Synchronizing input pins in directshow

Question

I am creating a directshow filter which's purpose is to take 3 input pins and create a video which shows alternately vidoe from the first source, the second source and the third source, in a fixed time internal.

So if i have three webcam connected to my filter, i want the final video for example to show 5 seconds of the first cam, five seconds of the second cam, and so on...

I have tried two approaches:

Approach one

I use a class TimeManager. This class has a function isItPinsTurn(pinname). This functions returns true or false regarding if the pin is supposed to send sample to the output. To do this the TimeManager creates a new thread which sleeps every x seconds. After it slept it changes to the current active inputpin to the next. The result is that every x seconds the isItPinSTurn(pinname) function returns another pin. This way every pin only seconds output to the outputpin when it is its turn, hence i get the desired videos with x intervalls between the input cam.

The problem with this approach

Sleep doesn't seem to work in directshow filters. I get a runtime error:

abort() has been called

Approach two

I use the samples GetMediaTime method and a buffer which keeps track of how much video samples in terms of its mediatime, has already been sent to the output pin. This is best illustrated with code:

void MyFilter::acceptFilterInput(LPCWSTR pinname, IMediaSample* sample)
{

    mylogger->LogDebug("In acceptFIlterInput", L"D:\\TEMP\\yc.log");
    if (wcscmp(pinname, this->currentInputPin) == 0)
    {
        outpin->Deliver(sample);
        LONGLONG timestart;
        LONGLONG timeend;
        sample->GetTime(&timestart, &timeend);

        *mediaTimeBuffer += timeend - timestart;
        if (*mediaTimeBuffer > this->MEDIATIME)
        {
            this->SetNextPinActive(pinname);
            *mediaTimeBuffer = 0;
        }

    }

}

When the filter starts the currentInputPin is set to pin0 (the first). Calls to acceptFilterInput (which is called by the the input pins receie function) adjust the mediaTimeBUffer with the size of the MediaSample-MediaTime. If this buffer is higher than MEDIATIME (which can for example be 5 (seconds)), the buffer is set back to zero and the next pin is set active.

Problems with this approach I am not even sure if CMediaSample->GetMediaTime returns the data i need, as it seems to return negative numbers, which doesn't seem to make much sense. I didn't find useful information about the return value of GetMediaTime on the web.

Roman R. Roman R. · Accepted Answer · 2014-03-21T13:00:37

You are expected to block execution (incoming calls to IPin::Receive) on input streams so that other streams could catch up on their own streaming threads. You typically achieve this by either using wait/synchronization APIs and functions, or by holding references on media samples so that input peer would block on empty allocator waiting for a media sample (buffer) to get available.

Yes Sleep works well, although polling is the worst of possible options.

Approach two does not make sense for me because I don't see any real synchronization there: there is no execution blocking, and there is no making pin active. You cannot force data on the input pin, you only can wait to get called with new media sample. So you should block accepting data on one input stream/pin until you get data on another.

Some useful relevant information on multiplexing:

How to make a DirectShow Muxer Filter - Part 1
How to make a DirectShow Muxer Filter - Part 2
GDCL MPEG-4 Multiplexer - available in source, and can multiplex data from 2+ streams

Synchronizing input pins in directshow

1 Answers