An image processing apparatus comprising: a storage configured to store a dither table in which a plurality of dither patterns are recorded, each of the dither patterns being formed with a halftone dot screen in which an output area ratio that is a proportion of void halftone dots per unit area corresponds to any one of gradation values of image data; and processing circuitry configured to read a gradation value for each region in image data input for each toner color, decide a dither pattern to be applied to the read gradation value, read out the decided dither pattern from the dither table, and perform dither processing of applying the read dither pattern to the region in the image data, wherein the output area ratio is equivalent to the gradation value and indicates an opening ratio of the screen, and the processing circuitry is configured to form the dither pattern by a dot growth sequence defined by the steps of: placing toner around growth centers (dc) to form first dot blocks (db); placing dots (da) each in the vicinity of the intersection of diagonal lines of a square formed with the growth centers (dc) adjacent to each other; placing dots around the placed dots (da) to form second dot blocks (db); generating bridges (dkl) by connecting the second dot blocks (db); generating bridges by connecting the first dot blocks (db); and filling the void halftone dots (ws) from the periphery until the output area ratio reaches a predetermined value, and wherein the dither pattern is formed by performing dot growth for image data of each of toner colors cyan, magenta, and black such that the number of screen lines of one or more of the toner colors is greater than the number of screen lines of the other toner colors. a storage configured to store a dither table in which a plurality of dither patterns are recorded, each of the dither patterns being formed with a halftone dot screen in which an output area ratio that is a proportion of void halftone dots per unit area corresponds to any one of gradation values of image data; and processing circuitry configured to read a gradation value for each region in image data input for each toner color, decide a dither pattern to be applied to the read gradation value, read out the decided dither pattern from the dither table, and perform dither processing of applying the read dither pattern to the region in the image data, wherein the output area ratio is equivalent to the gradation value and indicates an opening ratio of the screen, and the processing circuitry is configured to form the dither pattern by a dot growth sequence defined by the steps of: placing toner around growth centers (dc) to form first dot blocks (db); placing dots (da) each in the vicinity of the intersection of diagonal lines of a square formed with the growth centers (dc) adjacent to each other; placing dots around the placed dots (da) to form second dot blocks (db); generating bridges (dkl) by connecting the second dot blocks (db); generating bridges by connecting the first dot blocks (db); and filling the void halftone dots (ws) from the periphery until the output area ratio reaches a predetermined value, and read a gradation value for each region in image data input for each toner color, decide a dither pattern to be applied to the read gradation value, read out the decided dither pattern from the dither table, and perform dither processing of applying the read dither pattern to the region in the image data, wherein the output area ratio is equivalent to the gradation value and indicates an opening ratio of the screen, and the processing circuitry is configured to form the dither pattern by a dot growth sequence defined by the steps of: placing toner around growth centers (dc) to form first dot blocks (db); placing dots (da) each in the vicinity of the intersection of diagonal lines of a square formed with the growth centers (dc) adjacent to each other; placing dots around the placed dots (da) to form second dot blocks (db); generating bridges (dkl) by connecting the second dot blocks (db); generating bridges by connecting the first dot blocks (db); and filling the void halftone dots (ws) from the periphery until the output area ratio reaches a predetermined value, and placing toner around growth centers (dc) to form first dot blocks (db); placing dots (da) each in the vicinity of the intersection of diagonal lines of a square formed with the growth centers (dc) adjacent to each other; placing dots around the placed dots (da) to form second dot blocks (db); generating bridges (dkl) by connecting the second dot blocks (db); generating bridges by connecting the first dot blocks (db); and filling the void halftone dots (ws) from the periphery until the output area ratio reaches a predetermined value, and wherein the dither pattern is formed by performing dot growth for image data of each of toner colors cyan, magenta, and black such that the number of screen lines of one or more of the toner colors is greater than the number of screen lines of the other toner colors.