A graphical user interface for a computer program enables a user to define a model representing a predefined ideal pattern of objects, the model being useful in analyzing an image of an actual pattern of discrete objects with respect to the model. In the interface, a fragment of the ideal pattern of objects is displayed, the displayed fragment including a graphical indicator of a key distance between objects in the fragment. A device which is visually associated in the fragment with the indicator enables the user to specify a value for the distance. The interface also includes a displayed image representing the pattern of objects, and an interactive device that enables a user to turn on or turn off an object in the image as an indication of its inclusion or exclusion in the pattern.