Media Foundation set video interlacing and decode - c++

Question

I have an MOV file and I want to decode it and have all frames as separate images.
So I try to configure an uncompressed media type in the following way:
// configure the source reader
IMFSourceReader* m_pReader;
MFCreateSourceReaderFromURL(filePath, NULL, &m_pReader);
// get the compressed media type
IMFMediaType* pFileVideoMediaType;
m_pReader->GetCurrentMediaType(MF_SOURCE_READER_FIRST_VIDEO_STREAM, &pFileVideoMediaType);
// create new media type for uncompressed type
IMFMediaType* pTypeUncomp;
MFCreateMediaType(&pTypeUncomp);
// copy all settings from compressed to uncompressed type
pFileVideoMediaType->CopyAllItems(pTypeUncomp);
// set the uncompressed video attributes
pTypeUncomp->SetGUID(MF_MT_SUBTYPE, MFVideoFormat_RGB8);
pTypeUncomp->SetUINT32(MF_MT_ALL_SAMPLES_INDEPENDENT, TRUE);
pTypeUncomp->SetUINT32(MF_MT_INTERLACE_MODE, MFVideoInterlace_Progressive);
// set the new uncompressed type to source reader
m_pReader->SetCurrentMediaType(MF_SOURCE_READER_FIRST_VIDEO_STREAM, 0, pTypeUncomp);
// get the full uncompressed media type
m_pReader->GetCurrentMediaType(MF_SOURCE_READER_FIRST_VIDEO_STREAM, &pTypeUncomp);
I noticed that even I explicitly set the MF_MT_INTERLACE_MODE to MFVideoInterlace_Progressive the final configuration is still configured with the old mode MFVideoInterlace_MixedInterlaceOrProgressive.
Afterwards, I loop through all samples and look at their size:
IMFSample* videoSample = nullptr;
IMFMediaBuffer* mbuffer = nullptr;
LONGLONG llTimeStamp;
DWORD streamIndex, flags;
m_pReader->ReadSample(
MF_SOURCE_READER_FIRST_VIDEO_STREAM,
0, // Flags.
&streamIndex, // Receives the actual stream index.
&flags, // Receives status flags.
&llTimeStamp, // Receives the time stamp.
&videoSample) // Receives the sample or NULL.
videoSample->ConvertToContiguousBuffer(&mbuffer);
BYTE* videoData = nullptr;
DWORD sampleBufferLength = 0;
mbuffer->Lock(&videoData, nullptr, &sampleBufferLength);
cout << sampleBufferLength << endl;
And I get quite different sizes for the samples: from 31bytes to 18000bytes.
Even changing the format to MFVideoFormat_RGB32 does not change affect the sample sizes.
This question seems to have the same issue but the solution is not fixing it.
Any help on why I can't change the interlacing and how to properly decode video frames and get image data out of samples?
Many thanks in advance.

Answer 1

In order to make SourceReader convert the samples to RGB you need to create it like this:
IMFAttributes* pAttr = NULL;
MFCreateAttributes(&pAttr, 1);
pAttr->SetUINT32(MF_READWRITE_ENABLE_HARDWARE_TRANSFORMS, TRUE);
pAttr->SetUINT32(MF_SOURCE_READER_ENABLE_VIDEO_PROCESSING, TRUE);
IMFSourceReader* m_pReader;
throwIfFailed(MFCreateSourceReaderFromURL(filePath, pAttr, &m_pReader), Can't create source reader from url");
pAttr->Release();
Later, you shouldn't break from the cycle when MF_SOURCE_READERF_CURRENTMEDIATYPECHANGED occurs. Now you'll have all samples with the same size.
Otherwise you can use MFVideoFormat_NV12 subtype and then you won't need to specify MF_SOURCE_READER_ENABLE_VIDEO_PROCESSING attribute when creating the reader.