A method comprises: obtaining object model data defining an object or objects to be built by a three-dimensional printing apparatus; determining an effect of carrying out a post-build process on the object or objects, wherein the determining uses the object model data and a computer-implemented evaluation of the post-build process and is carried out prior to building the object or objects, identify portions of the object or objects that would not be processable by the cleaning process; and outputting an indication of non-processability of portions of the object or objects are determined not to be processable by the post-build process.