[FFmpeg-user] Transcoding from mp4 to avi is too slow

NDJORE BORIS ndjoreboris at gmail.com
Tue Feb 12 16:17:04 EET 2019

Hello Team,
I have trouble and I know you can help me.

I use transcoding.c example to transcode from mp4 to avi.
All is good but the duration of the output vidéo is two time
long than the input video duration.
For example, if I the duration of the input vidéo is 10s the
duration of the output will be 20s (10sx2) and the output video
is slow.
When I transcode from avi to avi, I have not this problem.
But it occur when I transcode from mp4 to avi.

See following ma transcoding parameters and configure options.
=> Transcoding parameters in open_output_file() function :

static int open_output_file(const char *filename)
AVStream *out_stream;
    AVStream *in_stream;
    AVCodecContext *dec_ctx, *enc_ctx;
    AVCodec *encoder;
    int ret;
    unsigned int i;

    ofmt_ctx = NULL;
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);
    if (!ofmt_ctx) {
        av_log(NULL, AV_LOG_ERROR, "Could not create output context\n");
        return AVERROR_UNKNOWN;
    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        out_stream = avformat_new_stream(ofmt_ctx, NULL);
        if (!out_stream) {
            av_log(NULL, AV_LOG_ERROR, "Failed allocating output stream\n");
            return AVERROR_UNKNOWN;

        in_stream = ifmt_ctx->streams[i];
        dec_ctx = stream_ctx[i].dec_ctx;

        if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO ||
dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO)

            /* in this example, we choose transcoding to same codec */
               encoder = dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO ?
            if (!encoder) {
                av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");
                return AVERROR_INVALIDDATA;
enc_ctx = avcodec_alloc_context3(encoder);
            if (!enc_ctx) {
                av_log(NULL, AV_LOG_FATAL, "Failed to allocate the encoder
                return AVERROR(ENOMEM);

            /* In this example, we transcode to same properties (picture
             * sample rate etc.). These properties can be changed for output
             * streams easily using filters */
            if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO)
                enc_ctx->height = dec_ctx->height;
                enc_ctx->width = dec_ctx->width;
                enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;


                /* take first format from list of supported formats */
                if (encoder->pix_fmts)
                    enc_ctx->pix_fmt = encoder->pix_fmts[0];
                    enc_ctx->pix_fmt = dec_ctx->pix_fmt;
                /* video time_base can be set to whatever is handy and
supported by encoder */
                enc_ctx->time_base = av_inv_q(dec_ctx->framerate);
            } else {
                enc_ctx->sample_rate = dec_ctx->sample_rate;
                enc_ctx->channel_layout = dec_ctx->channel_layout;
                enc_ctx->channels =
                /* take first format from list of supported formats */
                enc_ctx->sample_fmt = encoder->sample_fmts[0];
                enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate};

            /* Third parameter can be used to pass settings to encoder */
            ret = avcodec_open2(enc_ctx, encoder, NULL);
 if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for
stream #%u\n", i);
                return ret;
            ret = avcodec_parameters_from_context(out_stream->codecpar,
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to copy encoder
parameters to output stream #%u\n", i);
                return ret;
            if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
                enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

            out_stream->time_base = enc_ctx->time_base;
            stream_ctx[i].enc_ctx = enc_ctx;
        } else if (dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {
            av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown
type, cannot proceed\n", i);
            return AVERROR_INVALIDDATA;
        } else {
            /* if this stream must be remuxed */
            ret = avcodec_parameters_copy(out_stream->codecpar,
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Copying parameters for stream
#%u failed\n", i);
                return ret;
            out_stream->time_base = in_stream->time_base;

    av_dump_format(ofmt_ctx, 0, filename, 1);

    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Could not open output file '%s'",
            return ret;
ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0)
        av_log(NULL, AV_LOG_ERROR, "Error occurred when opening output
        return ret;

    return 0;
=> configure options :

            --extra-libs=-lalgolib\ -lpthread
        BUILD_COMMAND make -j8
        INSTALL_COMMAND make install

I need your help, please

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