DecideBufferSize method sets the buffer requirements.
virtual HRESULT DecideBufferSize( IMemAllocator *pAlloc, ALLOCATOR_PROPERTIES *ppropInputRequest ) = 0;
Pointer to the allocator's IMemAllocator interface.
Pointer to an ALLOCATOR_PROPERTIES structure that contains the input pin's buffer requirements. If the input pin does not have any requirements, the caller should zero out the members of this structure before calling the method.
Returns S_OK if successful, or an HRESULT value indicating the cause of the error.
Override this method in your derived class. Call the IMemAllocator::SetProperties method to specify your buffer requirements. Typically, the derived class will honor the input pin's buffer requirements, but it is not required to.