Patent ID: 7424151

Claim:
A method for classifying an image block of a printed image into contone, halftone, or error diffusion classes, comprising: scanning the printed image; selecting an n by n block of pixels from the scanned image; calculating an array of DCT coefficients of the pixel block, wherein the calculated DCT coefficients are representative of spatial frequency and spatial orientation of the pixel block; inputting the array of DCT coefficients into a classifier which has been configured to classify printed images into contone, halftone and error diffusion classifications using DCT coefficients; and wherein inputting the array of DCT coefficients into a classifier comprises comparing the array of DCT coefficients with an array of predetermined values, wherein the array of predetermined values are indicative of contone, halftone and error diffusion classes; grouping the array of DCT coefficients into a DC term, segments of DCT coefficients having similar radial spatial frequencies, and segments of DCT coefficients having similar spatial orientations; for each segment, calculating a sum of absolute values of all DCT coefficients in the segment to produce a feature of the pixel block, wherein the feature set includes the DC term and all segments features; comparing the feature set with the array of predetermined values to determine the image classification of the pixel block; wherein n=8 and the feature set compares the DC term, 7 spatial frequency terms and 5 spatial orientation terms; and determining whether the image classification of the pixel block is contone, halftone or error diffusion based on input DCT coefficients.