1. An outline definition apparatus comprising: an edge detection unit configured to detect pixels located at edge positions of a still image, each of the pixels located at the edge positions having a data value between a value representing a first level and a value representing a second level that is larger than the value representing the first level; a pixel extraction unit configured to extract the pixels located at the edge positions which are detected using the edge detection unit and extract pixels in the vicinity of the pixels located at the edge positions; a boundary-line generation unit configured to generate, in accordance with a data value of each of the pixels extracted using the pixel extraction unit, a boundary line indicating a boundary between a region of the first level and a region of the second level in each of the pixels which are located at the edge positions and are detected using the edge detection unit; and a link-processing unit configured to obtain an outline indicating a boundary between a region of the first level and a region of the second level in the still image by linking the boundary lines which are generated in the boundary-line generation unit and which are generated for the pixels which are located at the edge positions and are detected using the edge detection unit.