A robot caricaturist that can draw an accurate likeness of its subject in just three minutes was wowing the crowds on Wednesday at the world's biggest IT fair, but not every customer was satisfied.

The robot, designed by Germany's Fraunhofer Institute, takes a black-and-white picture of the subject, its computer brain then calculates the contrasts and contours of the face and it sketches what it "sees".

But, in contrast to a human artist, this robot is scrupulously honest, which can leave some punters feeling they have been poorly treated.

"Some people go away disappointed," admitted Karin Stein, a researcher on the project.

"For example, if you have very small lips, the computer registers very small lips. There is hardly any contrast or contour, so what you end up with is a short, straight line. People aren't happy," she added.

"The robot could do the portrait in three minutes flat, but we wanted to recreate the experience in Montmartre," she explained, with most portraits taking around 10 minutes.

"In fact, the younger you are, the quicker it is, because it takes more time for the computer to recognise and draw wrinkles."

Like most gadgets at this year's CeBIT, the world's biggest high-tech fair in the northern German city of Hanover, what may appear frivolous has a higher purpose.

In fact, the robot is just showing off its skills. Its day job is to work on measuring how light reflects off various surfaces.

This is crucial for research into reflective clothing for road workers or street signs, for example, when it is preferable to have as much light beamed back as strongly as possible, or camouflage when the opposite is needed.

But the street artists of the world can rest easy for the moment, as they still have the advantage of price.

Whereas a caricature might set you back $20 or so, this robot is worth at least 40,000 euros ($52,000).

"And that's just the hardware," Stein said, smiling.