一、avcodec_send_frame()
libavcodec\avcodec.h
/*** 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);libavcodec\encode.c
int attribute_align_arg avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame)
{if (!avcodec_is_open(avctx) || !av_codec_is_encoder(avctx->codec))return AVERROR(EINVAL);if (avctx->internal->draining)return AVERROR_EOF;if (!frame) {avctx->internal->draining = 1;if (!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY))return 0;}if (avctx->codec->send_frame)return avctx->codec->send_frame(avctx, frame);// Emulation via old API. Do it here instead of avcodec_receive_packet, because:// 1. if the AVFrame is not refcounted, the copying will be much more// expensive than copying the packet data// 2. assume few users use non-refcounted AVPackets, so usually no copy is// neededif (avctx->internal->buffer_pkt_valid)return AVERROR(EAGAIN);return do_encode(avctx, frame, &(int){0});
}从avcodec_send_frame()中可以看到，先判断AVCodec是否存在send_frame函数指针，如果存在就调用该函数，如果不存在则调用do_encode函数。本文以h264编码为例，如下：
AVCodec ff_libx264_encoder = {.name = "libx264",.long_name = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),.type = AVMEDIA_TYPE_VIDEO,.id = AV_CODEC_ID_H264,.priv_data_size = sizeof(X264Context),.init = X264_init,.encode2 = X264_frame,.close = X264_close,.capabilities = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS |AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE,.priv_class = &x264_class,.defaults = x264_defaults,.init_static_data = X264_init_static,.caps_internal = FF_CODEC_CAP_INIT_CLEANUP,.wrapper_name = "libx264",
};从ff_libx264_encoder定义发现，没有定义send_frame函数指针，所以调用do_encode。如下所示：
static int do_encode(AVCodecContext *avctx, const AVFrame *frame, int *got_packet)
{int ret;*got_packet = 0;av_packet_unref(avctx->internal->buffer_pkt);avctx->internal->buffer_pkt_valid = 0;if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {ret = avcodec_encode_video2(avctx, avctx->internal->buffer_pkt,frame, got_packet);} else if (avctx->codec_type == AVMEDIA_TYPE_AUDIO) {ret = avcodec_encode_audio2(avctx, avctx->internal->buffer_pkt,frame, got_packet);} else {ret = AVERROR(EINVAL);}if (ret >= 0 && *got_packet) {// Encoders must always return ref-counted buffers.// Side-data only packets have no data and can be not ref-counted.av_assert0(!avctx->internal->buffer_pkt->data || avctx->internal->buffer_pkt->buf);avctx->internal->buffer_pkt_valid = 1;ret = 0;} else {av_packet_unref(avctx->internal->buffer_pkt);}return ret;
}do_encode()函数最终视频会调用avcodec_encode_video2()，音频会调用avcodec_encode_audio2()函数。
二、avcodec_receive_packet()
avcodec_receive_packet()申明在文件libavcodec\avcodec.h中，如下所示。
/*** 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);avcodec_receive_packet()定义在文件libavcodec\encode.c中，如下所示。
int attribute_align_arg avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt)
{av_packet_unref(avpkt);if (!avcodec_is_open(avctx) || !av_codec_is_encoder(avctx->codec))return AVERROR(EINVAL);if (avctx->codec->receive_packet) {if (avctx->internal->draining && !(avctx->codec->capabilities & AV_CODEC_CAP_DELAY))return AVERROR_EOF;return avctx->codec->receive_packet(avctx, avpkt);}// Emulation via old API.if (!avctx->internal->buffer_pkt_valid) {int got_packet;int ret;if (!avctx->internal->draining)return AVERROR(EAGAIN);ret = do_encode(avctx, NULL, &got_packet);if (ret < 0)return ret;if (ret >= 0 && !got_packet)return AVERROR_EOF;}av_packet_move_ref(avpkt, avctx->internal->buffer_pkt);avctx->internal->buffer_pkt_valid = 0;return 0;
