1. A method of inspecting a photolithographic reticle, the method comprising: using an inspection system at one or more operating modes, obtaining images of a plurality of training regions of a reticle, wherein the training regions are identified as defect-free; deriving three or more basis training images from the images of the training regions; forming a classifier based on the three or more basis training images by: mapping the three or more basis training images to a plurality of voxels of a three or more dimensional space; and assigning an observation indicator to each voxel so as to specify whether such voxel is defined as a non-defective voxel that is observed in the training set; using the inspection system at the one or more operating modes, obtaining images of a plurality of test regions of a reticle; and deriving three or more basis test images from the test regions; and applying the classifier to the three or more basis test images to find defects in the test regions.