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Envelope Segments

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Microsoft DirectShow 9.0

Envelope Segments

A parameter curve consists of one or more envelope segments, defined using the MP_ENVELOPE_SEGMENT structure. This structure contains the following information:

  • The start and end times.
  • The starting and ending values.
  • The curve type (linear, square, and so forth).
  • Optional flags, described shortly.

The client adds envelope segments to a parameter by calling the IMediaParams::AddEnvelope method and passing in an array of MP_ENVELOPE_SEGMENT structures. The client should sort the segments into ascending time order before calling the method. As the DMO processes data, you can imagine the parameter traveling over each envelope segment, like a car driving over a series of hills. The IMediaParams::GetParam method returns the most recent value.

Two adjacent segments can have a gap between them. During gaps, the parameter retains its previous value, as follows:

  • Before the first segment, the value is the neutral value.
  • Between segments, the value is the ending value of the previous segment.
  • After the last segment, the value remains at the ending value of that segment.
  • If the client flushes the DMO, the value reverts to the neutral value.

You can alter a segment by setting either of the following flags:

  • MPF_ENVLP_BEGIN_CURRENTVAL. The DMO uses the most recent value of the parameter as the starting value for the segment. This might be the neutral value, or the ending value from the previous segment. The DMO ignores the valStart member of the MP_ENVELOPE_SEGMENT structure.
  • MPF_ENVLP_BEGIN_NEUTRALVAL. The DMO uses the neutral value of the parameter as the starting value for the segment. It ignores valStart.

You can think of these flags as grabbing the starting point of the segment and moving it up or down, while the ending value remains fixed. The segment will "stretch" accordingly.

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