Generating previews of 3d objects

Processing apparatus comprising:

• an interface to receive control data for generating a three-dimensional object and specifying, for voxels of the object, at least one print material to be deposited at that voxel during object generation;
• an image processing module to determine which of the voxels is visible in a viewing frustum and to determine, from the control data, at least one voxel appearance parameter for a voxel;
• a mapping module to map each visible voxel to a voxel set, wherein each voxel set is to be represented in a preview display pixel;
• a combining module to combine at least one voxel appearance parameter of the voxels in each voxel set and to generate at least one pixel appearance parameter;
• a display control data module to determine display control data to control a pixel of a display according to the pixel appearance parameter.

A non-transitory computer readable storage medium encoded with instructions, executable by a processor, the machine readable storage medium comprising:

• instructions to identity control data for an additive manufacturing apparatus, the control data being to generate a three-dimensional object and specifying, for voxels to the object, at least one print material to be deposited at that voxel during object generation;
• instructions to determine, from the control data, at least one voxel appearance parameter of a voxel in the three-dimensional object;
• instructions to determine, for a viewing frustum, the visible voxels of the three dimensional object;
• instructions to determine, for a display pixel, a set of visible voxels to be represented by that display pixel;
• instructions to combine at least one voxel appearance parameter of the voxels in a set of visible voxels;
• instructions to determine control data to control a pixel of a display to display a representation of the combined appearance parameters.