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Parameter Information

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[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The IMediaParamInfo::GetParamInfo method returns an MP_PARAMINFO structure that describes a parameter. This structure contains the following information:

  • The mpType member indicates the data type of the parameter value. For efficiency, all parameters are implemented as 32-bit floating-point values. The MP_TYPE enumeration defines whether to interpret the value as an integer, floating-point value, Boolean, or enumeration (integer series).
  • The mopCaps member indicates which curves this parameter supports. If the data type is Boolean or enumeration, the only curve that the parameter can support is "Jump."
  • The mpdMinValue and mpdMaxValue members define the minimum and maximum values for this parameter. Any curves for this parameter must fall within this range.
  • The mpdNeutralValue member is the default value of the parameter.
  • The szLabel member is the name of the parameter, and the szUnitText member is the name for the unit of measure for the parameter. Examples might include "Volume" and "Decibels," or "Frequency" and "kHz." Both strings are English and are never localized. The DMO can provide localized versions through the IMediaParamInfo::GetParamText method.

The information for each parameter remains fixed throughout the lifetime of the DMO. Therefore, the client can query for this information once and then cache it.

Time Formats

The client must time stamp the input data, so that the DMO can calculate the corresponding parameter values. By default, time stamps represent units of 100 nanoseconds, also called reference time. This time unit is not convenient for every application, however, so a DMO has an option to support other time formats. Time formats are identified by GUID.

GUID Description
GUID_TIME_REFERENCE Reference time
GUID_TIME_MUSIC Parts per quarter note (PPQN)
GUID_TIME_SAMPLES Samples per second

 

Third parties are encouraged to define their own time formats as needed. However, all DMOs must support reference time. This provides a standard baseline that everyone can use. To determine how many time formats a DMO supports, call the IMediaParamInfo::GetNumTimeFormats method. To enumerate the supported formats, call the IMediaParamInfo::GetSupportedTimeFormat method.

To set the time format, call IMediaParams::SetTimeFormat. This method specifies the time format GUID and the time data, which is the number of units per clock tick. For example, if the time format is samples per second, and the time data is 32, then a time stamp value of 10 corresponds to 320 samples.

Media Parameters