Enforcing initial containment of a JQuery draggable

StackOverflow https://stackoverflow.com/questions/4902738

Question

For a photography webpage I am implementing an image viewer that allows for selecting an area of interest in a scaled-down reference image by dragging a semi-transparent rectangle over the reference image. The selected area is continuously "mirrored" to a second image viewer, which shows the area with at 100% magnification ratio.

My solution uses XHTML/CSS and jQuery and I have a basic version that is working already – but suffers from a problem at initialization, which I cannot solve. I have set up the area of interest as a div that is made draggable using jQuery UI draggable. To constrain the area of interest to the image, I have used the containment property.

Initially the div is however shown outside of the image. As soon as I start dragging the area of interest div, the div snaps to the interior of the image and containment is properly enforced henceforth. All sample code I found, demonstrates containment with nested divs, which does not apply to my case since to my knowledge an image (XHML img) cannot contain other elements.

Is there a way how I can enforce that the draggable area of interest div is already contained when the webpage is initially rendered?

I have extracted to minimal code to illustrate the problem from my code, see below.

I will be glad for any hint.

Cheers, Christian

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <script type="text/javascript" src="js/jquery.js"></script>
    <script type="text/javascript" src="js/jquery-ui-1.8.9.custom.min.js"></script>

    <script type="text/javascript">
    $(document).ready(function() {
        $("#zoom_area").draggable( {
            containment: '#the_image'
        });
    });

    </script>

<style type="text/css">

#the_image {
    width: 640px;
    height: 427px;
}

#zoom_area {
    width: 50px;
    height: 50px;
    background-color: #bbb;
    border: 1px dashed black;
    opacity: 0.6;
}

</style>

</head>
<body>

<div id="image_viewer">
    <img id="the_image" src="img/img1.jpg" alt="reference image" />
</div>

<div id="zoom_area">
</div>

</body>
</html>
Was it helpful?

Solution

There are two ways to do this. First, you can try doing it without an image tag. Try placing #zoom_area inside #image_viewer, and using a CSS background image to include the image rather than an image tage.

HTML:

<div id="the_image">    
    <div id="zoom_area"></div>
</div>

CSS:

#the_image {
    width: 640px;
    height: 427px;
    background-image:url('img/img1.jpg');
}

#zoom_area {
    width: 50px;
    height: 50px;
    background-color: #bbb;
    border: 1px dashed black;
    opacity: 0.6;
}

See this first example in action: http://jsfiddle.net/qUtEW/

If you need to use an image tag, then you can use the following code:

HTML

<div id="image_viewer"> 
        <img id="the_image" src="img/img1.jpg" alt="reference image" /> 
        <div id="zoom_area"></div>
</div>

CSS:

#the_image {
    width: 640px;
    height: 427px;
}

#zoom_area {
    position: absolute;
    z-index:10;
    top:10px;
    width: 50px;
    height: 50px;
    background-color: #bbb;
    border: 1px dashed black;
    opacity: 0.6;
}

The trick here is to set position absolute and z-index 10.

See this second example in action: http://jsfiddle.net/pBRgb/

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