1. A method comprising detecting the presence of an object in at least one image using an object detection algorithm that uses training image data to train the detection algorithm to detect an object based at least in part on a similarity of appearance of image data in a first area of an image to data derived at least in part from the training image data, the method comprising: providing a first output indicative of a relatively high similarity of appearance of at least part of a first area of an image to data derived at least in part from the training image data, the first output indicating that a first detection confidence is sufficiently high to indicate that a first object is likely to have been detected in the first area of an image; determining a characteristic of a second area of an image in which an object may be detected, the characteristic being derived from data relating to the first area and being capable of indicating a likelihood of presence of a second object in the second area; and based on the determined characteristic, and a similarity of appearance of at least part of the second area to data derived at least in part from the training image data, providing a second output indicating that a second detection confidence is sufficiently high to indicate that the second object is likely to be present in the second area.