Filter是Directshow中最基本的概念。Directshow使用filter graph來管理filter。filter graph是filter的容器。

Filter一般由一個或者幾個Pin組成。filter之間通過Pin來連線，組成一條鏈。

PIN也是一種COM元件，每一個PIN都實現了IPin介面。

試圖連結的兩個Pin必須在一個filter graph中。

連線過程如下：

1.Filter Graph Manager在輸出pin上呼叫IPin::Connect

2.如果輸出Pin接受連線，則呼叫IPin::ReceiveConnection

3.如果輸入Pin也接受此次連線，則雙方連線成功

首先來分析基類函式CBasePin的Connect實現：

CBasePin繼承了IPin。

<pre name="code" class="cpp">HRESULT __stdcall CBasePin::Connect(IPin * pReceivePin, __in_opt const AM_MEDIA_TYPE *pmt   // optional media type)
{
    CheckPointer(pReceivePin,E_POINTER);
    ValidateReadPtr(pReceivePin,sizeof(IPin));
    CAutoLock cObjectLock(m_pLock);
    DisplayPinInfo(pReceivePin);

    /* 檢查該Pin是否早已連線 */
    if (m_Connected) {
        DbgLog((LOG_TRACE, CONNECT_TRACE_LEVEL, TEXT("Already connected")));
        return VFW_E_ALREADY_CONNECTED;
    }

    /*一般Filter只在停止狀態下接受連線*/
    if (!IsStopped() && !m_bCanReconnectWhenActive) {
        return VFW_E_NOT_STOPPED;
    }

    /*開始媒體型別檢查，找出一種雙方均支援的型別*/
    const CMediaType * ptype = (CMediaType*)pmt;
    HRESULT hr = AgreeMediaType(pReceivePin, ptype);
    if (FAILED(hr)) {
        DbgLog((LOG_TRACE, CONNECT_TRACE_LEVEL, TEXT("Failed to agree type")));

        // Since the procedure is already returning an error code, there
        // is nothing else this function can do to report the error.
        EXECUTE_ASSERT( SUCCEEDED( BreakConnect() ) );

#ifdef DXMPERF
        PERFLOG_CONNECT( (IPin *) this, pReceivePin, hr, pmt );
#endif // DXMPERF

        return hr;
    }

    DbgLog((LOG_TRACE, CONNECT_TRACE_LEVEL, TEXT("Connection succeeded")));

#ifdef DXMPERF
    PERFLOG_CONNECT( (IPin *) this, pReceivePin, NOERROR, pmt );
#endif // DXMPERF

    return NOERROR;
}
 

事實上，以上函式並沒有進行真正的連線，只是進行了型別檢查和狀態檢查。並且這個函式是從輸出Pin進入的。

Connect兩個引數分別代表：輸出Pin試圖連結的輸入Pin和指定連線用的媒體型別。

真正的連結是CBase::AgreeMediaType來實現的。

<pre name="code" class="cpp">HRESULT CBasePin::AgreeMediaType(IPin *pReceivePin, const CMediaType *pmt)
{
    ASSERT(pReceivePin);
    IEnumMediaTypes *pEnumMediaTypes = NULL;

    // 判斷pmt是不是一個完全指定的媒體型別
    if ( (pmt != NULL) && (!pmt->IsPartiallySpecified())) {
	//用這個指定的媒體型別去做連線，如果失敗不做任何處理。
        return AttemptConnection(pReceivePin, pmt);
    }


    /* 進行pin上支援的媒體型別的列舉 ，開始媒體型別的協商過程*/
    HRESULT hrFailure = VFW_E_NO_ACCEPTABLE_TYPES;

    for (int i = 0; i < 2; i++) {
        HRESULT hr;
        if (i == (int)m_bTryMyTypesFirst) {
            hr = pReceivePin->EnumMediaTypes(&pEnumMediaTypes);
        } else {
            hr = EnumMediaTypes(&pEnumMediaTypes);
        }
        if (SUCCEEDED(hr)) {
            ASSERT(pEnumMediaTypes);
            hr = TryMediaTypes(pReceivePin,pmt,pEnumMediaTypes);
            pEnumMediaTypes->Release();
            if (SUCCEEDED(hr)) {
                return NOERROR;
            } else {
                // try to remember specific error codes if there are any
                if ((hr != E_FAIL) &&
                    (hr != E_INVALIDARG) &&
                    (hr != VFW_E_TYPE_NOT_ACCEPTED)) {
                    hrFailure = hr;
                }
            }
        }
    }

