シーク、早送り、リバースプレイ

[アーティクル]
06/12/2023

このトピックでは、再生にメディアセッションを使用する場合のシークとレートの変更を管理するコード例を示します。

再生にメディアセッションを使用する場合、アプリケーションは次のように再生速度をシークして変更できます。

シークするには、 IMFMediaSession::Start を呼び出し、シーク位置を指定します。
再生レートを変更するには、「レート制御」の説明に従って IMFRateControl インターフェイスを使用します。
シーク中に更新されたビデオフレームを取得するには、 IMFMediaSession::Start を呼び出す前に再生速度を 0 に設定します。この操作は スクラブと呼ばれます。 (「スクラブを実行する方法」を参照してください)。

ただし、最適なユーザーエクスペリエンスを作成するには、次の動作を考慮する必要があります。

シークは非同期であり、メディアセッションは FIFO キューのすべてのシーク要求をキューに入れます。複数のシーク要求を送信すると、UI が実際の再生状態よりも前に実行される可能性があります。たとえば、アプリケーションがシークバーを実装するとします。ユーザーがシークバーを前後にドラッグすると、前方シークが実行されている間にラグが発生する可能性があります。シークの進行中は、ユーザーのシーク要求をキャッシュする必要があります。現在のシーク操作が完了したら、ユーザーの最新のシーク要求を送信し、他の要求を破棄します。
一部の転送状態では、一部のレート遷移は許可されません。たとえば、再生中に前方再生から逆再生への切り替えは許可されません。サポートされている遷移については、「 IMFRateControl::SetRate」を参照してください。レートの変更も非同期です。

次のヘルパークラスを使用して、シーク要求とレート変更を管理できます。非同期操作が保留中の間、クラスはシーク要求またはレート変更要求をキャッシュします。現在の操作が完了すると、クラスは最新の要求 (存在する場合) を送信します。クラスは、無効なレート変更を回避するためにトランスポート状態も管理します。

PlayerSeeking クラスの宣言を次に示します。

// Implements seeking and rate control functionality.

/*
    Usage:

    - Call SetTopology when you get the MF_TOPOSTATUS_READY session event.
    - Call SessionEvent for each session event.
    - Call Clear before closing the Media Session.
    - To coordinate rate-change requests with transport state, delegate all 
      stop, pause, and play commands to the PlayerSeeking class.

*/

class PlayerSeeking 
{
    class CritSec
    {
    private:
        CRITICAL_SECTION m_criticalSection;
    public:
        CritSec()
        {
            InitializeCriticalSection(&m_criticalSection);
        }
        ~CritSec()
        {
            DeleteCriticalSection(&m_criticalSection);
        }
        void Lock()
        {
            EnterCriticalSection(&m_criticalSection);
        }
        void Unlock()
        {
            LeaveCriticalSection(&m_criticalSection);
        }
    };

    class AutoLock
    {
    private:
        CritSec *m_pCriticalSection;
    public:
        AutoLock(CritSec& crit)
        {
            m_pCriticalSection = &crit;
            m_pCriticalSection->Lock();
        }
        ~AutoLock()
        {
            m_pCriticalSection->Unlock();
        }
    };

public:

    PlayerSeeking();
    virtual ~PlayerSeeking();

    HRESULT SetTopology(IMFMediaSession *pSession, IMFTopology *pTopology);
    HRESULT Clear();
    HRESULT SessionEvent(MediaEventType type, HRESULT hrStatus, IMFMediaEvent *pEvent);

    HRESULT CanSeek(BOOL *pbCanSeek);
    HRESULT GetDuration(MFTIME *phnsDuration);
    HRESULT GetCurrentPosition(MFTIME *phnsPosition);
    HRESULT SetPosition(MFTIME hnsPosition);

    HRESULT CanScrub(BOOL *pbCanScrub);
    HRESULT Scrub(BOOL bScrub);

    HRESULT CanFastForward(BOOL *pbCanFF);
    HRESULT CanRewind(BOOL *pbCanRewind);
    HRESULT SetRate(float fRate);
    HRESULT FastForward();
    HRESULT Rewind();

    HRESULT Start();
    HRESULT Pause();
    HRESULT Stop();

private:

    enum Command
    {
        CmdNone = 0,
        CmdStop,
        CmdStart,
        CmdPause,
        CmdSeek,
    };

    HRESULT SetPositionInternal(const MFTIME &hnsPosition);
    HRESULT CommitRateChange(float fRate, BOOL bThin);
    float   GetNominalRate();

    HRESULT OnSessionStart(HRESULT hr);
    HRESULT OnSessionStop(HRESULT hr);
    HRESULT OnSessionPause(HRESULT hr);
    HRESULT OnSessionEnded(HRESULT hr);

    HRESULT UpdatePendingCommands(Command cmd);

private:

    // Describes the current or requested state, with respect to seeking and 
    // playback rate.
    struct SeekState
    {
        Command command;
        float   fRate;      // Playback rate
        BOOL    bThin;      // Thinned playback?
        MFTIME  hnsStart;   // Start position
    };

    BOOL        m_bPending;     // Is a request pending?

