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FFmpeg的庫函式原始碼分析文章列表：

【架構圖】

【通用】

【解碼】

【編碼】

【其它】

【指令碼】

【H.264】

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本文簡單分析FFmpeg的avcodec_encode_video2()函式。該函式用於編碼一幀視訊資料。avcodec_encode_video2()函式的宣告位於libavcodec\avcodec.h，如下所示。

/** * 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_free_packet() (i.e. avpkt->destruct will be *                  called to free the user supplied buffer). * @param[in] frame AVFrame containing the raw video data to be encoded. *                  May be NULL when flushing an encoder that has the *                  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 */
 
int avcodec_encode_video2(AVCodecContext *avctx, AVPacket *avpkt,                          const AVFrame *frame, int *got_packet_ptr);

該函式每個引數的含義在註釋裡面已經寫的很清楚了，在這裡用中文簡述一下：

avctx：編碼器的AVCodecContext。
avpkt：編碼輸出的AVPacket。
frame：編碼輸入的AVFrame。
got_packet_ptr：成功編碼一個AVPacket的時候設定為1。

函式返回0代表編碼成功。

函式呼叫關係圖

函式的呼叫關係如下圖所示。
 

avcodec_encode_video2()

avcodec_encode_video2()的定義位於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(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&CODEC_FLAG_PASS1) && avctx->stats_out)        avctx->stats_out[0] = '\0';    if (!(avctx->codec->capabilities & CODEC_CAP_DELAY) && !frame) {        av_free_packet(avpkt);        av_init_packet(avpkt);        avpkt->size = 0;        return 0;    }    //檢查輸入    if (av_image_check_size(avctx->width, avctx->height, 0, avctx))        return AVERROR(EINVAL);    av_assert0(avctx->codec->encode2);    //編碼    ret = avctx->codec->encode2(avctx, avpkt, frame, got_packet_ptr);    av_assert0(ret <= 0);    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;#if FF_API_DESTRUCT_PACKETFF_DISABLE_DEPRECATION_WARNINGS            avpkt->destruct = user_pkt.destruct;FF_ENABLE_DEPRECATION_WARNINGS#endif        } else {            if (av_dup_packet(avpkt) < 0) {                ret = AVERROR(ENOMEM);            }        }    }    if (!ret) {        if (!*got_packet_ptr)            avpkt->size = 0;        else if (!(avctx->codec->capabilities & CODEC_CAP_DELAY))            avpkt->pts = avpkt->dts = frame->pts;        if (needs_realloc && avpkt->data) {            ret = av_buffer_realloc(&avpkt->buf, avpkt->size + FF_INPUT_BUFFER_PADDING_SIZE);            if (ret >= 0)                avpkt->data = avpkt->buf->data;        }        avctx->frame_number++;    }    if (ret < 0 || !*got_packet_ptr)        av_free_packet(avpkt);    else        av_packet_merge_side_data(avpkt);    emms_c();    return ret;}

從函式的定義可以看出，avcodec_encode_video2()首先呼叫了av_image_check_size()檢查設定的寬高參數是否合理，然後呼叫了AVCodec的encode2()呼叫具體的解碼器。

av_image_check_size()

av_image_check_size()是一個很簡單的函式，用於檢查影象寬高是否正常，它的定義如下所示。

int av_image_check_size(unsigned int w, unsigned int h, int log_offset, void *log_ctx){    ImgUtils imgutils = { &imgutils_class, log_offset, log_ctx };    if ((int)w>0 && (int)h>0 && (w+128)*(uint64_t)(h+128) < INT_MAX/8)        return 0;    av_log(&imgutils, AV_LOG_ERROR, "Picture size %ux%u is invalid\n", w, h);    return AVERROR(EINVAL);}

從程式碼中可以看出，av_image_check_size()主要是要求影象寬高必須為正數，而且取值不能太大。

AVCodec->encode2()

AVCodec的encode2()是一個函式指標，指向特定編碼器的編碼函式。在這裡我們以libx264為例，看一下它對應的AVCodec的結構體的定義，如下所示。

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     = CODEC_CAP_DELAY | CODEC_CAP_AUTO_THREADS,    .priv_class       = &x264_class,    .defaults         = x264_defaults,    .init_static_data = X264_init_static,};

從ff_libx264_encoder的定義可以看出，encode2()函式指向的是X264_frame()函式。

X264_frame()

