1. A method of correcting a threshold array having an array of thresholds for reproducing a gradation with a dot pattern as a clustered pattern of dots each made up of one or more blackening pixels, comprising the steps of: (A) determining at least one non-contact point candidate position to be turned into a non-contact point, among contact points of the dots at a given gradation; (B) determining at least one contact point candidate position to be turned into a contact point, among non-contact points of the dots at said given gradation; and (C) correcting said threshold array by switching around a threshold for the non-contact point candidate position determined in said step (A) and a threshold for the contact point candidate position determined in said step (B), whereby the layout of contact points of said dots can be corrected.