    SeekState   m_state;        // Current nominal state.
    SeekState   m_request;      // Pending request.

    CritSec     m_critsec;      // Protects the seeking and rate-change states.

    DWORD       m_caps;         // Session caps.
    BOOL        m_bCanScrub;    // Does the current session support rate = 0.

    MFTIME      m_hnsDuration;  // Duration of the current presentation.
    float       m_fPrevRate;

    IMFMediaSession         *m_pSession;
    IMFRateControl          *m_pRate;
    IMFRateSupport          *m_pRateSupport;
    IMFPresentationClock    *m_pClock;
};

PlayerSeeking クラスの実装を次に示します。

#include <assert.h>
#include <windows.h>
#include <mfapi.h>
#include <mfidl.h>
#include <mferror.h>
#include "seeking.h"

template <class T> void SafeRelease(T **ppT)
{
    if (*ppT)
    {
        (*ppT)->Release();
        *ppT = NULL;
    }
}

#define CMD_PENDING      0x01
#define CMD_PENDING_SEEK 0x02
#define CMD_PENDING_RATE 0x04

// Given a topology, returns a pointer to the presentation descriptor.

HRESULT GetPresentationDescriptorFromTopology(
    IMFTopology *pTopology,
    IMFPresentationDescriptor **ppPD
    )
{
    HRESULT hr = S_OK;

    IMFCollection *pCollection = NULL;
    IUnknown *pUnk = NULL;
    IMFTopologyNode *pNode = NULL;
    IMFPresentationDescriptor *pPD = NULL;

    // Get the collection of source nodes from the topology.
    hr = pTopology->GetSourceNodeCollection(&pCollection);
    if (FAILED(hr))
    {
        goto done;
    }

    // Any of the source nodes should have the PD, so take the first
    // object in the collection.

    hr = pCollection->GetElement(0, &pUnk);
    if (FAILED(hr))
    {
        goto done;
    }

    hr = pUnk->QueryInterface(IID_PPV_ARGS(&pNode));
    if (FAILED(hr))
    {
        goto done;
    }

    // Get the PD, which is stored as an attribute.
    hr = pNode->GetUnknown(
        MF_TOPONODE_PRESENTATION_DESCRIPTOR, IID_PPV_ARGS(&pPD));
    if (FAILED(hr))
    {
        goto done;
    }

    *ppPD = pPD;
    (*ppPD)->AddRef();

done:
    SafeRelease(&pCollection);
    SafeRelease(&pUnk);
    SafeRelease(&pNode);
    SafeRelease(&pPD);
    return hr;
}


PlayerSeeking::PlayerSeeking()
    : m_pClock(NULL),
      m_pSession(NULL),
      m_pRate(NULL),
      m_pRateSupport(NULL),
      m_bPending(FALSE)
{
    Clear();
}

PlayerSeeking::~PlayerSeeking()
{
    SafeRelease(&m_pClock);
    SafeRelease(&m_pSession);
    SafeRelease(&m_pRate);
    SafeRelease(&m_pRateSupport);
}

// Clears any resources for the current topology.

HRESULT PlayerSeeking::Clear()
{
    m_caps = 0;
    m_bCanScrub = FALSE;

    m_hnsDuration = 0;
    m_fPrevRate = 1.0f;

    m_bPending = FALSE;

    SafeRelease(&m_pClock);
    SafeRelease(&m_pSession);
    SafeRelease(&m_pRate);
    SafeRelease(&m_pRateSupport);

    ZeroMemory(&m_state, sizeof(m_state));
    m_state.command = CmdStop;
    m_state.fRate = 1.0f;

    ZeroMemory(&m_request, sizeof(m_request));
    m_request.command = CmdNone;
    m_request.fRate = 1.0f;

    return S_OK;
}


// Call when the full playback topology is ready.

