Image processing apparatus, image processing method and computer-readable medium

An image processing apparatus which performs boundary line extraction for raster image data, comprising:

• a detection unit configured to detect a color region formed from a plurality of pixels connected in an oblique direction of an 8-neighbor connection in the image data;
• a determination unit configured to determine whether the oblique 8-neighbor connection in the color region detected by said detection unit is to be modified to a 4-neighbor connection, in accordance with a connection state of the pixels connected by the oblique 8-neighbor connection and pixels surrounding the pixels connected by the oblique 8-neighbor connection;
• a modification unit configured to modify the oblique 8-neighbor connection in the color region, a modification of which is determined by said determination unit, by converting a pixel value of a pixel neighboring the pixels connected by the oblique 8-neighbor connection;
• and
• an extraction unit configured to extract a boundary line from image data modified by said modification unit.