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• ffmpeg api alternate transcoding and remuxing for same file

  21 juin 2022, par Alexandre Novius

  Context

  


  Hello !

  


  I'm currently working on the development of a small library allowing to cut an h.264 video on any frame, but without re-encoding (transcoding) the whole video. The idea is to re-encode only the GOP on which we want to cut, and to rewrite (remux) directly the others GOP.

  


  The avcut project (https://github.com/anyc/avcut) allows to do that, but requires a systematic decoding of each package, and seems to not work with the recent versions of ffmpeg from the tests I could do and from the recent feedbacks in the github issues.

  


  As a beginner, I started from the code examples provided in the ffmpeg documentation, in particular : transcoding.c and remuxing.c.

  


  Problem encountered

  


  The problem I'm having is that I can't get both transcoding and remuxing to work properly at the same time. In particular, depending on the method I use to initialize the AVCodecParameters of the output video stream, transcoding works, or remuxing works :

  


  

  • avcodec_parameters_copy works well for remuxing

  


  • avcodec_parameters_from_context works well for transcoding

  


  


  In case I choose avcodec_parameters_from_context, the transcoded GOP are correctly read by my video player (parole), but the remuxed packets are not read, and ffprobe does not show/detect them.

  


  In case I choose avcodec_parameters_from_context, the remuxing GOP are correctly read by my video player, but the transcoding key_frame are bugged (I have the impression that the b-frame and p-frame are ok), and ffprobe -i return an error about the NAL of the key-frames :

  


  [h264 @ 0x55ec8a079300] sps_id 32 out of range
[h264 @ 0x55ec8a079300] Invalid NAL unit size (1677727148 > 735).
[h264 @ 0x55ec8a079300] missing picture in access unit with size 744


  


  I suspect that the problem is related to the extradata of the packets. Through some experiments on the different attributes of the output AVCodecParameters, it seems that it is the extradata and extradata_size attributes that are responsible for the functioning of one method or the other.

  


  Version

  


  ffmpeg development branch retrieved on 2022-05-17 from https://github.com/FFmpeg/FFmpeg.

  


  Compiled with --enable-libx264 --enable-gpl --enable-decoder=png --enable-encoder=png

  


  Code

  


  My code is written in c++ and is based on two classes : a class defining the parameters and methods on the input file (InputContexts) and a class defining them for the output file (OutputContexts). The code of these two classes is defined in the following files :

  


  

  • contexts.h

  


  • contexts.cpp

  


  


  The code normally involved in the problem is the following :

  


  

  • stream initialization

  


  


  int OutputContexts::init(const char* out_filename, InputContexts* input_contexts){
    int ret;
    int stream_index = 0;

    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);
    if (!ofmt_ctx) {
        fprintf(stderr, "Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        return ret;
    }

    av_dump_format(ofmt_ctx, 0, out_filename, 1);
 
    encoders.resize(input_contexts->ifmt_ctx->nb_streams, nullptr);
    codecs.resize(input_contexts->ifmt_ctx->nb_streams, nullptr);
  
    // stream mapping
    for (int i = 0; i < input_contexts->ifmt_ctx->nb_streams; i++) {
        AVStream *out_stream;
        AVStream *in_stream = input_contexts->ifmt_ctx->streams[i];
        AVCodecContext* decoder_ctx = input_contexts->decoders[i];
 
        // add new stream to output context
        out_stream = avformat_new_stream(ofmt_ctx, NULL);
        if (!out_stream) {
            fprintf(stderr, "Failed allocating output stream\n");
            ret = AVERROR_UNKNOWN;
            return ret;
        }

        // from avcut blog
        av_dict_copy(&out_stream->metadata, in_stream->metadata, 0);

        out_stream->time_base = in_stream->time_base;

        // encoder
        if (decoder_ctx->codec_type == AVMEDIA_TYPE_VIDEO){
            ret = prepare_encoder_video(i, input_contexts);
            if (ret < 0){
                fprintf(stderr, "Error while preparing encoder for stream #%u\n", i);
                return ret;
            }

            // from avcut
            out_stream->sample_aspect_ratio = in_stream->sample_aspect_ratio;

            // works well for remuxing
            ret = avcodec_parameters_copy(out_stream->codecpar, in_stream->codecpar);
            if (ret < 0) {
                fprintf(stderr, "Failed to copy codec parameters\n");
                return ret;
            }

            // works well for transcoding
            // ret = avcodec_parameters_from_context(out_stream->codecpar, encoders[i]);
            // if (ret < 0) {
            //     av_log(NULL, AV_LOG_ERROR, "Failed to copy encoder parameters to output stream #%u\n", i);
            //     return ret;
            // }

        } else if (decoder_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            ...
        } else {
            ...
        }

        // TODO useful ???
        // set current stream position to 0
        // out_stream->codecpar->codec_tag = 0;
    }

    // opening output file in write mode with the ouput context
    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            fprintf(stderr, "Could not open output file '%s'", out_filename);
            return ret;
        }
    }
 
    // write headers from output context in output file
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        fprintf(stderr, "Error occurred when opening output file\n");
        return ret;
    }

    return ret;
}


  


  

  • AVCodecContext initialization for encoder

  


