Abstract: A method and apparatus are provided for classifying areas on a raster-type display unit. The problem is to identify areas defined by a closed boundary--e.g. for filling in such areas. Boundary data are entered into a first "polygon" store which has a plurality of memory locations each corresponding to a respective dot position of the display field, and at least one seed point is identified. Each dot position is then examined to see whether there exists a connected path, not including a boundary position, from it to a seed point, and if there is, this fact is recorded. The criterion for deciding whether or not a path is connected depends on the definition of what constitutes a closed boundary.