参考链接

• FFmpeg源代码简单分析：avcodec_encode_video()_雷霄骅的博客-CSDN博客_avcodec_encode_video2

avcodec_encode_video()

• 该函数用于编码一帧视频数据。
• 函数已被弃用
• 参考链接：FFmpeg 新旧版本编码 API 的区别_zouzhiheng的博客-CSDN博客

send_frame 和 receive_packet 例子

static void encode(AVCodecContext *enc_ctx, AVFrame *frame, AVPacket *pkt,
                   FILE *outfile)
{
    int ret;
 
    /* send the frame to the encoder */
    if (frame)
        printf("Send frame %3"PRId64"\n", frame->pts);
 
    ret = avcodec_send_frame(enc_ctx, frame);
    if (ret < 0) {
        fprintf(stderr, "Error sending a frame for encoding\n");
        exit(1);
    }
 
    while (ret >= 0) {
        ret = avcodec_receive_packet(enc_ctx, pkt);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            return;
        else if (ret < 0) {
            fprintf(stderr, "Error during encoding\n");
            exit(1);
        }
 
        printf("Write packet %3"PRId64" (size=%5d)\n", pkt->pts, pkt->size);
        fwrite(pkt->data, 1, pkt->size, outfile);
        av_packet_unref(pkt);
    }
}

• avcodec_encode_video一个函数即可完成编码操作，编码成功后可直接使用压缩后的数据。新版 API 需要两个函数一起使用，一个 send，一个 receive，分别用于发送原始视频数据、获取编码后的数据；具体在哪里完成了编码动作，暂时未知。
• avcodec_encode_video 一次编码动作对应 0 个或 1 个 AVFrame 和 0 个或 1 个 AVPacket。新本 API 一次编码动作对应 0 个或 1 个 AVFrame 和 0 个或多个 AVPacket。 

avcodec_send_frame

• avcodec_send_frame 的声明如下：

/**
 * 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, 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);

• 从注释中可以看出，这个函数用于发送原始的视频/音频数据给编码器编码，参数 AVFrame 同样可以为 NULL 以刷新编码器。

int attribute_align_arg avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame)
{
    AVCodecInternal *avci = avctx->internal;
    int ret;
 
    if (!avcodec_is_open(avctx) || !av_codec_is_encoder(avctx->codec))
        return AVERROR(EINVAL);
 
    if (avci->draining)
        return AVERROR_EOF;
 
    if (avci->buffer_frame->buf[0])
        return AVERROR(EAGAIN);
 
    if (!frame) {
        avci->draining = 1;
    } else {
        ret = encode_send_frame_internal(avctx, frame);
        if (ret < 0)
            return ret;
    }
 
    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;
}

avcodec_receive_packet

• avcodec_receive_packet 则用于获取编码后的视频/音频数据。它的声明如下：

/**
 * 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_packet_unref(avpkt) 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 a decoder
 *      other errors: legitimate encoding errors
 */
int avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt);

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;
}

 注意事项

• 旧版本视频编码使用 avcodec_encode_video2，音频编码使用 avcodec_encode_audio2；新版本音视频编码统一使用 avcodec_send_frame 和 avcodec_receive_packet
• 旧版本 API 内部直接调用了 AVCodec 的函数指针 encode2；新版本 API 首先会判断编码器是否实现了函数指针 send_frame 和 receive_packet，如果实现了，优先使用send_frame 和 receive_packet，否则使用旧版本的 encode2    未找到代码证明，每个版本之间差异较大
• 目前仅发现编码器 ff_hevc_nvenc_encoder 实现了新版本的 API（send_frame 和 receive_packet），libx264、AAC 等编码器依然使用了旧版本的 API（encode2）

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