  Fused filament fabrication is the method of choice for printing 3D models at
low cost and is the de-facto standard for hobbyists, makers, and schools.
Unfortunately, filament printers cannot truly reproduce colored objects. The
best current techniques rely on a form of dithering exploiting occlusion, that
was only demonstrated for shades of two base colors and that behaves
differently depending on surface slope.
  We explore a novel approach for 3D printing colored objects, capable of
creating controlled gradients of varying sharpness. Our technique exploits
off-the-shelves nozzles that are designed to mix multiple filaments in a small
melting chamber, obtaining intermediate colors once the mix is stabilized.
  We apply this property to produce color gradients. We divide each input layer
into a set of strata, each having a different constant color. By locally
changing the thickness of the stratum, we change the perceived color at a given
location. By optimizing the choice of colors of each stratum, we further
improve quality and allow the use of different numbers of input filaments.
  We demonstrate our results by building a functional color printer using low
cost, off-the-shelves components. Using our tool a user can paint a 3D model
and directly produce its physical counterpart, using any material and color
available for fused filament fabrication.
