範例：DownloadPreviewImage

DownloadPreviewImage 函式會呼叫預覽元件的 IWiaPreview：：GetNewPreview 方法，從掃描儀下載影像數據。 然後，如果應用程式使用者想要叫用分割篩選器，它會呼叫 DetectSubregions 函式，這會針對偵測到的每個區域，在 pWiaItem2 底 下建立子專案。 如需 DetectSubregions的相關資訊，請參閱 IWiaSegmentationFilter::DetectRegions 方法。

在此範例中，應用程式使用者按下複選框來設定 m_bUseSegmentationFilter 參數。 如果應用程式支援此功能，它應該先呼叫 IWiaItem2：：CheckExtension來檢查驅動程式是否有分割篩選。 如需了解在此範例中使用的 CheckImgFilter的相關信息，請參閱 Microsoft Windows SDK 檔案中的 IWiaPreview::GetNewPreview 方法。

HRESULT
DownloadPreviewImage(
  IN IWiaItem2 *pWiaFlatbedItem2)
{
  HRESULT              hr = S_OK;
  BOOL                 bHasImgFilter  = FALSE;
  IWiaTransferCallback *pAppWiaTransferCallback = NULL;

  hr = CheckImgFilter(pWiaFlatbedItem2, &bHasImgFilter)

  if (SUCCEEDED(hr))
  {
     if (bHasImgFilter)
     {
        IWiaPreview *pWiaPreview = NULL;

        // In this example, the AppWiaTransferCallback class 
        // implements the IWiaTransferCallback interface.
         // The constructor of AppWiaTransferCallback sets the 
         // reference count to 1.
         pAppWiaTransferCallback = new AppWiaTransferCallback();

         hr = pAppWiaTransferCallback ? S_OK : E_OUTOFMEMORY;

         if (SUCCEEDED(hr))
         {
            // Acquire image from scanner
            hr = m_pWiaPreview->GetNewPreview(pWiaFlatbedItem2,
                                              0,
                                              pAppWiaTransferCallback);    
         }

         // m_FlatbedPreviewStream is the stream that
         // AppWiaTransferCallback::GetNextStream returned for the
         // flatbed item.
         // This stream is where the image data is stored after
         // the successful return of GetNewPreview.
         // The stream is passed into the segmentation filter
         // for region detection.
         if (SUCCEEDED(hr) && m_bUseSegmentationFilter)
         {
            DetectSubregions(m_FlatbedPreviewStream, pWiaFlatbedItem2);
         }

         if (pAppWiaTransferCallback)
         {
            // If the call to GetNewPreview was successful, the
            // preview component calls AddRef on the callback so
            // this call doesn't delete the object.

            pAppWiaTransferCallback->Release();
         }

      }
      else
      {
         // Do not create an instance of preview component if the 
         // driver does not come with an image-processing filter.
         // You can use a segmentation filter, however, if the driver
         // comes with one (omitted here).
      }
   }

   return hr;
}