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Shape, color and brilliance.
These are the three most salient visual characteristics of an object. Currently, 3D printers can reproduce shape and color reasonably well. The shine, however, remains a challenge. This is because 3D printing hardware is not designed to handle the different viscosities of paints that give surfaces a glossy or matte appearance.
MIT researcher Michael Foshey and his colleagues may have a solution. They developed a combined hardware and software printing system that uses standard paints to finish objects with realistic and spatially varying gloss patterns. Foshey calls the progress “a chapter in the book on how to perform high fidelity appearance reproduction using a 3D printer”.
It provides a range of applications for the technology. It could be used to faithfully reproduce fine art, allowing near-flawless replicas to be distributed to museums without access to the originals. It could also help create more realistic looking dentures. Foshey hopes the progress represents a step towards a visually perfect 3D printing, “in which you almost couldn’t tell the difference between the object and the reproduction.”
Foshey, a mechanical engineer from MIT Computer Science and Artificial Intelligence Laboratory (CSAIL), will present the paper at next month’s SIGGRAPH Asia conference, together with lead author Michal Piovarči of the University of Lugano in Switzerland. Co-authors include Wojciech Matusik of MIT, Vahid Babaei of the Max Planck Institute, Szymon Rusinkiewicz of Princeton University, and Piotr Didyk of the University of Lugano.
Gloss is simply a measure of the amount of light reflected off a surface. A shiny surface is reflective, like a mirror. A dull or dull surface is not reflective, such as concrete. Paints that impart a glossy finish tend to be less viscous and dry to a smooth surface. Paints that give a matte finish are more viscous, closer to honey than water. They contain large polymers which, when dried, randomly protrude from the surface and absorb light. “You have a lot of these particles poking out of the surface, which gives you that roughness,” says Foshey.
But those polymers pose a dilemma for 3D printers, whose thin fluid channels and nozzles aren’t built for honey. “They are very small and can become clogged easily,” says Foshey.
The state-of-the-art way to reproduce a surface with variable gloss in space is laborious: the object is initially printed with a high gloss and with support structures that cover where a matte finish is desired. Then the support material is removed to give roughness to the final surface. “There is no way to instruct the printer to produce a matte finish in one area or a glossy finish in another,” says Foshey. So, his team came up with one.
They designed a printer with large nozzles and the ability to deposit droplets of paint of various sizes. The paint is stored in the pressurized tank of the printer and a needle valve opens and closes to release droplets of paint on the print surface. A variety of droplet sizes are achieved by controlling factors such as tank pressure and the speed of the needle valve movements. The greater the quantity of paint released, the greater the droplet deposited. The same goes for the drop release rate. “The faster it goes, the more it spreads once it hits the surface,” says Foshey. “So, essentially, by varying all of these parameters to get the size of the drop we want.”
The printer achieves a variable gloss in space through halftones. In this technique, the discrete droplets of paint are arranged in patterns which, when viewed from a distance, appear as a continuous surface. “Our eyes actually do the blending themselves,” says Foshey. The printer uses only three standard paints: one glossy, one matte and one in between. By incorporating these paints into its preprogrammed halftone pattern, the printer can produce continuous and spatially varying shades of gloss on the print surface.
Along with the hardware, Foshey’s team produced a software pipeline to control printer output. First, the user indicates the desired glossy pattern on the surface to be printed. Next, the printer performs a calibration, testing various halftone patterns of the three paints provided. Based on the reflectance of these calibration patterns, the printer determines the correct halftone pattern to use in the final print job to achieve the best possible reproduction. The researchers demonstrated their results on a variety of “2.5D” objects – mostly flat prints with textures that varied by half a centimeter in height. “They were impressive,” says Foshey. “They definitely have more of a feel for what you’re actually trying to reproduce.”
The team plans to continue developing hardware for use on fully 3D objects. Didyk says “the system is designed in such a way that future integration with commercial 3D printers is possible.”
This work was supported by the National Science Foundation and the European Research Council.
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