JavaScript Bible, Gold Edition part 67. This book will bring programmers and non-technical professionals, including casual programmers and scripters, painlessly up to speed on all aspects of mastering JavaScript. Key topics include programming fundamentals, JavaScript language elements and how to use them effectively, and how to easily and efficiently add powerful new functionality to HTML documents and Java applets. | 508 Part III Document Objects Reference Depending on the type and size of image you will be amazed at the speedy response of this kind of loading. With small-palette graphics the image displays instantaneously. A popular user-interface technique is to change the appearance of an image that represents a clickable button when the user rolls the mouse pointer atop that art. This action assumes that a mouse event fires on an element associated with the object. Prior to IE4 and NN6 IMG element objects did not respond to mouse events on their own. The required technique was to encase the IMG element inside an A element. This allowed the events associated with rollovers onMouseOver and onMouseOut and a user click on the image to effect some change usually to navigate to another page . While IE4 and NN6 provide these events directly for IMG element objects you can guarantee your pages to be backward-compatible if you continue to surround your images with A elements. You can see examples of these kinds of actions in Chapters 12 and 22. Image rollovers are most commonly accomplished in two different image states normal and highlighted. But you may want to increase the number of states to more closely simulate the way clickable buttons work in application programs. In some instances a third state signifies that the button is switched on. For example if you use rollovers in a frame for navigational purposes and the user clicks a button to navigate to the Products area that button stays selected but in a different style than the rollover highlights. Some designers go one step further by providing a fourth state that appears briefly when the user mouses down an image. Each one of these states requires the download of yet another image so you have to gauge the effect of the results against the delay in loading the page. The speed with which image swapping takes place may lead you to consider using this method for animation. Though this method may be practical for brief bursts of
