Media Foundation: Getting a MediaSink from a SinkWriter - c++

Question

I'm trying to add an MP4 file sink to a Topology. When my MediaSource is already MP4, I use MFCreateMPEG4MediaSink and MF_MPEG4SINK_SPSPPS_PASSTHROUGH. When my MediaSource isn't MP4 (so raw YUV from a webcam), I want to use MFCreateSinkWriterFromURL so that I don't have to figure out MP4 headers and other complex stuff.
According to the MSDN Docs I should be able to use GetServiceForStream to get at the MediaSink, since the input type is different from the output type. However it always returns MF_E_UNSUPPORTED_SERVICE.
How can I get the underlying MediaSink out of a MediaSinkWriter?
Alternatively, how can I easily create a MP4 media sink for an arbitrary topology?
HRESULT CreateVideoFileSink(
IMFStreamDescriptor *pSourceSD, // Pointer to the stream descriptor.
LPCWSTR pFilename, // Name of file to save to.
IMFStreamSink **ppStream) // Receives a pointer to the stream sink.
HRESULT hr = S_OK;
CComPtr<IMFAttributes> pAttr;
CComPtr<IMFMediaTypeHandler> pHandler;
CComPtr<IMFMediaType> pType;
CComPtr<IMFMediaSink> pSink;
CComPtr<IMFStreamSink> pStream;
CComPtr<IMFSinkWriter> pSinkWriter;
CComPtr<IMFByteStream> pByteStream;
*ppStream = nullptr;
// Get the media type handler for the stream.
IFR(pSourceSD->GetMediaTypeHandler(&pHandler));
// Get the major media type.
GUID guidMajorType;
IFR(pHandler->GetMajorType(&guidMajorType));
IFR(MFCreateAttributes(&pAttr, 1));
IFR(pAttr->SetUINT32(MF_READWRITE_ENABLE_HARDWARE_TRANSFORMS, TRUE));
// Create an output file
if (MFMediaType_Video == guidMajorType)
{
GUID guidSubType;
IFR(pHandler->GetCurrentMediaType(&pType));
IFR(pType->GetGUID(MF_MT_SUBTYPE, &guidSubType));
if (MFVideoFormat_H264 == guidSubType)
{
// ... use MFCreateMPEG4MediaSink
}
else
{
IFR(MFCreateSinkWriterFromURL(pFilename, nullptr, pAttr, &pSinkWriter));
DWORD streamIdx;
IFR(pSinkWriter->AddStream(pType, &streamIdx));
IFR(pSinkWriter->GetServiceForStream(MF_SINK_WRITER_MEDIASINK, GUID_NULL, IID_PPV_ARGS(&pSink)));
IFR(pSink->GetStreamSinkByIndex(streamIdx, &pStream));
}
}
else
{
// Don't use this stream
IFR(E_FAIL)
}
// Return IMFStreamSink pointer to caller.
*ppStream = pStream.Detach();
return S_OK;
}

Answer 1

Figured it out right after writing the question - of course. The SinkWriter doesn't have a MediaSink until you call BeginWriting.
IFR(MFCreateSinkWriterFromURL(pFilename, nullptr, pAttr, &pSinkWriter));
DWORD streamIdx;
IFR(pSinkWriter->AddStream(pType, &streamIdx));
IFR(pSinkWriter->BeginWriting()); // <<----
IFR(pSinkWriter->GetServiceForStream(MF_SINK_WRITER_MEDIASINK, GUID_NULL, IID_PPV_ARGS(&pSink)));
IFR(pSink->GetStreamSinkByIndex(streamIdx, &pStream));
(Make sure you don't let the SinkWriter get Released while you're using the StreamSink)