A method for additive fabrication by 3D printing includes processing model data representing material transition boundaries of an object to be printed to form build data for use in controlling printing of a plurality of successive layers to form the object, the build data comprising, for each location of a plurality of locations in a two-dimensional arrangement, material transition data for representing heights of material transitions in a third dimension, and repeating for each layer of the plurality of successive layers, receiving surface height data representing a height of a partial fabrication of the object at respective locations of a plurality of locations on a surface of the partial fabrication for each location of the plurality of locations, using the height at the location to access the material transition data corresponding to the location in the build data, and using the material transition data to determine material to be deposited at that location, and causing emission of the determined material at each location of the plurality of locations, thereby causing printing of the layer.