FFmpeg在libavcodec模块，旧版本提供avcodec_decode_video2()作为视频解码函数，avcodec_decode_audio4()作为音频解码函数。在FFmpeg 3.1版本新增avcodec_send_packet()与avcodec_receive_frame()作为音视频解码函数。后来，在3.4版本把avcodec_decode_video2()和avcodec_decode_audio4()标记为过时API。版本变更描述如下：
FFmpeg 3.1
2016-04-21 - 7fc329e - lavc 57.37.100 - avcodec.h
Add a new audio/video encoding and decoding API with decoupled input
and output -- avcodec_send_packet(), avcodec_receive_frame(),
avcodec_send_frame() and avcodec_receive_packet().
FFmpeg 3.4
2017-09-26 - b1cf151c4d - lavc 57.106.102 - avcodec.h
Deprecate AVCodecContext.refcounted_frames. This was useful for deprecated
API only (avcodec_decode_video2/avcodec_decode_audio4). The new decode APIs
(avcodec_send_packet/avcodec_receive_frame) always work with reference
counted frames.avcodec_send_packet()和avcodec_receive_frame()函数声明位于libavcodec/avcodec.h。我们来看看函数声明介绍：
/**
* Supply raw packet data as input to a decoder.
*
* @param avctx codec context
* @param[in] avpkt The input AVPacket. Usually, this will be a single video
* frame, or several complete audio frames.
*
* @return 0 on success, otherwise negative error code:
* AVERROR(EAGAIN): input is not accepted in the current state - user
* must read output with avcodec_receive_frame()
* AVERROR_EOF: the decoder has been flushed, and no new packets
* AVERROR(EINVAL): codec not opened, it is an encoder, or requires flush
* AVERROR(ENOMEM): failed to add packet to internal queue, or similar
*/
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);
/**
* Return decoded output data from a decoder.
*
* @param avctx codec context
* @param frame This will be set to a reference-counted video or audio
* frame (depending on the decoder type) allocated by the
* decoder. Note that the function will always call
* av_frame_unref(frame) before doing anything else.
*
* @return
* 0: success, a frame was returned
* AVERROR(EAGAIN): output is not available in this state
* AVERROR_EOF: the decoder has been fully flushed
* AVERROR(EINVAL): codec not opened, or it is an encoder
* AVERROR_INPUT_CHANGED: current decoded frame has changed parameters
*/
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);由描述可知，avcodec_send_packet()负责把AVpacket数据包发送给解码器，avcodec_receive_frame()则从解码器取出一帧AVFrame数据。返回0，代表解码成功；返回EAGAIN，代表当前状态无可输出的数据；返回EOF，代表到达文件流结尾；返回INVAL，代表解码器未打开或者当前打开的是编码器；返回INPUT_CHANGED，代表输入参数发生变化，比如width和height改变。
avcodec_send_packet()和avcodec_receive_frame()的解码流程图如下：
一、avcodec_send_packet发送AVPacket
1、avcodec_send_packet
avcodec_send_packet()函数位于libavcodec/decode.c，具体如下：
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt)
{
AVCodecInternal *avci = avctx->internal;
int ret;
// 判断解码器是否打开，是否为解码器
if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))
return AVERROR(EINVAL);
if (avctx->internal->draining)
return AVERROR_EOF;
if (avpkt && !avpkt->size && avpkt->data)
return AVERROR(EINVAL);
av_packet_unref(avci->buffer_pkt);
if (avpkt && (avpkt->data || avpkt->side_data_elems)) {
ret = av_packet_ref(avci->buffer_pkt, avpkt);
if (ret < 0)
return ret;
}
// 发送packet到bitstream滤波器
ret = av_bsf_send_packet(avci->bsf, avci->buffer_pkt);
if (ret < 0) {
av_packet_unref(avci->buffer_pkt);
return ret;
}
if (!avci->buffer_frame->buf[0]) {
ret = decode_receive_frame_internal(avctx, avci->buffer_frame);
if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
return ret;
}
return 0;
}2、av_bsf_send_packet
由此可见，内部是调用av_bsf_send_packet()函数把packet发送给bitstream滤波器，代码位于libavcodec/bsf.c，具体如下：
int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt)
