After rounds of trial and error, he found the key in the theory of deformation, which, he says, accounts for 70 percent of what it takes to make a 3D painting.

The technique of paper-cutting is sometimes applied to enhance the effect, he adds.

"To put it very simply, you need to draw your subject higher and longer," Cao says. "For example, the mouth of a cup is elliptical in 2D drawings, but should be round for a 3D effect. The subject must also have a bigger head and small bottom. Finally, you choose the right angle to take a photograph," he explains.

It took Cao more than a month to acquire working skills. He started with Chinese words before delving into more complex patterns. "No one was there to teach me. I had to explore on my own, so there were many detours. I guess the best way to learn the art is through constant study and practice," he says.