1. A computer-implemented method of forming a high dynamic range image comprising: receiving a plurality of digital images of the same scene, each digital image having been captured at a different exposure level; for each received image, computing a seed mask comprising, for each image element of the received image, an indication as to whether that image element is appropriate for the exposure level of the received image; for each received image computing a geodesic distance map using the associated seed mask, the geodesic distance map having a geodesic distance value for each image element of the received image to an image element of the seed mask which is indicated as appropriate for the exposure level of the received image; combining the received digital images according to the geodesic distance maps to form the high dynamic range image.