Patent ID: 7869094
Filing Date: 2011-01-11
Classification: H04N

Abstract:
1. A method of processing a digital image tangibly stored on a first computer-readable medium, the method performed by a computer processor executing computer program instructions tangibly stored on a second computer-readable medium, the method comprising: (A) identifying at least one non-monotonic portion of a transfer function of a printer, the transfer function relating input digits to output density, the at least one portion having at least one range of digital values which is less than the full range of digital values; and (B) dithering only that subset of the digital image having digits within the at least one range of digital values to produce a dithered image tangibly stored on a third computer-readable medium, wherein the step (B) comprises: applying a nonlinear transformation to the digital image to produce a transformed image; selectively adding a dither pattern to the transformed image to produce a dithered pre-image; quantizing the dithered pre-image to produce a quantized image; and applying an inverse of the nonlinear transformation to the image to produce the dithered image, wherein the nonlinear transformation comprises: dividing a transfer function of a printer into a plurality of portions, each portion defined by a pair of anchor points; for each portion of the transfer function, selecting a slope less than 1 if the portion corresponds to a first part of the digital image where the dither pattern will be added; and for each portion of the transfer function, selecting a slope equal to 1 if the portion corresponds to a second part of the digital image where the dither pattern will not be added.