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' For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
Using doc As Document = Document.Load(Utilities.MapSourceFilePath(PptFilePath))
    ' Initialize criteria with the image
    Dim imageSearchCriteria As ImageSearchCriteria = New ImageDctHashSearchCriteria("D:\watermark.jpg")

    'Set maximum allowed difference between images
    imageSearchCriteria.MaxDifference = 0.9

    Dim possibleWatermarks As PossibleWatermarkCollection = doc.FindWatermarks(imageSearchCriteria)
End Using