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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (SlidesDocument doc = Document.Load<SlidesDocument>(@"D:\test.pptx"))
{
    using (var watermark = new ImageWatermark(@"D:\logo.png"))
    {
        SlidesImageEffects effects = new SlidesImageEffects();
        effects.Brightness = 0.7;
        effects.Contrast = 0.6;
        effects.ChromaKey = Color.Red;
        effects.BorderLineFormat.Enabled = true;
        effects.BorderLineFormat.Weight = 1;
        doc.AddImageWatermark(watermark, effects);
    }
    doc.Save();
}