Media Buffer Objects

DirectX Media Objects (DMOs) manage buffer memory using a COM object that supports the IMediaBuffer interface, called a media buffer.

Media buffers serve a function similar to media samples in DirectShow, except that media samples are created and managed by an allocator object, which itself is provided by a filter. With DMOs, the client of the DMO is responsible for creating the media buffer object.

The following code show an minimal implementation of a media buffer object.

//  CMediaBuffer object
#include <new>
#include <dmo.h>
class CMediaBuffer : public IMediaBuffer
{
private:
    DWORD        m_cbLength;
    const DWORD  m_cbMaxLength;
    LONG         m_cRef;
    BYTE         *m_pbData;

public:
    CMediaBuffer(DWORD cbMaxLength) :
        m_cRef(0),
        m_cbMaxLength(cbMaxLength),
        m_cbLength(0),
        m_pbData(NULL)
    {
        m_pbData = new BYTE[cbMaxLength];
        if (!m_pbData) throw std::bad_alloc();
    }

    ~CMediaBuffer()
    {
        if (m_pbData) {
            delete [] m_pbData;
        }
    }

    // Creation function
    static HRESULT CreateBuffer(long cbMaxLength, REFIID iid, void **ppUnk)
    {
        try {
            CMediaBuffer *pBuffer = new CMediaBuffer(cbMaxLength);
            return pBuffer->QueryInterface(iid, ppUnk);
        }
        catch (std::bad_alloc)
        {
            return E_OUTOFMEMORY;
        }
    }

    // IUnknown methods
    STDMETHODIMP QueryInterface(REFIID riid, void **ppv)
    {
        if (ppv == NULL) {
            return E_POINTER;
        }
        if (riid == IID_IMediaBuffer || riid == IID_IUnknown) {
            *ppv = static_cast<IMediaBuffer *>(this);
            AddRef();
            return S_OK;
        }
        *ppv = NULL;
        return E_NOINTERFACE;
    }

    STDMETHODIMP_(ULONG) AddRef()
    {
        return InterlockedIncrement(&m_cRef);
    }

    STDMETHODIMP_(ULONG) Release()
    {
        LONG lRef = InterlockedDecrement(&m_cRef);
        if (lRef == 0) {
            delete this;
        }
        return lRef;
    }

    // IMediaBuffer methods
    STDMETHODIMP SetLength(DWORD cbLength)
    {
        if (cbLength > m_cbMaxLength) {
            return E_INVALIDARG;
        } else {
            m_cbLength = cbLength;
            return S_OK;
        }
    }

    STDMETHODIMP GetMaxLength(DWORD *pcbMaxLength)
    {
        if (pcbMaxLength == NULL) {
            return E_POINTER;
        }
        *pcbMaxLength = m_cbMaxLength;
        return S_OK;
    }

    STDMETHODIMP GetBufferAndLength(BYTE **ppbBuffer, DWORD *pcbLength)
    {
        if (ppbBuffer == NULL || pcbLength == NULL) {
            return E_POINTER;
        }
        *ppbBuffer = m_pbData;
        *pcbLength = m_cbLength;
        return S_OK;
    }
};