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jQuery in XPages #6 – watermark.js

In this article I will demonstrate how to implement watermark.js by Patrick Wied. Using the power of HTML5 image manipulation we are able to add a watermark graphic to another image and determine position and opacity - even with a JPG!! Of course that means no IE but hey we’re looking forward not backward people!


This week I am only going to use one article because this hopefully won’t be too long. I am also going to try and break the article down into a simpler format than before, highlighting the XPages techniques as well as the jQuery techniques. Thanks to David Leedy for the feedback on article format and content.


The effect is demonstrated here


The demonstration database can be downloaded from the link above or from here

Opacity in images

JPEG has been the standard web format for many years and was fantastic when we were able to reduce the size of images to (good enough) over a slow connection. Nowadays though the need to have image capability is opacity. PNG files are now the go to image format for the web and like GIF they support an A(lpha) channel which allows for an opacity to be applied to the image directly.


Looping through all pixels in an image – using JavaScript!

watermark.js uses a new feature in HTML5 whereby each pixel of a graphic can be manipulated from JavaScript, including the addition of an alpha (opactiy) value. The original article by Patrick explains how some very simple loop code can be written to go through each pixel in an image and apply an opacity.

First we have to create a canvas element, add it to the document’s body, set the canvas size to the image size (we have a 500×200 img) and add the image to the canvas.

// get the image
var img = document.getElementById("your-image");
// create and customize the canvas
var canvas = document.createElement("canvas");
canvas.width = 500;
canvas.height = 200;
// get the context
var ctx = canvas.getContext("2d");
// draw the image into the canvas
ctx.drawImage(img, 0, 0);

“Ok this is the more interesting part of the solution. We have to access the image data, loop thru all pixels of the image and set the alpha values to the value of our choice. A pixel in the imagedata is represented by four values: red, green, blue, alpha (rgba) therefore we have to change every fourth value.”

// get the image data object
var image = ctx.getImageData(0, 0, 500, 200);
// get the image data values
var imageData = image.data,
length = imageData.length;
// set every fourth value to 50
for(var i=3; i < length; i+=4){
    imageData[i] = 50;
// after the manipulation, reset the data
image.data = imageData;
// and put the imagedata back to the canvas
ctx.putImageData(image, 0, 0);

And here’s what we are going to build – an image with another opaque image surfaced on top of is as a watermark.

Our watermarked image

Our watermarked image

The watermark custom control

We are going to build an XPages watermark custom control whereby the developer can determine all the parameters of the effect through the custom properties of the control and therefore not have to change any of the underlying code.

There are 5 parameters which go with the jQuery plugin for watermark.js

  1. “className”: the class of the elements which should get a mark. (Optionally) Default: “watermark”
  2. “position”: “top-left”|”top-right”|”bottom-right”|”bottom-left”. (Optionally) Default: “bottom-right”
  3. “watermark”: an img element which represents your watermark. (Optionally)
  4. “path”: the path to your watermark image. (Optionally)
  5. “opacity”: opacity percentage, no “%” only the number [0-100] (Optionally) Default: 50

So we create the custom control in our database and under the “Property Definition” section we add our “properties”. Each property is a string except the position which is a combobox with pre-defined parameters.

Property Definition of a Custom Control

formWatermark Property Definitions

formWatermark Property Definitions

Entering custom properties to your custom control

And when we add the custom control to our XPage we are able to fill in the parameters for the control

Adding parameter values to our custom control in the XPage

Adding parameter values to our custom control in the XPage

Accessing the custom properties through the XPage

Accessing the custom properties through the XPage is achieved using compositeData.whateverParameterName. As you can see from the above example though not all the fields need to be filled in – so we need to compute their inclusion on the plugin instantiation.

This is the code used to create the effect. It must be added through and output script in the XPage because we need access to the Server Side variable computation which is not available through a Client Side library.

<xp:scriptBlock id="scriptBlock1">
			path: '#{compositeData.path}',
			className: '#{compositeData.className}',
			opacity: '#{compositeData.opacity}',
			position: '#{compositeData.position}',
			watermark: '#{compositeData.watermark}'

Watermark these images
Unless otherwise stated in the parameters the watermark.js effect is applied to all images with the class=”watermark”. Here are my images from the database.

	<xp:table style="width:100%" id="table1">
			<xp:td style="width:50%">
				<xp:image url="/Nin9TV.jpg" id="image1" styleClass="watermark">
				<xp:image url="/xpages.jpg" id="image2" styleClass="watermark">


The demonstration site shows a number of possibilities and allows the developer to change the variables and see the effect.

Demonstration of using watermark.js in an XPage

Demonstration of using watermark.js in an XPage

Apr 17, 2012
68 hits

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