X264_frame()函式的定義位於libavcodec\libx264.c，如下所示。

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};    AVFrameSideData *side_data;    x264_picture_init( &x4->pic );    x4->pic.img.i_csp   = x4->params.i_csp;    if (x264_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->pic.i_type =            frame->pict_type == AV_PICTURE_TYPE_I ? X264_TYPE_KEYFRAME :            frame->pict_type == AV_PICTURE_TYPE_P ? X264_TYPE_P :            frame->pict_type == AV_PICTURE_TYPE_B ? X264_TYPE_B :                                            X264_TYPE_AUTO;        if (x4->avcintra_class < 0) {        if (x4->params.b_interlaced && x4->params.b_tff != frame->top_field_first) {            x4->params.b_tff = frame->top_field_first;            x264_encoder_reconfig(x4->enc, &x4->params);        }        if (x4->params.vui.i_sar_height != ctx->sample_aspect_ratio.den ||            x4->params.vui.i_sar_width  != ctx->sample_aspect_ratio.num) {            x4->params.vui.i_sar_height = ctx->sample_aspect_ratio.den;            x4->params.vui.i_sar_width  = ctx->sample_aspect_ratio.num;            x264_encoder_reconfig(x4->enc, &x4->params);        }        if (x4->params.rc.i_vbv_buffer_size != ctx->rc_buffer_size / 1000 ||            x4->params.rc.i_vbv_max_bitrate != ctx->rc_max_rate    / 1000) {            x4->params.rc.i_vbv_buffer_size = ctx->rc_buffer_size / 1000;            x4->params.rc.i_vbv_max_bitrate = ctx->rc_max_rate    / 1000;            x264_encoder_reconfig(x4->enc, &x4->params);        }        if (x4->params.rc.i_rc_method == X264_RC_ABR &&            x4->params.rc.i_bitrate != ctx->bit_rate / 1000) {            x4->params.rc.i_bitrate = ctx->bit_rate / 1000;            x264_encoder_reconfig(x4->enc, &x4->params);        }        if (x4->crf >= 0 &&            x4->params.rc.i_rc_method == X264_RC_CRF &&            x4->params.rc.f_rf_constant != x4->crf) {            x4->params.rc.f_rf_constant = x4->crf;            x264_encoder_reconfig(x4->enc, &x4->params);        }        if (x4->params.rc.i_rc_method == X264_RC_CQP &&            x4->cqp >= 0 &&            x4->params.rc.i_qp_constant != x4->cqp) {            x4->params.rc.i_qp_constant = x4->cqp;            x264_encoder_reconfig(x4->enc, &x4->params);        }        if (x4->crf_max >= 0 &&            x4->params.rc.f_rf_constant_max != x4->crf_max) {            x4->params.rc.f_rf_constant_max = x4->crf_max;            x264_encoder_reconfig(x4->enc, &x4->params);        }        }        side_data = av_frame_get_side_data(frame, AV_FRAME_DATA_STEREO3D);        if (side_data) {            AVStereo3D *stereo = (AVStereo3D *)side_data->data;            int fpa_type;            switch (stereo->type) {            case AV_STEREO3D_CHECKERBOARD:                fpa_type = 0;                break;            case AV_STEREO3D_COLUMNS:                fpa_type = 1;                break;            case AV_STEREO3D_LINES:                fpa_type = 2;                break;            case AV_STEREO3D_SIDEBYSIDE:                fpa_type = 3;                break;            case AV_STEREO3D_TOPBOTTOM:                fpa_type = 4;                break;            case AV_STEREO3D_FRAMESEQUENCE:                fpa_type = 5;                break;            default:                fpa_type = -1;                break;            }            if (fpa_type != x4->params.i_frame_packing) {                x4->params.i_frame_packing = fpa_type;                x264_encoder_reconfig(x4->enc, &x4->params);            }        }    }    do {        if (x264_encoder_encode(x4->enc, &nal, &nnal, frame? &x4->pic: NULL, &pic_out) < 0)            return -1;        ret = encode_nals(ctx, pkt, nal, nnal);        if (ret < 0)            return -1;    } while (!ret && !frame && x264_encoder_delayed_frames(x4->enc));    pkt->pts = pic_out.i_pts;    pkt->dts = pic_out.i_dts;    switch (pic_out.i_type) {    case X264_TYPE_IDR:    case X264_TYPE_I:        ctx->coded_frame->pict_type = AV_PICTURE_TYPE_I;        break;    case X264_TYPE_P:        ctx->coded_frame->pict_type = AV_PICTURE_TYPE_P;        break;    case X264_TYPE_B:    case X264_TYPE_BREF:        ctx->coded_frame->pict_type = AV_PICTURE_TYPE_B;        break;    }    pkt->flags |= AV_PKT_FLAG_KEY*pic_out.b_keyframe;    if (ret)        ctx->coded_frame->quality = (pic_out.i_qpplus1 - 1) * FF_QP2LAMBDA;    *got_packet = ret;    return 0;}

有關X264編碼的程式碼在以後分析X264的時候再進行詳細分析。在這裡我們可以我們可以簡單看出該函式中有一個do while迴圈，其中呼叫了x264_encoder_encode()完成了編碼的工作。


雷霄驊
[email protected]
http://blog.csdn.net/leixiaohua1020