    return hrFailure;
}
 

從AgreeMediaType看到，該函式首先判斷媒體型別的有效性，如果pmt是一種完全指定的媒體型別，那麼就以這種媒體型別呼叫內部

函式AttempConnection進行連線。

但是如果pmt是一個空指標或者不是完全指定的媒體型別，那麼真正的協商過程也就開始了。注意，for迴圈的次數為2，輸出Pin上的成員變數

m_bTryMyTypesFirst初始值為false，也就是說，連線過程進行到這裡，會首先得到輸入pin上的媒體型別列舉器的試連線，如果連線不成功，再去得到

輸出Pin上的媒體型別列舉器的試連線。

下面看一下TryMediaTypes函式的實現：

HRESULT CBasePin::TryMediaTypes(IPin *pReceivePin, __in_opt const CMediaType *pmt, IEnumMediaTypes *pEnum)
{
    /* 復位列舉器內部狀態 */

    HRESULT hr = pEnum->Reset();
    if (FAILED(hr)) {
        return hr;
    }

    CMediaType *pMediaType = NULL;
    ULONG ulMediaCount = 0;

    // attempt to remember a specific error code if there is one
    HRESULT hrFailure = S_OK;

    for (;;) {

        /* Retrieve the next media type NOTE each time round the loop the
           enumerator interface will allocate another AM_MEDIA_TYPE structure
           If we are successful then we copy it into our output object, if
           not then we must delete the memory allocated before returning */

        hr = pEnum->Next(1, (AM_MEDIA_TYPE**)&pMediaType,&ulMediaCount);
        if (hr != S_OK) {
            if (S_OK == hrFailure) {
                hrFailure = VFW_E_NO_ACCEPTABLE_TYPES;
            }
            return hrFailure;
        }


        ASSERT(ulMediaCount == 1);
        ASSERT(pMediaType);

        // 檢查當前列舉得到的型別是否與不完全指定的型別引數匹配

        if (pMediaType &&
            ((pmt == NULL) ||
            pMediaType->MatchesPartial(pmt))) {
<span style="white-space:pre">	</span>    //進行試連線
            hr = AttemptConnection(pReceivePin, pMediaType);

            // attempt to remember a specific error code
            if (FAILED(hr) &&
            SUCCEEDED(hrFailure) &&
            (hr != E_FAIL) &&
            (hr != E_INVALIDARG) &&
            (hr != VFW_E_TYPE_NOT_ACCEPTED)) {
                hrFailure = hr;
            }
        } else {
            hr = VFW_E_NO_ACCEPTABLE_TYPES;
        }

        if(pMediaType) {
            DeleteMediaType(pMediaType);
            pMediaType = NULL;
        }

        if (S_OK == hr) {
            return hr;
        }
    }
}

當連線程序進入TryMediaTypes函式後，會使用媒體型別列舉器列舉Pin上提供的所有的媒體型別，然後一種一種的進行試連線。如果有一種成功，則整個Pin連線成功。

最後需要看一下AttempConnection函式的跟Pin連線相關的虛擬函式的呼叫的順序，對與自己實現filter有指導意義：

<pre name="code" class="cpp">HRESULT CBasePin::AttemptConnection(IPin* pReceivePin,      // connect to this pin
  					const CMediaType* pmt   // using this type
)
{
    // The caller should hold the filter lock becasue this function
    // uses m_Connected.  The caller should also hold the filter lock
    // because this function calls SetMediaType(), IsStopped() and
    // CompleteConnect().
    ASSERT(CritCheckIn(m_pLock));

    // Check that the connection is valid  -- need to do this for every
    // connect attempt since BreakConnect will undo it.
    HRESULT hr = CheckConnect(pReceivePin);
    if (FAILED(hr)) {
        DbgLog((LOG_TRACE, CONNECT_TRACE_LEVEL, TEXT("CheckConnect failed")));

        // Since the procedure is already returning an error code, there
        // is nothing else this function can do to report the error.
        EXECUTE_ASSERT( SUCCEEDED( BreakConnect() ) );

        return hr;
    }
    //可以顯示媒體型別，有時候挺有用
    DisplayTypeInfo(pReceivePin, pmt);

    /* Pin上的媒體型別檢查 */

    hr = CheckMediaType(pmt);
    if (hr == NOERROR) {

        /*  Make ourselves look connected otherwise ReceiveConnection
            may not be able to complete the connection
        */
        m_Connected = pReceivePin;
        m_Connected->AddRef();
        hr = SetMediaType(pmt);//在Pin上儲存媒體型別
        if (SUCCEEDED(hr)) {
            /* 詢問連線對方Pin是否能接受當前的媒體型別 */

            hr = pReceivePin->ReceiveConnection((IPin *)this, pmt);
	    //連線成功
            if (SUCCEEDED(hr)) {
                /* Complete the connection */

                hr = CompleteConnect(pReceivePin);
                if (SUCCEEDED(hr)) {
                    return hr;
                } else {
                    DbgLog((LOG_TRACE,
                            CONNECT_TRACE_LEVEL,
                            TEXT("Failed to complete connection")));
                    pReceivePin->Disconnect();
                }
            }
        }
    } else {
        // we cannot use this media type

