What clock does IMFSourceReader::ReadSample use for the time stamp?

Elias Guestrin 0

Hi,

I need to know what clock IMFSourceReader::ReadSample uses for the time stamp that it provides (argument pllTimestamp in https://learn.microsoft.com/en-us/windows/win32/api/mfreadwrite/nf-mfreadwrite-imfsourcereader-readsample). Is it a monotonic / steady clock or a non-monotonic / system / wall clock? How does it relate to std::chrono::steady_clock or std::chrono::system_clock?

Thank you very much!

2 answers

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Limitless Technology 43,966

Hello there,

This can be of any source but it must meet the below conditions.

An application that uses the Media Session should never start, stop, or pause the presentation clock; change the clock rate; or shut down the clock.

To get a pointer to the presentation clock, call IMFMediaSession::GetClock on the Media Session. The presentation clock does not return a valid time until the Media Session sends the MESessionTopologyStatus event with the MF_TOPOSTATUS_READY flag. Until then, GetClock returns MF_E_CLOCK_NO_TIME_SOURCE.

Hope this resolves your Query !!

--If the reply is helpful, please Upvote and Accept it as an answer–

Elias Guestrin 0

Hi,

Thanks for the response but it does not answer my question. I am working with functional code written by someone else, and I don't see any reference to a Media Session or presentation clock anywhere in the code. To be more specific about the context of my question, I'm providing some code snippets. I'll greatly appreciate getting a concrete, clear answer.

Thank you very much!

// Error checking and other formalities are not shown here for simplicity of presentation

// *******
// Startup
// *******

MFStartup(MF_VERSION, MFSTARTUP_NOSOCKET);

MFEnumDeviceSources(...)

// Get the friendly name and symbolic links of all enumerated sources
// (code not shown here)


// **************
// Initialization
// **************

IMFMediaSource* pMediaSource = NULL;
IMFSourceReader* pSourceReader = NULL;

IMFAttributes* pAttributes = NULL;

MFCreateAttributes(&pAttributes, 2);

pAttributes->SetGUID(MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE, MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE_VIDCAP_GUID);

pAttributes->SetString(MF_DEVSOURCE_ATTRIBUTE_SOURCE_TYPE_VIDCAP_SYMBOLIC_LINK, symLink);

MFCreateDeviceSource(pAttributes, &pMediaSource);

pAttributes->Release();

MFCreateSourceReaderFromMediaSource(pMediaSource, NULL, &pSourceReader);

IMFMediaType* pBestType = NULL;
// Iterate through the supported camera modes (resolution and frame rate combinations)
// and pick the one that matches requirements. The code is omitted here for brevity
// but the required camera mode will be pointed to by pBestType

pSourceReader->SetCurrentMediaType((DWORD)MF_SOURCE_READER_FIRST_VIDEO_STREAM, NULL, pBestType);

pBestType->Release();

pSourceReader->SetStreamSelection(DWORD(MF_SOURCE_READER_ALL_STREAMS), false);
pSourceReader->SetStreamSelection(DWORD(MF_SOURCE_READER_FIRST_VIDEO_STREAM), true);


// ******************
// On a regular basis
// ******************

pSourceReader->ReadSample(
    (DWORD)MF_SOURCE_READER_FIRST_VIDEO_STREAM,
    0, // Flags.
    &streamIndex, // Receives the actual stream index.
    &flags, // Receives status flags.
    &timeStamp, // Receives the time stamp, 100ns resolution.
    &pSample // Receives the sample or NULL.
)

// The above works fine and provides valid time stamps that look like the count of 100 ns
// intervals since the last system reboot.
// BIG QUESTION: Is the clock used for `timeStamp` above a monotonic / steady clock or
// a non-monotonic / system / wall clock?
// How does it relate to