Reconnecting Your Input to Ensure Specific Output Types
[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]
Filters implement the IAMStreamConfig::SetFormat method to set the audio or video format before the filter's pins are connected. If your output pin is already connected and you can provide a new type, then reconnect your pin, but only if the other filter can accept the new type. If the other filter cannot accept the media type, fail the call to SetFormat and leave your connection alone.
A transform filter may not have any preferred output types unless their input pin is connected. In that case, the SetFormat and IAMStreamConfig::GetStreamCaps methods should return VFW_E_NOT_CONNECTED until the input pin is connected. Otherwise, these methods can function as usual.
In certain cases it is useful to reconnect pins when you are offering a format on an established connection. For example, suppose that a filter can compress 24-bit RGB video into format X, and that it can compress 8-bit RGB video into format Y. The output pin can do either of the following:
- Always offer both X and Y in GetStreamCaps, and always accept both X and Y in SetFormat.
- Offer and accept just format X if the input type is 24-bit RGB. Offer and accept just format Y if the input type 8-bit RGB. Fail both methods if the input pin is not connected.
In either case, you will need some reconnecting code that looks like this:
HRESULT MyOutputPin::CheckMediaType(const CMediaType *pmtOut)
{
// Fail if the input pin is not connected.
if (!m_pFilter->m_pInput->IsConnected()) {
return VFW_E_NOT_CONNECTED;
}
// (Not shown: Reject any media types that you know in advance your
// filter cannot use. Check the major type and subtype GUIDs.)
// (Not shown: If SetFormat was previously called, check whether
// pmtOut exactly matches the format that was specified in SetFormat.
// Return S_OK if they match, or VFW_E_INVALIDMEDIATYPE otherwise.)
// Now do the normal check for this media type.
HRESULT hr;
hr = m_pFilter->CheckTransform(
&m_pFilter->m_pInput->CurrentMediaType(), // The input type.
pmtOut // The proposed output type.
);
if (hr == S_OK)
{
// This format is compatible with the current input type.
return S_OK;
}
// This format is not compatible with the current input type.
// Maybe we can reconnect the input pin with a new input type.
// Enumerate the upstream filter's preferred output types, and
// see if one of them will work.
CMediaType *pmtEnum;
BOOL fFound = FALSE;
IEnumMediaTypes *pEnum;
hr = m_pFilter->m_pInput->GetConnected()->EnumMediaTypes(&pEnum);
if (hr != S_OK)
{
return E_FAIL;
}
while (hr = pEnum->Next(1, (AM_MEDIA_TYPE **)&pmtEnum, NULL), hr == S_OK)
{
// Check this input type against the proposed output type.
hr = m_pFilter->CheckTransform(pmtEnum, pmtOut);
if (hr != S_OK)
{
DeleteMediaType(pmtEnum);
continue; // Try the next one.
}
// This input type is a possible candidate. But, we have to make
// sure that the upstream filter can switch to this type.
hr = m_pFilter->m_pInput->GetConnected()->QueryAccept(pmtEnum);
if (hr != S_OK)
{
// The upstream filter will not switch to this type.
DeleteMediaType(pmtEnum);
continue; // Try the next one.
}
fFound = TRUE;
DeleteMediaType(pmtEnum);
break;
}
pEnum->Release();
if (fFound)
{
// This output type is OK, but if we are asked to use it, we will
// need to reconnect our input pin. (See SetFormat, below.)
return S_OK;
}
else
{
return VFW_E_INVALIDMEDIATYPE;
}
}
HRESULT MyOutputPin::SetFormat(AM_MEDIA_TYPE *pmt)
{
CheckPointer(pmt, E_POINTER);
HRESULT hr;
// Hold the filter state lock, to make sure that streaming isn't
// in the middle of starting or stopping:
CAutoLock cObjectLock(&m_pFilter->m_csFilter);
// Cannot set the format unless the filter is stopped.
if (m_pFilter->m_State != State_Stopped)
{
return VFW_E_NOT_STOPPED;
}
// The set of possible output formats depends on the input format,
// so if the input pin is not connected, return a failure code.
if (!m_pFilter->m_pInput->IsConnected())
{
return VFW_E_NOT_CONNECTED;
}
// If the pin is already using this format, there's nothing to do.
if (IsConnected() && CurrentMediaType() == *pmt)
{
return S_OK;
}
// See if this media type is acceptable.
if ((hr = CheckMediaType((CMediaType *)pmt)) != S_OK)
{
return hr;
}
// If we're connected to a downstream filter, we have to make
// sure that the downstream filter accepts this media type.
if (IsConnected())
{
hr = GetConnected()->QueryAccept(pmt);
if (hr != S_OK)
{
return VFW_E_INVALIDMEDIATYPE;
}
}
// Now make a note that from now on, this is the only format allowed,
// and refuse anything but this in the CheckMediaType code above.
// Changing the format means reconnecting if necessary.
if (IsConnected())
{
m_pFilter->m_pGraph->Reconnect(this);
}
return NOERROR;
}
// Override CTransformFilter::SetMediaType to reconnect the input pin.
// This method is called immediately after the media type is set on a pin.
HRESULT MyFilter::SetMediaType(
PIN_DIRECTION direction,
const CMediaType *pmt
)
{
HRESULT hr;
if (direction == PINDIR_OUTPUT)
{
// Before we set the output type, we might need to reconnect
// the input pin with a new type.
if (m_pInput && m_pInput->IsConnected())
{
// Check if the current input type is compatible.
hr = CheckTransform(
&m_pInput->CurrentMediaType(),
&m_pOutput->CurrentMediaType());
if (SUCCEEDED(hr))
{
return S_OK;
}
// Otherwise, we need to reconnect the input pin.
// Note: The CheckMediaType method has already called
// QueryAccept on the upstream filter.
hr = m_pGraph->Reconnect(m_pInput);
return hr;
}
}
return S_OK;
}