Build material switching

A method for three-dimension fabrication using an extruder having a feed to receive a build material and a nozzle to extrude the build material, the method comprising:

• receiving a tool path for the extruder to fabricate an object from a three-dimensional model, the three-dimensional model including a pattern of colors on a surface thereof and the tool path defining interstitial void space between infill formed by the tool path for use in fabricating the object;
• identifying, within the tool path, a change from a first color of the pattern of colors to a second color of the pattern of colors that results in a portion of the pattern of colors on the surface;
• calculating a transition having a transitional length including a beginning and an end for the change from the first color to the second color to occur, the calculation of the transition incorporating a safety margin to ensure that the transition is completed over the transitional length;
• altering the tool path to cause a continuous excursion from the surface of the object for the transition having the transitional length including the safety margin, the transitional length beginning after the continuous excursion from the surface of the object and completing before a return to the surface of the object, wherein the continuous excursion includes a length located within the interstitial void space between infill formed by the tool path;
• changing the build material supplied to the extruder from the first color to the second color;
• and
• fabricating a portion of the object by moving the extruder along the continuous excursion after changing the build material supplied to the extruder.