Sharing memory allocators between input and output pins is an important concept to keep performance of filter graph. Unlike more frequent scenario with different allocators, a filter (referred to as “middle filter” below) which has equal media types on input and output pins has an advantage to avoid memory-to-memory copy operation for every frame processed, by delivering downstream the buffer obtained from an upstream filter. With a high resolution video, at high rate, multiple streams running simultaneously this is the expense one would try to avoid for performance reasons.

Memory allocators are (or can be) shared by well known filters, such as Sample Grabber Filter, Infinite Tee Pin Filter and in-place transformation base filters (CTransInPlaceFilter Class).

Still handling Dynamic Format Changes (not only from video renderer filter) filters that share memory allocators may run into the problem of being notified of media type change. Because allocator are typically owned by another filter (e.g. Video Mixing Renderer Filter) and originally its buffer is queried by an upstream filter, the upstream filter obtains allocated buffer independently from the middle filter that shares memory allocators. If the upstream filter decides to never deliver this buffer, however the buffer has a media type attached (see AM_SAMPLE2_PROPERTIES::pMediaType), there is no way for the middle filter to learn about dynamic format change completed.

As a workaround for handling Format Changes from the Video Renderer, when resolution is not changed and it is only stride which might be extended, middle filter might be checking data size in lActual field and learn about the change from an increase in this value.

To be reliably notified on media type change the middle filter is to take extra measures while sharing the allocator. Instead using raw allocator obtained from one pin on another pin (typically output pin’s allocator to be used on an input pin), middle filter may be using an internal proxy object, which implements IMemAllocator interface and forward calls to internal IMemAllocator, obtained originally. Additionally to that, the proxy can check for attached media types on every buffer taken from the allocator, and once the change is noticed – at the moment upstream filter is requesting the buffer – the proxy has a timely chance to remember the new media type so that in the following IMemInputPin::Receive call this media type can be checked for the case upstream buffer decided to not deliver the buffer with attached media type.

if(IsSharingMemAllocators())
{
    // ...
    ATLASSERT((InputMediaSampleProperties.pMediaType != NULL) ^ !(InputMediaSampleProperties.dwSampleFlags & AM_SAMPLE_TYPECHANGED));
    {
        CRoCriticalSectionLock DataLock(GetDataCriticalSection());
        const CObjectPtr<CProxyMemAllocator>& pInputProxyMemAllocator = m_pInputPin->GetProxyMemAllocatorReference();
        CMediaType pMediaType;
        if(pInputProxyMemAllocator && pInputProxyMemAllocator->GetDynamicallyChangedMediaType(pMediaType, TRUE))
        {
            m_pInputPin->SetMediaType(pMediaType);
            m_pOutputPin->SetMediaType(pMediaType);
            // ...
        }
    }
    if(InputMediaSampleProperties.pMediaType)
    {
        m_pInputPin->SetMediaType(InputMediaSampleProperties.pMediaType);
        m_pOutputPin->SetMediaType(InputMediaSampleProperties.pMediaType);
        // ...
    }
    DeliverMediaSample(pMemInputPin, pInputMediaSample);
}