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- Norberg, Ole, 1970-, et al.
(författare)
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Extending color primary set in spectral vector error diffusion by multilevel halftoning
- 2013
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Ingår i: Color Imaging XVIII. - Bellingham : SPIE - International Society for Optical Engineering. - 9780819494252 ; , s. 8652OM-1-8652OM-9
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Konferensbidrag (refereegranskat)abstract
- Ever since its origin in the late 19th century, a color reproduction technology has relied on a trichromatic color reproduction approach. This has been a very successful method and also fundamental for the development of color reproduction devices. Trichromatic color reproduction is sufficient to approximate the range of colors perceived by the human visual system. However, tricromatic systems only have the ability to match colors when the viewing illumination for the reproduction matches that of the original. Furthermore, the advancement of digital printing technology has introduced printing systems with additional color channels. These additional color channels are used to extend the tonal range capabilities in light and dark regions and to increase color gamut. By an alternative approach the addition color channels can also be used to reproduce the spectral information of the original color. A reproduced spectral match will always correspond to original independent of lighting situation. On the other hand, spectral color reproductions also introduce a more complex color processing by spectral color transfer functions and spectral gamut mapping algorithms. In that perspective, spectral vector error diffusion (sVED) look like a tempting approach with a simple workflow where the inverse color transfer function and halftoning is performed simultaneously in one single operation. Essential for the sVED method are the available color primaries, created by mixing process colors. Increased numbers of as well as optimal spectral characteristics of color primaries are expected to significantly improve the color accuracy of the spectral reproduction. In this study, sVED in combination with multilevel halftoning has been applied on a ten channel inkjet system. The print resolution has been reduced and the underlying physical high resolution of the printer has been used to mix additional primaries. With ten ink channels and halfton cells built-up by 2x2 micro dots where each micro dot can be a combination of all ten inks the number of possible ink combinations gets huge. Therefore, the initial study has been focused on including lighter colors to the intrinsic primary set. Results from this study shows that by this approach the color reproduction accuracy increases significantly. The RMS spectral difference to target color for multilevel halftoning is less than 1/6 of the difference achieved by binary halftoning. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
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| 2. |
- Norberg, Ole, 1970-, et al.
(författare)
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Spectral Vector Error Diffusion - Promising Road or Dead End?
- 2012
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Ingår i: Twentieth Color and Imaging Conference. - Springfield. VA, USA : The Society for Imaging Science and Technology. - 9780892083039 ; , s. 329-334
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Konferensbidrag (refereegranskat)abstract
- The interest for spectral color reproduction has increased with the growing field of multispectral imaging and the increasing use of multi-colorant printing systems. Spectral color reproduction, i.e. aiming at reproducing the spectral reflectance of an original, first requires a colorant separation for a multi-colorant printing system, followed by halftoning of each the color channels. Spectral vector error diffusion, sVED, has previously been introduced as a tempting alternative for spectral color reproduction, since the method combines the colorant separation and the halftoning in a single step. Only the spectral properties of the Neugebauer primaries are needed as input, and there is no need to invert a complex printer model for the colorant separation. Previously, spectral vector error diffusion has been positively evaluated for simulated prints, assuming a perfect printer and no dot gain. In this study, we evaluate the performance of sVED in practice, for real prints.Spectral vector error diffusion has been used to reproduce 1000 spectral targets, all within the spectral gamut of the printing system. The resulting color patches have been printed in various print resolutions, using a 10-colorant inkjet printing system. The experimental results reveal a remarkably large difference between the reproduction errors for the printed samples compared to the simulated spectra from the digital halftones. The results show a strong relation between the print resolution and the magnitude of the reproduction error, with lower resolutions giving smaller errors, due to the effect of dot gain in the printing process. The experimental results imply that in its current form, without compensation for physical and optical dot gain, spectral vector error diffusion produces unacceptable spectral and colorimetric reproduction errors, for any print resolutions used in practice.The results further show that the sVED method in many cases produces color patches that appear noisy and visually unpleasant. By replacing the spectral RMS difference with the ΔE94 color difference as criterion in the sVED algorithm, the graininess as well as the resulting color difference was decreased. However, the improvements in colorimetric performance and more visually pleasant reproductions, comes at the cost of an increase in spectral reproduction errors.
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| 3. |
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| 4. |
- Nyström, Daniel, 1974-, et al.
