Apparatus and methods are provided, including displaying an image of one or more lumens of a subject. A processor includes lumen-identification functionality configured to automatically identify one or more lumens in the image. Envelope-designation functionality of the processor designates as lumen envelopes, portions of the image in vicinities of respective lumens. In response to the user designating a location inside a lumen envelope of a given lumen, via an input device, the location corresponding to a given longitudinal location within the given lumen, feature-generation functionality of the processor generates a feature on the image that corresponds to the given longitudinal location. An output is displayed in response to the generated feature. Other embodiments are also described.