从前面一篇文章ffmpeg内部组件总结可以知道encoder的具体使用方法:

char *filename = "/data/record/test.mp4"AVFormatContext *format_ctx = NULL;//通过输入url找到对应的muxer即format_ctx->oformat并malloc AVFormatContextformat_ctx = avformat_alloc_output_context2(&format_ctx, NULL, NULL, filename);//通过url找到并初始化IO模块avio_open2(&format_ctx->pb, filename, AVIO_FLAG_WRITE, NULL, NULL);//通过codec找到对应的encoderAVCodec *enc = avcodec_find_encoder(format_ctx->oformat->video_codec);AVCodecContext *enc_ctx = avcodec_alloc_context3(enc);//打开encoderavcodec_open2(enc_ctx, enc, NULL);AVStream *stream = avformat_new_stream(format_ctx, enc);avcodec_parameters_from_context(stream->codecpar, enc_ctx);while(!exit) {//将输入源的数据送到encoder中编码avcodec_send_frame(enc_ctx, frame);AVPacket *pkt = av_packet_alloc();//从encoder中获取编码后的数据avcodec_receive_packet(enc_ctx, pkt);//将编码后的数据送到muxer中av_write_frame(format_ctx, pkt);}

本篇文章将以struct FFCodec ff_mpeg4_encoder深入分析下，

encoder需要外部传入什么参数？

输入/输出buffer的处理

encoder内部是否有数据缓存？
接下来重点分析下这四个接口: avcodec_open2(…), avcodec_parameters_from_context(…), avcodec_send_frame(…), avcodec_receive_packet(…)分别干了什么。

avcodec_open2

int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options){int ret = 0;AVCodecInternal *avci;const FFCodec *codec2;........................;if (!codec)codec = avctx->codec;//将struct AVCodec*转换成struct FFCodec*codec2 = ffcodec(codec);avctx->codec_type = codec->type;avctx->codec_id = codec->id;avctx->codec = codec;........................;//分配AVCodecInternal并赋值给AVCodecContext.internalavci = av_mallocz(sizeof(*avci));avctx->internal = avci;//后面重点关注下这个AVCodecInternal中的buffer_frame和buffer_pkt是干啥的？avci->buffer_frame = av_frame_alloc();avci->buffer_pkt = av_packet_alloc();........................;avci->skip_samples_multiplier = 1;if (codec2->priv_data_size > 0) {if (!avctx->priv_data) {//以FFCodec ff_mpeg4_encoder为例子，这里malloc的codec2->priv_data_size实际上是struct MpegEncContext//const FFCodec ff_mpeg4_encoder = {// .p.name = "mpeg4",// .p.long_name = NULL_IF_CONFIG_SMALL("MPEG-4 part 2"),// .p.type = AVMEDIA_TYPE_VIDEO,// .p.id = AV_CODEC_ID_MPEG4,// .priv_data_size = sizeof(MpegEncContext),// .init = encode_init,// FF_CODEC_ENCODE_CB(ff_mpv_encode_picture),// .close = ff_mpv_encode_end,// .p.pix_fmts = (const enum AVPixelFormat[]) { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE },// .p.capabilities = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS,// .caps_internal = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,// .p.priv_class = &mpeg4enc_class,// };avctx->priv_data = av_mallocz(codec2->priv_data_size);if (!avctx->priv_data) {ret = AVERROR(ENOMEM);goto free_and_end;}if (codec->priv_class) {*(const AVClass **)avctx->priv_data = codec->priv_class;av_opt_set_defaults(avctx->priv_data);}}if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, options)) < 0)goto free_and_end;} else {avctx->priv_data = NULL;}..........................;if (av_codec_is_encoder(avctx->codec))ret = ff_encode_preinit(avctx);elseret = ff_decode_preinit(avctx);if (ret < 0)goto free_and_end;.....................................;//调用codec2->init为具体encoder的init函数if (!(avctx->active_thread_type & FF_THREAD_FRAME) ||avci->frame_thread_encoder) {if (codec2->init) {lock_avcodec(codec2);ret = codec2->init(avctx);unlock_avcodec(codec2);if (ret < 0) {avci->needs_close = codec2->caps_internal & FF_CODEC_CAP_INIT_CLEANUP;goto free_and_end;}}avci->needs_close = 1;}if (av_codec_is_decoder(avctx->codec)) {......................;}...................................;}

从上面avcodec_open2(…)针对encoder来说主要做了以下4件事情:

malloc具体encoder的context

malloc AVCodecInternal

调用ff_encode_preinit(…) 即调用encode_preinit_video(…)检查AVPixFmtDescriptor里面的参数并赋值到AVCodecContext中的成员。

调用具体encoder的init接口

有一个全局数组AVPixFmtDescriptor av_pix_fmt_descriptors[] 里面定义了这种像素格式的参数:

static const AVPixFmtDescriptor av_pix_fmt_descriptors[AV_PIX_FMT_NB] = {[AV_PIX_FMT_YUV420P] = {.name = "yuv420p",.nb_components = 3,.log2_chroma_w = 1,.log2_chroma_h = 1,.comp = {{ 0, 1, 0, 0, 8 }, /* Y */{ 1, 1, 0, 0, 8 }, /* U */{ 2, 1, 0, 0, 8 }, /* V */},.flags = AV_PIX_FMT_FLAG_PLANAR,},[AV_PIX_FMT_YUYV422] = {.name = "yuyv422",.nb_components = 3,.log2_chroma_w = 1,.log2_chroma_h = 0,.comp = {{ 0, 2, 0, 0, 8 }, /* Y */{ 0, 4, 1, 0, 8 }, /* U */{ 0, 4, 3, 0, 8 }, /* V */},},[AV_PIX_FMT_YVYU422] = {.name = "yvyu422",.nb_components = 3,.log2_chroma_w = 1,.log2_chroma_h = 0,.comp = {{ 0, 2, 0, 0, 8 }, /* Y */{ 0, 4, 3, 0, 8 }, /* U */{ 0, 4, 1, 0, 8 }, /* V */},},................;}

这里面的具体含义，后面再encode用到的时候再反过来查看。

typedef struct AVPixFmtDescriptor {const char *name;uint8_t nb_components; ///< The number of components each pixel has, (1-4)/*** Amount to shift the luma width right to find the chroma width.* For YV12 this is 1 for example.* chroma_width = AV_CEIL_RSHIFT(luma_width, log2_chroma_w)* The note above is needed to ensure rounding up.* This value only refers to the chroma components.*/uint8_t log2_chroma_w;/*** Amount to shift the luma height right to find the chroma height.* For YV12 this is 1 for example.* chroma_height= AV_CEIL_RSHIFT(luma_height, log2_chroma_h)* The note above is needed to ensure rounding up.* This value only refers to the chroma components.*/uint8_t log2_chroma_h;/*** Combination of AV_PIX_FMT_FLAG_... flags.*/uint64_t flags;/*** Parameters that describe how pixels are packed.* If the format has 1 or 2 components, then luma is 0.* If the format has 3 or 4 components:* if the RGB flag is set then 0 is red, 1 is green and 2 is blue;* otherwise 0 is luma, 1 is chroma-U and 2 is chroma-V.** If present, the Alpha channel is always the last component.*/AVComponentDescriptor comp[4];/*** Alternative comma-separated names.*/const char *alias;} AVPixFmtDescriptor; typedef struct AVComponentDescriptor {/*** Which of the 4 planes contains the component.*/int plane;/*** Number of elements between 2 horizontally consecutive pixels.* Elements are bits for bitstream formats, bytes otherwise.*/int step;/*** Number of elements before the component of the first pixel.* Elements are bits for bitstream formats, bytes otherwise.*/int offset;/*** Number of least significant bits that must be shifted away* to get the value.*/int shift;/*** Number of bits in the component.*/int depth;} AVComponentDescriptor;

avcodec_parameters_from_context

将中AVCodecContext 的参数赋值给AVStream中的AVCodecParameters

int avcodec_parameters_from_context(AVCodecParameters *par,const AVCodecContext *codec){int ret;codec_parameters_reset(par);par->codec_type = codec->codec_type;par->codec_id = codec->codec_id;par->codec_tag = codec->codec_tag;par->bit_rate = codec->bit_rate;par->bits_per_coded_sample = codec->bits_per_coded_sample;par->bits_per_raw_sample = codec->bits_per_raw_sample;par->profile = codec->profile;par->level = codec->level;switch (par->codec_type) {case AVMEDIA_TYPE_VIDEO:par->format = codec->pix_fmt;par->width = codec->width;par->height = codec->height;par->field_order = codec->field_order;par->color_range = codec->color_range;par->color_primaries = codec->color_primaries;par->color_trc = codec->color_trc;par->color_space = codec->colorspace;par->chroma_location = codec->chroma_sample_location;par->sample_aspect_ratio = codec->sample_aspect_ratio;par->video_delay = codec->has_b_frames;break;case AVMEDIA_TYPE_AUDIO:par->format = codec->sample_fmt;#if FF_API_OLD_CHANNEL_LAYOUTFF_DISABLE_DEPRECATION_WARNINGS// if the old/new fields are set inconsistently, prefer the old onesif ((codec->channels && codec->channels != codec->ch_layout.nb_channels) ||(codec->channel_layout && (codec->ch_layout.order != AV_CHANNEL_ORDER_NATIVE ||codec->ch_layout.u.mask != codec->channel_layout))) {if (codec->channel_layout)av_channel_layout_from_mask(&par->ch_layout, codec->channel_layout);else {par->ch_layout.order = AV_CHANNEL_ORDER_UNSPEC;par->ch_layout.nb_channels = codec->channels;}FF_ENABLE_DEPRECATION_WARNINGS} else {#endifret = av_channel_layout_copy(&par->ch_layout, &codec->ch_layout);if (ret < 0)return ret;#if FF_API_OLD_CHANNEL_LAYOUTFF_DISABLE_DEPRECATION_WARNINGS}par->channel_layout = par->ch_layout.order == AV_CHANNEL_ORDER_NATIVE ?par->ch_layout.u.mask : 0;par->channels = par->ch_layout.nb_channels;FF_ENABLE_DEPRECATION_WARNINGS#endifpar->sample_rate = codec->sample_rate;par->block_align = codec->block_align;par->frame_size = codec->frame_size;par->initial_padding = codec->initial_padding;par->trailing_padding = codec->trailing_padding;par->seek_preroll = codec->seek_preroll;break;case AVMEDIA_TYPE_SUBTITLE:par->width = codec->width;par->height = codec->height;break;}if (codec->extradata) {par->extradata = av_mallocz(codec->extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);if (!par->extradata)return AVERROR(ENOMEM);memcpy(par->extradata, codec->extradata, codec->extradata_size);par->extradata_size = codec->extradata_size;}return 0;}

avcodec_send_frame

下面来看看send接口把需要编码的数据送到什么地方了？

int attribute_align_arg avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame){AVCodecInternal *avci = avctx->internal;int ret;......................................;if (!frame) {avci->draining = 1;} else {//重点看下ret = encode_send_frame_internal(avctx, frame);if (ret < 0)return ret;}//如果avci->buffer_pkt->data为null，执行一次encode动作//相当于调用了一次avcodec_receive_packet(.......)if (!avci->buffer_pkt->data && !avci->buffer_pkt->side_data) {ret = encode_receive_packet_internal(avctx, avci->buffer_pkt);if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)return ret;}avctx->frame_number++;return 0;} static int encode_send_frame_internal(AVCodecContext *avctx, const AVFrame *src){AVCodecInternal *avci = avctx->internal;AVFrame *dst = avci->buffer_frame;int ret;if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {................;}//将src中的buffer 转移到AVCodecInternal.buffer_frame中ret = av_frame_ref(dst, src);if (ret < 0)return ret;return 0;}

