Converting a multipage PDF to a multipage TIFF using color detection in VB.NET

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'We assume that GdPicture has been correctly installed and unlocked.
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Const  INPUT_DOCUMENT As String = "input.pdf"
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Const  OUTPUT_DOCUMENT As String = "output.tif"
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Const  DPI As Single = 200F
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Dim oGdPicturePDF As New GdPicturePDF()
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Dim oGdPictureImaging As New GdPictureImaging()
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If oGdPicturePDF.LoadFromFile(INPUT_DOCUMENT, False) = GdPictureStatus.OK Then
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    Dim tiffID As Integer = 0
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    For i As Integer = 0 To oGdPicturePDF.GetPageCount() - 1
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        oGdPicturePDF.SelectPage(i + 1)
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        'Rasterizing a page or extracting a page bitmap.
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        Dim rasterPage As Integer = oGdPicturePDF.RenderPageToGdPictureImageEx(DPI, True, System.Drawing.Imaging.PixelFormat.Format24bppRgb)
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        If oGdPicturePDF.GetStat() = GdPictureStatus.OK Then
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            'Defining the compression of the tiff page. Let's use CCITT4 for bitonal pages and JPEG for others.
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            Dim tiffCompression As TiffCompression
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            'Applying color detection and color depth automatic conversion.
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            oGdPictureImaging.ColorDetection(rasterPage, True, False, False)
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            If oGdPictureImaging.GetBitDepth(rasterPage) = 1 Then
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                tiffCompression = TiffCompression.TiffCompressionCCITT4
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            Else
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                tiffCompression = TiffCompression.TiffCompressionJPEG
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            End If
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            If i = 0 Then
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                If oGdPictureImaging.TiffSaveAsMultiPageFile(rasterPage, OUTPUT_DOCUMENT, tiffCompression) = GdPictureStatus.OK Then
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                    'Firstly rasterized page's GdPicture ID becomes the multipage tiff id.
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                    tiffID = rasterPage
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                Else
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                    MessageBox.Show("Error occurred when saving the file. Status: " + oGdPictureImaging.GetStat().ToString(), "Color Detection Example", MessageBoxButtons.OK, MessageBoxIcon.Error)
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                    Exit For
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                End If
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            Else
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                If oGdPictureImaging.TiffAddToMultiPageFile(tiffID, rasterPage, tiffCompression) <> GdPictureStatus.OK Then
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                    MessageBox.Show("Error occurred when adding a page. Status: " + oGdPictureImaging.GetStat().ToString(), "Color Detection Example", MessageBoxButtons.OK, MessageBoxIcon.Error)
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                    Exit For
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                End If
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                oGdPictureImaging.ReleaseGdPictureImage(rasterPage)
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            End If
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        Else
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            MessageBox.Show("Error occurred when rendering the page. Status: " + oGdPicturePDF.GetStat().ToString(), "Color Detection Example", MessageBoxButtons.OK, MessageBoxIcon.Error)
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        End If
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    Next
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    oGdPictureImaging.TiffCloseMultiPageFile(tiffID)
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    oGdPictureImaging.ReleaseGdPictureImage(tiffID)
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Else
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    MessageBox.Show("The file can't be opened. Status: " + oGdPicturePDF.GetStat().ToString(), "Color Detection Example", MessageBoxButtons.OK, MessageBoxIcon.Error)
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End If
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oGdPicturePDF.Dispose()
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oGdPictureImaging.Dispose()