Watermark Every Page

Read the page count from the byte-backed reader, then edit each one-based Recipe page and draw semi-transparent centered text.

import { createMuhammaraWasm, createRecipe } from "@muhammara/wasm";

var muhammara = await createMuhammaraWasm();
var reader = muhammara.createReader(inputBytes);
var pageCount = reader.getPagesCount();
reader.end();

var Recipe = await createRecipe();
await Recipe.registerFontAsync("watermark-font", fontFile);
var pdf = new Recipe(inputBytes);

for (var pageNumber = 1; pageNumber <= pageCount; pageNumber++) {
  var page = pdf.pageInfo(pageNumber);
  pdf
    .editPage(pageNumber)
    .text("WATERMARK", page.width / 2, page.height / 2, {
      font: "watermark-font",
      fontSize: 60,
      color: "#0000ff",
      align: "center center",
      opacity: 0.3,
    })
    .endPage();
}

var outputBytes = pdf.endPDF();

The reader uses zero-based page indexes for page operations, while Recipe editing uses one-based page numbers. pageInfo() accounts for rotated page dimensions before the watermark center is calculated.

The browser example's Watermark tab runs a diagonal variant of this workflow on an uploaded PDF, or on a built-in sample when none is chosen, on the main thread or in a module Worker.

Watermark In Place

The WebAssembly package is byte-first, so there is no second file to avoid: endPDF() returns the finished document and the source bytes stay untouched. "In place" means replacing whatever held the input with the returned bytes.

var pdf = new Recipe(inputBytes);

for (var pageNumber = 1; pageNumber <= pageCount; pageNumber++) {
  var page = pdf.pageInfo(pageNumber);
  pdf
    .editPage(pageNumber)
    .text("WATERMARK", page.width / 2, page.height / 2, {
      font: "watermark-font",
      fontSize: 60,
      align: "center center",
      opacity: 0.3,
    })
    .endPage();
}

inputBytes = pdf.endPDF();
Recipe.unregisterFont("watermark-font");

The result is a separate Uint8Array; the input is never modified, so both documents are briefly in memory. Drop your reference to the source, as above, when only the watermarked bytes are needed. Unregister the font once every page that uses it has been watermarked.

Editing an existing document appends an incremental update rather than rewriting it, so feeding the output back in as the next input grows the document each round. Watermark once from the original when size matters.

In Node, write the bytes back over the file the input came from:

await writeFile("input.pdf", pdf.endPDF());

In the browser, hand them to a download or upload as described in Preview, Download, or Upload a PDF. The native package can instead overwrite the source file directly by omitting the Recipe output path; see Differences and Restrictions.