从头用脚分析FFmpeg源码 - avcodec_send_frame | avcodec_receive_packet

avcodec_send_frame和avcodec_receive_packet 作用

相对应avcodec_send_packet | avcodec_receive_frame而言，avcodec_send_frame ｜ avcodec_receive_packet 是编码用的。

/**
 * Supply a raw video or audio frame to the encoder. Use avcodec_receive_packet()
 * to retrieve buffered output packets.
 * @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 encoding errors
 */
int avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame);

/**
 * Read encoded data from the encoder.
 * @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 a decoder
 *      other errors: legitimate encoding errors
 */
int avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt);

avcodec_send_frame源码

把frame送到avci->buffer_frame中，然后根据avci->buffer_pkt中是否存在编码完成后的cache，判断是否调用encode_receive_packet_internal来编码。

int attribute_align_arg avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame)
{
    AVCodecInternal *avci = avctx->internal;
    int ret;

	//检测是否open，是否为编码器
    if (!avcodec_is_open(avctx) || !av_codec_is_encoder(avctx->codec))
        return AVERROR(EINVAL);

	//frame为空过，这个时候不能再次调用avcodec_send_frame编码，需要flush
	//对于x264来说，调用flush无效，需要重新创建AVCodecContext
    if (avci->draining)
        return AVERROR_EOF;

    if (avci->buffer_frame->data[0])
        return AVERROR(EAGAIN);

	//为null的话，就标记draining为1，之后会在编码阶段使用
    if (!frame) {
        avci->draining = 1;
    } else {
    	//avctx->internal->buffer_frame引用frame，之后编码阶段就可以直接使用buffer_frame
        ret = encode_send_frame_internal(avctx, frame);
        if (ret < 0)
            return ret;
    }

	//当前没有存放的编码完成后的cache
    if (!avci->buffer_pkt->data && !avci->buffer_pkt->side_data) {
    	//调用编码方法
        ret = encode_receive_packet_internal(avctx, avci->buffer_pkt);
        if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
            return ret;
    }

    return 0;
}

avcodec_receive_packet 源码

如果avctx->internal->buffer_pkt中存在已经编码后的cache，就用buffer_pkt，不存在就调用encode_receive_packet_internal编码。

int attribute_align_arg avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt)
{
    AVCodecInternal *avci = avctx->internal;
    int ret;

    av_packet_unref(avpkt);

	//检测是否open，是否为编码器
    if (!avcodec_is_open(avctx) || !av_codec_is_encoder(avctx->codec))
        return AVERROR(EINVAL);

	//是否使用cache
    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;
}

encode_send_frame_internal源码

主要作用是把输入的AVFrame复制到avctx->internal->buffer_frame中。
如果是音频，还会根据codec的capabilities类型，判断输入的AVFrame的采样数量是否足够，不足够就返回AVERROR(EINVAL)。

static int encode_send_frame_internal(AVCodecContext *avctx, const AVFrame *src)
{
    AVCodecInternal *avci = avctx->internal;
    AVFrame *dst = avci->buffer_frame;
    int ret;

	//检测输入的音频Fram采样数量是否满足编码器的capabilities
    if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
        /* extract audio service type metadata */
        AVFrameSideData *sd = av_frame_get_side_data(src, AV_FRAME_DATA_AUDIO_SERVICE_TYPE);
        if (sd && sd->size >= sizeof(enum AVAudioServiceType))
            avctx->audio_service_type = *(enum AVAudioServiceType*)sd->data;

        /* check for valid frame size */
        if (avctx->codec->capabilities & AV_CODEC_CAP_SMALL_LAST_FRAME) {
            if (src->nb_samples > avctx->frame_size) {
                av_log(avctx, AV_LOG_ERROR, "more samples than frame size\n");
                return AVERROR(EINVAL);
            }
        } else if (!(avctx->codec->capabilities & AV_CODEC_CAP_VARIABLE_FRAME_SIZE)) {
            /* if we already got an undersized frame, that must have been the last */
            if (avctx->internal->last_audio_frame) {
                av_log(avctx, AV_LOG_ERROR, "frame_size (%d) was not respected for a non-last frame\n", avctx->frame_size);
                return AVERROR(EINVAL);
            }

            if (src->nb_samples < avctx->frame_size) {
                ret = pad_last_frame(avctx, dst, src);
                if (ret < 0)
                    return ret;

                avctx->internal->last_audio_frame = 1;
            } else if (src->nb_samples > avctx->frame_size) {
                av_log(avctx, AV_LOG_ERROR, "nb_samples (%d) != frame_size (%d)\n", src->nb_samples, avctx->frame_size);
                return AVERROR(EINVAL);
            }
        }
    }

	//将输入的src引用到avctx->internal->buffer_frame中
    if (!dst->data[0]) {
        ret = av_frame_ref(dst, src);
        if (ret < 0)
             return ret;
    }

    return 0;
}

encode_receive_packet_internal 源码

编码阶段重要函数，这个函数会判断所使用的AVCodec是否已经实现了receive_packet函数，从而判断是否是调用receive_packet编码还是调用encode_simple_receive_packet编码。

static int encode_receive_packet_internal(AVCodecContext *avctx, AVPacket *avpkt)
{
    AVCodecInternal *avci = avctx->internal;
    int ret;

	// 已经输入null，且已经返回了，这个时候就不能在调用编码方法
    if (avci->draining_done)
        return AVERROR_EOF;

    av_assert0(!avpkt->data && !avpkt->side_data);

    if (avctx->codec->type == AVMEDIA_TYPE_VIDEO) {
        if ((avctx->flags & AV_CODEC_FLAG_PASS1) && avctx->stats_out)
            avctx->stats_out[0] = '\0';
        if (av_image_check_size2(avctx->width, avctx->height, avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx))
            return AVERROR(EINVAL);
    }