HRESULT PlayerSeeking::SetTopology(IMFMediaSession *pSession, IMFTopology *pTopology)
{
    HRESULT hr = S_OK;
    HRESULT hrTmp = S_OK;   // For non-critical failures.

    IMFClock *pClock = NULL;
    IMFPresentationDescriptor *pPD = NULL;

    Clear();

    // Get the session capabilities.
    hr = pSession->GetSessionCapabilities(&m_caps);
    if (FAILED(hr))
    {
        goto done;
    }

    // Get the presentation descriptor from the topology.
    hr = GetPresentationDescriptorFromTopology(pTopology, &pPD);
    if (FAILED(hr))
    {
        goto done;
    }

    // Get the duration from the presentation descriptor (optional)
    (void)pPD->GetUINT64(MF_PD_DURATION, (UINT64*)&m_hnsDuration);

    // Get the presentation clock (optional)
    hrTmp = pSession->GetClock(&pClock);
    if (SUCCEEDED(hrTmp))
    {
        hr = pClock->QueryInterface(IID_PPV_ARGS(&m_pClock));
        if (FAILED(hr))
        {
            goto done;
        }
    }

    // Get the rate control interface (optional)
    hrTmp = MFGetService(
        pSession, MF_RATE_CONTROL_SERVICE, IID_PPV_ARGS(&m_pRate));

    // Get the rate support interface (optional)
    if (SUCCEEDED(hrTmp))
    {
        hrTmp = MFGetService(
            pSession, MF_RATE_CONTROL_SERVICE, IID_PPV_ARGS(&m_pRateSupport));
    }
    if (SUCCEEDED(hrTmp))
    {
        // Check if rate 0 (scrubbing) is supported.
        hrTmp = m_pRateSupport->IsRateSupported(TRUE, 0, NULL);
    }
    if (SUCCEEDED(hrTmp))
    {
        m_bCanScrub = TRUE;
    }

    // if m_pRate is NULL, m_bCanScrub must be FALSE.
    assert(m_pRate || !m_bCanScrub); 

    // Cache a pointer to the session.
    m_pSession = pSession;
    m_pSession->AddRef();

done:
    SafeRelease(&pPD);
    SafeRelease(&pClock);
    return hr;
}

// Call when media session fires an event.

HRESULT PlayerSeeking::SessionEvent(
    MediaEventType type, 
    HRESULT hrStatus, 
    IMFMediaEvent *pEvent
    )
{
    PROPVARIANT var;
    HRESULT hr = S_OK;

    switch (type)
    {
    case MESessionStarted:
        OnSessionStart(hrStatus);
        break;

    case MESessionStopped:
        OnSessionStop(hrStatus);
        break;

    case MESessionPaused:
        OnSessionPause(hrStatus);
        break;

    case MESessionRateChanged:
        // If the rate change succeeded, we've already got the rate
        // cached. If it failed, try to get the actual rate.
        if (FAILED(hrStatus))
        {
            PropVariantInit(&var);

            hr = pEvent->GetValue(&var);

            if (SUCCEEDED(hr) && (var.vt == VT_R4))
            {
                m_state.fRate = var.fltVal;
            }
        }
        break;

    case MESessionEnded:
        OnSessionEnded(hrStatus);
        break;

    case MESessionCapabilitiesChanged:
        // The session capabilities changed. Get the updated capabilities.
        m_caps = MFGetAttributeUINT32(pEvent, MF_EVENT_SESSIONCAPS, m_caps);
        break;
    }
    return S_OK;
}


// Starts playback.

HRESULT PlayerSeeking::Start()
{
    HRESULT hr = S_OK;

    AutoLock lock(m_critsec);

    // If another operation is pending, cache the request.
    // Otherwise, start the media session.
    if (m_bPending)
    {
       m_request.command = CmdStart;
    }
    else
    {
        PROPVARIANT varStart;
        PropVariantInit(&varStart);

        hr =  m_pSession->Start(NULL, &varStart);

        m_state.command = CmdStart;
        m_bPending = CMD_PENDING;
    }
    return hr;
}


// Pauses playback.

HRESULT PlayerSeeking::Pause()
{
    HRESULT hr = S_OK;

    AutoLock lock(m_critsec);

    // If another operation is pending, cache the request.
    // Otherwise, pause the media session.
    if (m_bPending)
    {
        m_request.command = CmdPause;
    }
    else
    {
        hr =  m_pSession->Pause();

        m_state.command = CmdPause;
        m_bPending = CMD_PENDING;
    }
    return hr;
}


// Stops playback.