avcodec_receive_packet

int attribute_align_arg avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt){AVCodecInternal *avci = avctx->internal;int ret;av_packet_unref(avpkt);if (!avcodec_is_open(avctx) || !av_codec_is_encoder(avctx->codec))return AVERROR(EINVAL);if (avci->buffer_pkt->data || avci->buffer_pkt->side_data) {av_packet_move_ref(avpkt, avci->buffer_pkt);} else {ret = encode_receive_packet_internal(avctx, avpkt);if (ret < 0)return ret;}return 0;} static int encode_receive_packet_internal(AVCodecContext *avctx, AVPacket *avpkt){AVCodecInternal *avci = avctx->internal;int ret;.............................;if (ffcodec(avctx->codec)->cb_type == FF_CODEC_CB_TYPE_RECEIVE_PACKET) {ret = ffcodec(avctx->codec)->cb.receive_packet(avctx, avpkt);//这种方式一般没有codec实现if (ret < 0)av_packet_unref(avpkt);else// Encoders must always return ref-counted buffers.// Side-data only packets have no data and can be not ref-counted.av_assert0(!avpkt->data || avpkt->buf);} elseret = encode_simple_receive_packet(avctx, avpkt);//都走这里return ret;} static int encode_simple_receive_packet(AVCodecContext *avctx, AVPacket *avpkt){int ret;while (!avpkt->data && !avpkt->side_data) {//执行编码动作ret = encode_simple_internal(avctx, avpkt);if (ret < 0)return ret;}return 0;} int ff_encode_get_frame(AVCodecContext *avctx, AVFrame *frame){AVCodecInternal *avci = avctx->internal;if (avci->draining)return AVERROR_EOF;if (!avci->buffer_frame->buf[0])return AVERROR(EAGAIN);av_frame_move_ref(frame, avci->buffer_frame);return 0;}static int encode_simple_internal(AVCodecContext *avctx, AVPacket *avpkt){AVCodecInternal *avci = avctx->internal;AVFrame *frame = avci->in_frame;const FFCodec *const codec = ffcodec(avctx->codec);int got_packet;int ret;if (avci->draining_done)return AVERROR_EOF;if (!frame->buf[0] && !avci->draining) {av_frame_unref(frame);//将avcodec_send_frame下来的frame取出来ret = ff_encode_get_frame(avctx, frame);if (ret < 0 && ret != AVERROR_EOF)return ret;}if (CONFIG_FRAME_THREAD_ENCODER &&avci->frame_thread_encoder && (avctx->active_thread_type & FF_THREAD_FRAME))/* This might modify frame, but it doesn't matter, because* the frame properties used below are not used for video* (due to the delay inherent in frame threaded encoding, it makes* no sense to use the properties of the current frame anyway). */ret = ff_thread_video_encode_frame(avctx, avpkt, frame, &got_packet);else {//调用具体encoder的编码函数，传入frame，传出编码好的avpktret = codec->cb.encode(avctx, avpkt, frame, &got_packet);if (avctx->codec->type == AVMEDIA_TYPE_VIDEO && !ret && got_packet &&!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY))avpkt->pts = avpkt->dts = frame->pts;}av_assert0(ret <= 0);emms_c();if (!ret && got_packet) {if (avpkt->data) {//将pkt->data里面的数据拷贝到pkt->buf->data//看起来是encoder只是将数据指向了pkt->dataret = av_packet_make_refcounted(avpkt);if (ret < 0)goto end;}............;return ret;}

