Computer animations have become increasingly realistic over the years, but a few lingering nuances still trigger our brains to recognize that a character is not flesh and blood. It's the subtle details -- a facial expression or muscle movement -- that usually gives it away. But now researchers from the Bournemouth University in the U.K., have developed easy-to-use tools that make skin and the muscles underneath it move more realistically.

In the figure on the right, the major muscles on the left half of its body have been extracted using the researchers’ technique. The muscles are used to deform the skin surface when the character is animated. (Credit: Jian Jun Zhang)

When animators build a character, they usually start with its external appearance, rather than internal features such as skeletal structure and musculature, says lead researcher Jian Zhang, professor of computer graphics at the University of Bournemouth. Details such as the way a muscle looks when flexed are added after the basic look of the character is complete.

This approach, called skin-based, is popular in the animation industry because it's the most intuitive for animators to use, he says. And, indeed, drawing an arm with a bulging biceps is easy enough -- if the character isn't moving.

However, the skin-based approach isn't the most accurate way to capture the nuanced movement of skin as muscles contract below: that requires information about the shape of the muscles themselves. And ill-fitting skin can make an animated character look fake.

A more accurate method, Zhang says, is to build a character's muscles first and then add the skin on top of them. It's more natural looking, he says, but it requires detailed knowledge of anatomy, and it's counterintuitive for an artist who has the external appearance of a character in mind. For those reasons, he says, the muscle-based approach has been mostly an academic curiosity.

The researchers' algorithm combines the realism of the muscle-based approach with the intuitive appeal of the skin-based model. It allows an animator to start with a character's outward appearance and modify the movement of major muscle groups -- without having to build the musculature from scratch.

The problem is more complex than it may sound, says Zhang. As soon as an arm is flexed, for instance, the animation algorithm needs to use information about the shape of the muscle under the skin to make the muscle contract and the skin deform accurately. When the only information about the muscle comes from the shape of the skin around it, it's challenging to make flexing look realistic.