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FFmpeg编解码处理系列笔记：
0. FFmpeg时间戳详解
1. FFmpeg编解码处理1-转码全流程简介
2. FFmpeg编解码处理3-编解码API详解
3. FFmpeg编解码处理4-视频编码
4. FFmpeg编解码处理5-音频编码
基于FFmpeg 4.1版本。
4. 编解码API详解
解码使用avcodec_send_packet()和avcodec_receive_frame()两个函数。
编码使用avcodec_send_frame()和avcodec_receive_packet()两个函数。
4.1 API定义
4.1.1 avcodec_send_packet()
代码语言：javascript复制/**
* 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.
*
* @warning Do not mix this API with the legacy API (like avcodec_decode_video2())
* on the same AVCodecContext. It will return unexpected results now
* or in future libavcodec versions.
*
* @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.
*
* @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() (once
* all output is read, the packet should be resent, and
* the call will not fail with EAGAIN).
* 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)
* AVERROR(EINVAL): codec not opened, it is an encoder, or requires flush
* AVERROR(ENOMEM): failed to add packet to internal queue, or similar
* other errors: legitimate decoding errors
*/
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);4.1.2 avcodec_receive_frame()
代码语言：javascript复制/**
* 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 - user must try
* to send new input
* AVERROR_EOF: the decoder has been fully flushed, and there will be
* no more output frames
* AVERROR(EINVAL): codec not opened, or it is an encoder
* other negative values: legitimate decoding errors
*/
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);4.1.3 avcodec_send_frame()
代码语言：javascript复制/**
* Supply a raw video or audio frame to the encoder. Use avcodec_receive_packet()
* to retrieve buffered output packets.
*
* @param avctx codec context
* @param[in] frame AVFrame containing the raw audio or video frame to be encoded.
* Ownership of the frame remains with the caller, and the
* encoder will not write to the frame. The encoder may create
* a reference to the frame data (or copy it if the frame is
* not reference-counted).
* It can be NULL, in which case it is considered a flush
* packet. This signals the end of the stream. If the encoder
* still has packets buffered, it will return them after this
* call. Once flushing mode has been entered, additional flush
* packets are ignored, and sending frames will return
* AVERROR_EOF.
*
* For audio:
* If AV_CODEC_CAP_VARIABLE_FRAME_SIZE is set, then each frame
* can have any number of samples.
* If it is not set, frame->nb_samples must be equal to
* avctx->frame_size for all frames except the last.
* The final frame may be smaller than avctx->frame_size.
* @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_packet() (once
* all output is read, the packet should be resent, and
* the call will not fail with EAGAIN).
* AVERROR_EOF: the encoder has been flushed, and no new frames can
* be sent to it
* AVERROR(EINVAL): codec not opened, refcounted_frames not set, it is a
* decoder, or requires flush
* AVERROR(ENOMEM): failed to add packet to internal queue, or similar
* other errors: legitimate decoding errors
*/
int avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame);4.1.4 avcodec_receive_packet()
代码语言：javascript复制/**
* Read encoded data from the encoder.
*
* @param avctx codec context
* @param avpkt This will be set to a reference-counted packet allocated by the
* encoder. Note that the function will always call
* av_frame_unref(frame) before doing anything else.
