{'fraction_non_alphanumeric': 0.022551928783382788, 'fraction_numerical': 0.0, 'mean_word_length': 4.833910034602076, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Spatial non-uniformity correction for color printer calibration

Color printer calibration is always performed in the presence of noise. A major part of this noise is due to the spatial non-uniformity of the printer. In many cases, the spatial non-uniformity does not repeat the same pattern page by page. Some calibration techniques use randomizing the locations of patches and averaging a large numbers of measurements to reduce the noise. Since the non-uniformity is a kind of systematic errors, it should be estimated and corrected in a more effective way. This presentation describes a method to provide a more accurate color calibration/characterization result by estimating the spatial non-uniformity presented on the printed target page and applying a correction to the very same page itself. Instead of defining the noise in the color measurement space, e.g. the CIE Lab space, the new method specifies the noise in the printer-dependent color space, e.g. the CMY space. After the first-round color calibration, an inverse transform, from CIE Lab to CMY, is derived from measurements of the entire printed target. The noise dC, dM, or dY is determined as the difference between the original CMY values and the output of the inverse transform with the measured Lab values as the transform input. Since color patch locations are stochastically arranged, any "noticeable" spatial pattern of the noise is most likely due to the printer non-uniformity in the corresponding channel. The nonuniformity can be estimated by spatially smoothing the noise terms and the result can be subtracted from the original CMY input for a second-round calibration to achieve higher color accuracy.