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How to dynamic add image watermark when rendering

I would like to use GroupDocs.Viewer-for-.NET-UI as codebase and reivse it for our customer need. How to dynamic add image watermark(not text watermark) to each view page(png view) when rendering?

@jefflin0322

Adding image watermark is not supported out of the box but since you’re going to use Viewer UI as a code base you can add image watermark with a bit of custom code in this method. There are two parameters that you can use page number and page stream. Viewer populates a page stream and then calls this method. You can read the stream, apply image watermark and write the data back to the same stream similar to:

var viewOptions = new PngViewOptions(_ => pageStream,
    (int pageNumber, Stream stream) =>
    {
        // 1. read `stream`

        // 2. apply watermark

        // 3. write data back to `stream`

        //NOTE: do not dispose/close stream here
    });

You can also implement IPageStreamFactory interface and pass the instance as PngViewOptions constructor parameter instead of using delegates.