目录
FFmpeg相关记录：
示例工程：
【FFmpeg】调用ffmpeg库实现264软编
【FFmpeg】调用ffmpeg库实现264软解
【FFmpeg】调用ffmpeg库进行RTMP推流和拉流
【FFmpeg】调用ffmpeg库进行SDL2解码后渲染
流程分析：
【FFmpeg】编码链路上主要函数的简单分析
【FFmpeg】解码链路上主要函数的简单分析
结构体分析：
【FFmpeg】AVCodec结构体
【FFmpeg】AVCodecContext结构体
【FFmpeg】AVStream结构体
【FFmpeg】AVFormatContext结构体
【FFmpeg】AVIOContext结构体
【FFmpeg】AVPacket结构体
函数分析：
【通用】
【FFmpeg】avcodec_find_encoder和avcodec_find_decoder
【FFmpeg】关键结构体的初始化和释放（AVFormatContext、AVIOContext等）
【FFmpeg】avcodec_open2函数
【推流】
【FFmpeg】avformat_open_input函数
【FFmpeg】avformat_find_stream_info函数
【FFmpeg】avformat_alloc_output_context2函数
【FFmpeg】avio_open2函数
【FFmpeg】avformat_write_header函数
【FFmpeg】av_write_frame函数
【编码】
【FFmpeg】avcodec_send_frame函数
1.avcodec_send_packet
函数的主要功能是将一个压缩包送入到解码器当中进行解码，定义位于libavcodec\decode.c中
/**
* Supply raw packet data as input to a decoder.
*
* Internally, this call will copy relevant AVCodecContext fields, which can
* influence decoding per-packet, and apply them when the packet is actually
* decoded. (For example AVCodecContext.skip_frame, which might direct the
* decoder to drop the frame contained by the packet sent with this function.)
*
* @warning The input buffer, avpkt->data must be AV_INPUT_BUFFER_PADDING_SIZE
* larger than the actual read bytes because some optimized bitstream
* readers read 32 or 64 bits at once and could read over the end.
*
* @note The AVCodecContext MUST have been opened with @ref avcodec_open2()
* before packets may be fed to the decoder.
*
* @param avctx codec context
* @param[in] avpkt The input AVPacket. Usually, this will be a single video
* frame, or several complete audio frames.
* Ownership of the packet remains with the caller, and the
* decoder will not write to the packet. The decoder may create
* a reference to the packet data (or copy it if the packet is
* not reference-counted).
* Unlike with older APIs, the packet is always fully consumed,
* and if it contains multiple frames (e.g. some audio codecs),
* will require you to call avcodec_receive_frame() multiple
* times afterwards before you can send a new packet.
* It can be NULL (or an AVPacket with data set to NULL and
* size set to 0); in this case, it is considered a flush
* packet, which signals the end of the stream. Sending the
* first flush packet will return success. Subsequent ones are
* unnecessary and will return AVERROR_EOF. If the decoder
* still has frames buffered, it will return them after sending
* a flush packet.
*
* @retval 0 success
* @retval AVERROR(EAGAIN) input is not accepted in the current state - user
* must read output with avcodec_receive_frame() (once
* all output is read, the packet should be resent,
* and the call will not fail with EAGAIN).
* @retval AVERROR_EOF the decoder has been flushed, and no new packets can be
* sent to it (also returned if more than 1 flush
* packet is sent)
* @retval AVERROR(EINVAL) codec not opened, it is an encoder, or requires flush
* @retval AVERROR(ENOMEM) failed to add packet to internal queue, or similar
* @retval "another negative error code" legitimate decoding errors
*/
// 提供原始数据包数据作为解码器的输入
int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt)
{
AVCodecInternal *avci = avctx->internal;
DecodeContext *dc = decode_ctx(avci);
int ret;
// 1.输入信息的检查
if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))
return AVERROR(EINVAL);
if (dc->draining_started)
return AVERROR_EOF;
if (avpkt && !avpkt->size && avpkt->data)
return AVERROR(EINVAL);
if (avpkt && (avpkt->data || avpkt->side_data_elems)) {
if (!AVPACKET_IS_EMPTY(avci->buffer_pkt))
return AVERROR(EAGAIN);
ret = av_packet_ref(avci->buffer_pkt, avpkt);
if (ret < 0)
return ret;
} else
dc->draining_started = 1;
// 2.解码入口
if (!avci->buffer_frame->buf[0] && !dc->draining_started) {
ret = decode_receive_frame_internal(avctx, avci->buffer_frame);
if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
return ret;
}
return 0;
}
1.1 解码入口函数（decode_receive_frame_internal）
函数的定义位于libavcodec\decode.c中，如下所示。这里最重要的地方在于检查cb_type，如果使用硬件解码器，则使用receive_frame，否则使用decode_simple_receive_frame执行软解过程
static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
{
AVCodecInternal *avci = avctx->internal;
DecodeContext *dc = decode_ctx(avci);
const FFCodec *const codec = ffcodec(avctx->codec);
int ret, ok;
av_assert0(!frame->buf[0]);
// 如果使用硬件解码器，cb_type才会是FF_CODEC_CB_TYPE_RECEIVE_FRAME
// 这里使用函数指针，是调用硬件的解码器
if (codec->cb_type == FF_CODEC_CB_TYPE_RECEIVE_FRAME) {
ret = codec->cb.receive_frame(avctx, frame);
