【FFmpeg】avcodec_receive

【FFmpeg】avcodec_receive_frame函数

1.avcodec_receive_frame

函数的主要功能是从解码器或编码器返回解码后的输出数据(当使用AV_CODEC_FLAG_RECON_FRAME标志时)

/*** Return decoded output data from a decoder or encoder (when the* @ref AV_CODEC_FLAG_RECON_FRAME flag is used).** @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*              codec. Note that the function will always call*              av_frame_unref(frame) before doing anything else.** @retval 0                success, a frame was returned* @retval AVERROR(EAGAIN)  output is not available in this state - user must*                          try to send new input* @retval AVERROR_EOF      the codec has been fully flushed, and there will be*                          no more output frames* @retval AVERROR(EINVAL)  codec not opened, or it is an encoder without the*                          @ref AV_CODEC_FLAG_RECON_FRAME flag enabled* @retval "other negative error code" legitimate decoding errors*/
int attribute_align_arg avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{av_frame_unref(frame);// 1.如果是解码器，返回decoded frame（一般情况）if (av_codec_is_decoder(avctx->codec))return ff_decode_receive_frame(avctx, frame);// 2.如果是编码器，返回重建帧return ff_encode_receive_frame(avctx, frame);
}

1.1 返回解码帧（ff_decode_receive_frame）

函数的主要功能是返回解码的帧，定义位于libavcodec\avcodec.c中。如果代码需要从已编码的缓冲区中取出数据，会调用这个函数。这个函数的主要流程是检查缓冲区是否有数据，如果有则直接取出，否则尝试进行解码，随后会进行一些其他的检查和处理

/*** avcodec_receive_frame() implementation for decoders.*/
int ff_decode_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{AVCodecInternal *avci = avctx->internal;int ret;if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))return AVERROR(EINVAL);// 1.如果缓冲区有数据，则直接取出if (avci->buffer_frame->buf[0]) {av_frame_move_ref(frame, avci->buffer_frame);} else {// 2.如果缓冲区没有数据，尝试进行解码ret = decode_receive_frame_internal(avctx, frame);if (ret < 0)return ret;}// 3.确保返回给调用者的帧是有效的// 主要是检查frame的format, width, height这些信息ret = frame_validate(avctx, frame);if (ret < 0)goto fail;if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {// Video decode only// 4.某些视频编解码器支持裁剪，这意味着只有被解码帧的子矩形用于显示, 这个选项控制libavcodec如何处理裁剪ret = apply_cropping(avctx, frame);if (ret < 0)goto fail;}avctx->frame_num++;#if FF_API_DROPCHANGED// AV_CODEC_FLAG_DROPCHANGED表示不要输出与流中第一个解码帧参数不同的帧if (avctx->flags & AV_CODEC_FLAG_DROPCHANGED) {if (avctx->frame_num == 1) {avci->initial_format = frame->format;switch(avctx->codec_type) {case AVMEDIA_TYPE_VIDEO:avci->initial_width  = frame->width;avci->initial_height = frame->height;break;case AVMEDIA_TYPE_AUDIO:avci->initial_sample_rate = frame->sample_rate ? frame->sample_rate :avctx->sample_rate;ret = av_channel_layout_copy(&avci->initial_ch_layout, &frame->ch_layout);if (ret < 0)goto fail;break;}}if (avctx->frame_num > 1) {int changed = avci->initial_format != frame->format;switch(avctx->codec_type) {case AVMEDIA_TYPE_VIDEO:changed |= avci->initial_width  != frame->width ||avci->initial_height != frame->height;break;case AVMEDIA_TYPE_AUDIO:changed |= avci->initial_sample_rate    != frame->sample_rate ||avci->initial_sample_rate    != avctx->sample_rate ||av_channel_layout_compare(&avci->initial_ch_layout, &frame->ch_layout);break;}if (changed) {avci->changed_frames_dropped++;av_log(avctx, AV_LOG_INFO, "dropped changed frame #%"PRId64" pts %"PRId64" drop count: %d \n",avctx->frame_num, frame->pts,avci->changed_frames_dropped);ret = AVERROR_INPUT_CHANGED;goto fail;}}}
#endifreturn 0;
fail:av_frame_unref(frame);return ret;
}

1.2 返回重建帧（ff_encode_receive_frame）

如果不是decoder，会进入到下面函数，去调重建帧

/*** avcodec_receive_frame() implementation for encoders.*/
int ff_encode_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{AVCodecInternal *avci = avctx->internal;// 不存在重建帧，报错if (!avci->recon_frame)return AVERROR(EINVAL);if (!avci->recon_frame->buf[0])return avci->draining_done ? AVERROR_EOF : AVERROR(EAGAIN);// 返回重建帧av_frame_move_ref(frame, avci->recon_frame);return 0;
}