Method of generating gradation-corrected image data, method of generating gradation-corrected halftone dot percentage data, method of generating gradation-corrected threshold data, image output apparatus, and method of calculating amount of exposure
A gradation-corrected image data is generated by converting an input gradation image data depending on conditions of an output machine and thereafter scanning a recording medium in the output machine with a light beam based on the gradation-corrected image data. Specifically, an amount-of-exposure distribution is calculated based on a shape of recording dots corresponding to input gradations and specification details of the light beam step. Then, a density distribution of an image on the recording medium is calculated from the calculated amount-of-exposure distribution and characteristics of density characteristics regarding an amount of exposure of the recording medium step. Output gradations recorded on the recording medium are calculated based on the calculated density distribution, generating an input/output gradation characteristic curve, from which a gradation corrective curve is generated as an inverse function. The gradation image data is then converted into the gradation-corrected image data according to the gradation corrective curve.
A halftone dot threshold data rewriting unit reads halftone dot threshold data corresponding to the output resolution of a printed image generating apparatus which has been set by an output resolution setting unit, from a halftone dot threshold data storage unit, rewrites the read halftone dot threshold data into halftone dot threshold data corresponding to the output resolution of a proof image generating apparatus, and supplies the rewritten halftone dot threshold data to a binary image generator. The binary image generator compares continuous tone image data with the halftone dot threshold data, generates binary image data, and supplies the generated binary image data to an exposure recorder, which generates a proof image corresponding to the output resolution of the printed image generating apparatus.
A halftone dot threshold data rewriting unit reads halftone dot threshold data corresponding to the output resolution of a printed image generating apparatus which has been set by an output resolution setting unit, from a halftone dot threshold data storage unit, rewrites the read halftone dot threshold data into halftone dot threshold data corresponding to the output resolution of a proof image generating apparatus, and supplies the rewritten halftone dot threshold data to a binary image generator. The binary image generator compares continuous tone image data with the halftone dot threshold data, generates binary image data, and supplies the generated binary image data to an exposure recorder, which generates a proof image corresponding to the output resolution of the printed image generating apparatus.
In a digital printing process employing a CTP device 14, hues can easily be changed in a process of generating printing plates without going back to a prior process. A plurality of gradation conversion curves whose gradation characteristics are different stepwise from each other, which are stored in a gradation conversion curve saving unit 22, are displayed on a display unit 36. The user selects a desired of the displayed gradation conversion curves with respect to at least one color element with a keyboard/mouse 34. The selected gradation conversion curve is set in an LUT converter 26. Input image data G can easily be converted into output image data Ga by converting the gradations of the input image data G according to the gradation conversion curve set in the LUT converter 26. The image data Ga is converted by a binarizing unit 30 into binary image data H, which is supplied to the CTP device 14. The CTP device 14 outputs printing plates each bearing a binary image represented by the binary image data H.
