When Steve Jobs takes the stage Monday at Apple's programmers conference, he's likely to give the world a glimpse of an upgraded Mac operating system that could herald the biggest changes to the machine's interface in 30 years.

At the annual Worldwide Developers Conference in San Francisco, Jobs will probably show off Leopard, a Mac OS X update due in October that he has promised contains "top secret" features. But perhaps the most important feature is one that has been overlooked by many Apple fans: a new set of tools for building program interfaces called Core Animation.

Core Animation will allow programmers to give their applications flashy, animated interfaces. Some developers think Core Animation is so important, it will usher in the biggest changes to computer interfaces since the original Mac shipped three decades ago. "The revolution coming with Core Animation is akin to the one that came from the original Mac in 1984," says Wil Shipley, developer of the personal media-cataloging application Delicious Library. "We're going to see a whole new world of user-interface metaphors with Core Animation."

Shipley predicts that Core Animation will kick-start a new era of interface experimentation, and may lead to an entirely new visual language for designing desktop interfaces. The traditional desktop may become a multilayered three-dimensional environment where windows flip around or zoom in and out. Double-clicks and keystrokes could give way to mouse gestures and other forms of complex user input. The Core Animation "revolution" is already starting to happen. Apple's iPhone at the end of the month will see people using their fingers to flip through media libraries, and pinching their fingers together to resize photos.