emms_c();
if (!ret) {
if (avctx->codec->type == AVMEDIA_TYPE_VIDEO)
ret = (frame->flags & AV_FRAME_FLAG_DISCARD) ? AVERROR(EAGAIN) : 0;
else if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
int64_t discarded_samples = 0;
ret = discard_samples(avctx, frame, &discarded_samples);
}
}
} else // 使用软件解码器
ret = decode_simple_receive_frame(avctx, frame);
if (ret == AVERROR_EOF)
avci->draining_done = 1;
/* preserve ret */
// 检测colorspace，这里函数还没实现，默认返回为0
ok = detect_colorspace(avctx, frame);
if (ok < 0) {
av_frame_unref(frame);
return ok;
}
if (!ret) {
if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
if (!frame->width)
frame->width = avctx->width;
if (!frame->height)
frame->height = avctx->height;
} else
frame->flags |= AV_FRAME_FLAG_KEY;
ret = fill_frame_props(avctx, frame);
if (ret < 0) {
av_frame_unref(frame);
return ret;
}
#if FF_API_FRAME_KEY
FF_DISABLE_DEPRECATION_WARNINGS
frame->key_frame = !!(frame->flags & AV_FRAME_FLAG_KEY); // 配置key_frame
FF_ENABLE_DEPRECATION_WARNINGS
#endif
#if FF_API_INTERLACED_FRAME
FF_DISABLE_DEPRECATION_WARNINGS
frame->interlaced_frame = !!(frame->flags & AV_FRAME_FLAG_INTERLACED);
frame->top_field_first = !!(frame->flags & AV_FRAME_FLAG_TOP_FIELD_FIRST);
FF_ENABLE_DEPRECATION_WARNINGS
#endif
frame->best_effort_timestamp = guess_correct_pts(dc,
frame->pts,
frame->pkt_dts);
/* the only case where decode data is not set should be decoders
* that do not call ff_get_buffer() */
// 不设置decode数据的唯一情况应该是解码器不调用ff_get_buffer()
av_assert0((frame->private_ref && frame->private_ref->size == sizeof(FrameDecodeData)) ||
!(avctx->codec->capabilities & AV_CODEC_CAP_DR1));
if (frame->private_ref) {
FrameDecodeData *fdd = (FrameDecodeData*)frame->private_ref->data;
if (fdd->post_process) {
ret = fdd->post_process(avctx, frame);
if (ret < 0) {
av_frame_unref(frame);
return ret;
}
}
}
}
/* free the per-frame decode data */
av_buffer_unref(&frame->private_ref);
return ret;
}
1.1.1 软解入口（decode_simple_receive_frame）
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;
}
函数最重要的是调用了decode_simple_internal，函数的定义如下
/*
* 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.
**/
// 用于实现简单API的解码器的receive_frame_wrapper的核心。某些解码器可能会消耗部分数据包而不返回任何输出
// 因此需要在循环中调用此函数，直到它返回EAGAIN
static inline int decode_simple_internal(AVCodecContext *avctx, AVFrame *frame, int64_t *discarded_samples)
{
AVCodecInternal *avci = avctx->internal;
AVPacket *const pkt = avci->in_pkt;
const FFCodec *const codec = ffcodec(avctx->codec);
int got_frame, consumed;
int ret;
// 如果pkt没有data，获取下一个pkt
if (!pkt->data && !avci->draining) {
av_packet_unref(pkt);
ret = ff_decode_get_packet(avctx, pkt);
if (ret < 0 && ret != AVERROR_EOF)
return ret;
}
// Some codecs (at least wma lossless) will crash when feeding drain packets
// after EOF was signaled.
// 一些编解码器(至少是wma无损的)在EOF信号发出后输入漏包时会崩溃
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;
// 执行解码过程
if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME) {
// 线程解码
consumed = ff_thread_decode_frame(avctx, frame, &got_frame, pkt);
} else {
// 根据使用的解码器进行解码
consumed = codec->cb.decode(avctx, frame, &got_frame, pkt);
if (!(codec->caps_internal & FF_CODEC_CAP_SETS_PKT_DTS))
frame->pkt_dts = pkt->dts;
if (avctx->codec->type == AVMEDIA_TYPE_VIDEO) {
#if FF_API_FRAME_PKT
FF_DISABLE_DEPRECATION_WARNINGS
if(!avctx->has_b_frames)
frame->pkt_pos = pkt->pos;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
}
}
emms_c();
if (avctx->codec->type == AVMEDIA_TYPE_VIDEO) {
ret = (!got_frame || frame->flags & AV_FRAME_FLAG_DISCARD)
? AVERROR(EAGAIN)
: 0;
} else if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
ret = !got_frame ? AVERROR(EAGAIN)
: discard_samples(avctx, frame, discarded_samples);
}
if (ret == AVERROR(EAGAIN))
av_frame_unref(frame);
// FF_CODEC_CB_TYPE_DECODE decoders must not return AVERROR EAGAIN
// code later will add AVERROR(EAGAIN) to a pointer
av_assert0(consumed != AVERROR(EAGAIN));
if (consumed < 0)
ret = consumed;
if (consumed >= 0 && avctx->codec->type == AVMEDIA_TYPE_VIDEO)
consumed = pkt->size;
if (!ret)
av_assert0(frame->buf[0]);
if (ret == AVERROR(EAGAIN))
ret = 0;
/* do not stop draining when got_frame != 0 or ret < 0 */
if (avci->draining && !got_frame) {
if (ret < 0) {
/* prevent infinite loop if a decoder wrongly always return error on draining */
/* reasonable nb_errors_max = maximum b frames + thread count */
int nb_errors_max = 20 + (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME ?
