1. An image-processing apparatus comprising: a processor comprising: an extraction unit configured to extract data on an area subjected to thinning-out processing from image data based on a result of a comparison made between the image data and a predetermined image pattern; a thinning-out unit configured to reduce a signal level at a predetermined ratio for at least one pixel provided in the extracted data on the area subjected to thinning-out processing; and an addition unit configured to add dot data to the image data at a position away from an end of the area subjected to thinning-out processing by a predetermined distance, wherein the addition unit adds the dot data to the image data that are not adjacent to the image data subjected to the thinning-out processing; and a user interface for selecting a thinning-out level from a plurality of thinning-out levels, a thinning-out pattern from a plurality of thinning-out patterns, an additional-dot pattern from a plurality of additional-dot patterns, and selecting a distance between an additional-dot-data item and the image data.