(författare)
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Improved Spectral Vector Error Diffusion by Dot Gain Compensation
- 2013
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Ingår i: Color Imaging XVIII. - Bellingham : SPIE - International Society for Optical Engineering. - 9780819494252 ; , s. 8652OL-1-8652OL-11
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Konferensbidrag (refereegranskat)abstract
- Abstract: Spectral Vector Error Diffusion, sVED, is an interesting approach to achieve spectral color reproduction, i.e. reproducing the spectral reflectance of an original, creating a reproduction that will match under any illumination. For each pixel in the spectral image, the colorant combination producing the spectrum closest to the target spectrum is selected, and the spectral error is diffused to surrounding pixels using an error distribution filter. However, since the colorant separation and halftoning is performed in a single step in sVED, compensation for dot gain cannot be made for each color channel independently, as in a conventional workflow where the colorant separation and halftoning is performed sequentially. In this study, we modify the sVED routine to compensate for the dot gain, applying the Yule-Nielsen n-factor to modify the target spectra, i.e. performing the computations in (1/n)-space. A global n-factor, optimal for each print resolution, reduces the spectral reproduction errors by approximately a factor of 4, while an n-factor that is individually optimized for each target spectrum reduces the spectral reproduction error to 7% of that for the unmodified prints. However, the improvements when using global n-values are still not sufficient for the method to be of any real use in practice, and to individually optimize the n-values for each target is not feasible in a real workflow. The results illustrate the necessity to properly account for the dot gain in the printing process, and that further developments is needed in order to make Spectral Vector Error Diffusion a realistic alternative for spectral color reproduction.
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| 5. |
- Nyström, Daniel, 1974-
(författare)
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Micro-reflectance Measurements of Multiple Colorants in Halftone Prints
- 2011
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Ingår i: TAGA (Technical Association of the Graphic Arts) 2011. - Sewickley, PA, USA : TAGA - Technical Association of the Graphic Arts. - 9781935185031 ; , s. 157-176
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Modeling color reproduction in halftone prints is difficult, mainly because of light scattering in the substrate, causing optical dot gain. Most available models are limited to macroscopic color measurements, averaging the reflectance over an area that is large relative the halftone dot size. The reflectance values for the full tone ink and the unprinted paper are used as input, and these values are assumed to be constant. An experimental imaging system, combining the accuracy of color measurement instruments with a high spatial resolution, allows us to measure the individual halftone dots, as well as the paper between them. Microscopic reflectance measurements reveal that the micro-reflectance of the printed dots and the paper between them is not constant, but varies with the dot area coverage. By incorporating the varying micro-reflectance values of the ink and paper in an expanded Murray-Davies model, we have previously shown that the resulting prediction errors are smaller than for the famous Yule-Nielsen model. Moreover, unlike Yule-Nielsen, the expanded Murray-Davies model takes into account the varying micro-reflectance for the printed dots and the paper, thus providing a better physical description of optical dot gain in halftone reproduction.In this study, we further extend the methodology to handle color prints, predicting tristimulus values for prints with multiple and overlapping colorants. After converting the microscopic images of halftone prints into CIEXYZ color space, 3D histograms are computed. In the 3D histograms, the paper and the inks appear as clusters, with the transitions between the clusters corresponding to the edges of halftone dots. The tristimulus values for the paper and the different combinations of ink are computed as the centers of gravity for the clusters in the 3D histogram. From the microscopic images we can also compute the physical dot area coverage for each of the Neugebauer primaries, which typically differ from the nominal one, due to physical dot gain. The result is an expanded Neugebauer model, employing the varying tristimulus values of the paper and primary inks, as well as for overlapping, secondary colors. Experimental results confirm the accuracy of the proposed methodology, whencompared to measurements using a spectrophotometer. Further, the results have shown that the variation of the micro-reflectance of the Neugebauer primaries is large, and depends strongly on the total dot area coverage.
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| 6. |
- Nyström, Daniel, 1974-
(författare)
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Microscopic Color Measurement of Halftone Prints
- 2010
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Ingår i: NIP26: The 26th International Conference on Digital Printing Technologies. - Sprinfield, VA, USA : The Society for Imaging Science and Technology. - 9780892082926 ; , s. 459-462
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Modeling halftone print reproduction is difficult, mainlybecause of light scattering, causing optical dot gain. Mostavailable models are based on macroscopic color measurements,integrating the reflectance over an area that is large relative thehalftone dot size. The reflectance values for the full tone and theunprinted paper are used as input, and these values are assumedto be constant. An experimental imaging system, combining theaccuracy of color measurement instruments with a high spatialresolution, allows us to measure the individual halftone dots, aswell as the paper between them. Microscopic color measurementsreveal that the micro-reflectance of the printed dots and the paperis not constant, but varies with the dot area fraction. Byincorporating the varying reflectance of the ink and paper in anexpanded Murray-Davies model, the resulting prediction errorsare smaller than for the Yule-Nielsen model. However, unlikeYule-Nielsen, the expanded Murray-Davies model preserves thelinear additivity of reflectance, thus providing a better physicaldescription of optical dot gain. The microscopic colormeasurements further show that the color shift of the ink andpaper depends on the halftone geometry and the print resolution.In this study, we measure and characterize the varying microreflectanceof ink and paper with respect to properties of thehalftones, using AM and FM prints of various print resolutions.
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