  


  int OutputContexts::prepare_encoder_video(int stream_index, InputContexts* input_contexts){
    int ret;
    const AVCodec* encoder;
    AVCodecContext* decoder_ctx = input_contexts->decoders[stream_index];
    AVCodecContext* encoder_ctx;

    if (video_index >= 0){
        fprintf(stderr, "Impossible to mark stream #%u as video, stream #%u is already registered as video stream.\n", 
                stream_index, video_index);
        return -1; //TODO change this value for correct error code
    }
    video_index = stream_index;

    if(decoder_ctx->codec_id == AV_CODEC_ID_H264){
        encoder = avcodec_find_encoder_by_name("libx264");
        if (!encoder) {
            av_log(NULL, AV_LOG_FATAL, "Encoder libx264 not found\n");
            return AVERROR_INVALIDDATA;
        }
        fmt::print("Encoder libx264 will be used for stream {}.\n", stream_index);
    } else {
        std::string s = fmt::format("No video encoder found for the given codec_id: {}\n", avcodec_get_name(decoder_ctx->codec_id));
        av_log(NULL, AV_LOG_FATAL, s.c_str());
        return AVERROR_INVALIDDATA;
    }
    
    encoder_ctx = avcodec_alloc_context3(encoder);
    if (!encoder_ctx) {
        av_log(NULL, AV_LOG_FATAL, "Failed to allocate the encoder context\n");
        return AVERROR(ENOMEM);
    }

    // from avcut
    encoder_ctx->time_base = decoder_ctx->time_base;
    encoder_ctx->ticks_per_frame = decoder_ctx->ticks_per_frame;
    encoder_ctx->delay = decoder_ctx->delay;
    encoder_ctx->width = decoder_ctx->width;
    encoder_ctx->height = decoder_ctx->height;
    encoder_ctx->pix_fmt = decoder_ctx->pix_fmt;
    encoder_ctx->sample_aspect_ratio = decoder_ctx->sample_aspect_ratio;
    encoder_ctx->color_primaries = decoder_ctx->color_primaries;
    encoder_ctx->color_trc = decoder_ctx->color_trc;
    encoder_ctx->colorspace = decoder_ctx->colorspace;
    encoder_ctx->color_range = decoder_ctx->color_range;
    encoder_ctx->chroma_sample_location = decoder_ctx->chroma_sample_location;
    encoder_ctx->profile = decoder_ctx->profile;
    encoder_ctx->level = decoder_ctx->level;

    encoder_ctx->thread_count = 1; // spawning more threads causes avcodec_close to free threads multiple times
    encoder_ctx->codec_tag = 0;
    
    // correct values ???
    encoder_ctx->qmin = 16;
    encoder_ctx->qmax = 26;
    encoder_ctx->max_qdiff = 4;
    // end from avcut

    // according to avcut, should not be set
    // if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER){
    //     encoder_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    // }

    ret = avcodec_open2(encoder_ctx, encoder, NULL);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", stream_index);
        return ret;
    }
    
    codecs[stream_index] = encoder;
    encoders[stream_index] = encoder_ctx;

    return ret;
}


  


  Example

  


  To illustrate my problem, I provide here a test code using the two classes that alternates between transcoding and remuxing at each key-frame encountered in the file using my classes.

  


  

  • trans_remux.cpp

  


  


  To compile the code :

  


  g++ -o trans_remux trans_remux.cpp contexts.cpp -D__STDC_CONSTANT_MACROS `pkg-config --libs libavfilter` -lfmt -g


  


  Currently the code is using avcodec_parameters_copy (contexts.cpp:333), so it works well for remuxing. If you want to test the version with avcodec_parameters_from_context, pls comment from line 333 to 337 in contexts.cpp and uncomment from line 340 to 344 and recompile.

  


• FFMPEG : Overlay Capture Time (w/Counter)

  27 juin 2022, par AdamK

  I am using the Shutter Encoder application on Windows to batch convert .MOV files, which provides the option of injecting custom FFMPEG commands for each file. The app natively offers overlay (drawtext) of timecode starting at 00:00:00:00. I also see that it knows and preserves the metadata time for each file as this is included in the commands "-metadata creation_time="2022-06-27T16:00:30.730888500Z"

  


  I would like to have the timecode start at the creation time, and was wondering how I might be able to offset the timecode as such. Or...is there another way of overlaying (drawtext-ing) a time counter, starting at creation time ? I would also like to overlay the creation date as well. Thanks in advance for your advice.

  


• opus : add a native Opus encoder

  11 février 2017, par Rostislav Pehlivanov

  opus : add a native Opus encoder
  This marks the first time anyone has written an Opus encoder without
  
  using any libopus code. The aim of the encoder is to prove how far
  
  the format can go by writing the craziest encoder for it.
  Right now the encoder’s basic, it only supports CBR encoding, however
  
  internally every single feature the CELT layer has is implemented
  
  (except the pitch pre-filter which needs to work well with the rest of
  
  whatever gets implemented). Psychoacoustic and rate control systems are
  
  under development.
  The encoder takes in frames of 120 samples and depending on the value of
  
  opus_delay the plan is to use the extra buffered frames as lookahead.
  
  Right now the encoder will pick the nearest largest legal frame size and
  
  won’t use the lookahead, but that’ll change once there’s a
  
  psychoacoustic system.
  Even though its a pretty basic encoder its already outperforming
  
  any other native encoder FFmpeg has by a huge amount.
  The PVQ search algorithm is faster and more accurate than libopus’s
  
  algorithm so the encoder’s performance is close to that of libopus
  
  at zero complexity (libopus has more SIMD).
  
  The algorithm might be ported to libopus or other codecs using PVQ in
  
  the future.
  The encoder still has a few minor bugs, like desyncs at ultra low
  
  bitrates (below 9kbps with 20ms frames).
  Signed-off-by : Rostislav Pehlivanov <atomnuker@gmail.com>
  

  • [DH] configure
  • [DH] libavcodec/Makefile
  • [DH] libavcodec/allcodecs.c
  • [DH] libavcodec/opus_celt.h
  • [DH] libavcodec/opus_pvq.c
  • [DH] libavcodec/opus_pvq.h
  • [DH] libavcodec/opusenc.c