Add watermarking example

samples/watermark.js

@@ -0,0 +1,174 @@
+/*
+
+This example writes a transparent watermark/stamp across every page in the supplied pdf.
+It writes the watermark from the bottom left to the top right  at the fontSize specified.
+If it is calculated that the text is too long to fit then the font size is reduced.
+
+
+Assistance from:
+https://github.com/galkahana/HummusJS/issues/242
+https://github.com/AndreMocke91
+https://github.com/richard-kurtosys
+
+*/
+
+var hummus = require('../hummus');
+
+var pdfWriter = hummus.createWriterToModify('./multipage.pdf', {
+    modifiedFilePath: './multipage_watermarked.pdf',
+});
+
+var font = pdfWriter.getFontForFile('./Arial.ttf');
+var fontSize = 45;  //Initial font size - may be decreased in getTextMatric if the text is too long to fit the diagonal.
+var watermarkText = "Watermark Text"
+var matrixCache = {};
+var objectCache = {};
+
+var pdfReader = hummus.createReader('./multipage.pdf');
+var isEncrypted = pdfReader.isEncrypted();
+
+for (var i = 0; i < pdfReader.getPagesCount(); ++i) {
+
+    //Get page size [left, bottom, right, top]
+    const pageDimensions = pdfReader.parsePage(i).getCropBox();
+
+    //calculate the text size and angle
+    var result = getTextMatrix(matrixCache, watermarkText, font, 45, pageDimensions[2], pageDimensions[3], 0.1);
+    const tmMatrix = result.tMatrix;
+    matrixCache = result.matrixCache;
+
+    result = createXObject(tmMatrix, objectCache, watermarkText, font, pageDimensions, pdfWriter);
+
+    var xObject = result.xObject;
+    objectCache = result.objectCache;
+
+
+    //get the page from the pdf in a modifier context
+    var pageModifier = new hummus.PDFPageModifier(pdfWriter, i);
+
+    //Write the "page object" onto the existing pdf page.
+    var modifier = pageModifier.startContext().getContext();
+    modifier.q()
+        .cm(1, 0, 0, 1, 0, 35) // Set Current Matrix - scale to 100% (x and y), translate 0,35
+        .doXObject(xObject)
+        .Q();
+
+    pageModifier.endContext().writePage();
+
+}
+pdfWriter.end();
+
+//Calculate the angle the text should be displayed at from bottom left.
+//This takes into consideration the orientation of the page - landscape vs portrait will have different angles from bottom left to top right.
+function getTextMatrix(matrixCache, text, font, fontSize, pageRight, pageTop) {
+
+    pageTop = pageTop;
+    pageRight = pageRight;
+    let cachedMatrix = matrixCache[`${pageRight} ${pageTop}`];
+
+    if (cachedMatrix) {
+        return { tMatrix: cachedMatrix, matrixCache: matrixCache };;
+    }
+
+    const hypot = Math.hypot(pageTop, pageRight);
+    let angle = 0;
+    let textDimensions = font.calculateTextDimensions(text, fontSize);
+    let lesserFontSize = fontSize;
+
+    while (textDimensions.width > hypot * 0.8) {
+        fontSize--;
+        lesserFontSize = fontSize;
+        textDimensions = font.calculateTextDimensions(text, fontSize);
+    }
+
+    const textWidth = textDimensions.width;
+    const textHeight = textDimensions.height;
+    const diagonalOffset = (hypot - textWidth) / 2;
+
+    angle = -(Math.atan(pageTop / pageRight)).toFixed(2);
+
+    const rightOffset = (diagonalOffset * Math.sin(-angle));
+    const bottomOffset = (diagonalOffset * Math.cos(-angle));
+
+    let tMatrix = {
+        a: Math.cos(angle),
+        b: -Math.sin(angle),
+        c: Math.sin(angle),
+        d: Math.cos(angle),
+        e: bottomOffset,
+        f: rightOffset,
+        lesserFontSize,
+    }
+
+    matrixCache[`${pageRight} ${pageTop}`] = tMatrix;
+
+    return { tMatrix: tMatrix, matrixCache: matrixCache };
+}
+
+
+/* Create the transparent graphic state with 0.5 transparency */
+function createTransparencyObject(pdfWriter) {
+    var objCxt = pdfWriter.getObjectsContext();
+    var gsId = objCxt.startNewIndirectObject();
+    var dict = objCxt.startDictionary()
+    dict.writeKey("type");
+    dict.writeNameValue("ExtGState");
+    dict.writeKey("ca");
+    objCxt.writeNumber(0.5);
+    objCxt.endLine();
+    objCxt.endDictionary(dict);
+    return gsId;
+}
+
+
+// #242: Use xObjectForm to get the gsName (graphic state name)
+function assignGsStateToResource(xObject, gsId) {
+    var resourcesDict = xObject.getResourcesDictinary(); // This is not a typo =~=
+    return resourcesDict.addExtGStateMapping(gsId);
+}
+
+
+//Make an object once that can be reused on many pages
+function setXObject(xObject, gsName, font, tmMatrix, text) {
+    xObject.getContentContext()
+        .q()
+        .gs(gsName) //Use the graphic state we created earlier
+        .BT() // Begin Text
+        .k(0, 0, 0, 0.3) // Set Color (CMYK, 0-1)
+        .Tf(font, tmMatrix.lesserFontSize) // Set font and font size
+        .Tm(tmMatrix.a, tmMatrix.b, tmMatrix.c, tmMatrix.d, tmMatrix.e, tmMatrix.f) // Set the text matrix to the angle calculated in getTextMatrix
+        .Tj(text) // Write the text
+        .ET() // End Text
+        .Q();
+}
+
+function createXObject(tmMatrix, objectCache, watermarkText, font, pageDimensions, pdfWriter) {
+
+    //Check if we have created an object for this page size already
+    let cachedObject = objectCache[`${pageDimensions[2]} ${pageDimensions[3]}`];
+    if (cachedObject) {
+        return { xObject: cachedObject, objectCache: objectCache };
+    }
+
+    //Get the correct text matrix to display the text diagnoally across the page from corner to corner
+    //This caters for portrait, landscape and anything in between
+
+
+    //Create the transparency object
+    var gsId = createTransparencyObject(pdfWriter);
+
+    //Create a "page object" that we can later assign the transparency object to
+    var xObject = pdfWriter.createFormXObject(0, 0, pageDimensions[2], pageDimensions[3]);
+
+    //Add the graphic state to the "page object" and get the name back
+    var gsName = assignGsStateToResource(xObject, gsId);
+
+    //Write the text to the "page object"
+    setXObject(xObject, gsName, font, tmMatrix, watermarkText);
+    pdfWriter.endFormXObject(xObject);
+
+    //Save the page size object to the cache
+    objectCache[`${pageDimensions[2]} ${pageDimensions[3]}`] = xObject;
+
+    return { xObject: xObject, objectCache: objectCache };
+}