HRESULT PlayerSeeking::Stop()
{
    HRESULT hr = S_OK;

    AutoLock lock(m_critsec);

    // If another operation is pending, cache the request.
    // Otherwise, stop the media session.
    if (m_bPending)
    {
        m_request.command = CmdStop;
    }
    else
    {
        hr =  m_pSession->Stop();

        m_state.command = CmdStop;
        m_bPending = CMD_PENDING;
    }
    return hr;
}

// Queries whether the current session supports seeking.

HRESULT PlayerSeeking::CanSeek(BOOL *pbCanSeek)
{
    if (pbCanSeek == NULL)
    {
        return E_POINTER;
    }

    // Note: The MFSESSIONCAP_SEEK flag is sufficient for seeking. However, to
    // implement a seek bar, an application also needs the duration (to get 
    // the valid range) and a presentation clock (to get the current position).

    *pbCanSeek = (
        ((m_caps & MFSESSIONCAP_SEEK) == MFSESSIONCAP_SEEK) &&
        (m_hnsDuration > 0) &&
        (m_pClock != NULL)
        );

    return S_OK;
}


// Gets the duration of the current presentation.

HRESULT PlayerSeeking::GetDuration(MFTIME *phnsDuration)
{
    if (phnsDuration == NULL)
    {
        return E_POINTER;
    }

    *phnsDuration = m_hnsDuration;

    if (m_hnsDuration == 0)
    {
        return MF_E_NO_DURATION;
    }
    else
    {
        return S_OK;
    }
}

// Gets the current playback position.

HRESULT PlayerSeeking::GetCurrentPosition(MFTIME *phnsPosition)
{
    if (phnsPosition == NULL)
    {
        return E_POINTER;
    }

    HRESULT hr = S_OK;

    AutoLock lock(m_critsec);

    if (m_pClock == NULL)
    {
        return MF_E_NO_CLOCK;
    }

    // Return, in order:
    // 1. Cached seek request (nominal position).
    // 2. Pending seek operation (nominal position).
    // 3. Presentation time (actual position).

    if (m_request.command == CmdSeek)
    {
        *phnsPosition = m_request.hnsStart;
    }
    else if (m_bPending & CMD_PENDING_SEEK)
    {
        *phnsPosition = m_state.hnsStart;
    }
    else
    {
        hr = m_pClock->GetTime(phnsPosition);
    }

    return hr;
}


// Sets the current playback position.

HRESULT PlayerSeeking::SetPosition(MFTIME hnsPosition)
{
    AutoLock lock(m_critsec);

    HRESULT hr = S_OK;

    if (m_bPending)
    {
        // Currently seeking or changing rates, so cache this request.
        m_request.command = CmdSeek;
        m_request.hnsStart = hnsPosition;
    }
    else
    {
        hr = SetPositionInternal(hnsPosition);
    }

    return hr;
}


// Queries whether the current session supports scrubbing.

HRESULT PlayerSeeking::CanScrub(BOOL *pbCanScrub)
{
    if (pbCanScrub == NULL)
    {
        return E_POINTER;
    }

    *pbCanScrub = m_bCanScrub;
    return S_OK;
}


// Enables or disables scrubbing.

HRESULT PlayerSeeking::Scrub(BOOL bScrub)
{
    // Scrubbing is implemented as rate = 0.

    AutoLock lock(m_critsec);

    if (!m_pRate)
    {
        return MF_E_INVALIDREQUEST;
    }
    if (!m_bCanScrub)
    {
        return MF_E_INVALIDREQUEST;
    }

    HRESULT hr = S_OK;

    if (bScrub)
    {
        // Enter scrubbing mode. Cache the current rate.

        if (GetNominalRate() != 0)
        {
            m_fPrevRate = m_state.fRate;
        }

        hr = SetRate(0.0f);
    }
    else
    {
        // Leaving scrubbing mode. Restore the old rate.

        if (GetNominalRate() == 0)
        {
            hr = SetRate(m_fPrevRate);
        }
    }

    return hr;
}


// Queries whether the current session supports fast-forward.

HRESULT PlayerSeeking::CanFastForward(BOOL *pbCanFF)
{
    if (pbCanFF == NULL)
    {
        return E_POINTER;
    }

    *pbCanFF = 
        ((m_caps & MFSESSIONCAP_RATE_FORWARD) == MFSESSIONCAP_RATE_FORWARD);
    return S_OK;
}


// Queries whether the current session supports rewind (reverse play).

