Imagine that the egg is a globe with lines of latitude and longitude. The X and Y carriages position the print head from about 85 degrees (near the top or north pole) to about -80 degrees in steps. At each step the egg is rotated in longitude 360 degrees while printing much like printing a line on a conventional inkjet printer. The printhead in positioned to the next latitude increment and egg is rotated 360 degrees in the opposite direction. Much like the conventional sheet printer. Note that the print head must be rotated at each latitude increment to keep it perpendicular to the egg surface.
The egg is held firmly on a suction cup connected to a vacuum pump. The egg cannot be rotated continuously because of the vacuum hose connection so hence the 0 to 360 degree limitation.
Since no two natural eggs will have the same shape, the shape of each egg must be determined prior to printing so the print head can be properly positioned. This is done by projecting a spot of light focused at the optimum distance for the print head. A small CCD camera with a resolution of 64x64 pixels observes the spot. First, the print head is positioned to maximize the roundness of the spot. An elongated spot is the result of the spot projector beam not being perpendicular to the egg surface. Next the print head is positioned for minimum spot diameter which occurs when the surface is a the focal point of the spot projector. The entire surface of the egg is thus sampled and mapped. During the print phase, this data is interpolated and used to smoothly position the print head. With molded artificial eggs, this profiling needs to be done only once.
This printer should also work for matte surface Christmas ornaments and golf balls (this ball property of ...).