Using the DirectShow Video Mixing Renderer 9 filter

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Introduction

This article shows the steps involved in creating and configuring DirectShow’s Video Mixing Renderer Filter 9 (VMR9). The two video streams, one on top of the other, are rendered on a single surface. This surface, in our case, is a PictureBox control. Each stream's alpha value, position and height/width can be adjusted at runtime.

How VMR9 is different

The following diagrams show the difference between rendering two videos with VMR9 and without VMR9.

Without VMR9

We notice that simply rendering two videos will result in two separate Video Renderers, which means that the videos are being played on two separate surfaces.

With VMR9

In this case, the VMR9 filter directs both video streams into its own input pins. This means there is only one renderer, and thus a single rendering surface for both video streams.

The Working

To enhance reusability and readability factors, the functionality of the VMR9 filter has been encapsulated inside a class named myVMR9.

The myVMR9 class

This class has the following private data members:

• VMR9NormalizedRect *r;
• IVMRWindowlessControl9 *pWC;
• IVMRMixerControl9 *pMix;
• IGraphBuilder *pGB;
• IBaseFilter *pVmr;
• IVMRFilterConfig9 *pConfig;
• IMediaControl *pMC;
• IMediaSeeking *pMS;

The constructor

The constructor receives a PictureBox's coordinates of type System::Drawing::Rectangle, along with its handler of type HWND. These two attributes are used by VMR9 for rendering purposes.

public: myVMR9(System::Drawing::Rectangle rect, HWND hwnd)
{
    // initialize video coordinates with normal values
    r = new VMR9NormalizedRect;
    r->left = 0;
    r->top = 0;
    r->right = 1;
    r->bottom = 1;

    pWC = NULL;
    pMix = NULL;
    pGB = NULL;
    pVmr = NULL;
    pConfig = NULL;
    pMC = NULL;
    pMS = NULL;
    // create an instance of the Filter Graph Manager
    CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER, 
        IID_IGraphBuilder, (void **)&pGB);
    // create an instance of the VMR9 filter
    CoCreateInstance(CLSID_VideoMixingRenderer9, NULL, CLSCTX_INPROC,
        IID_IBaseFilter, (void**)&pVmr);
    // add the VMR9 filter to the Graph Manager
    pGB->AddFilter(pVmr, L"Video");    
    // get a pointer to the IVMRFilterConfig9 interface
    pVmr->QueryInterface(IID_IVMRFilterConfig9, (void**)&pConfig);
    // make sure VMR9 is in windowless mode
    pConfig->SetRenderingMode(VMR9Mode_Windowless);
    // get a pointer to the IVMRWindowlessControl9 interface 
    pVmr->QueryInterface(IID_IVMRWindowlessControl9, (void**)&pWC);
    // explicitly convert System::Drawing::Rectangle type to RECT type
    RECT rcDest = {0};
    rcDest.bottom = rect.Bottom;
    rcDest.left = rect.Left;
    rcDest.right = rect.Right;
    rcDest.top = rect.Top;

    // set destination rectangle for the video
    pWC->SetVideoPosition(NULL, &rcDest);

    // specify the container window that the video should be clipped to    
    pWC->SetVideoClippingWindow(hwnd);
    // IVMRMixerControl manipulates video streams
    pVmr->QueryInterface(IID_IVMRMixerControl9, (void**)&pMix);
    // IMediaSeeking seeks to a position in the video stream
    pGB->QueryInterface(IID_IMediaSeeking, (void **)&pMS);
    // IMediaControl controls flow of data through the graph
    pGB->QueryInterface(IID_IMediaControl, (void **)&pMC);
}

The methods

HRESULT play()
{
    pMC->Run(); return
    S_OK;
}

HRESULT pause()
{
    pMC->Pause();
    return S_OK;
}

HRESULT stop()
{
    LONGLONG pos = 0;
    pMC->Stop();
    pMS->SetPositions(&pos, AM_SEEKING_AbsolutePositioning, 
                      NULL,AM_SEEKING_NoPositioning);
    pMC->Pause();
    return S_OK;
}

HRESULT close()
{
    // make sure resources are freed
    SAFE_RELEASE(pWC);
    SAFE_RELEASE(pMix);
    SAFE_RELEASE(pGB);
    SAFE_RELEASE(pVmr);
    SAFE_RELEASE(pConfig);
    SAFE_RELEASE(pMC);
    SAFE_RELEASE(pMS);
    return S_OK;
}

HRESULT setAlpha(DWORD stream, float alpha)
{
    // set alpha of specified video stream
    pMix->SetAlpha(stream, alpha);
    return S_OK;
}

HRESULT setX(DWORD stream, float x)
{
    // video displacement along x-axis
    r->right = x + (r->right - r->left);
    r->left = x;
    pMix->SetOutputRect(stream, r);
    return S_OK;
}

HRESULT setY(DWORD stream, float y)
{
    // video displacement along y-axis
    r->bottom = y + (r->bottom - r->top);
    r->top = y;
    pMix->SetOutputRect(stream, r);
    return S_OK;
}

HRESULT setW(DWORD stream, float w)
{
    // video stretching/shrinking along x-axis
    r->right = r->left + w;
    pMix->SetOutputRect(stream, r);
    return S_OK;
}

HRESULT setH(DWORD stream, float h)
{
    // video stretching/shrinking along y-axis
    r->bottom = r->top + h;
    pMix->SetOutputRect(stream, r);
    return S_OK;
}

HRESULT renderFiles(String* file1, String* file2)
{
    // convert String type to LPCSTR type and render the videos
    LPCTSTR lFile;
    lFile = 
      static_cast<LPCTSTR>(const_cast<void*>(static_cast<const void*>
      (System::Runtime::InteropServices::Marshal::StringToHGlobalAuto(file1))));
    pGB->RenderFile((LPCWSTR)lFile, NULL);
    lFile = 
      static_cast<LPCTSTR>(const_cast<void*>(static_cast<const void*>
      (System::Runtime::InteropServices::Marshal::StringToHGlobalAuto(file2))));
    pGB->RenderFile((LPCWSTR)lFile, NULL);
    System::Runtime::InteropServices::Marshal::FreeHGlobal
      (static_cast<IntPtr>(const_cast<void*>
      (static_cast<const void*>(lFile))));
    pMC->StopWhenReady();
    return S_OK;
}

Now that the VMR9's functionality has been separated from the GUI, Button and TrackBar handlers can simply create a pointer to a myVMR9 object and call the required methods.

Additional Information

• The second video stream opened is on top of the first one, i.e., file-2 video is rendered on top of file-1 video. Therefore, if the first video's alpha value is a 100% and the second video's alpha value is 50%, then both videos will be equally (50%) visible.
• It should be noted that the values of Width and Height of trackbars can run into negative values. So when a video stream's width is -100%, it is laterally inverted. Similarly, when a video stream's height is -100%, the video is upside down.