{
AVBSFInternal *bsfi = ctx->internal;
int ret;
if (!pkt || IS_EMPTY(pkt)) {
bsfi->eof = 1;
return 0;
}
if (bsfi->eof) {
return AVERROR(EINVAL);
}
if (!IS_EMPTY(bsfi->buffer_pkt))
return AVERROR(EAGAIN);
// 申请AVPacket内存，并拷贝数据
ret = av_packet_make_refcounted(pkt);
if (ret < 0)
return ret;
// 把pkt指针赋值给bsfi->buffer_pkt
av_packet_move_ref(bsfi->buffer_pkt, pkt);
return 0;
}
av_bsf_send_packet()函数内部调用av_packet_make_refcounted()来申请AVPacket内存，并拷贝数据。然后调用av_packet_move_ref()函数把pkt指针赋值给bsfi->buffer_pkt。
二、avcodec_receive_frame接收AVFrame
1、avcodec_receive_frame
avcodec_receive_frame()函数同样位于libavcodec/decode.c，主要是调用内部函数decode_receive_frame_internal()实现解码，具体代码如下：
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{
AVCodecInternal *avci = avctx->internal;
int ret, changed;
av_frame_unref(frame);
// 判断解码器是否打开，是否为解码器
if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))
return AVERROR(EINVAL);
if (avci->buffer_frame->buf[0]) {
av_frame_move_ref(frame, avci->buffer_frame);
} else {
ret = decode_receive_frame_internal(avctx, frame);
if (ret < 0)
return ret;
}
if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
ret = apply_cropping(avctx, frame);
if (ret < 0) {
av_frame_unref(frame);
return ret;
}
}
......
return 0;
}2、decode_receive_frame_internal
decode_receive_frame_internal()函数判断是否支持avctx->codec->receive_frame，如果支持就调用avctx->codec->receive_frame()函数进行解码，否则调用decode_simple_receive_frame()函数进行解码。具体如下：
static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
{
AVCodecInternal *avci = avctx->internal;
int ret;
if (avctx->codec->receive_frame) {
ret = avctx->codec->receive_frame(avctx, frame);
if (ret != AVERROR(EAGAIN))
av_packet_unref(avci->last_pkt_props);
} else {
ret = decode_simple_receive_frame(avctx, frame);
}
......
/* free the per-frame decode data */
av_buffer_unref(&frame->private_ref);
return ret;
}3、decode_simple_receive_frame
decode_simple_receive_frame()函数又调用decode_simple_internal()内部函数进行解码：
static int decode_simple_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{
int ret;
int64_t discarded_samples = 0;
while (!frame->buf[0]) {
if (discarded_samples > avctx->max_samples)
return AVERROR(EAGAIN);
ret = decode_simple_internal(avctx, frame, &discarded_samples);
if (ret < 0)
return ret;
}
return 0;
}4、decode_simple_internal
由描述可知，decode_simple_internal()函数是解码器的核心封装函数。我们应该循环调用avcodec_receive_frame()函数，直到返回EAGAIN。通过判断是否需要特定线程进行解码，如果需要就调用ff_thread_decode_frame()函数，否则调用avctx->codec->decode指向的解码函数。具体调用过程如下：
/*
* The core of the receive_frame_wrapper for the decoders implementing
* the simple API. Certain decoders might consume partial packets without
* returning any output, so this function needs to be called in a loop until it
* returns EAGAIN.
**/
static inline int decode_simple_internal(AVCodecContext *avctx,
AVFrame *frame,
int64_t *discarded_samples)
{
AVCodecInternal *avci = avctx->internal;
DecodeSimpleContext *ds = &avci->ds;
AVPacket *pkt = ds->in_pkt;
int got_frame, actual_got_frame;
int ret;
if (!pkt->data && !avci->draining) {
av_packet_unref(pkt);
ret = ff_decode_get_packet(avctx, pkt);
if (ret < 0 && ret != AVERROR_EOF)
return ret;
}
if (avci->draining_done)
return AVERROR_EOF;
if (!pkt->data &&
!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY ||
avctx->active_thread_type & FF_THREAD_FRAME))
return AVERROR_EOF;
got_frame = 0;
if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME) {
ret = ff_thread_decode_frame(avctx, frame, &got_frame, pkt);
} else {
ret = avctx->codec->decode(avctx, frame, &got_frame, pkt);
......
