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About IMediaObject

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[The feature associated with this page, Windows Media Player SDK, is a legacy feature. It has been superseded by MediaPlayer. MediaPlayer has been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer instead of Windows Media Player SDK, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The IMediaObject interface is the required interface for DMOs. IMediaObject contains the methods that a Windows Media Player DSP plug-in uses to get data from Windows Media Player, to process the data, and to return the processed data to Windows Media Player. For complete documentation of the IMediaObject interface, see the DirectShow section of the Windows SDK.

The methods of IMediaObject can be categorized as follows:

Methods that Handle Format Negotiation

These are the methods that Windows Media Player calls to get information about the data formats supported by the plug-in. These methods include:

  • GetInputMaxLatency
  • GetInputSizeInfo
  • GetInputStreamInfo
  • GetInputType
  • GetOutputSizeInfo
  • GetOutputStreamInfo
  • GetOutputType
  • GetStreamCount
  • SetInputMaxLatency
  • SetInputType
  • SetOutputType

Several of these methods, such as GetInputType and SetInputType, use the DMO_MEDIA_TYPE structure to describe the format of the data used by a stream. When Windows Media Player calls these methods, it provides a pointer to a DMO_MEDIA_TYPE structure. If a method such as SetInputType specifies the media type information, the plug-in should copy the DMO_MEDIA_TYPE structure to a member variable and inspect its data members to determine the type of data that Windows Media Player will provide in the input buffer. If a method such as GetInputType retrieves the media type information, the plug-in should copy the address of the member variable containing the DMO_MEDIA_TYPE structure to the pointer provided by Windows Media Player in the parameter list.

Windows Media Player mainly uses two members of the DMO_MEDIA_TYPE structure:

  • majortype: A globally unique identifier (GUID) that specifies the overall category of the media, such as audio or video.
  • subtype: A GUID that specifies a more detailed description of the media, such as PCM audio.

These GUIDs can be found in the header named uuids.h, which is included with the Windows SDK.

Methods such as GetInputSizeInfo provide information to Windows Media Player about how much memory is required to allocate the processing buffers. Methods such as GetStreamCount and GetOutputStreamInfo provide information to Windows Media Player about the number and character of the streams supported by the DSP plug-in.

GetInputMaxLatency and SetInputMaxLatency are implemented by DMOs in special cases. Windows Media Player DSP plug-ins should return E_NOTIMPL.

Methods that Specify or Retrieve State Information

These are the methods that Windows Media Player calls to get or set values related to the current state of the plug-in. These methods include:

  • GetInputCurrentType
  • GetInputStatus
  • GetOutputCurrentType

GetInputCurrentType and GetOutputCurrentType use the DMO_MEDIA_TYPE structure to return information to Windows Media Player about the media types previously set for the input and output streams. GetInputStatus returns a flag that tells Windows Media Player whether the DSP plug-in can accept input data.

Methods that Handle Buffering and Processing Data

These are the methods that Windows Media Player calls to initiate the various processes that the plug-in performs to do the digital signal processing. These methods include:

  • AllocateStreamingResources
  • Discontinuity
  • Flush
  • FreeStreamingResources
  • Lock
  • ProcessInput
  • ProcessOutput

Windows Media Player calls AllocateStreamingResources and FreeStreamingResources to provide the DSP plug-in with an opportunity to set up or release any additional buffers the plug-in may require for internal processing.

Windows Media Player DSP plug-ins do not need to use the DMO Discontinuity method.

Windows Media Player calls Flush to direct the DSP plug-in to flush all internally buffered data. The plug-in should release any references to IMediaBuffer interfaces, clear any values that specify the time stamp or sample length for the media buffer, and reinitialize any internal states that depend upon the contents of the media sample.

Windows Media Player calls ProcessInput to pass a pointer to an IMediaBuffer interface to the DSP plug-in. This interface provides access to the input buffer allocated by Windows Media Player to supply data to the plug-in. Windows Media Player subsequently calls ProcessOutput to pass a pointer to an IMediaBuffer interface that provides access to the output buffer allocated by Windows Media Player to receive the processed data from the DSP plug-in.

The IMediaObject interface includes a method named Lock. This method is designed to acquire or release a lock on the DMO to keep the DMO serialized when performing multiple operations. The version of IMediaObject::Lock in the wizard code overrides the ATL implementation of Lock. Because Windows Media Player serializes calls made to the DMO interface of a DSP plug-in, the implementation of Lock simply returns S_OK. For details about how to create a multithreaded DMO, refer to the DirectShow section of the Windows SDK.

Required Interfaces