Time Formats For Seek Commands

Time Formats For Seek Commands

Many of the methods in the IMediaSeeking interface have parameters that express position values, such as current position or stop position. By default, these parameters are expressed in units of 100 nanoseconds, also called reference time. Any filter that can seek must support seeking by reference time. Some filters can seek using other units of time as well, such as seeking to a particular frame number, or seeking to a given byte offset within a stream. Each of these time units is called a time format, and is defined by a globally unique identifier (GUID). For a list of the time formats that are defined by DirectShow, see Time Format GUIDs. Third parties can also define GUIDs for custom time formats.

To determine whether the filters currently in the filter graph support a particular time format, call the IMediaSeeking::IsFormatSupported method. The method returns S_OK if the format is supported. Otherwise, it returns S_FALSE or an error code. If a format is supported, you can switch to that format by calling the IMediaSeeking::SetTimeFormat method. If the SetTimeFormat method succeeds, subsequent seek commands are expressed using the new time format.

The follow example checks whether the graph supports seeking by frame number. If so, it seeks to frame number 20:

hr = pSeek->IsFormatSupported(&TIME_FORMAT_FRAME);
if (hr == S_OK)
{
    hr = pSeek->SetTimeFormat(&TIME_FORMAT_FRAME);
    if (SUCCEEDED(hr))
    {
        // Seek to frame number 20.
        LONGLONG rtNow = 20;
        hr = pSeek->SetPositions(
            &rtNow, AM_SEEKING_AbsolutePositioning, 
            0, AM_SEEKING_NoPositioning);
    }
}

Note that time formats only apply to seek commands. They do not affect graph playback or other actions.