Automatically extending a boundary for an image to fully divide the image

Some embodiments provide a method for editing an image. For a displayed image, the method draws a partial boundary based on movement of a cursor over the image. The partial boundary does not fully divide the image into a region of interest and a second region. Upon completion of the cursor movement, the method automatically extends at least one end of the boundary in order to divide the image into the region of interest and the second region. The method applies edits (e.g., color correction) to only the region of interest of the image. In some embodiments, automatically extending at least one end of the boundary involves (i) extending one or both of the ends to points at which the boundary ends intersect borders of the image and (ii) connecting the boundary from the intersection points along the borders of the image.