[Libav-user] Getting resulting width and height from scale filter

[Strahinja Radman]

From: Michael Bachmann
Sent: 21 February 2019 13:02
To: libav-user at ffmpeg.org
Subject: [Libav-user] Getting resulting width and height from scale filter

Is there a way to get the resulting width and height from a scale filter, so i can setup the Encoding-AVCodecContext correctly (with width and height)?


Here's my init filter method, which gets feeded with the filter description "scale=-1:'min(720,iw)'":


int FFMPEGCommands::init_filters(const char *filters_descr, int width, int height, int streamFrameRate) {
    char args[512];
    int ret = 0;
    const AVFilter *buffersrc = avfilter_get_by_name("buffer");
    const AVFilter *buffersink = avfilter_get_by_name("buffersink");
    AVFilterInOut *outputs = avfilter_inout_alloc();
    AVFilterInOut *inputs = avfilter_inout_alloc();
    AVRational time_base = { 1, streamFrameRate };
    //AVRational time_base = fmt_ctx->streams[video_stream_index]->time_base;
    enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE };
    filter_graph = avfilter_graph_alloc();
    if (!outputs || !inputs || !filter_graph) {
        ret = AVERROR(ENOMEM);
        goto end;
    }
    /* buffer video source: the decoded frames from the decoder will be inserted here. */
    snprintf(args, sizeof(args),
        "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
        width, height, STREAM_PIX_FMT,
        time_base.num, time_base.den,
        0, 1);
    ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
        args, NULL, filter_graph);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot create buffer source\n");
        goto end;
    }
    /* buffer video sink: to terminate the filter chain. */
    ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
        NULL, NULL, filter_graph);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot create buffer sink\n");
        goto end;
    }
    ret = av_opt_set_int_list(buffersink_ctx, "pix_fmts", pix_fmts,
        AV_PIX_FMT_NONE, AV_OPT_SEARCH_CHILDREN);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot set output pixel format\n");
        goto end;
    }
    /*
     * Set the endpoints for the filter graph. The filter_graph will
     * be linked to the graph described by filters_descr.
     */
     /*
      * The buffer source output must be connected to the input pad of
      * the first filter described by filters_descr; since the first
      * filter input label is not specified, it is set to "in" by
      * default.
      */
    outputs->name = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx = 0;
    outputs->next = NULL;
    /*
     * The buffer sink input must be connected to the output pad of
     * the last filter described by filters_descr; since the last
     * filter output label is not specified, it is set to "out" by
     * default.
     */
    inputs->name = av_strdup("out");
    inputs->filter_ctx = buffersink_ctx;
    inputs->pad_idx = 0;
    inputs->next = NULL;
    if ((ret = avfilter_graph_parse_ptr(filter_graph, filters_descr,
        &inputs, &outputs, NULL)) < 0)
        goto end;
    if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
        goto end;
end:
    avfilter_inout_free(&inputs);
    avfilter_inout_free(&outputs);
    return ret;
}


The info you are seeking should be available here filter_ctx.buffersink_ctx->inputs[0]->w  for width and filter_ctx.buffersink_ctx->inputs[0]->h for height. 
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