Au cours #3 vous avez appris ce qu'est le diagramme de chromaticity xyY. Vous avez appris qu'il permet de comparer les performances d'appareils. A ce cours-ci nous nous en servirons (dans une version simplifiée) pour expliquer les différents modes de rendu utilisés en gestion des couleurs. A la suite de l'explication vous comprendrez pourquoi le but de la gestion des couleurs n'est pas toujours de reproduire les couleurs à l'identique. Parfois dans une opération de conversion d'un espace colorimétrique à un autre toutes les couleurs changent… C'est voulu, et le résultat est meilleur ainsi !
des sites avec des explications et des schémas…
http://www.newsandtech.com/issues/2004/03-04/pt/03-04_rendering.htm
http://www.signindustry.com/computers/articles/2007-05-01-SGIA-Shell-RenderingIntents.php3
des sites avec des explications…
Perceptual If an image includes any colors that are out-of-gamut for the printer, the printer adjusts all the colors in the image, even those that are already in the gamut of the printer, so they are all in-gamut and maintain their color relationships to each other. The result is an image that is visually pleasing, but is not colorimetrically accurate. The perceptual rendering intent is useful for general reproduction of images, particularly photographs.
Saturation If a print job includes colors that are out-of-gamut for the printer, the printer replaces the out-of-gamut color with the nearest color in the gamut. It also adjusts the in-gamut colors so that they are more vivid. Saturation is the least used rendering intent, but it is useful for business graphics, such as images that contain charts or diagrams.
Media-relative colorimetric If a print job includes colors that are out-of-gamut for the printer, the printer substitutes the nearest in-gamut color; in-gamut colors are not adjusted. Colors printed on papers with different media white points might not match visually. The media white point is the color of the paper that the print job is printed on. For example, if you print an image on white paper, on off-white paper, and on blue paper using the media-relative colorimetric rendering intent, the printer uses the same amount of ink or toner for each one and the resulting color is technically the same. However, the images might seem different because your eyes adjust to the color of the background and interpret the color differently. This rendering intent is typically used for vector graphics.
Absolute colorimetric All colors are mapped using the same method as the media-relative colorimetric rendering intent, however, all colors are adjusted for the media white point. For example, if you print an image on white paper, on off-white paper, and on blue paper using the media-relative colorimetric rendering intent, the printer adjusts the ink or toner used for each one. The resulting color is technically not same, but the images might look the same because of the way your eyes interpret them in relationship to the color of the paper. The absolute colorimetric rendering intent is typically used for logos.
http://dba.med.sc.edu/price/irf/Adobe_tg/manage/renderintent.html
Perceptual Compresses the total gamut from one device's color space into the gamut of another device's color space when one or more colors in the original image is out of the gamut of the destination color space. This preserves the visual relationship between colors by shrinking the entire color space and shifting all colors – including those that were in gamut.
Saturation Reproduces the original image color saturation (vividness) when converting into the target device's color space. In this approach, the relative saturation of colors is maintained from gamut to gamut. This render intent is primarily designed for business graphics, where the exact relationship between colors (such as in a photographic image) is not as important as are bright saturated colors.
Relative Colorimetric When a color in the current color space is out of gamut in the target color space, it is mapped to the closest possible color within the gamut of the target color space, while colors that are in gamut are not affected. Only the colors that fall outside of the destination gamut are changed. This render intent can cause two colors, which appear different in the source color space, to be the same in the target color space. This is called "clipping." Relative colorimetric is the default method of color conversion built into Photoshop 4.0 and earlier.
Absolute Colorimetric Colors match exactly with no adjustment made for white point or black point that would alter the image's brightness. Absolute colorimetric is valuable for rendering "signature colors", those colors that are highly identified with a commercial product such as the yellow used by the Eastman Kodak Company, or the red used by the Coca-Cola Company.
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