* @return 0 on success, otherwise negative error code:
* AVERROR(EAGAIN): output is not available in the current state - user
* must try to send input
* AVERROR_EOF: the encoder has been fully flushed, and there will be
* no more output packets
* AVERROR(EINVAL): codec not opened, or it is an encoder
* other errors: legitimate decoding errors
*/
int avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt);4.2 API使用说明
4.2.1 解码API使用详解
关于avcodec_send_packet()与avcodec_receive_frame()的使用说明：
- 按dts递增的顺序向解码器送入编码帧packet，解码器按pts递增的顺序输出原始帧frame，实际上解码器不关注输入packet的dts(错值都没关系)，它只管依次处理收到的packet，按需缓冲和解码
- avcodec_receive_frame()输出frame时，会根据各种因素设置好frame->best_effort_timestamp(文档明确说明)，实测frame->pts也会被设置(通常直接拷贝自对应的packet.pts，文档未明确说明)用户应确保avcodec_send_packet()发送的packet具有正确的pts，编码帧packet与原始帧frame间的对应关系通过pts确定
- avcodec_receive_frame()输出frame时，frame->pkt_dts拷贝自当前avcodec_send_packet()发送的packet中的dts，如果当前packet为NULL(flush packet)，解码器进入flush模式，当前及剩余的frame->pkt_dts值总为AV_NOPTS_VALUE。因为解码器中有缓存帧，当前输出的frame并不是由当前输入的packet解码得到的，所以这个frame->pkt_dts没什么实际意义，可以不必关注
- avcodec_send_packet()发送第一个NULL会返回成功，后续的NULL会返回AVERROR_EOF。
- avcodec_send_packet()多次发送NULL并不会导致解码器中缓存的帧丢失，使用avcodec_flush_buffers()可以立即丢掉解码器中缓存帧。因此播放完毕时应avcodec_send_packet(NULL)来取完缓存的帧，而SEEK操作或切换流时应调用avcodec_flush_buffers()来直接丢弃缓存帧。
- 解码器通常的冲洗方法：调用一次avcodec_send_packet(NULL)(返回成功)，然后不停调用avcodec_receive_frame()直到其返回AVERROR_EOF，取出所有缓存帧，avcodec_receive_frame()返回AVERROR_EOF这一次是没有有效数据的，仅仅获取到一个结束标志。
4.2.2 编码API使用详解
关于avcodec_send_frame()与avcodec_receive_packet()的使用说明：
- 按pts递增的顺序向编码器送入原始帧frame，编码器按dts递增的顺序输出编码帧packet，实际上编码器关注输入frame的pts不关注其dts，它只管依次处理收到的frame，按需缓冲和编码
- avcodec_receive_packet()输出packet时，会设置packet.dts，从0开始，每次输出的packet的dts加1，这是视频层的dts，用户写输出前应将其转换为容器层的dts
- avcodec_receive_packet()输出packet时，packet.pts拷贝自对应的frame.pts，这是视频层的pts，用户写输出前应将其转换为容器层的pts
- avcodec_send_frame()发送NULL frame时，编码器进入flush模式
- avcodec_send_frame()发送第一个NULL会返回成功，后续的NULL会返回AVERROR_EOF
- avcodec_send_frame()多次发送NULL并不会导致编码器中缓存的帧丢失，使用avcodec_flush_buffers()可以立即丢掉编码器中缓存帧。因此编码完毕时应使用avcodec_send_frame(NULL)来取完缓存的帧，而SEEK操作或切换流时应调用avcodec_flush_buffers()来直接丢弃缓存帧。
- 编码器通常的冲洗方法：调用一次avcodec_send_frame(NULL)(返回成功)，然后不停调用avcodec_receive_packet()直到其返回AVERROR_EOF，取出所有缓存帧，avcodec_receive_packet()返回AVERROR_EOF这一次是没有有效数据的，仅仅获取到一个结束标志。
- 对音频来说，如果AV_CODEC_CAP_VARIABLE_FRAME_SIZE(在AVCodecContext.codec.capabilities变量中，只读)标志有效，表示编码器支持可变尺寸音频帧，送入编码器的音频帧可以包含任意数量的采样点。如果此标志无效，则每一个音频帧的采样点数目(frame->nb_samples)必须等于编码器设定的音频帧尺寸(avctx->frame_size)，最后一帧除外，最后一帧音频帧采样点数可以小于avctx->frame_size
4.3 API使用例程
4.3.1 解码API例程
代码语言：javascript复制// retrun 0: got a frame success
// AVERROR(EAGAIN): need more packet
// AVERROR_EOF: end of file, decoder has been flushed
// <0: error
