1. A method of pre-processing an image to identify processes for subsequent processing of the image, comprising the steps of: a) investigating portions of the image using a spatial filter; b) calculating for a first plurality of regions within a portion of the image under investigation respective metrics as a function of intensity within those regions; c) selecting combinations of regions within the portion of the image under investigation and processing them to obtain a second plurality of filter values, where the second plurality is greater than the first plurality; and d) comparing the filter values with process thresholds for subsequent processes so as to identify subsequent processes that can be skipped.