Question

I have a task to wrap a 3rd party video decoder library in a direct show transform filter.

I believe this is going to be a problem as the 3rd party library uses an asynchronous callback when a full frame is ready i.e.

// on main thread
lib->RegisterCallback(callback function)
lib->write(raw data bytes)

void callback(frame)
{
  // here is your frame (on a worker thread)
}

When I look at the pure virtual CTransformFilter.Transform function it expects the transform to be synchronous. Now I could set this up so it blocks on a event that is Set in the callback but what happens if the data coming in to the Transform function is not sufficient to generate a full new frame? I would deadlock the function.

Is my only option to go back to the library developer and ask for a synchronous decoder?

I’ve just been looking at the CTransformFilter Receive function. This is what calls the (overridden) Transform function. It then calls m_pOutput->m_pInputPin->Receive(pOutSample) to pass the sample onwards. Can I call this Receive function from the worker thread in the callback or do I have to keep everything on the same thread?

Thanks

Answer 1

You can still achieve what you want. Note that a filter is not required to output a media sample on the same thread and/or within the call it receives an input media sample. CTransformFilter is however made with this assumption in mind.

So basically your straightforward choices are:

• step back from CTransformFilter and use its ancestor to inherit from to implement output media sample delivery from [a worker thread’s] callback call
• wait within Transform function for asynchronous completion (makes sense if the inner library still decodes 1 output frame for 1 input frame) and catch up then delivering the output; you will also have to wait for pause requests there and abort your wait in order to not block the execution