int av_decode_frame(AVCodecContext *dec_ctx, AVPacket *packet, bool *new_packet, AVFrame *frame)
{
int ret = AVERROR(EAGAIN);
while (1)
{
// 2. 从解码器接收frame
if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO)
{
// 2.1 一个视频packet含一个视频frame
// 解码器缓存一定数量的packet后，才有解码后的frame输出
// frame输出顺序是按pts的顺序，如IBBPBBP
// frame->pkt_pos变量是此frame对应的packet在视频文件中的偏移地址，值同pkt.pos
ret = avcodec_receive_frame(dec_ctx, frame);
if (ret >= 0)
{
if (frame->pts == AV_NOPTS_VALUE)
{
frame->pts = frame->best_effort_timestamp;
printf("set video pts %dn", frame->pts);
}
}
}
else if (dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO)
{
// 2.2 一个音频packet含一至多个音频frame，每次avcodec_receive_frame()返回一个frame，此函数返回。
// 下次进来此函数，继续获取一个frame，直到avcodec_receive_frame()返回AVERROR(EAGAIN)，
// 表示解码器需要填入新的音频packet
ret = avcodec_receive_frame(dec_ctx, frame);
if (ret >= 0)
{
if (frame->pts == AV_NOPTS_VALUE)
{
frame->pts = frame->best_effort_timestamp;
printf("set audio pts %dn", frame->pts);
}
}
}
if (ret >= 0) // 成功解码得到一个视频帧或一个音频帧，则返回
{
return ret;
}
else if (ret == AVERROR_EOF) // 解码器已冲洗，解码中所有帧已取出
{
avcodec_flush_buffers(dec_ctx);
return ret;
}
else if (ret == AVERROR(EAGAIN))// 解码器需要喂数据
{
if (!(*new_packet)) // 本函数中已向解码器喂过数据，因此需要从文件读取新数据
{
//av_log(NULL, AV_LOG_INFO, "decoder need more packetn");
return ret;
}
}
else // 错误
{
av_log(NULL, AV_LOG_ERROR, "decoder error %dn", ret);
return ret;
}
/*
if (packet == NULL || (packet->data == NULL && packet->size == 0))
{
// 复位解码器内部状态/刷新内部缓冲区。当seek操作或切换流时应调用此函数。
avcodec_flush_buffers(dec_ctx);
}
*/
// 1. 将packet发送给解码器
// 发送packet的顺序是按dts递增的顺序，如IPBBPBB
// pkt.pos变量可以标识当前packet在视频文件中的地址偏移
// 发送第一个 flush packet 会返回成功，后续的 flush packet 会返回AVERROR_EOF
ret = avcodec_send_packet(dec_ctx, packet);
*new_packet = false;
if (ret != 0)
{
av_log(NULL, AV_LOG_ERROR, "avcodec_send_packet() error, return %dn", ret);
return ret;
}
}
return -1;
}4.3.2 编码API例程
代码语言：javascript复制int av_encode_frame(AVCodecContext *enc_ctx, AVFrame *frame, AVPacket *packet)
{
int ret = -1;
// 第一次发送flush packet会返回成功，进入冲洗模式，可调用avcodec_receive_packet()
// 将编码器中缓存的帧(可能不止一个)取出来
// 后续再发送flush packet将返回AVERROR_EOF
ret = avcodec_send_frame(enc_ctx, frame);
if (ret == AVERROR_EOF)
{
//av_log(NULL, AV_LOG_INFO, "avcodec_send_frame() encoder flushedn");
}
else if (ret == AVERROR(EAGAIN))
{
//av_log(NULL, AV_LOG_INFO, "avcodec_send_frame() need output read outn");
}
else if (ret < 0)
{
//av_log(NULL, AV_LOG_INFO, "avcodec_send_frame() error %dn", ret);
return ret;
}
ret = avcodec_receive_packet(enc_ctx, packet);
if (ret == AVERROR_EOF)
{
av_log(NULL, AV_LOG_INFO, "avcodec_recieve_packet() encoder flushedn");
}
else if (ret == AVERROR(EAGAIN))
{
//av_log(NULL, AV_LOG_INFO, "avcodec_recieve_packet() need more inputn");
}
return ret;
}
