probe_codec(s, st, NULL)) < 0)
return err;
av_assert0(st->request_probe <= 0);
}
continue;
}
//如果AVFormatContext的flags设置了丢弃损坏包，并且此包是损坏的，则丢弃
if ((s->flags & AVFMT_FLAG_DISCARD_CORRUPT) &&
(pkt->flags & AV_PKT_FLAG_CORRUPT)) {
av_log(s, AV_LOG_WARNING,
"Dropped corrupted packet (stream = %d)\n",
pkt->stream_index);
av_free_packet(pkt);
continue;
}
//读取到的pkt，流index不合法，抛弃
if (pkt->stream_index >= (unsigned)s->nb_streams) {
av_log(s, AV_LOG_ERROR, "Invalid stream index %d\n", pkt->stream_index);
continue;
}
st = s->streams[pkt->stream_index];
if (update_wrap_reference(s, st, pkt->stream_index, pkt) && st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {
// correct first time stamps to negative values
if (!is_relative(st->first_dts))
st->first_dts = wrap_timestamp(st, st->first_dts);
if (!is_relative(st->start_time))
st->start_time = wrap_timestamp(st, st->start_time);
if (!is_relative(st->cur_dts))
st->cur_dts = wrap_timestamp(st, st->cur_dts);
}
pkt->dts = wrap_timestamp(st, pkt->dts);
pkt->pts = wrap_timestamp(st, pkt->pts);
//把codec的code_id
force_codec_ids(s, st);
/* TODO: audio: time filter; video: frame reordering (pts != dts) */
if (s->use_wallclock_as_timestamps)
pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);
//如果不需要探测，则返回，如果需要探测，且raw_packet_buffer为空，则把刚读到的包加进去。
if (!pktl && st->request_probe <= 0)
return ret;
//把刚读的包，加入raw_packet_buffer
add_to_pktbuf(&s->internal->raw_packet_buffer, pkt,
&s->internal->raw_packet_buffer_end);
s->internal->raw_packet_buffer_remaining_size -= pkt->size;
if ((err = probe_codec(s, st, pkt)) < 0)
return err;
}
}
看下yuv demuxer的read_packet：
static int rawvideo_read_packet(AVFormatContext *s, AVPacket *pkt)
{
int packet_size, ret, width, height;
AVStream *st = s->streams[0];
width = st->codec->width;
height = st->codec->height;
packet_size = avpicture_get_size(st->codec->pix_fmt, width, height);
if (packet_size < 0)
return -1;
ret = av_get_packet(s->pb, pkt, packet_size);
pkt->pts = pkt->dts = pkt->pos / packet_size;
pkt->stream_index = 0;
if (ret < 0)
return ret;
return 0;
}/**
* Raw packets from the demuxer, prior to parsing and decoding.
* This buffer is used for buffering packets until the codec can
* be identified, as parsing cannot be done without knowing the
* codec.
*/
struct AVPacketList *raw_packet_buffer;
struct AVPacketList *raw_packet_buffer_end;
在av_read_frame函数里缓存则为AVFormatInternal::packet_buffer，注释为：
/**
* This buffer is only needed when packets were already buffered but
* not decoded, for example to get the codec parameters in MPEG
* streams.