}从avcodec_receive_packet()中可以看到，先判断AVCodec是否存在receive_packet函数指针，如果存在就调用该函数，如果不存在也是调用do_encode函数。h264编码中也会调用do_encode()函数。从前面分析得知，do_encode()函数最终视频会调用avcodec_encode_video2()，音频会调用avcodec_encode_audio2()函数。
三、avcodec_encode_video2()
/*** Encode a frame of video.** Takes input raw video data from frame and writes the next output packet, if* available, to avpkt. The output packet does not necessarily contain data for* the most recent frame, as encoders can delay and reorder input frames* internally as needed.** @param avctx codec context* @param avpkt output AVPacket.* The user can supply an output buffer by setting* avpkt->data and avpkt->size prior to calling the* function, but if the size of the user-provided data is not* large enough, encoding will fail. All other AVPacket fields* will be reset by the encoder using av_init_packet(). If* avpkt->data is NULL, the encoder will allocate it.* The encoder will set avpkt->size to the size of the* output packet. The returned data (if any) belongs to the* caller, he is responsible for freeing it.** If this function fails or produces no output, avpkt will be* freed using av_packet_unref().* @param[in] frame AVFrame containing the raw video data to be encoded.* May be NULL when flushing an encoder that has the* AV_CODEC_CAP_DELAY capability set.* @param[out] got_packet_ptr This field is set to 1 by libavcodec if the* output packet is non-empty, and to 0 if it is* empty. If the function returns an error, the* packet can be assumed to be invalid, and the* value of got_packet_ptr is undefined and should* not be used.* @return 0 on success, negative error code on failure** @deprecated use avcodec_send_frame()/avcodec_receive_packet() instead*/
attribute_deprecated
int avcodec_encode_video2(AVCodecContext *avctx, AVPacket *avpkt,const AVFrame *frame, int *got_packet_ptr);该函数每个参数的含义在注释里面已经写的很清楚了，在这里用中文简述一下：
@ avctx：编码器的AVCodecContext。
@ avpkt：编码输出的AVPacket。
@ frame：编码输入的AVFrame。
@ got_packet_ptr：成功编码一个AVPacket的时候设置为1。
avcodec_encode_video2()的定义位于libavcodec\utils.c，如下所示。
int attribute_align_arg avcodec_encode_video2(AVCodecContext *avctx,AVPacket *avpkt,const AVFrame *frame,int *got_packet_ptr)
{int ret;AVPacket user_pkt = *avpkt;int needs_realloc = !user_pkt.data;*got_packet_ptr = 0;if (!avctx->codec->encode2) {av_log(avctx, AV_LOG_ERROR, "This encoder requires using the avcodec_send_frame() API.\n");return AVERROR(ENOSYS);}if(CONFIG_FRAME_THREAD_ENCODER &&avctx->internal->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))return ff_thread_video_encode_frame(avctx, avpkt, frame, got_packet_ptr);if ((avctx->flags&AV_CODEC_FLAG_PASS1) && avctx->stats_out)avctx->stats_out[0] = '\0';if (!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY) && !frame) {av_packet_unref(avpkt);return 0;}if (av_image_check_size2(avctx->width, avctx->height, avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx))return AVERROR(EINVAL);if (frame && frame->format == AV_PIX_FMT_NONE)av_log(avctx, AV_LOG_WARNING, "AVFrame.format is not set\n");if (frame && (frame->width == 0 || frame->height == 0))av_log(avctx, AV_LOG_WARNING, "AVFrame.width or height is not set\n");av_assert0(avctx->codec->encode2);ret = avctx->codec->encode2(avctx, avpkt, frame, got_packet_ptr);av_assert0(ret <= 0);emms_c();if (avpkt->data && avpkt->data == avctx->internal->byte_buffer) {needs_realloc = 0;if (user_pkt.data) {if (user_pkt.size >= avpkt->size) {memcpy(user_pkt.data, avpkt->data, avpkt->size);} else {av_log(avctx, AV_LOG_ERROR, "Provided packet is too small, needs to be %d\n", avpkt->size);avpkt->size = user_pkt.size;ret = -1;}avpkt->buf = user_pkt.buf;avpkt->data = user_pkt.data;} else if (!avpkt->buf) {ret = av_packet_make_refcounted(avpkt);if (ret < 0)return ret;}}if (!ret) {if (!