On Thu, Mar 28, 2019 at 16:30:27 +0100, Moritz Barsnick wrote: > > av_dict_set(&frame->metadata, "my-custom-key", data, 0); // data: 23-char string > IIUC, the MP4 container doesn't support arbitrary metadata keys. Sorry, I missed the "frame level". Ignore my remark. Moritz