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c++directshowclock

DirectShow IReferenceClock implementation

How exactly are you meant to implement an IReferenceClock that can be set via IMediaFilter::SetSyncSource?

I have a system that implements GetTime and AdviseTime, UnadviseTime. When a stream starts playing it sets a base time via AdviseTime and then increases Stream Time for each subsequent advise.

However how am I supposed to know when a new graph has run? I need to set a zero point for a given reference clock. Otherwise if I create a reference clock and then, 10 seconds later, I start the graph I am now in the position that I don't know whether I should be 10 seconds down the playback or whether I should be starting from 0. Obviously the base time will say that I am starting from 0 but have I just stalled for 10 seconds and do I need to drop a bunch of frames?

I really can't seem to figure out how to write a proper IReferenceClock so any hints or ideas would be hugely appreciated.

Edit: One example of a problem I am having is that I have 2 graphs and 2 videos. The audio from both videos is going to a null renderer. The Video to a standard CLSID_VideoRenderer. Now If i set the same reference clock to both and then Run graph 1 all seems to be fine. However if 10 seconds down the line I run graph 2 then it will run as though the SetSyncSource is NULL for the first 10 seconds or so until it has caught up with the other video.

Obviously if the graphs called GetTime to get their "base time" this would solve the problem but this is not what I'm seeing happening. Both videos end up with a base time of 0 because thats the point I run them from.

Its worth noting that if I set no clock at all (or call SetDefaultSyncSource) then both graphs run as fast as they can. I assume this is due to the lack of an Audio Renderer ...

Solution

  • However how am I supposed to know when a new graph has run?

    The clock runs on its own, it is the graph that aligns its operation against the clock and not otherwise. The graph receives outer Run call, then it checks current clock time and assigns base time, which is distributed among filters, as "current clock time + some time for the things to take off". The clock itself doesn't have to have a faintest idea about all this and its task is to keep running and keep incrementing time.

    In particular, clock time does not have to reset to zero at any time.

    From documentation:

    The clock's baseline—the time from which it starts counting—depends on the implementation, so the value returned by GetTime is not inherently meaningful. What matters is the delta from when the graph started running.

    When an application calls IMediaControl::Run to run the filter graph, the Filter Graph Manager calls IMediaFilter::Run on each filter. To compensate for the slight amount of time it takes for the filters to start running, the Filter Graph Manager specifies a start time slightly in the future.

    BaseClasses offer CBaseReferenceClock class, which you can use as reference implementation (in refclock.*).

    Comment to your edit:

    You obviously not describing the case in full and you are omitting important details. There is a simple test: you can instantiate standard clock (CLSID_SystemClock) and use it on two regular graphs - they WILL run fine, even with time-separated Run times.

    I suspect that you are doing some sync'ing or matching between the graphs and you are time stamping the samples, also using the clock. Presumably you are doing something wrong at that point and then you have hard time fixing it through the clock.