3-D printing is driving a huge revolution in the world of design and technology. In the process, it is changing the way we think about the design, prototyping, and manufacturing of just about everything.  

But anyone who has played with a 3-D printer will be aware of one significant problem. This 800-pound gorilla is the issue of color. 3-D prints can be magnificent copies of more or less any shape. But in terms of color, they are mere shadows of the originals.

Today, that looks set to change thanks to the work of Alan Brunton and pals at the Fraunhofer Institute for Computer Graphics Research in Germany, who have worked out how to produce accurate colors in a 3-D print for the first time. Their work promises to take 3-D printing to an entirely new level.

The new approach takes advantage of a relatively new way to make 3-D prints. In general, these objects are made one layer at a time by fusing powder or laying down extruded plastic. Neither approach gives anything but rudimentary control over an object’s color.

What’s needed instead is a way of creating objects in the same way as 2-D printers make images, pixel by pixel. In other words, this requires 3-D prints to be laid down, not in layers, but voxel by voxel.

In the last year or so, exactly this technology has come to market. It works using a number of inkjets that lay down an object, droplet by droplet. These droplets are instantly cured by UV light to form a solid.

That immediately allows the possibility of much more accurate control of color, since each droplet can be thought of as a voxel. This is the approach that Brunton and pals have taken, but it is easier said than done for a number of reasons.

The first is the sheer volume of data and number crunching involved in creating a virtual color 3-D object, even before the printing begins. The droplets from inkjets are tiny—there are some 18 million of them in a solid cubic centimeter. So any decent-sized object must be made up of tens of billions of voxels and the impact that each one has on the final color has to be calculated.

The second is that the droplets are translucent because UV light must be able to pass through to cure them. This has a significant impact on their visual appearance since light ends up passing through several layers of voxels, being scattered along the way.

That means droplet color has to be carefully controlled to a depth of several voxels throughout the object. And this dramatically increases the complexity of the algorithms needed to calculate their required colors.

The final challenge comes from the nature of 3-D printing. In 2-D printing, it is possible to combine up to three different inks at any point on an image. In a 3-D print, each droplet must be a single material and that places important constraints on what is possible colorwise.

Nevertheless, Brunto and co have made significant advances by bringing to bear the many decades of research that has been done on color management for 2-D printing and for color imaging in general.

Their approach is to combine two techniques. The first is the 3-D equivalent of a 2-D printing technique called half-toning. This is where continuous shade and color is replaced by an arrangement of dots of different sizes and spacing. The second is a way of calculating the color of a surface given the way light has been scattered for several layers of voxels below.

And the results look impressive. In the pictures above, three apples and the thumb are real. The rest are 3-D prints but it is no easy task to tell them apart.

And Brunton and co say the results should get better in the near future as materials scientists develop less translucent printing materials and as printers become even higher resolution. In both these respects, the team’s algorithms are future proof. Less translucent inks should be easier to handle and the higher resolution should be manageable too.

The ability to combine translucent and opaque inks should even make it possible to reproduce the surface appearance of many biological materials that are also semi-translucent, such as skin.

That’s fascinating work. It will usher in a new generation of printing application. And it will make the current generation of printers look thoroughly old-fashioned in just a few years.

Ref: arxiv.org/abs/1506.02400 : Pushing the Limits of 3-D Color Printing: Error Diffusion with Translucent Materials