Apparatus and methods are described, including at least one tool configured to be placed inside a subject's mouth. A first optically-visible marker is coupled to the tool. A second optically-visible marker is coupled to a structure inside the subject's mouth. A plurality of optical cameras are coupled to the tool, and are placed inside the subject's mouth by the tool being placed inside the subject's mouth. The cameras acquire respective images of the inside of the subject's mouth, in which the second optically-visible marker is identifiable. A plurality of optical cameras that are disposed outside the subject's mouth acquire respective images of the tool, while the tool is inside the subject's mouth, in which images the first optically-visible marker is identifiable. Other embodiments are also described.