Forum Samples, Tips and Tricks


Here is an example of how to make changebars using the new JavaScript box tracking API with a two-pass process where Prince is run once to generate the changebar markup and then run a second time to generate the final PDF.

Changebars are vertical bars in the margin used to highlight areas of the text that have been modified since the previous revision of a document. This example uses <ins> tags but you could also use <del> or your own class names that apply to blocks as well as spans.

Since Prince runs JavaScript before doing layout, it's not possible to figure out where to put the changebars in advance, which is why we need a two-pass process. The first pass uses the Prince oncomplete event to call a JavaScript function when layout is finished, then inspects the box tree to find out where the <ins> elements ended up.

Each <ins> generates multiple span boxes, one for each line that they appear on. The box objects have fields like pageNum, x, y, w, h, and the code aggregates adjacent boxes on the same page into a single changebar.

(Note that box coordinates are given in pt units (1/72in) and the origin is the bottom-left corner of the page, so the Y coordinate decreases as you go down the page).

At this point the JavaScript can't modify the document as layout has already finished, so it writes some markup to the output log which can be captured by a calling process and inserted into the original document. Then when Prince is run again, the changebars will be there! :D

Although it's a little tricky to setup, this two-pass approach is a powerful technique for changing the document in ways that depend upon the layout of other content.
  1. changebars.html5.7 kB
  2. changebars.pdf28.9 kB
Awesome. Thanks!
Does princexml support document.getElementsByClassName() method? Our XML uses class="rev" to denote revised content. I modified the script to use document.getElementsByClassName("rev") and added the class="rev" to the first <p> tag but the first-pass output comes back empty.
Yes this should work, here is a sample:
window.onload = function() {
    var es = document.getElementsByClassName("rev");
    for (var i = 0; i < es.length; ++i) {
        es[i].style.color = "red";
<p class="rev">
not red
Here is the full file I modified.
Running prince.exe --javascript content.html returns only:
  1. content.html5.7 kB
    Use class name
The issue is that the first box type is not "SPAN", it's "BOX", which has 12 children "LINE" boxes which contain "SPAN" boxes.
Thank you very much!