1. A method for defining a selection region in a source image, the method comprising: displaying, on a display device, a selection cursor over the source image, the selection cursor having an interior region and a peripheral region; responsive to user input, moving the selection cursor from a first region of the source image to a second region of the source image; and as the selection cursor moves, locating an initial position of a selection mask boundary in a portion of the source image that is overlaid by the peripheral region of the selection cursor, wherein the initial position of the selection mask boundary is at least partially based on a boundary that is detected as a result of selectively applying a boundary detection algorithm to a sub-region of the source image, wherein the boundary detection algorithm is not applied in a portion of the source image that is overlaid by the interior region of the selection cursor.