}
......
return ret < 0 ? ret : 0;
}5、ff_thread_decode_frame
ff_thread_decode_frame()函数位于libavcodec/pthread_frame.c。首先是调用submit_packet()函数提交packet到下一个解码线程，然后调用数组中最前面线程进行解码。相关代码如下：
int ff_thread_decode_frame(AVCodecContext *avctx,
AVFrame *picture,
int *got_picture_ptr,
AVPacket *avpkt)
{
FrameThreadContext *fctx = avctx->internal->thread_ctx;
int finished = fctx->next_finished;
PerThreadContext *p;
int err;
async_unlock(fctx);
/*
* Submit a packet to the next decoding thread.
*/
p = &fctx->threads[fctx->next_decoding];
err = submit_packet(p, avctx, avpkt);
if (err)
goto finish;
......
/* Return the next available frame from the oldest thread.*/
do {
p = &fctx->threads[finished++];
if (atomic_load(&p->state) != STATE_INPUT_READY) {
pthread_mutex_lock(&p->progress_mutex);
while (atomic_load_explicit(&p->state, memory_order_relaxed) != STATE_INPUT_READY)
pthread_cond_wait(&p->output_cond, &p->progress_mutex);
pthread_mutex_unlock(&p->progress_mutex);
}
av_frame_move_ref(picture, p->frame);
*got_picture_ptr = p->got_frame;
picture->pkt_dts = p->avpkt->dts;
err = p->result;
p->got_frame = 0;
p->result = 0;
if (finished >= avctx->thread_count) finished = 0;
} while (!avpkt->size && !*got_picture_ptr && err >= 0 && finished != fctx->next_finished);
update_context_from_thread(avctx, p->avctx, 1);
if (fctx->next_decoding >= avctx->thread_count)
fctx->next_decoding = 0;
fctx->next_finished = finished;
/* return the size of the consumed packet if no error occurred */
if (err >= 0)
err = avpkt->size;
finish:
async_lock(fctx);
return err;
}6、h264_decode_frame
以h264解码器为例，位于libavcodec/h264dec.c，对应的AVCodec如下：
AVCodec ff_h264_decoder = {
.name = "h264",
.long_name = NULL_IF_CONFIG_SMALL("H.264/AVC/MPEG-4 AVC/MPEG-4 part10"),
.type = AVMEDIA_TYPE_VIDEO,
.id = AV_CODEC_ID_H264,
.priv_data_size = sizeof(H264Context),
.init = h264_decode_init,
.close = h264_decode_end,
.decode = h264_decode_frame,
.capabilities = AV_CODEC_CAP_DR1 |
AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
AV_CODEC_CAP_FRAME_THREADS,
.flush = h264_decode_flush,
.update_thread_context = ONLY_IF_THREADS_ENABLED(ff_h264_update_thread_context),
.profiles = NULL_IF_CONFIG_SMALL(ff_h264_profiles),
.priv_class = &h264_class,
};此时， avctx->codec->decode函数指针指向的是h264_decode_frame。如果存在extradata就调用ff_h264_decode_extradata()函数解析，最关键是调用decode_nal_units()函数来解码NAL单元。相关代码如下：
static int h264_decode_frame(AVCodecContext *avctx, void *data,
int *got_frame, AVPacket *avpkt)
{
......
