Tech

The difference between a matt and a glossy painted surface shows that there's more to the appearance of real-world objects than colour alone. But reproducing that variation in printed images has been beyond the capabilities of even the best colour printers. Now an international team of computer scientists says that could soon change thanks to a printer that can reproduce sheen as well as colour.

Spot the difference

Some modern printers can use matt, glossy or metallic inks to change the reflectivity of an image, but the inks are always used on their own, as so-called spot colours. But by carefully mixing a range of such metallic inks, Fabio Pellacini at Adobe Systems and Dartmouth College in Hanover, New Hampshire, says it's possible to reproduce subtle differences in reflectivity in the same way that mixing cyan, magenta and yellow can reproduce a range of colours.

Pellacini worked with colleagues including Wojciech Matusik and Szymon Rusinkiewicz at Adobe Systems. The team used a colour thermal printer, which is versatile enough to print many metallic inks and foils as well as standard inks.

Mixing the inks

The printer they used can accept up to seven ink cartridges at one time, but to produce a full range of reflectivity the team needed 12 inks: cyan, metallic cyan, magenta, metallic magenta, yellow, metallic gold, black, metallic silver, gold foil, silver foil, finish and primer.

They swapped individual cartridges and printed over images several times to produce the final product. By overlaying up to six inks, the team worked out how to represent a spectrum of colours with a wide range of reflectivities.

But being able to control the reflectivity of a printed image solves only half the problem. To reproduce real-world objects on the printed page, the team first had to assess the reflectivity of the original object. For this, they used an automated process that takes several pictures of an object from different angles and lit from different positions.

The team then used this information to determine the ratio between the amounts of incoming and reflected light, says Pellacini. "[The process] captures not only the matt colour of a surface but its highlights, allowing us to represent a wide range of materials like plastics, metals and paints."

3D printing

The first prints using the new technique (see image, above right) are relatively poor quality because the measurement technique has limited resolution, Pellacini says. But future developments should improve the technique. The goal is to combine the technique with 3D printing to create 3D objects that look more realistic, he says.

Roger Hersch at the Swiss Federal Institute of Technology in Lausanne, who was not involved in the study, says there is a trend towards adding more than pure colour to prints. "It provides an additional dimension which may be used by designers to create new effects," he says.

A job for the specialists

But he doubts that the new technology will appeal to home computer users yet. "It's mainly graphic art professionals who will be interested in the technology," he says.

Pellacini agrees. "There is still the issue of the 'input' to our printer," he says – very few labs have the equipment to measure the reflectivity of an object, and without that information it's not possible to reproduce an object's surface texture in print.

Pellacini's team will present their work at the Siggraph Asia conference in Yokohama, Japan, in December.

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