A method and apparatus for creating a print file for printing a 3-D print using a 3-D printing system is described, the 3-D print comprising a plurality of 3-D structures printed on a substrate, each 3-D print structure having a height with respect to the substrate. A two-dimensional source image is input comprising a plurality of image pixel areas. A filtered image is obtained by applying a topographic operator to the source image to generate for every image pixel a representation of a pixel height profile, the pixel height profile corresponding to cross-sections through a 3-D print structure which is to be formed by 3-D printing. Thereby definitions of a plurality of image layers are generated from the filtered image for printing using the 3-D printing system. The print file is output based on the plurality of image layers.;The topographic operator has the following property:;any cross-section through a solid section of a 3-D structure at a second level in the 3-D print structure which is closer to the substrate than a first level has an area which is equal to or larger than the area of the cross-section of the 3-D structure at the first level.
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