	//硬编码器一般都会实现receive_packet。
    if (avctx->codec->receive_packet) {
        ret = avctx->codec->receive_packet(avctx, avpkt);
        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);
    } else
    	//软编码基本都走这里
        ret = encode_simple_receive_packet(avctx, avpkt);

    if (ret == AVERROR_EOF)
        avci->draining_done = 1;

    return ret;
}

encode_simple_receive_packet 源码

这个函数通过while循环不停的调用encode_simple_internal编码。

static int encode_simple_receive_packet(AVCodecContext *avctx, AVPacket *avpkt)
{
    int ret;

    while (!avpkt->data && !avpkt->side_data) {
        ret = encode_simple_internal(avctx, avpkt);
        if (ret < 0)
            return ret;
    }

    return 0;
}

encode_simple_internal源码

软编码的大部分都会调用到这个函数中，在这种函数中，通过判断是否是多线程编码，调用不同的编码函数。

static int encode_simple_internal(AVCodecContext *avctx, AVPacket *avpkt)
{
    AVCodecInternal   *avci = avctx->internal;
    EncodeSimpleContext *es = &avci->es;
    AVFrame          *frame = es->in_frame;
    int got_packet;
    int ret;

    if (avci->draining_done)
        return AVERROR_EOF;

    if (!frame->buf[0] && !avci->draining) {
        av_frame_unref(frame);
        // avctx->internal->buffer_frame送到frame中。
        ret = ff_encode_get_frame(avctx, frame);
        if (ret < 0 && ret != AVERROR_EOF)
            return ret;
    }

    if (!frame->buf[0]) {
        if (!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY ||
              (avci->frame_thread_encoder && avctx->active_thread_type & FF_THREAD_FRAME)))
            return AVERROR_EOF;

        // Flushing is signaled with a NULL frame
        frame = NULL;
    }

    got_packet = 0;

    av_assert0(avctx->codec->encode2);

	//多线程编码
    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 {
    	//单线程，直接调用AVCodec中的encode2方法编码，对应的实现方式，就是对应的编码器实现
        ret = avctx->codec->encode2(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) {
    	// 返回正常且got_packet，这个是否就引用一下 avpkt，然后直接到end，不设置draining_done
        if (avpkt->data) {
            ret = av_packet_make_refcounted(avpkt);
            if (ret < 0)
                goto end;
        }

        if (frame && !(avctx->codec->capabilities & AV_CODEC_CAP_DELAY)) {
            if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
                if (avpkt->pts == AV_NOPTS_VALUE)
                    avpkt->pts = frame->pts;
                if (!avpkt->duration)
                    avpkt->duration = ff_samples_to_time_base(avctx,
                                                              frame->nb_samples);
            }
        }
        if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
            /* NOTE: if we add any audio encoders which output non-keyframe packets,
             *       this needs to be moved to the encoders, but for now we can do it
             *       here to simplify things */
            avpkt->flags |= AV_PKT_FLAG_KEY;
            avpkt->dts = avpkt->pts;
        }
    }

	//如果这个时候是输入了null，且这次没有得到编码的buffer，
	//即所有的编码完成的cache都已经输出了，这个时候就标识draining_done，不再允许调用avcodec_receive_packet
    if (avci->draining && !got_packet)
        avci->draining_done = 1;

end:
    if (ret < 0 || !got_packet)
        av_packet_unref(avpkt);

    if (frame) {
        if (!ret)
            avctx->frame_number++;
        av_frame_unref(frame);
    }

    if (got_packet)
        // 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);

    return ret;
}

ff_thread_video_encode_frame 源码

多线程编码，配合调用avcodec_open2 时，打开的编码线程。

int ff_thread_video_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
                                 AVFrame *frame, int *got_packet_ptr)
{
    ThreadContext *c = avctx->internal->frame_thread_encoder;
    //多线程编码中，是通过Task传递编码Frame和编码完成后Packet的
    Task *outtask;

    av_assert1(!*got_packet_ptr);

    if(frame){
    	//引用 frame
        av_frame_move_ref(c->tasks[c->task_index].indata, frame);

        pthread_mutex_lock(&c->task_fifo_mutex);
        c->task_index = (c->task_index + 1) % c->max_tasks;
        //发送task_fifo_cond信号量， 编码线程中等待task_fifo_cond这个信号量编码。
        pthread_cond_signal(&c->task_fifo_cond);
        pthread_mutex_unlock(&c->task_fifo_mutex);
    }

    outtask = &c->tasks[c->finished_task_index];
    pthread_mutex_lock(&c->finished_task_mutex);
    /* The access to task_index in the following code is ok,
     * because it is only ever changed by the main thread. */
    if (c->task_index == c->finished_task_index ||
        (frame && !outtask->finished &&
         (c->task_index - c->finished_task_index + c->max_tasks) % c->max_tasks <= avctx->thread_count)) {
            pthread_mutex_unlock(&c->finished_task_mutex);
            return 0;
        }
    //等待finished_task_cond信号量
    while (!outtask->finished) {
        pthread_cond_wait(&c->finished_task_cond, &c->finished_task_mutex);
    }
    pthread_mutex_unlock(&c->finished_task_mutex);
    /* We now own outtask completely: No worker thread touches it any more,
     * because there is no outstanding task with this index. */
    outtask->finished = 0;
    //引用编码后的pkt
    av_packet_move_ref(pkt, outtask->outdata);
    if(pkt->data)
        *got_packet_ptr = 1;
    c->finished_task_index = (c->finished_task_index + 1) % c->max_tasks;

    return outtask->return_code;
}