A method comprising: acquiring three-dimensional surface data from a subject as a sequence of three-dimensional images, each one of the three-dimensional images of the sequence fitted to at least one previous one of the three-dimensional images of the sequence, acquiring an image set for a next three-dimensional image; when the image set can be converted into the next three-dimensional image and the next three-dimensional image can be fitted to the three-dimensional surface data, superimposing the three-dimensional surface data and the next three-dimensional image onto a two-dimensional image of the subject in a display; and when the image set cannot be converted into the next three-dimensional image or the next three-dimensional image cannot be fitted to the three-dimensional surface data, entering a recover mode comprising the steps of: superimposing a current two-dimensional image of the subject onto a previous two-dimensional image of the subject in the display, the previous two-dimensional image representing a view of the subject from a position from which one of the three-dimensional images of the sequence was acquired, and the current two-dimensional image representing a view of the subject from a position from which the image set was acquired; acquiring at least one subsequent three-dimensional image; and test fitting the at least one subsequent three-dimensional image to one or more of the three-dimensional images of the sequence. acquiring three-dimensional surface data from a subject as a sequence of three-dimensional images, each one of the three-dimensional images of the sequence fitted to at least one previous one of the three-dimensional images of the sequence, acquiring an image set for a next three-dimensional image; when the image set can be converted into the next three-dimensional image and the next three-dimensional image can be fitted to the three-dimensional surface data, superimposing the three-dimensional surface data and the next three-dimensional image onto a two-dimensional image of the subject in a display; and when the image set cannot be converted into the next three-dimensional image or the next three-dimensional image cannot be fitted to the three-dimensional surface data, entering a recover mode comprising the steps of: superimposing a current two-dimensional image of the subject onto a previous two-dimensional image of the subject in the display, the previous two-dimensional image representing a view of the subject from a position from which one of the three-dimensional images of the sequence was acquired, and the current two-dimensional image representing a view of the subject from a position from which the image set was acquired; acquiring at least one subsequent three-dimensional image; and test fitting the at least one subsequent three-dimensional image to one or more of the three-dimensional images of the sequence. superimposing a current two-dimensional image of the subject onto a previous two-dimensional image of the subject in the display, the previous two-dimensional image representing a view of the subject from a position from which one of the three-dimensional images of the sequence was acquired, and the current two-dimensional image representing a view of the subject from a position from which the image set was acquired; acquiring at least one subsequent three-dimensional image; and test fitting the at least one subsequent three-dimensional image to one or more of the three-dimensional images of the sequence.