*got_packet_ptr)avpkt->size = 0;else if (!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY))avpkt->pts = avpkt->dts = frame->pts;if (needs_realloc && avpkt->data) {ret = av_buffer_realloc(&avpkt->buf, avpkt->size + AV_INPUT_BUFFER_PADDING_SIZE);if (ret >= 0)avpkt->data = avpkt->buf->data;}if (frame)avctx->frame_number++;}if (ret < 0 || !*got_packet_ptr)av_packet_unref(avpkt);return ret;
}从函数的定义可以看出，avcodec_encode_video2()首先调用了av_image_check_size()检查设置的宽高参数是否合理，然后调用了AVCodec的encode2()调用具体的解码器。
3.1 av_image_check_size2()
int av_image_check_size2(unsigned int w, unsigned int h, int64_t max_pixels, enum AVPixelFormat pix_fmt, int log_offset, void *log_ctx)
{ImgUtils imgutils = {.class = &imgutils_class,.log_offset = log_offset,.log_ctx = log_ctx,};int64_t stride = av_image_get_linesize(pix_fmt, w, 0);if (stride <= 0)stride = 8LL*w;stride += 128*8;if ((int)w<=0 || (int)h<=0 || stride >= INT_MAX || stride*(uint64_t)(h+128) >= INT_MAX) {av_log(&imgutils, AV_LOG_ERROR, "Picture size %ux%u is invalid\n", w, h);return AVERROR(EINVAL);}if (max_pixels < INT64_MAX) {if (w*(int64_t)h > max_pixels) {av_log(&imgutils, AV_LOG_ERROR,"Picture size %ux%u exceeds specified max pixel count %"PRId64", see the documentation if you wish to increase it\n",w, h, max_pixels);return AVERROR(EINVAL);}}return 0;
}从代码中可以看出，av_image_check_size()主要是要求图像宽高必须为正数，而且取值不能太大。
3.2 AVCodec->encode2()
从ff_libx264_encoder的定义可以看出，encode2()函数指向的是X264_frame()函数。
static int X264_frame(AVCodecContext *ctx, AVPacket *pkt, const AVFrame *frame,int *got_packet)
{X264Context *x4 = ctx->priv_data;x264_nal_t *nal;int nnal, i, ret;x264_picture_t pic_out = {0};int pict_type;int bit_depth;int64_t *out_opaque;AVFrameSideData *sd;x264_picture_init( &x4->pic );x4->pic.img.i_csp = x4->params.i_csp;
#if X264_BUILD >= 153bit_depth = x4->params.i_bitdepth;
#elsebit_depth = x264_bit_depth;
#endifif (bit_depth > 8)x4->pic.img.i_csp |= X264_CSP_HIGH_DEPTH;x4->pic.img.i_plane = avfmt2_num_planes(ctx->pix_fmt);if (frame) {for (i = 0; i < x4->pic.img.i_plane; i++) {x4->pic.img.plane[i] = frame->data[i];x4->pic.img.i_stride[i] = frame->linesize[i];}x4->pic.i_pts = frame->pts;x4->reordered_opaque[x4->next_reordered_opaque] = frame->reordered_opaque;x4->pic.opaque = &x4->reordered_opaque[x4->next_reordered_opaque];x4->next_reordered_opaque++;x4->next_reordered_opaque %= x4->nb_reordered_opaque;switch (frame->pict_type) {case AV_PICTURE_TYPE_I:x4->pic.i_type = x4->forced_idr > 0 ? X264_TYPE_IDR: X264_TYPE_KEYFRAME;break;case AV_PICTURE_TYPE_P:x4->pic.i_type = X264_TYPE_P;break;case AV_PICTURE_TYPE_B:x4->pic.i_type = X264_TYPE_B;break;default:x4->pic.i_type = X264_TYPE_AUTO;break;}reconfig_encoder(ctx, frame);if (x4->a53_cc) {void *sei_data;size_t sei_size;ret = ff_alloc_a53_sei(frame, 0, &sei_data, &sei_size);if (ret < 0) {av_log(ctx, AV_LOG_ERROR, "Not enough memory for closed captions, skipping\n");} else if (sei_data) {x4->pic.extra_sei.payloads = av_mallocz(sizeof(x4->pic.extra_sei.payloads[0]));if (x4->pic.extra_sei.payloads == NULL) {av_log(ctx, AV_LOG_ERROR, "Not enough memory for closed captions, skipping\n");av_free(sei_data);} else {x4->pic.extra_sei.sei_free = av_free;x4->pic.extra_sei.payloads[0].payload_size = sei_size;x4->pic.extra_sei.payloads[0].payload = sei_data;x4->pic.extra_sei.num_payloads = 1;x4->pic.extra_sei.payloads[0].payload_type = 4;}}}sd = av_frame_get_side_data(frame, AV_FRAME_DATA_REGIONS_OF_INTEREST);if (sd) {if (x4->params.rc.i_aq_mode == X264_AQ_NONE) {av_log(ctx, AV_LOG_WARNING, "Adaptive quantization must be enabled to use ROI encoding, skipping ROI.