Patent ID: 8005305

Claim:
A method of generating a halftoned image in a computerized digital image forming apparatus in which pixel values can have only two possible levels from an image compressed by domain transformation yielding coefficients for resulting domain components and by a coding operation by which said coefficients are coded, the method comprising: performing a decoding operation on the compressed image by which a decoded DC coefficient and decoded AC coefficients (y p ) are obtained, wherein the decoded coefficients are not transferred back into input pixels; selecting the decoded DC coefficient; selecting decoded AC coefficients (y p ) having an absolute value equal to or larger than a predetermined value; and generating the halftoned image by performing a thresholding operation to obtain a bitmap using only the selected decoded DC coefficient and the selected decoded AC coefficients, wherein the thresholding operation is expressed by: b d = 1 ⁢ ⁢ if ⁢ ⁢ ∑ p = 1 63 ⁢ ⁢ y p ⁢ FDCT p d ≥ m d - y 0 , else ⁢ ⁢ b d = 0 , where b d is an element of the output bitmap for a pixel with index d, m d is a threshold level for a pixel with index d; y 0 is the decoded DC coefficient; y p are the decoded AC coefficients, and FDCT p d are inverse domain transformation coefficients, and wherein the quality of the halftoned image is dependent on the selected decoded coefficient.