HRESULT PlayerSeeking::CanRewind(BOOL *pbCanRewind)
{
    if (pbCanRewind == NULL)
    {
        return E_POINTER;
    }

    *pbCanRewind = 
        ((m_caps & MFSESSIONCAP_RATE_REVERSE) == MFSESSIONCAP_RATE_REVERSE);
    return S_OK;
}


// Switches to fast-forward playback, as follows:
// - If the current rate is < 0 (reverse play), switch to 1x speed.
// - Otherwise, double the current playback rate.
//
// Note: This method is for convenience; the application can also call SetRate.

HRESULT PlayerSeeking::FastForward()
{
    if (!m_pRate)
    {
        return MF_E_INVALIDREQUEST;
    }

    HRESULT hr = S_OK;
    float   fTarget = GetNominalRate() * 2;

    if (fTarget <= 0.0f)
    {
        fTarget = 1.0f;
    }

    hr = SetRate(fTarget);

    return hr;
}


// Switches to reverse playback, as follows:
// - If the current rate is > 0 (forward playback), switch to -1x speed.
// - Otherwise, double the current (reverse) playback rate.
//
// Note: This method is for convenience; the application can also call SetRate.

HRESULT PlayerSeeking::Rewind()
{
    if (!m_pRate)
    {
        return MF_E_INVALIDREQUEST;
    }

    HRESULT hr = S_OK;
    float   fTarget = GetNominalRate() * 2;

    if (fTarget >= 0.0f)
    {
        fTarget = -1.0f;
    }

    hr = SetRate(fTarget);

    return hr;
}


// Sets the playback rate.

HRESULT PlayerSeeking::SetRate(float fRate)
{
    HRESULT hr = S_OK;
    BOOL bThin = FALSE;

    AutoLock lock(m_critsec);

    if (fRate == GetNominalRate())
    {
        return S_OK; // no-op
    }

    if (m_pRateSupport == NULL)
    {
        return MF_E_INVALIDREQUEST;
    }

    // Check if this rate is supported. Try non-thinned playback first,
    // then fall back to thinned playback.

    hr = m_pRateSupport->IsRateSupported(FALSE, fRate, NULL);

    if (FAILED(hr))
    {
        bThin = TRUE;
        hr = m_pRateSupport->IsRateSupported(TRUE, fRate, NULL);
    }

    if (FAILED(hr))
    {
        // Unsupported rate.
        return hr;
    }

    // If there is an operation pending, cache the request.
    if (m_bPending)
    {
        m_request.fRate = fRate;
        m_request.bThin = bThin;

        // Remember the current transport state (play, paused, etc), so that we
        // can restore it after the rate change, if necessary. However, if 
        // anothercommand is already pending, that one takes precedent.

        if (m_request.command == CmdNone)
        {
            m_request.command = m_state.command;
        }

    }
    else
    {
        // No pending operation. Commit the new rate.
        hr  = CommitRateChange(fRate, bThin);
    }

    return hr;

}

// Sets the playback position.

HRESULT PlayerSeeking::SetPositionInternal(const MFTIME &hnsPosition)
{
    assert (!m_bPending);

    if (m_pSession == NULL)
    {
        return MF_E_INVALIDREQUEST;
    }

    HRESULT hr = S_OK;

    PROPVARIANT varStart;
    varStart.vt = VT_I8;
    varStart.hVal.QuadPart = hnsPosition;

    hr =  m_pSession->Start(NULL, &varStart);

    if (SUCCEEDED(hr))
    {
        // Store the pending state.
        m_state.command = CmdStart;
        m_state.hnsStart = hnsPosition;
        m_bPending = CMD_PENDING_SEEK;
    }
    return hr;
}




// Sets the playback rate.

HRESULT PlayerSeeking::CommitRateChange(float fRate, BOOL bThin)
{
    assert (!m_bPending);

    // Caller holds the lock.

    HRESULT hr = S_OK;
    MFTIME  hnsSystemTime = 0;
    MFTIME  hnsClockTime = 0;

    Command cmdNow = m_state.command;

    IMFClock *pClock = NULL;

    // Allowed rate transitions:

    // Positive <-> negative:   Stopped
    // Negative <-> zero:       Stopped
    // Postive <-> zero:        Paused or stopped

    if ((fRate > 0 && m_state.fRate <= 0) || (fRate < 0 && m_state.fRate >= 0))
    {
        // Transition to stopped.
        if (cmdNow == CmdStart)
        {
            // Get the current clock position. This will be the restart time.
            hr = m_pSession->GetClock(&pClock);
            if (FAILED(hr))
            {
                goto done;
            }

            (void)pClock->GetCorrelatedTime(0, &hnsClockTime, &hnsSystemTime);

            assert(hnsSystemTime != 0);

            // Stop and set the rate
            hr = Stop();
            if (FAILED(hr))
            {
                goto done;
            }

            // Cache Request: Restart from stop.
            m_request.command = CmdSeek;
            m_request.hnsStart = hnsClockTime;
        }
        else if (cmdNow == CmdPause)
        {
            // The current state is paused.

