Thursday, August 14, 2008

Javascript image magnifier

A friend of mine has asked me if she can put a script on her image gallery that magnifies the pictures the way that desktop software does when you zoom in. The main reason for this was that some of the people, browsing the gallery, have found it difficult to read some funny stuff like a road sign in the background of the picture. She needed something to work on the page itself, so the users won't need to save the picture on their pc and then zoom in to read this funny road sign. So, here it is, a javascript image magnifier:

I've made some settings for the the script to be more easily adjustable to what you need. The code starts with the declaration of the magnifier_settings array:

var magnifier_settings = new Array();
magnifier_settings['widht'] = 100;
magnifier_settings['height'] = 100;
magnifier_settings['zoom_level'] = 2;

The width and height indexes of the array are used to manage how big the magnifier should be. The zoom_level is, well, a zoom level :) The value in this var will be used to determine how much times the magnified image will be biger than the original one.

What fallows is a function that I've already posted about, which gets the offset coordinates of the cursor, according to the image on which it is called. This offsets will be used to determine where should the magnifier be displayed.

function get_image_offset(e, id)
 offset_x = e.offsetX ? e.offsetX : e.pageX-document.getElementById(id).offsetLeft;
 offset_y = e.offsetY ? e.offsetY : e.pageY-document.getElementById(id).offsetTop;

The new addition to the code in this function is the call to magnify(), which is the function that does the magnification.

function magnify(elmnt)
 //check if the element exists; if it does not - create it
  //create the div that will hold the magnifier
  var magnifier = document.createElement('div'); = 'magnifier'; = magnifier_settings['widht']; = magnifier_settings['height']; = 'hidden'; = '2px solid #000'; = 'relative';
  //magnified image. The original image is used to create the new one, which means that this function 
  //will work on any number of images on the same page, without the need to declare them or anything else..
  var magnifier_img = document.createElement('img'); = 'magnifier_img';
  magnifier_img.src = elmnt.src;
  magnifier_img.width = parseInt(elmnt.width)*magnifier_settings['zoom_level'];
  magnifier_img.height = parseInt(elmnt.height)*magnifier_settings['zoom_level'];
  //if the element already existst we don't need to recreate it. Just declare the same variables, as used when creating the elements. 
  //This saves a lot of work for the browser and makes the script run smoother even on older PCs
  var magnifier = document.getElementById('magnifier');
  var magnifier_img = document.getElementById('magnifier_img');
 //Adjust the magnifier position. Basically, it is set to the offset of the cursor plus a random value(i use 25) 
 //that will move the magnifier a little bit to the right  bottom of the page. This is used with two purposes: 
 //1. the magnifier will not pop over the cursor, so the user will actually see what will be magnified :)
 //2. this resolves an issue with the onmouseover trigger of the original image. If the value is not altered by 25, 
 //the instant when the magnifier is displayed, a onmouseout event is triggered on the original image, cause 
 //the mouse is now over the magnifier and not the original image. This causes the magnifier to be destoyed 
 //and in general is quite the oposite of what we want, cause this adds a big load to the client machine.
 //The top value is altered by the elements height, cause if the image is not at the top of the page,  
 //the magnifier will be misplaced somewhere above the image = offset_x+25; = offset_y+25-elmnt.height; 
 //Put negative margins to the magnified image, so the will show at the right place in the magnifier div = '-'+offset_x*magnifier_settings['zoom_level']+'px'; = '-'+offset_y*magnifier_settings['zoom_level']+'px';
 //Check again, if the magnifier is not created, cause if it's not we need to append it to the page elements
  //The only way I figured this magnification thing would work is using an image 'holder', which is a div 
  //with the same id as the  element plus the part '_holder'.
  var id ='_holder';

//Removes the magnifier from tha page. This can be changed to just alter the magnifier's display property to 'none'
//or visibility to 'hidden', but i figured removing it is the most appropriate way
function hide_magnifier(id)
 id += '_holder';


And now, the HTML code that will display the images on the page:

<div id='test_holder'>
<img id='test' src='test.jpg' onmousemove='get_image_offset(event,;' onmouseout='hide_magnifier('>
<div id='test2_holder'>
<img id='test2' src='test2.jpg' onmousemove='get_image_offset(event,;' onmouseout='hide_magnifier('>
<div id='test3_holder'>
<img id='test3' src='test.jpg' onmousemove='get_image_offset(event,;' onmouseout='hide_magnifier('>

You can put as many images as you wish, they will be handled by the same function automatically. I've used 3 images to show how the script works on more that one image. You can put the code on you entire gallery or just a single image, it's your call :)

The two things required for this script to work are as fallows:

  • You need to have an id on each image. This is very important, cause otherwise the script won't work and a handful of javascript errors will occur when the user mouseover's an image that doesn't have an 'id' property. You can use whatever id you want, just use one :)
  • The second thing is a div that holds the images. This is also very important, cause the script again won't work, and again there will be many many javascript errors when you mouseover the images. The requirement here is that the id for the div is the same as the id of the image it holds, plus the text '_holder'. You can use whatever styles you want on that div. If you want to, you can also put some other stuff in it, not just the image. Again - just put the images in divs and use the same ids with the text '_holder' added to them.

I made two demos for this script with different settings. The first one is here and it uses a 100x100 pixels magnifier, with a zoom level of 2. You can download the code from here. The second one can be seen here and it uses 200x200 pixels magnifier with the zoom level of 4. You can download this code from here.

No comments: