Rotate watermark text at 45 degree angle across the center Apache PDFBox

Question

I want to add a text to the PDF using PDFBox API and rotate it by 45 Degree and place it at the center of the page, The text is dynamic and should be placed in the center always, I got everything else to work except centering piece, I'll appreciate any help. I have this code:

Point2D.Float pageCenter = getCenter(page);

float stringWidth = getStringWidth(watermarkText, font, fontSize);
float textX = pageCenter.x - stringWidth / 2F + center.x;
System.out.println(textX);
float textY = pageCenter.y + center.y;
//System.out.println("Inside cross"+textX+", "+textY);
fontSize = 110.0f;  
cs.transform(Matrix.getRotateInstance(Math.toRadians(45), textX, textY));
cs.moveTo(0, 0);
cs.lineTo(125, 0);
r0.setNonStrokingAlphaConstant(0.20f);

This is the result i want: Output PDF

Answer 1

What I do is to first rotate based on the calculated angle. In this "rotated world" I do a horizontal offset so that the text is in the middle, and also move the text vertically a bit lower, so that it is in the "vertical" middle of an imagined diagonal line (horizontal in the "rotated world").

try (PDDocument doc = new PDDocument())
{
    PDPage page = new PDPage();
    doc.addPage(page);

    PDFont font = PDType1Font.HELVETICA_BOLD;
    try (PDPageContentStream cs =
            new PDPageContentStream(doc, page, PDPageContentStream.AppendMode.APPEND, true, true))
        // use this long constructor when working on existing PDFs
    {
        float fontHeight = 110;
        String text = "Watermark";

        float width = page.getMediaBox().getWidth();
        float height = page.getMediaBox().getHeight();
        int rotation = page.getRotation();
        switch (rotation)
        {
            case 90:
                width = page.getMediaBox().getHeight();
                height = page.getMediaBox().getWidth();
                cs.transform(Matrix.getRotateInstance(Math.toRadians(90), height, 0));
                break;
            case 180:
                cs.transform(Matrix.getRotateInstance(Math.toRadians(180), width, height));
                break;
            case 270:
                width = page.getMediaBox().getHeight();
                height = page.getMediaBox().getWidth();
                cs.transform(Matrix.getRotateInstance(Math.toRadians(270), 0, width));
                break;
            default:
                break;
        }
        float stringWidth = font.getStringWidth(text) / 1000 * fontHeight;
        float diagonalLength = (float) Math.sqrt(width * width + height * height);
        float angle = (float) Math.atan2(height, width);
        float x = (diagonalLength - stringWidth) / 2; // "horizontal" position in rotated world
        float y = -fontHeight / 4; // 4 is a trial-and-error thing, this lowers the text a bit
        cs.transform(Matrix.getRotateInstance(angle, 0, 0));
        cs.setFont(font, fontHeight);
        //cs.setRenderingMode(RenderingMode.STROKE); // for "hollow" effect

        PDExtendedGraphicsState gs = new PDExtendedGraphicsState();
        gs.setNonStrokingAlphaConstant(0.2f);
        gs.setStrokingAlphaConstant(0.2f);
        gs.getCOSObject().setItem(COSName.BM, COSName.MULTIPLY);
        // gs.setBlendMode(BlendMode.MULTIPLY); // will work in 2.0.14
        cs.setGraphicsStateParameters(gs);

        // some API weirdness here. When int, range is 0..255.
        // when float, this would be 0..1f
        cs.setNonStrokingColor(255, 0, 0);
        cs.setStrokingColor(255, 0, 0);

        cs.beginText();
        cs.newLineAtOffset(x, y);
        cs.showText(text);
        cs.endText();
    }
    doc.save("watermarked.pdf");
}

Note that I've set both stroking and non stroking (= fill). This is useful for people who want to try the (disabled) "hollow" appearance, that one uses stroking only. The default mode is fill, i.e. non-stroking.