下面来看下FFCodec ff_mpeg4_encoder的encode函数: ff_mpv_encode_picture

int ff_mpv_encode_picture(AVCodecContext *avctx, AVPacket *pkt,const AVFrame *pic_arg, int *got_packet){MpegEncContext *s = avctx->priv_data;int i, stuffing_count, ret;int context_count = s->slice_context_count;s->vbv_ignore_qmax = 0;s->picture_in_gop_number++;//将AVFrame *pic_arg里面的内容拷贝picture中，然后将picture存放到MpegEncContext.input_picture//然后通过reorder后将此picture存放到MpegEncContext.reordered_input_pictureif (load_input_picture(s, pic_arg) < 0)return -1;//先将MpegEncContext.input_picture中picture按照规则再重排order到MpegEncContext.reordered_input_picture//然后选择MpegEncContext.reordered_input_picture中的第一个作为new_picture，即要编码的数据if (select_input_picture(s) < 0) {return -1;}/* output? */if (s->new_picture->data[0]) {int growing_buffer = context_count == 1 && !s->data_partitioning;size_t pkt_size = 10000 + s->mb_width * s->mb_height *(growing_buffer ? 64 : (MAX_MB_BYTES + 100));if (CONFIG_MJPEG_ENCODER && avctx->codec_id == AV_CODEC_ID_MJPEG) {ret = ff_mjpeg_add_icc_profile_size(avctx, s->new_picture, &pkt_size);if (ret < 0)return ret;}//给AVPacket.data分配内存空间if ((ret = ff_alloc_packet(avctx, pkt, pkt_size)) < 0)return ret;pkt->size = avctx->internal->byte_buffer_size - AV_INPUT_BUFFER_PADDING_SIZE;if (s->mb_info) {s->mb_info_ptr = av_packet_new_side_data(pkt,AV_PKT_DATA_H263_MB_INFO,s->mb_width*s->mb_height*12);s->prev_mb_info = s->last_mb_info = s->mb_info_size = 0;}for (i = 0; i < context_count; i++) {int start_y = s->thread_context[i]->start_mb_y;int end_y = s->thread_context[i]-> end_mb_y;int h = s->mb_height;uint8_t *start = pkt->data + (size_t)(((int64_t) pkt->size) * start_y / h);uint8_t *end = pkt->data + (size_t)(((int64_t) pkt->size) * end_y / h);init_put_bits(&s->thread_context[i]->pb, start, end - start);}s->pict_type = s->new_picture->pict_type;//emms_c();ret = frame_start(s);vbv_retry://开始编码ret = encode_picture(s, s->picture_number);if (growing_buffer) {//pkt->data在ff_alloc_packet中是指向avctx->internal->byte_bufferav_assert0(s->pb.buf == avctx->internal->byte_buffer);//此处如果buffer size有增长，则重新赋值一下pkt->data = s->pb.buf;pkt->size = avctx->internal->byte_buffer_size;}........................;frame_end(s);if ((CONFIG_MJPEG_ENCODER || CONFIG_AMV_ENCODER) && s->out_format == FMT_MJPEG)ff_mjpeg_encode_picture_trailer(&s->pb, s->header_bits);..............................;//设置pts 和dtspkt->pts = s->current_picture.f->pts;if (!s->low_delay && s->pict_type != AV_PICTURE_TYPE_B) {if (!s->current_picture.f->coded_picture_number)pkt->dts = pkt->pts - s->dts_delta;elsepkt->dts = s->reordered_pts;s->reordered_pts = pkt->pts;} elsepkt->dts = pkt->pts;if (s->current_picture.f->key_frame)pkt->flags |= AV_PKT_FLAG_KEY;if (s->mb_info)av_packet_shrink_side_data(pkt, AV_PKT_DATA_H263_MB_INFO, s->mb_info_size);} else {s->frame_bits = 0;}.......................;pkt->size = s->frame_bits / 8;*got_packet = !!pkt->size;return 0;}

AVFrame编码之后编程AVPacket过程