ff_h264_unref_picture(h, &h->last_pic_for_ec);
/* end of stream, output what is still in the buffers */
if (buf_size == 0)
return send_next_delayed_frame(h, pict, got_frame, 0);
if (av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, NULL)) {
buffer_size_t side_size;
uint8_t *side = av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, &side_size);
ff_h264_decode_extradata(side, side_size,
&h->ps, &h->is_avc, &h->nal_length_size,
avctx->err_recognition, avctx);
}
if (h->is_avc && buf_size >= 9 && buf[0]==1 && buf[2]==0 && (buf[4]&0xFC)==0xFC) {
// 解析avcc对应的extradata
if (is_avcc_extradata(buf, buf_size))
return ff_h264_decode_extradata(buf, buf_size,
&h->ps, &h->is_avc, &h->nal_length_size,
avctx->err_recognition, avctx);
}
// 解码NAL单元
buf_index = decode_nal_units(h, buf, buf_size);
if (buf_index < 0)
return AVERROR_INVALIDDATA;
if (!h->cur_pic_ptr && h->nal_unit_type == H264_NAL_END_SEQUENCE) {
av_assert0(buf_index <= buf_size);
return send_next_delayed_frame(h, pict, got_frame, buf_index);
}
if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) && (!h->cur_pic_ptr || !h->has_slice)) {
if (avctx->skip_frame >= AVDISCARD_NONREF ||
buf_size >= 4 && !memcmp("Q264", buf, 4))
return buf_size;
return AVERROR_INVALIDDATA;
}
if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) ||
(h->mb_y >= h->mb_height && h->mb_height)) {
if ((ret = ff_h264_field_end(h, &h->slice_ctx[0], 0)) < 0)
return ret;
/* Wait for second field. */
if (h->next_output_pic) {
ret = finalize_frame(h, pict, h->next_output_pic, got_frame);
if (ret < 0)
return ret;
}
}
ff_h264_unref_picture(h, &h->last_pic_for_ec);
return get_consumed_bytes(buf_index, buf_size);
}三、avcodec_decode_video2视频解码
由于avcodec_decode_video2()函数已经过时，所以内部提供compat_decode()来兼容旧版本，具体代码如下：
int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,
int *got_picture_ptr,
const AVPacket *avpkt)
{
return compat_decode(avctx, picture, got_picture_ptr, avpkt);
}我们来看看compat_decode()函数源码，其实内部也是调用avcodec_send_packet()和avcodec_receive_frame()进行解码，具体如下：
static int compat_decode(AVCodecContext *avctx, AVFrame *frame,
int *got_frame, const AVPacket *pkt)
{
AVCodecInternal *avci = avctx->internal;
int ret = 0;
*got_frame = 0;
if (avci->draining_done && pkt && pkt->size != 0) {
avcodec_flush_buffers(avctx);
}
if (avci->compat_decode_partial_size > 0 &&
avci->compat_decode_partial_size != pkt->size) {
ret = AVERROR(EINVAL);
goto finish;
}
if (!avci->compat_decode_partial_size) {
// 调用avcodec_send_packet发送packet
ret = avcodec_send_packet(avctx, pkt);
if (ret == AVERROR_EOF)
ret = 0;
else if (ret == AVERROR(EAGAIN)) {
/* we fully drain all the output in each decode call, so this should not
* ever happen */
ret = AVERROR_BUG;
goto finish;
} else if (ret < 0)
goto finish;
}
while (ret >= 0) {
// 循环调用avcodec_receive_frame接收frame
ret = avcodec_receive_frame(avctx, frame);
if (ret < 0) {
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
ret = 0;
goto finish;
}
if (frame != avci->compat_decode_frame) {
if (!avctx->refcounted_frames) {
ret = unrefcount_frame(avci, frame);
if (ret < 0)
goto finish;
}
*got_frame = 1;
frame = avci->compat_decode_frame;
} else {
if (!avci->compat_decode_warned) {
avci->compat_decode_warned = 1;
}
}
if (avci->draining || (!avctx->codec->bsfs
&& avci->compat_decode_consumed < pkt->size)) {
break;
}
}
finish:
if (ret == 0) {
/* if there are any bsfs then assume full packet is always consumed */
if (avctx->codec->bsfs)
ret = pkt->size;
else
ret = FFMIN(avci->compat_decode_consumed, pkt->size);
}
avci->compat_decode_consumed = 0;
avci->compat_decode_partial_size = (ret >= 0) ? pkt->size - ret : 0;
return ret;
}四、avcodec_decode_audio4音频解码
和avcodec_decode_video2()函数一样，avcodec_decode_audio4()函数已经过时，内部也是提供compat_decode()来兼容旧版本，具体代码如下：
int avcodec_decode_audio4(AVCodecContext *avctx,
AVFrame *frame,
int *got_frame_ptr,
const AVPacket *avpkt)
{
return compat_decode(avctx, frame, got_frame_ptr, avpkt);
}至此，avcodec_send_packet()和avcodec_receive_frame()组成的解码函数已经分析完毕。文章来源：https://www.toymoban.com/news/detail-437490.html
学习FFmpeg与代码实践，可参考：FFmpegAndroid文章来源地址https://www.toymoban.com/news/detail-437490.html
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