[Libav-user] videorate realtime-adjusting

[Strahinja Radman]

On Mon, Dec 2, 2019 at 9:29 AM 田浩杨 <734512183 at qq.com> wrote:

> I have a question. How can I modify the video-encode bitrate in real time
> during coding after the video encoder has been turned on? Here's what I did
> (take x264-encoder as an example) :
>
> step1.
> Modified the definition of structural AVCoder to add a function pointer.
> int (*valid_context)(AVCodecContext*);
>
> step2.
> Added function definition to enable the codec rate of x264.
> static int X264_config(AVCodecContext* avctx){
>     X264Context* x4 = avctx->priv_data;
>     if(avctx->bit_rate){
>         x4->params.rc.i_bitrate = avctx->bit_rate / 1000;
>     }
>     avctx->bit_rate = x4->params.rc.i_bitrate*1000;
>     return 0;
> }
>
> step3.
> build AVCodec.
> 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,
>     //
>     .valid_context    = X264_config,
>     //
>     .encode2          = X264_frame,
>     .close            = X264_close,
>     .capabilities     = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS,
>     .priv_class       = &x264_class,
>     .defaults         = x264_defaults,
>     .init_static_data = X264_init_static,
>     .caps_internal    = FF_CODEC_CAP_INIT_THREADSAFE |
>                         FF_CODEC_CAP_INIT_CLEANUP,
>     .wrapper_name     = "libx264",
> };
>
> step4.
> FFmpeg interface layer.
> int attribute_align_arg avcodec_valid_codec_Context(AVCodecContext* avctx){
>     int ret = 0;
>     ret = avctx->codec->valid_context(avctx);
>     if(ret){
>         av_log(avctx, AV_LOG_ERROR, "valid context error, ERROR
> CODE:%d\n",
> ret);
>     }
>     return ret;
> }
>
> step5.
> Whenever the application layer needs to modify the bit rate, then App
> should modify the AVCodecContext-> bit_rate, and then call the above
> interface to make the Context take effect in the encoder.
>
> Because of the limited understanding of ffmpeg. So if there is a better
> way, also hope to give instruction.
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Well, according to the libx264.c wrapper and x264's encoder.c code, one can
simply change the value
of bitrate, bufsize, maxrate or crf in AVCodecContext, and reconfig_encoder
will invoke x264's internal
API that will reconfigure the encoder with one or more of the previously
mentioned values. That should
be it. The change will be triggered on the next sent frame. Checkout the
source of libx264.c line 186.

-- 

Regards
Strahinja Radman
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