        // return a specific media type error if there is one
        // or map a general failure code to something more helpful
        // (in particular S_FALSE gets changed to an error code)
        if (SUCCEEDED(hr) ||
            (hr == E_FAIL) ||
            (hr == E_INVALIDARG)) {
            hr = VFW_E_TYPE_NOT_ACCEPTED;
        }
    }

    // BreakConnect and release any connection here in case CheckMediaType
    // failed, or if we set anything up during a call back during
    // ReceiveConnection.

    // Since the procedure is already returning an error code, there
    // is nothing else this function can do to report the error.
    EXECUTE_ASSERT( SUCCEEDED( BreakConnect() ) );

    /*  If failed then undo our state */
    if (m_Connected) {
        m_Connected->Release();
        m_Connected = NULL;
    }

    return hr;
}

至此：連線雙方Pin上的使用的媒體型別協商完了，但是並不能真正的傳輸資料。因為記憶體還沒分配唄。

這些均是在輸出Pin上的CompleteConnect中完成的。

CBaseOutPin繼承自CBasePin

HRESULT CBaseOutputPin::CompleteConnect(IPin *pReceivePin)
{
    UNREFERENCED_PARAMETER(pReceivePin);
    return DecideAllocator(m_pInputPin, &m_pAllocator);
}

DecideAllocator就是Pin之間資料傳送所使用的記憶體分配器的協商。在dshow中資料傳輸的單元叫做Sample(也是一種COM元件，管理記憶體用的)。而Sample是由分配器Allocator(也是COM元件)來管理的。連線雙方必須使用同一個分配器，該分配器由誰來建立也需要協商。

<pre name="code" class="cpp">HRESULT CBaseOutputPin::DecideAllocator(IMemInputPin *pPin, __deref_out IMemAllocator **ppAlloc)
{
    HRESULT hr = NOERROR;
    *ppAlloc = NULL;

    // get downstream prop request
    // the derived class may modify this in DecideBufferSize, but
    // we assume that he will consistently modify it the same way,
    // so we only get it once
    ALLOCATOR_PROPERTIES prop;
    ZeroMemory(&prop, sizeof(prop));

    // 詢問輸入Pin對於分配器的需求
    pPin->GetAllocatorRequirements(&prop);

    // if he doesn't care about alignment, then set it to 1
    if (prop.cbAlign == 0) {
        prop.cbAlign = 1;
    }

    /* 詢問輸入Pin是否提供一個分配器 */

    hr = pPin->GetAllocator(ppAlloc);
    if (SUCCEEDED(hr)) {
       //決定Sample使用的記憶體的大小，以及分配器管理的Sample的數量
        hr = DecideBufferSize(*ppAlloc, &prop);
        if (SUCCEEDED(hr)) {
            //通知輸入Pin最終使用的分配器的物件
            hr = pPin->NotifyAllocator(*ppAlloc, FALSE);
            if (SUCCEEDED(hr)) {
                return NOERROR;
            }
        }
    }

    /* 如果輸入Pin上不提供分配器，則必須在輸出Pin上提供 */

    if (*ppAlloc) {
        (*ppAlloc)->Release();
        *ppAlloc = NULL;
    }

    /* 建立一個輸出Pin上的分配器 */

    hr = InitAllocator(ppAlloc);
    if (SUCCEEDED(hr)) {

        // note - the properties passed here are in the same
        // structure as above and may have been modified by
        // the previous call to DecideBufferSize
        hr = DecideBufferSize(*ppAlloc, &prop);
        if (SUCCEEDED(hr)) {
            hr = pPin->NotifyAllocator(*ppAlloc, FALSE);
            if (SUCCEEDED(hr)) {
                return NOERROR;
            }
        }
    }

    /* Likewise we may not have an interface to release */

    if (*ppAlloc) {
        (*ppAlloc)->Release();
        *ppAlloc = NULL;
    }
    return hr;
}

當Pin上的資料傳送記憶體分配器協商成功後，並沒有馬上分配記憶體，實際上是等Filter Graph執行後，呼叫輸出Pin上的Active函式時進行的。

HRESULT CBaseOutputPin::Active(void)
{
    if (m_pAllocator == NULL) {
        return VFW_E_NO_ALLOCATOR;
    }
    return m_pAllocator->Commit();
}

至此整個Pin的連結才算真正完成。