*/
struct AVPacketList *packet_buffer;
struct AVPacketList *packet_buffer_end;
函数probe_codec是用来探测codec的，看下它的代码：
static int probe_codec(AVFormatContext *s, AVStream *st, const AVPacket *pkt)
{
if (st->request_probe>0) {
AVProbeData *pd = &st->probe_data;
int end;
av_log(s, AV_LOG_DEBUG, "probing stream %d pp:%d\n", st->index, st->probe_packets);
--st->probe_packets;
if (pkt) {
uint8_t *new_buf = av_realloc(pd->buf, pd->buf_size+pkt->size+AVPROBE_PADDING_SIZE);
if (!new_buf) {
av_log(s, AV_LOG_WARNING,
"Failed to reallocate probe buffer for stream %d\n",
st->index);
goto no_packet;
}
pd->buf = new_buf;
memcpy(pd->buf + pd->buf_size, pkt->data, pkt->size);
pd->buf_size += pkt->size;
memset(pd->buf + pd->buf_size, 0, AVPROBE_PADDING_SIZE);
} else {
no_packet:
st->probe_packets = 0;
if (!pd->buf_size) {
av_log(s, AV_LOG_WARNING,
"nothing to probe for stream %d\n", st->index);
}
}
end= s->internal->raw_packet_buffer_remaining_size <= 0
|| st->probe_packets<= 0;
if (end || av_log2(pd->buf_size) != av_log2(pd->buf_size - pkt->size)) {
int score = set_codec_from_probe_data(s, st, pd);
if ( (st->codec->codec_id != AV_CODEC_ID_NONE && score > AVPROBE_SCORE_STREAM_RETRY)
|| end) {
pd->buf_size = 0;
av_freep(&pd->buf);
st->request_probe = -1;
if (st->codec->codec_id != AV_CODEC_ID_NONE) {
av_log(s, AV_LOG_DEBUG, "probed stream %d\n", st->index);
} else
av_log(s, AV_LOG_WARNING, "probed stream %d failed\n", st->index);
}
force_codec_ids(s, st);
}
}
return 0;
}static int set_codec_from_probe_data(AVFormatContext *s, AVStream *st,
AVProbeData *pd)
{
static const struct {
const char *name;
enum AVCodecID id;
enum AVMediaType type;
} fmt_id_type[] = {
{ "aac", AV_CODEC_ID_AAC, AVMEDIA_TYPE_AUDIO },
{ "ac3", AV_CODEC_ID_AC3, AVMEDIA_TYPE_AUDIO },
{ "dts", AV_CODEC_ID_DTS, AVMEDIA_TYPE_AUDIO },
{ "dvbsub", AV_CODEC_ID_DVB_SUBTITLE,AVMEDIA_TYPE_SUBTITLE },
{ "eac3", AV_CODEC_ID_EAC3, AVMEDIA_TYPE_AUDIO },
{ "h264", AV_CODEC_ID_H264, AVMEDIA_TYPE_VIDEO },
{ "hevc", AV_CODEC_ID_HEVC, AVMEDIA_TYPE_VIDEO },
{ "loas", AV_CODEC_ID_AAC_LATM, AVMEDIA_TYPE_AUDIO },
{ "m4v", AV_CODEC_ID_MPEG4, AVMEDIA_TYPE_VIDEO },
{ "mp3", AV_CODEC_ID_MP3, AVMEDIA_TYPE_AUDIO },
{ "mpegvideo", AV_CODEC_ID_MPEG2VIDEO, AVMEDIA_TYPE_VIDEO },
{ 0 }
};
int score;
AVInputFormat *fmt = av_probe_input_format3(pd, 1, &score);
if (fmt && st->request_probe <= score) {
int i;
av_log(s, AV_LOG_DEBUG,
"Probe with size=%d, packets=%d detected %s with score=%d\n",
pd->buf_size, MAX_PROBE_PACKETS - st->probe_packets,
fmt->name, score);
for (i = 0; fmt_id_type[i].name; i++) {
if (!strcmp(fmt->name, fmt_id_type[i].name)) {
st->codec->codec_id = fmt_id_type[i].id;
st->codec->codec_type = fmt_id_type[i].type;
return score;
}
}
}
return 0;
}AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
int *score_ret)
{
AVProbeData lpd = *pd;
AVInputFormat *fmt1 = NULL, *fmt;
int score, nodat = 0, score_max = 0;
const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
if (!lpd.buf)
lpd.buf = zerobuffer;
//检测ID3v2
if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {
int id3len = ff_id3v2_tag_len(lpd.buf);
if (lpd.buf_size > id3len + 16) {
lpd.buf += id3len;
lpd.buf_size -= id3len;
} else if (id3len >= PROBE_BUF_MAX) {
nodat = 2;
} else
nodat = 1;
}
fmt = NULL;//循环调用av_iformat_next匹配格式，如果当前得分和最高分一样，那么格式匹配就不对，fmt为null，继续下一个循环匹配
while ((fmt1 = av_iformat_next(fmt1))) {
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))//为什么要和image2做对比
continue;
score = 0;
if (fmt1->read_probe) {
score = fmt1->read_probe(&lpd);
if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
if (nodat == 0) score = FFMAX(score, 1);
else if (nodat == 1) score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
else score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
}
} else if (fmt1->extensions) {
if (av_match_ext(lpd.filename, fmt1->extensions))
score = AVPROBE_SCORE_EXTENSION;
}
if (av_match_name(lpd.mime_type, fmt1->mime_type))
score = FFMAX(score, AVPROBE_SCORE_MIME);
if (score > score_max) {
score_max = score;
fmt = fmt1;
} else if (score == score_max)
fmt = NULL;
}
if (nodat == 1)
score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
*score_ret = score_max;
return fmt;
}