How to get null audio packet?

user_18892861 0
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I want to add null audio stream. In the ffmpeg command line tool I could do:

-re -f lavfi -i anullsrc

But I don't know how to do it in the code. Based on this: https://ffmpeg.org/doxygen/trunk/filter_audio_8c-example.html#a12 I reckon I could create a fake frame, feed it to a filter graph, read the output, use some encoder (?) and maybe I would get it working. But that would seem like a rather ugly workaround.

Edit:

Essentially I just want to create AVPacket instance with audio coming from anullsrc filter/input.

ffmpeg c
1 Answers

You might be able to use AVFilter in our output context, although I haven't tried it. Here looks to be an example of someone using a basic implementation of AVFilter https://blog.birost.com/a?ID=01500-3825be8d-e578-4030-9e2e-a4f7160d1410. Specifically the following code (I realize it is related to video, but you should be able to modify to audio and may be able to get rid of a lot of that code).

static int init_filters(const char *filters_descr)
{
    char args[512];
    int ret;
    AVFilter *buffersrc  = avfilter_get_by_name("buffer");
    AVFilter *buffersink = avfilter_get_by_name("ffbuffersink");
    AVFilterInOut *outputs = avfilter_inout_alloc();
    AVFilterInOut *inputs  = avfilter_inout_alloc();
    enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE };
    AVBufferSinkParams *buffersink_params;

    filter_graph = avfilter_graph_alloc();

   /* 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",
            pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt,
            pCodecCtx->time_base.num, pCodecCtx->time_base.den,
            pCodecCtx->sample_aspect_ratio.num, pCodecCtx->sample_aspect_ratio.den);

    ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                                       args, NULL, filter_graph);
    if (ret < 0) {
        printf("Cannot create buffer source\n");
        return ret;
    }

   /* buffer video sink: to terminate the filter chain. */
    buffersink_params = av_buffersink_params_alloc();
    buffersink_params->pixel_fmts = pix_fmts;
    ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
                                       NULL, buffersink_params, filter_graph);
    av_free(buffersink_params);
    if (ret < 0) {
        printf("Cannot create buffer sink\n");
        return ret;
    }

   /* Endpoints for the filter graph. */
    outputs->name       = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx    = 0;
    outputs->next       = NULL;

    inputs->name       = av_strdup("out");
    inputs->filter_ctx = buffersink_ctx;
    inputs->pad_idx    = 0;
    inputs->next       = NULL;

AVFilter has the following filters registered:

REGISTER_FILTER(ANULLSRC, anullsrc, asrc);
REGISTER_FILTER(ANULLSINK, anullsink, asink);

Which you might be able to use in the same way the example code is using. I would think an empty AVPacket could work then. Its possible this approach doesn't work (haven't had to deal with this directly), but something you could try if you haven't already. Would love to hear the results if you do.

user_32806440
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