CODEWITHSUNDEEP
androidc++ffmpeg
Sowry
Sowry

Reputation: 105

FFmpeg - How do i get the extradata required to open the ALAC Codec?

I am wondering how to get the extradata from an ALAC media file using the FFmpeg library without having to manually parse the file?

I setup initially with:

avformat_open_input(&formatContext, pszFileName, 0, 0);
avformat_find_stream_info(formatContext, NULL);
av_find_best_stream(formatContext, AVMEDIA_TYPE_AUDIO, -1, -1, &codec, 0);
codecContext = avcodec_alloc_context3(codec);

Currently I am able to detect and find the ALAC codec but it fails to open the codec returning AVERROR_INVALIDDATA, which comes from the extradata and extradata_size not being set.

avcodec_open2(codecContext, codec, NULL);

The FFmpeg documentation states that some codecs require extradata and extradata_size to be set to the codec's specifications. But shouldn't this data be set by avformat_find_stream_info?

Upvotes: 0

Views: 2467

Answers (1)

Ronald S. Bultje
Ronald S. Bultje

Reputation: 11184

Yes, extradata is populated during either avformat_open_input() or avformat_find_stream_info(). However, it populates it in a field you didn't use. You need the following code:

avformat_open_input(&formatContext, pszFileName, 0, 0);
avformat_find_stream_info(formatContext, NULL);
int audioStreamIndex = av_find_best_stream(formatContext, AVMEDIA_TYPE_AUDIO, -1, -1, &codec, 0);
codecContext = avcodec_alloc_context3(codec);
avcodec_copy_context(codecContext, formatContext->streams[audioStreamIndex]->codec);
avcodec_open2(codecContext, codec, NULL);

The relevant extra line is avcodec_copy_context(), which copies the data from the libavformat demuxer (in formatContext->streams[]) to the copy of that context (codecContext) that you will use for decoding using the decoder in libavcodec.

Upvotes: 2

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