IWiaTransferCallback::GetNextStream method

Gets a new stream for the specified item.

Syntax


HRESULT GetNextStream(
  [in]  LONG    lFlags,
  [in]  BSTR    bstrItemName,
  [in]  BSTR    bstrFullItemName,
  [out] IStream **ppDestination
);

Parameters

lFlags [in]

Type: LONG

Currently unused. Should be set to zero.

bstrItemName [in]

Type: BSTR

Specifies the name of the item to create stream for.

bstrFullItemName [in]

Type: BSTR

Specifies the full name of the item to create stream for.

ppDestination [out]

Type: IStream**

Receives the address of a pointer to the new IStream object.

Return value

Type: HRESULT

If this method succeeds, it returns S_OK. Otherwise, it returns an HRESULT error code.

Remarks

When this method is implemented by an image processing filter, the Windows Image Acquisition (WIA) 2.0 minidriver calls it during image acquisition to get the destination stream from the client.

A filter's IWiaTransferCallback::GetNextStream must delegate to the application's callback method. The filter uses the stream returned by the application callback's IWiaTransferCallback::GetNextStream implementation to create its own stream that it passes back to the WIA 2.0 service. The filtering is done when the filter's stream calls the IStream::Write method.

The filter's stream cannot make any assumptions on the number of bytes that are written to it on each write, since the unfiltered image data may come from the WIA 2.0 Preview Component rather than the driver. The WIA 2.0 Preview Component always writes the whole unfiltered image data into the filter's stream only once, which means that the filter's stream has one source writing into it. If both the driver and the preview component write into the filter's stream, the filter's stream cannot assume, for example, that it will receive the full header the first time IStream::Write is called although its corresponding driver always writes the header data first in one write. Nor can it assume that a subsequent write contains exactly one scan line. So the filtering stream may have to count the number of bytes written to it to determine, for example, where the image data starts.

The image processing filter's IWiaTransferCallback::GetNextStream implementation should read the properties needed for its image processing from the item for which the image is being acquired. The filter does not read the properties directly from the pWiaItem2 passed into InitializeFilter. Instead the filter must call FindItemByName on this WIA 2.0 item to obtain the actual WIA 2.0 item. The reason for this is that the image being acquired may actually be a child item of pWiaItem2. For example, during a folder acquisition the filter uses pWiaItem2 to obtain pWiaItem2's child items in IWiaTransferCallback::GetNextStream (during a folder acquisition the driver returns the images represented by the child items of pWiaItem2). The same is true when the WIA 2.0 Preview Component calls into the image processing filter passing a child WIA 2.0 item.

Requirements

Minimum supported client

Windows Vista [desktop apps only]

Minimum supported server

Windows Server 2008 [desktop apps only]

Header

Wia.h

IDL

Wia.idl

Library

Wiaguid.lib

 

 

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