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To Write Samples

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 It has been superseded by Source Reader and Sink Writer.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

When you have identified and configured the inputs for the file you are writing, you can begin passing samples to the writer. You should pass samples in presentation-time order, if possible, to make the writing process more efficient.

Before passing any samples, you must set the writer to accept them by calling IWMWriter::BeginWriting.

To pass a sample to the writer, perform the following steps:

  1. Allocate a buffer and retrieve a pointer to the INSSBuffer interface by calling IWMWriter::AllocateSample.
  2. Retrieve the address of the buffer created in step 1 by calling INSSBuffer::GetBuffer.
  3. Copy your sample data to the buffer location, making sure that the sample passed will fit in the allocated buffer. You can use any memory copying function to copy your data. A common choice is memcpy, which is included in the standard C run-time library.
  4. Update the amount of data used in the buffer to reflect the actual size of the sample by calling INSSBuffer::SetLength.
  5. Pass the buffer interface to the writer along with the input number and sample time using the IWMWriter::WriteSample method. All audio samples for an input represent the same duration of content, so you can figure the sample time by adding the sample duration to a running total. For video, you need to calculate the time based on the frame rate.

WriteSample works asynchronously and might not finish writing the data from the buffer before your application is ready to call the method again. Therefore, it is important to call AllocateSample once for each call to WriteSample. However, you can release the INSSBuffer interface immediately after calling WriteSample.

When you have finished passing samples, call IWMWriter::EndWriting.

Note It is important that samples from all streams in the file are passed to the writer in synchronization with one another. That is, whenever possible you should pass samples to the writer in presentation-time order within the sync tolerance specified in IWMWriterAdvanced::SetSyncTolerance. Best results are achieved when data is delivered to each stream in units of one second or less.

Streams should also end at approximately the same time. For example, you should not write a file with an audio stream that is 45 seconds long and a video stream that is 50 seconds long. If you encode such a file with unaltered inputs, some of the audio data at the end of the stream will be dropped (even though it is the shorter stream). To make the file encoding work, you should add 5 seconds of silence to the audio input so that one stream does not end several seconds before another. It is not necessary for stream types with intermittent samples, like text or image streams, to be padded in this way. Script command streams should also follow all these rules.

INSSBuffer Interface

IWMWriter Interface

Writing ASF Files