// sets up libavformat state: creates the AVFormatContext, the frames, etc. to start decoding, but does not actually start the decodingLoop
private void prepareDecoding()
{
const int context_buffer_size = 4096;
readPacketCallback = readPacket;
seekCallback = streamSeekCallbacks;
// we shouldn't keep a reference to this buffer as it can be freed and replaced by the native libs themselves.
// https://ffmpeg.org/doxygen/4.1/aviobuf_8c.html#a853f5149136a27ffba3207d8520172a5
byte *contextBuffer = (byte *)ffmpeg.av_malloc(context_buffer_size);
ioContext = ffmpeg.avio_alloc_context(contextBuffer, context_buffer_size, 0, (void *)handle.Handle, readPacketCallback, null, seekCallback);
var fcPtr = ffmpeg.avformat_alloc_context();
formatContext = fcPtr;
formatContext->pb = ioContext;
formatContext->flags |= AGffmpeg.AVFMT_FLAG_GENPTS; // required for most HW decoders as they only read `pts`
int openInputResult = ffmpeg.avformat_open_input(&fcPtr, "dummy", null, null);
inputOpened = openInputResult >= 0;
if (!inputOpened)
{
throw new InvalidOperationException($"Error opening file or stream: {getErrorMessage(openInputResult)}");
}
int findStreamInfoResult = ffmpeg.avformat_find_stream_info(formatContext, null);
if (findStreamInfoResult < 0)
{
throw new InvalidOperationException($"Error finding stream info: {getErrorMessage(findStreamInfoResult)}");
}
int streamIndex = ffmpeg.av_find_best_stream(formatContext, AVMediaType.AVMEDIA_TYPE_VIDEO, -1, -1, null, 0);
if (streamIndex < 0)
{
throw new InvalidOperationException($"Couldn't find video stream: {getErrorMessage(streamIndex)}");
}
stream = formatContext->streams[streamIndex];
timeBaseInSeconds = stream->time_base.GetValue();
if (stream->duration > 0)
{
Duration = stream->duration * timeBaseInSeconds * 1000;
}
else
{
Duration = formatContext->duration / AGffmpeg.AV_TIME_BASE * 1000;
}
}