Concept# Noir

Résumé

Le noir est un champ chromatique regroupant les teintes les plus obscures. Les objets noirs n'émettent ou ne reflètent qu'une part négligeable du spectre de la lumière visible. Le noir s'oppose ainsi à toutes les couleurs, mais surtout au blanc, la plus claire de toutes les couleurs. Certains auteurs, qui prennent le mot « couleur » dans un sens restreint, estiment que le noir n'est pas une couleur.
La culture occidentale associe principalement le noir au renoncement, involontaire dans le deuil ou volontaire dans la sobriété.
Définition
La couleur noire n'est pas l'obscurité totale. Quand l'éclairement lumineux est inférieur à , l'être humain ne distingue pas les couleurs. On parle de vision scotopique. Quand la luminance d'un objet est moindre de , on ne perçoit rien du tout . Cependant, cette absence de perception n'est pas du noir ; l'eigengrau représente, en quelque sorte, le bruit de fond de la perception visuelle.
Le noir absolu peut plutôt se définir comme une lu

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Concepts associés (14)

Couleur

vignette|Peinture de Claude Monet.
La couleur est la perception visuelle de l'aspect d'une surface ou d'une lumière, basée, sans lui être rigoureusement liée, sur la répartition spectrale de la

Jaune

vignette|Jaune d'œuf(œuf au plat).
Le jaune est un champ chromatique correspondant à des couleurs claires situées entre l'orange et le vert.
Une des trois couleurs primaires de la synthèse sou

Bleu

vignette|Le ciel et la mer dans diverses nuances de bleu.
Le bleu est un champ chromatique, regroupant les teintes rappelant celles du ciel et de la mer par temps clair.
C'est aujourd'hui la couleur l

Unités associées (1)

Séances de cours associées (73)

Mathieu Brichon, Thomas Bugnon, Roger Hersch

The Yule-Nielsen modified Spectral Neugebauer reflection prediction model enhanced with an ink spreading model provides high accuracy when predicting reflectance spectra from ink surface coverages. In the present contribution, we try to inverse the model, i.e. to deduce the surface coverages of a printed color halftone patch from its measured reflectance spectrum. This process yields good results for cyan, magenta, and yellow inks, but unstable results when simultaneously fitting cyan, magenta, yellow, and black inks due to redundancy between these four inks: black can be obtained by printing either the black ink or similar amounts of the cyan, magenta, and yellow inks. To overcome this problem, we use the fact that the black pigmented ink absorbs light in the infrared domain, whereas cyan, magenta, and yellow inks do not. Therefore, with reflection spectra measurements spanning both the visible and infrared domain, it is possible to accurately deduce the black ink coverage. Since there is no redundancy anymore, the cyan, magenta, yellow, and pigmented black ink coverages can be recovered with high accuracy.

2007,

The present invention facilitates the calibration of printers and the color separation of input images into a set of inks by disclosing methods and systems for populating device-calibration lookup tables. The disclosed methods and systems rely on a comprehensive spectral prediction model which is capable of predicting at a high accuracy the reflectance spectra of halftone ink patches. The comprehensive spectral prediction model is composed of a first part predicting the reflection spectra as a function of physical (mechanical) surface coverages an of a second part comprising functions mapping nominal surface coverages to effective surface coverages. These mapping functions are calibrated by halftone patch wedges printed alone and by halftone patch wedges printed in superposition with one or several solid inks. The part of the comprehensive spectral prediction model predicting the reflection spectra relies on a weighted average between one component behaving according to the Clapper-Yule model and another component behaving as the spectral Neugebauer model, extended to include multiple internal reflections at the paper-air boundary. The disclosed methods and systems can perform the color separation as well as the calibration of printers printing with standard cyan, magenta, and yellow inks as well as with inks comprising standard and non-standard inks such as Pantone inks (custom inks). They are also used for performing precise undercolour removal in order to carry out the color separation of images into cyan, magenta, yellow and black inks. The can further be used to carry out the color separation of images into cyan, magenta, yellow, black, light cyan and light magenta inks. In addition, the disclosed methods and systems can be used for printer control, i.e. to control printer actuation parameters in different types of printers e.g. liquid ink professional printers (offset, gravure, letterpress), electrophotographic printers, ink-jet printers, thermal transfer printers and in dye-sublimation printers.

