Configuring the ASF Splitter Object

The ASF splitter object is a WMContainer layer object that parses the ASF Data Object of an Advanced Systems Format (ASF) file. After the splitter is created and initialized to parse the ASF Data Object of a media file, the splitter must be configured to generate samples for specific streams. Call IMFASFSplitter::SelectStreams to select the required streams.

Optionally, an application can also configure it to generate samples in reverse order or generate samples for protected content. To set these options, call IMFASFSplitter::SetFlags and pass the required bitwise combination of the supported flags. Before calling this method, the client must complete the IMFASFSplitter::Initialize call successfully; otherwise, SetFlags fails with MF_E_NOT_INITIALIZED error code. For information about initializing the splitter, see Creating the ASF Splitter Object.

To check whether this flag is currently set on the splitter, call IMFASFSplitter::GetFlags.

Selecting Streams for Parsing

During the initialization process through IMFASFSplitter::Initialize call, the splitter detects the number of streams and the stream identifiers in the ASF file. By default, no streams are selected by the splitter. The application must select the streams by calling IMFASFSplitter::SelectStreams. This method takes an array of stream numbers. To get the stream number for a stream, call IMFASFProfile::GetStream on the ASF profile or call IMFStreamDescriptor::GetStreamIdentifier on the stream descriptor. (You can obtain both the ASF profile and the stream descriptor from the ContentInfo object.) If the client passes a stream number that is not recognized by the splitter, it fails with a MF_E_INVALIDSTREAMNUMBER error.

Calling SelectStreams clears the previous selections. Any stream that is not specified in the array is not selected. To get a list of streams that are currently selected, call IMFASFSplitter::GetSelectedStreams. This method takes a pointer to an array, which the method fills with the stream numbers. If the array size is less than the number of selected streams, the method fails with the MF_E_BUFFERTOOSMALL error. In this case, the method returns the number of selected streams in the pwNumStreams parameter. You can then use this number to allocate an array of the correct size and call the method again.

For example code, see "Select a Stream for Parsing" in Tutorial: Reading an ASF File.

Reverse Playback Setting

During the initialization process of the splitter, it determines if the ASF content supports reverse playback. If it does, the splitter can be configured to generate samples in reverse order by setting the MFASF_SPLITTER_REVERSE flag. If the content does not support reverse playback, the IMFASFSplitter::SetFlags returns MF_E_INVALIDREQUEST, but the flag is set on the splitter.

If the splitter is configured to parse in the reverse direction, then the splitter always starts parsing at the end of the buffer that contains the ASF Data Object. Therefore, for reverse parsing the data offset and the length of the data to parse must be set appropriately. For information about setting the correct values, see Generating Stream Samples from an Existing ASF Data Object.

Protected Content Setting

The splitter can be configured to work with packet-level encryption content by setting the MFASF_SPLITTER_WMDRM through IMFASFSplitter::SetFlags. This instructs the splitter to deliver samples for content that is protected by Windows Media Digital Rights Management (DRM). When this flag is set, the samples generated by the splitter contain information required to decrypt media data and reconstruct the frames, such as the MFSampleExtension_PacketCrossOffsets attribute. This attribute is a blob that contains an array of DWORDs. Each DWORD provides you the payload boundaries for the frame relative to the beginning of the frame. If this attribute is not present, the frame is contained in a single payload. Typically, the samples generated by the splitter contain multiple media buffers, the application can copy all the buffers into one contiguous buffer by calling IMFSample::ConvertToContiguousBuffer. The resulting buffer contains the frame and the attribute value contains offsets into this buffer.

Related topics

ASF Splitter