Patent ID: 8345310

Claim:
An image processing apparatus having a central processing unit, comprising: halftone frequency determining section performed by the central processing unit for determining a halftone frequency of an image that has been read from a document by an image reading apparatus, wherein: in a case where (A) first color information is information regarding color components read by the image reading apparatus, (B) second color information is information regarding color components which are in a relationship of complementary color with the first color information, and (C) a composite halftone consisting of halftones of at least two color components exists in the document, the halftone frequency determining section comprises a threshold value determining section for determining a threshold value for use in extraction of a feature of density transition between pixels, an extracting section for extracting the feature according to the threshold value determined by the threshold value determining section, and a halftone frequency estimating section for estimating the halftone frequency from the feature extracted by the extracting section; and in a case where a particular color component, among the color components of the first color information read by the image reading apparatus, is a color component which has the largest busyness which indicates a total sum of the differences in density between any adjacent pixels, the threshold value determining section determines the threshold value for the composite halftone by using an adjusting value that is predetermined in accordance with a reading property of the image reading apparatus so that the feature extracted by the extracting section does not include density transition of a color component of the second color information other than the color component, which is in the relationship of complementary color with the particular color component, of the second color information.