A 3D printer is disclosed herein. The 3D printer comprises a build material distributor, an agent delivery device and a controller. The controller obtains slice image data of a sliced virtual build volume including a 3D object to be generated, where the slice image data is indicative, for each slice, of the portions of corresponding build material layers to be solidified. The controller is to determine print data corresponding to each slice, the print data defining a density of printing agent to be applied to regions of each layer based on the slice image data; and to obtain additional data relating to a portion of a slice that corresponds to a sloped portion of the 3D object. The controller is further to modify the determined print data to modify the density of printing agent to be printed at a portion of a given build material layer based on the additional data. The controller is also to control the build material distributor to generate a build material layer; and the agent delivery device to selectively eject the printing agent based on the modified print data.