\n");} else {if (frame->interlaced_frame == 0) {int mbx = (frame->width + MB_SIZE - 1) / MB_SIZE;int mby = (frame->height + MB_SIZE - 1) / MB_SIZE;int qp_range = 51 + 6 * (bit_depth - 8);int nb_rois;const AVRegionOfInterest *roi;uint32_t roi_size;float *qoffsets;roi = (const AVRegionOfInterest*)sd->data;roi_size = roi->self_size;if (!roi_size || sd->size % roi_size != 0) {av_log(ctx, AV_LOG_ERROR, "Invalid AVRegionOfInterest.self_size.\n");return AVERROR(EINVAL);}nb_rois = sd->size / roi_size;qoffsets = av_mallocz_array(mbx * mby, sizeof(*qoffsets));if (!qoffsets)return AVERROR(ENOMEM);// This list must be iterated in reverse because the first// region in the list applies when regions overlap.for (int i = nb_rois - 1; i >= 0; i--) {int startx, endx, starty, endy;float qoffset;roi = (const AVRegionOfInterest*)(sd->data + roi_size * i);starty = FFMIN(mby, roi->top / MB_SIZE);endy = FFMIN(mby, (roi->bottom + MB_SIZE - 1)/ MB_SIZE);startx = FFMIN(mbx, roi->left / MB_SIZE);endx = FFMIN(mbx, (roi->right + MB_SIZE - 1)/ MB_SIZE);if (roi->qoffset.den == 0) {av_free(qoffsets);av_log(ctx, AV_LOG_ERROR, "AVRegionOfInterest.qoffset.den must not be zero.\n");return AVERROR(EINVAL);}qoffset = roi->qoffset.num * 1.0f / roi->qoffset.den;qoffset = av_clipf(qoffset * qp_range, -qp_range, +qp_range);for (int y = starty; y < endy; y++) {for (int x = startx; x < endx; x++) {qoffsets[x + y*mbx] = qoffset;}}}x4->pic.prop.quant_offsets = qoffsets;x4->pic.prop.quant_offsets_free = av_free;} else {av_log(ctx, AV_LOG_WARNING, "interlaced_frame not supported for ROI encoding yet, skipping ROI.\n");}}}}do {if (x264_encoder_encode(x4->enc, &nal, &nnal, frame? &x4->pic: NULL, &pic_out) < 0)return AVERROR_EXTERNAL;ret = encode_nals(ctx, pkt, nal, nnal);if (ret < 0)return ret;} while (!ret && !frame && x264_encoder_delayed_frames(x4->enc));pkt->pts = pic_out.i_pts;pkt->dts = pic_out.i_dts;out_opaque = pic_out.opaque;if (out_opaque >= x4->reordered_opaque &&out_opaque < &x4->reordered_opaque[x4->nb_reordered_opaque]) {ctx->reordered_opaque = *out_opaque;} else {// Unexpected opaque pointer on picture outputctx->reordered_opaque = 0;}switch (pic_out.i_type) {case X264_TYPE_IDR:case X264_TYPE_I:pict_type = AV_PICTURE_TYPE_I;break;case X264_TYPE_P:pict_type = AV_PICTURE_TYPE_P;break;case X264_TYPE_B:case X264_TYPE_BREF:pict_type = AV_PICTURE_TYPE_B;break;default:pict_type = AV_PICTURE_TYPE_NONE;}
#if FF_API_CODED_FRAME
FF_DISABLE_DEPRECATION_WARNINGSctx->coded_frame->pict_type = pict_type;
FF_ENABLE_DEPRECATION_WARNINGS
#endifpkt->flags |= AV_PKT_FLAG_KEY*pic_out.b_keyframe;if (ret) {ff_side_data_set_encoder_stats(pkt, (pic_out.i_qpplus1 - 1) * FF_QP2LAMBDA, NULL, 0, pict_type);#if FF_API_CODED_FRAME
FF_DISABLE_DEPRECATION_WARNINGSctx->coded_frame->quality = (pic_out.i_qpplus1 - 1) * FF_QP2LAMBDA;
FF_ENABLE_DEPRECATION_WARNINGS
#endif}*got_packet = ret;return 0;
} 有关X264编码的代码在以后分析X264的时候再进行详细分析。在这里我们可以我们可以简单看出该函数中有一个do while循环，其中调用了x264_encoder_encode()完成了编码的工作。