2005In the printing industry, one of the most common methods for reproducing halftone images using bilevel printing devices is clustered-dot ordered dithering. The images produced using this method are quite faithful to the original and are visually pleasing. Nevertheless, only rational angles are attainable with clustered-dot dithering, due to the discrete nature of the grids. This phenomenon can become detrimental in the case of four-color printing, when different screen angles and maybe even different screen frequencies are used for separate color planes, thus producing a so-called Moiré phenomenon. Another important drawback, the so-called banding or contouring effect, is related to the limited area of basic screen elements used in traditional dithering. In order to deal with these problems, we have developed, within the scope of our research, several new techniques for digital halftoning: (1) pseudo-random halftone screening, (2) a new method for generating clustered-dot halftone images having a number of reproducible gray or colour levels which is independent of the screen element size (CombiScreen), (3) rotated clustered-dot dithering, based on discrete one-to-one rotation, and (4) rotated dispersed-dot dither. A new method of pseudo-random halftone screening is described. It starts by obtaining the quasi-random distribution of tile centers according to some well-defined spectral characteristics. We then obtain the desired tesselation of the output device space by applying the Voronoi polygonization process. Then, an analytic black-dot curve is calculated according to the resampled input signal level and the area of each given tile. This analytic curve is scan-converted to obtain the blackened pixels. In the second approach, we associate threshold values to all pixels inside every tile according to some specially tailored analytic spot function. Then, the standard threshold comparison process is applied. Unlike known error-diffusion techniques, the pseudo-random halftone screening technique can be applied to a high resolution printing process. The characteristic screen element size can be properly chosen so as to ensure the best trade-off between the printing process constraints and the most precise printing. The described halftone algorithm seems to be appropriate for high-resolution color and black&white devices (above 1000 dpi). A new method (CombiScreen) is proposed for generating clustered-dot halftone images on raster printing devices having a number of reproducible gray or colour levels which is independent of the screen element size. The dither tiles generated by this method may contain several screen elements having any rational orientation and size. Threshold values are distributed among the cells of the dither tile so as to produce a large range of gray values, while at the same time preserving the clustered-dot behavior of individual screen elements. When rendering images at smoothly increasing intensity levels, this new method generates few contouring effects and other visible artifacts. The method works equally well for quadratic, rectangular, parallelogram and hexagonally shaped screen elements. Resulting dither tiles are generally either of parallelogram or of hexagonal shape. Since CombiScreen enables the screen dot frequency or orientation to be chosen independently of the number of gray levels, it has proven to be specially effective when printing at resolutions between 150 to 600 dpi with ink jet printers and at resolutions between 300 and 1200 dpi with xerographic printers. A new operator of discrete one-to-one rotation is described. It offers means previously unknown in the art for generating rotated screens which approximate irrational angles with high-precision, producing much less disturbing interferences and artifacts than other methods. Therefore, a carefully prepared dither tile incorporating screen elements with the desired period, initial orientation, and dither threshold values defining their screen dot shape growth behavior can be rotated by discrete one-to-one rotation and keep the desired screen element period, the number of cells per screen element and the threshold values associated with each screen element cell, thereby preserving the screen dot shape growth behavior of the original dither tile. Several different discrete one-to-one rotation variants are described: a small angle rotation technique valid for a subset of rational rotation angles, a rigid band technique and a technique based on discrete shearing transformations. The high-quality of the so rotated dither tile is due to the fact that discrete one-to-one rotation preserves the exact number of elementary cells per screen element and their exact dither threshold values. The described method provides a new range of solutions for obtaining high-quality digital angled halftone screens. High-quality solutions can be found for generating three digital angled halftone screens, each 30° apart from each other, as known from traditional photographic colour screening techniques. Further solutions minimizing Moiré effects may be obtained by halftone screens whose first order frequency component vectors sum up to zero. This new method has turned out to be particularly effective when printing with color ink jet printers at resolutions between 150 and 800 dpi as well as with xerographic printers at resolutions between 300 and 1200 dpi. Rotated dispersed-dot dither is based on the discrete one-to-one rotation of a Bayer dispersed-dot dither array. The halftone patterns produced by the rotated dither method therefore incorporate fewer disturbing artifacts than the horizontal and vertical components present in most of Bayer's halftone patterns. In grayscale wedges produced by rotated dither, texture changes at consecutive gray levels are much smoother than in error diffusion or in Bayer's dispersed-dot dither methods, thereby avoiding contouring effects. Due to its semi-clustering behavior at mid-tones, rotated dispersed-dot dither exhibits an improved tone reproduction behavior on printers having a significant dot gain, while maintaining the high detail rendition capabilities of dispersed-dot halftoning algorithms. This technique has successfully been applied to in-phase color reproduction on ink-jet printers as well as to black and white reproduction on laser printers.