FFMPEG Understanding AVFrame::linesize (Audio)

Question

As per the doucmentation of AVFrame, for audio, lineSize is size in bytes of each plane and only linesize[0] may be set. But however, am unsure whether lineszie[0] is holding per plane buffer size or is it the complete buffer size and we have to divide it by no of channels to get per plane buffer size.

For Example, when I call int data_size = av_samples_get_buffer_size(NULL, iDesiredNoOfChannels, iAudioSamples, (AVSampleFormat)iDesiredFormat, 0) ; For iDesiredNoOfChannels = 2, iAudioSamples = 1024 & iDesiredFormat = AV_SAMPLE_FMT_FLTP data_size=8192. Pretty straightforward, as each sample is 4 bytes and since there are 2 channels total memory will be (1024 * 4 * 2) bytes. As such lineSize[0] should be 4096 for planar audio. data[0] & data[1] should be each of size 4096. However, pFrame->lineSize[0] is giving 8192. So to get the size per plane, I have to do pFrame->lineSize[0] / pFrame->channels. Isn't this behaviour different from what the documentation suggests or is my understanding of the documentaion wrong.

I was bit by this recently! The ffmpeg docs have this note about linesize that says "The linesize may be larger than the size of usable data". My takeaway was... don't trust it, and compute the number of bytes yourself, with sizeof(sample) * channels * samples for packed data, or sizeof(sample) * samples for planar data. — Dave Ceddia

JimmyStack JimmyStack · Accepted Answer · 2021-04-11T04:29:51

Old question but thought I'd answer it anyway for people who may be wondering the same thing.

In all audio AVFrames, only linesize[0] may be set and they are all required to be the same size. You should not be using linesize[1], etc. I don't know why they chose to do things this way because it's not consistent with video frames, but whatever. Just remember whether interleaved or planar only linesize[0] matters so you have to divide by channel count for planar.

FFMPEG Understanding AVFrame::linesize (Audio)

1 Answers