            // For this rate change, the session must be stopped. However, the 
            // session cannot transition back from stopped to paused. 
            // Therefore, this rate transition is not supported while paused.

            hr = MF_E_UNSUPPORTED_STATE_TRANSITION;
            goto done;
        }
    }
    else if (fRate == 0 && m_state.fRate != 0)
    {
        if (cmdNow != CmdPause)
        {
           // Transition to paused.

            // This transition requires the paused state.

            // Pause and set the rate.
            hr = Pause();
            if (FAILED(hr))
            {
                goto done;
            }

            // Request: Switch back to current state.
            m_request.command = cmdNow;
        }
    }

    // Set the rate.
    hr = m_pRate->SetRate(bThin, fRate);
    if (FAILED(hr))
    {
        goto done;
    }

    // Adjust our current rate and requested rate.
    m_request.fRate = m_state.fRate = fRate;

done:
    SafeRelease(&pClock);
    return hr;
}


float PlayerSeeking::GetNominalRate()
{
    return m_request.fRate;
}


// Called when playback starts or restarts.

HRESULT PlayerSeeking::OnSessionStart(HRESULT hrStatus)
{
    HRESULT hr = S_OK;

    if (FAILED(hrStatus))
    {
        return hrStatus;
    }

    // The Media Session completed a start/seek operation. Check if there
    // is another seek request pending.
    UpdatePendingCommands(CmdStart);

    return hr;
}


// Called when playback stops.

HRESULT PlayerSeeking::OnSessionStop(HRESULT hrStatus)
{
    HRESULT hr = S_OK;

    if (FAILED(hrStatus))
    {
        return hrStatus;
    }

    // The Media Session completed a transition to stopped. This might occur
    // because we are changing playback direction (forward/rewind). Check if
    // there is a pending rate-change request.

    UpdatePendingCommands(CmdStop);

    return hr;
}


// Called when playback pauses.

HRESULT PlayerSeeking::OnSessionPause(HRESULT hrStatus)
{
    HRESULT hr = S_OK;

    if (FAILED(hrStatus))
    {
        return hrStatus;
    }

    hr = UpdatePendingCommands(CmdPause);

    return hr;
}



// Called when the session ends.

HRESULT PlayerSeeking::OnSessionEnded(HRESULT hr)
{
    // After the session ends, playback starts from position zero. But if the
    // current playback rate is reversed, playback would end immediately 
    // (reversing from position 0). Therefore, reset the rate to 1x.

    if (GetNominalRate() < 0.0f)
    {
        m_state.command = CmdStop;

        hr = CommitRateChange(1.0f, FALSE);
    }

    return hr;
}


// Called after an operation completes.
// This method executes any cached requests.

HRESULT PlayerSeeking::UpdatePendingCommands(Command cmd)
{
    HRESULT hr = S_OK;

    PROPVARIANT varStart;
    PropVariantInit(&varStart);

    AutoLock lock(m_critsec);

    if (m_bPending && m_state.command == cmd)
    {
        m_bPending = FALSE;

        // The current pending command has completed.

        // First look for rate changes.
        if (m_request.fRate != m_state.fRate)
        {
            hr = CommitRateChange(m_request.fRate, m_request.bThin);
            if (FAILED(hr))
            {
                goto done;
            }
        }

        // Now look for seek requests.
        if (!m_bPending)
        {
            switch (m_request.command)
            {
            case CmdNone:
                // Nothing to do.
                break;

            case CmdStart:
                Start();
                break;

            case CmdPause:
                Pause();
                break;

            case CmdStop:
                Stop();
                break;

            case CmdSeek:
                SetPositionInternal(m_request.hnsStart);
                break;
            }
            m_request.command = CmdNone;
        }
    }

done:
    return hr;
}

レート制御
オーディオ/ビデオの再生

次の方法で共有

シーク、早送り、リバースプレイ

フィードバック

その他のリソース

次の方法で共有

シーク、早送り、リバース プレイ

関連トピック

フィードバック

その他のリソース

シーク、早送り、リバースプレイ