How to make only the white part of a PDF stamp transparent?

Question

Using iTextSharp, I want to put a stamp on a PDF and make it transparent. The stamp has colored text on it (various colors) that becomes difficult to read when the entire image is transparent, so I don't want to make the colored text transparent - only the white background in the stamp's rectangle.

Based on this answer, I tried the following code:

public void addImage(PdfDictionary oldAnnot, string imagePath, 
                         int pageNumber,iTextSharp.text.Rectangle someRectangle) {
    Stream inputImageStream = new FileStream(imagePath, FileMode.Open, FileAccess.Read, FileShare.Read);
    iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance(inputImageStream);
    PdfAnnotation pdfStamp = PdfAnnotation.CreateStamp(pdfStamper.Writer, someRectangle, null, Guid.NewGuid().ToString());
    image.SetAbsolutePosition(0, 0);
    PdfAppearance app = pdfStamper.GetOverContent(pageNumber).CreateAppearance(image.Width, image.Height);
    app.SaveState();
    PdfGState state = new PdfGState();
    state.FillOpacity = 0.1f;
    app.SetGState(state);
    app.AddImage(image);
    app.RestoreState();
    pdfStamp.SetAppearance(PdfName.N, app);
    pdfStamp.SetPage();
    pdfStamper.AddAnnotation(pdfStamp, pageNumber);
}

However, this makes the colored parts of the image translucent. How can I make only the blank background of the image transparent and leave the colored parts opaque?

Thanks.

Answer 1

You essentially are asking for the Colour Key Masking PDF feature. Unfortunately

When colour key masking is specified, the use of a DCTDecode or lossy JPXDecode filter for the stream can produce unexpected results.

(section 8.9.6.4 – Colour Key Masking – ISO 32000-1)

As you have a JPEG and JPEGs usually are embedded in PDFs using the DCTDecode filter, Colour Key Masking might not work as desired.

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Instead of working with transparency you might want to try using a different blend mode, e.g. Darken or Multiply.

Multiply B(c_b, c_s) = c_b * c_s

NOTE 1 Multiplies the backdrop and source colour values.

NOTE 2 The result colour is always at least as dark as either of the two constituent colours. Multiplying any colour with black produces black; multiplying with white leaves the original colour unchanged. Painting successive overlapping objects with a colour other than black or white produces progressively darker colours.

Darken B(c_b, c_s) = min(c_b, c_s)

NOTE 6 Selects the darker of the backdrop and source colours.

NOTE 7 The backdrop is replaced with the source where the source is darker; otherwise, it is left unchanged.

(Table 136 – Standard separable blend modes – ISO 32000-1)

To select a blend mode, you can use a PdfGState like you already do but instead of setting the opacity

state.FillOpacity = 0.1f;

you set the blend mode

state.BlendMode = PdfGState.BM_MULTIPLY;

or

state.BlendMode = PdfGState.BM_DARKEN;