avctx->thread_count : 1);
if (decode_ctx(avci)->nb_draining_errors++ >= nb_errors_max) {
av_log(avctx, AV_LOG_ERROR, "Too many errors when draining, this is a bug. "
"Stop draining and force EOF.\n");
avci->draining_done = 1;
ret = AVERROR_BUG;
}
} else {
avci->draining_done = 1;
}
}
if (consumed >= pkt->size || ret < 0) {
av_packet_unref(pkt);
} else {
pkt->data += consumed;
pkt->size -= consumed;
pkt->pts = AV_NOPTS_VALUE;
pkt->dts = AV_NOPTS_VALUE;
if (!(codec->caps_internal & FF_CODEC_CAP_SETS_FRAME_PROPS)) {
#if FF_API_FRAME_PKT
// See extract_packet_props() comment.
avci->last_pkt_props->stream_index = avci->last_pkt_props->stream_index - consumed;
#endif
avci->last_pkt_props->pts = AV_NOPTS_VALUE;
avci->last_pkt_props->dts = AV_NOPTS_VALUE;
}
}
return ret;
}
代码最核心的地方在于调用codec->cb.decode执行解码过程，在FFmpeg中内嵌了H264的解码器，以这个为例进行记录，会使用h264_decode_frame进行解码
const FFCodec ff_h264_decoder = {
.p.name = "h264",
CODEC_LONG_NAME("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
.p.type = AVMEDIA_TYPE_VIDEO,
.p.id = AV_CODEC_ID_H264,
.priv_data_size = sizeof(H264Context),
.init = h264_decode_init,
.close = h264_decode_end,
FF_CODEC_DECODE_CB(h264_decode_frame), // 使用h264_decode_frame进行解码
.p.capabilities = AV_CODEC_CAP_DR1 |
AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
AV_CODEC_CAP_FRAME_THREADS,
.hw_configs = (const AVCodecHWConfigInternal *const []) {
// ...
NULL
},
.caps_internal = FF_CODEC_CAP_EXPORTS_CROPPING |
FF_CODEC_CAP_ALLOCATE_PROGRESS | FF_CODEC_CAP_INIT_CLEANUP,
.flush = h264_decode_flush,
UPDATE_THREAD_CONTEXT(ff_h264_update_thread_context),
UPDATE_THREAD_CONTEXT_FOR_USER(ff_h264_update_thread_context_for_user),
.p.profiles = NULL_IF_CONFIG_SMALL(ff_h264_profiles),
.p.priv_class = &h264_class,
};
h264_decode_frame的定义位于libavcodec\h264dec.c中，定义如下
static int h264_decode_frame(AVCodecContext *avctx, AVFrame *pict,
int *got_frame, AVPacket *avpkt)
{
const uint8_t *buf = avpkt->data;
int buf_size = avpkt->size;
H264Context *h = avctx->priv_data;
int buf_index;
int ret;
h->flags = avctx->flags;
h->setup_finished = 0;
h->nb_slice_ctx_queued = 0;
ff_h264_unref_picture(&h->last_pic_for_ec);
/* end of stream, output what is still in the buffers */
// 到达流的末尾，输出buffer当中现有的数据
if (buf_size == 0)
return send_next_delayed_frame(h, pict, got_frame, 0);
// 解码side data
if (av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, NULL)) {
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);
}
// 解码extra data
if (h->is_avc && buf_size >= 9 && buf[0]==1 && buf[2]==0 && (buf[4]&0xFC)==0xFC) {
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);
}
// 解码入口
buf_index = decode_nal_units(h, buf, buf_size);
if (buf_index < 0)
return AVERROR_INVALIDDATA;
// 到达文件末尾
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;
av_log(avctx, AV_LOG_ERROR, "no frame!\n");
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;
}
}
av_assert0(pict->buf[0] || !*got_frame);
ff_h264_unref_picture(&h->last_pic_for_ec);
// 获取使用的比特数
return get_consumed_bytes(buf_index, buf_size);
}
在上面的代码中，先检查是否需要解码side data和extra data，随后使用decode_nal_units进行数据包的解码，随后会进入H264格式的解码过程，这个过程链路比较长，就是另一个系列记录的内容了，这里不做进一步记录
CSDN : https://blog.csdn.net/weixin_42877471
Github : https://github.com/DoFulangChen