Image processing system for compressing image data including binary image data and continuous tone image data by a sub-band transform method with a high-compression rate

An image processing apparatus comprising:

• a buffer unit extracting n×m pixel matrix block data from image data, where n and m are integers;
• a sub-band transform unit transforming the n×m pixel matrix block data by a sub-band transform method so as to obtain a transform factor having a plurality of frequency components including a low-frequency component and a high-frequency component;
• an area discriminating unit discriminating a type of an image area corresponding to the n×m pixel matrix block data being processed so that the image area is determined as one of an edge area and a non-edge area, a discrimination being made based on whether or not an absolute value of each of the components of the transform factor exceeds a threshold value;
• and
• a quantizing unit quantizing the transform factor by adaptively changing a quantizing method so as to equalize a code length such that a number of bits of a first code obtained when the block data corresponds to the edge area is equal to a number of bits of a second code obtained when the block data corresponds to the non-edge area,
• wherein the first code includes flag information representing the edge area and information representing a subband factor quantized for the edge area, and the second code includes flag information representing the non-edge area and a subband factor quantized for the non-edge area.