1. An image processing apparatus comprising: a region segmentation unit configured to divide an input image into a plurality of regions; a selection unit configured to determine whether each of the regions is suitable for thinning processing and configured to select outlining processing to be executed based on a result of the determination; an outlining unit configured to generate outline data for each of the regions using the outlining processing selected by the selection unit; and an outputting unit configured to output outline data generated by the outlining unit.