1. A method of operating a machine that includes: image input circuitry for obtaining data defining images as input; memory for storing data; and a processor connected for receiving data defining images from the image input circuitry and for accessing the memory; the method comprising: operating the processor to receive input image data from the image input circuitry, the input image data defining an input image; operating the processor to use the input image data to obtain initial array data; the initial array data defining an initial array of value items, each value item indicating a value; storing the initial array data in the memory; operating the processor to access the initial array data in the memory and to use the initial array data to obtain gap data indicating one or more gaps between value items in the initial array; operating the processor to use the gap data to obtain threshold data indicating a threshold; each gap indicated by the gap data being above or below the threshold; the threshold data indicating a threshold that would produce a number of groups of the value items; the number of groups being stable across a range of thresholds; the range of thresholds meeting a criterion for largeness of a range; and operating the processor to use the threshold data to obtain grouping data indicating a grouping of the value items in the initial array.