videostreamer | Stream video to an HTML5 video element | Video Utils library

This was a silly mistake of ffmpeg's limited logging functionality plus my limited knowledge in video codecs.

The problem was that ffmpeg (with h264 input) when writes an atom to the output buffer:

• first puts 0 for the atom's size (movenc.c#L3981)
• then fills the rest of the buffer
• then at the end it seeks back to atom's size in buffer and update the size (movenc.c#L4049)

This is all good, but during filling the rest of the buffer, it uses avio_w8 and avio_w8 will flush the buffer, if it reaches the end of it.

If you are using custom IO with avio_alloc_context AND you don't define a buffer which is large enough AND you don't define the seek operation either AND the atom being written is larger than your buffer size, it will be flushed out and ffmpeg will not be able to seek back to update the atom's size.

And this will probably result in a corrupted (and unplayable) video output file or stream.

So the solution was to increase the buffer size from 512 to 4096 in this case moov atom can be fit even without seeking operation.

This is pretty straightforward if you know that the moov atom is larger that 512 bytes in length and look my sample code.

It was a silly mistake:

In the initialization part I called this: