For example, the color displayed on a monitor from RGB values of 0,0, is not readily reproducible in print, and the color produced in print using percent cyan ink cannot be displayed by most monitors. The rendering intent being used defines how out-of-gamut colors are mapped or not to colors that exist in the destination color space.
The ICC profile specification defines four different rendering intents: absolute colorimetric rendering, relative colorimetric rendering, perceptual rendering, and saturation rendering. To give us a good starting point for discussing each, in Figure 1 we show a full spectrum in the Adobe RGB color space. Figures 2 through 5 show that same spectrum converted to an inkjet space using each of the four rendering intents.
Absolute Colorimetric Rendering Absolute colorimetric rendering reproduces in-gamut colors exactly, and clips out-of-gamut colors to the nearest reproducible hue, sacrificing saturation and possibly lightness. When we judge color, our eyes seek out white, then judge other colors in relation to that white. Absolute colorimetric rendering tries to reproduce the source white exactly in the target space.
Second, our eyes are much better at evaluating color relationships than they are at evaluating absolute colors. When you clip the out-of-gamut colors in an image to their nearest reproducible hue, you change the relationship between the in-gamut and out-of-gamut colors, which often destroys the image.
Absolute colorimetric rendering is mostly useful for proofing, when our proofing device has a larger gamut than the final output we want the proofer to simulate. Since the source space the final output has a smaller gamut than the destination space the proofer , no gamut clipping takes place, and the absolute colorimetric rendering makes the proofer lay down ink in the white areas to simulate the paper color of the final output.
Relative Colorimetric Rendering The other three rendering intents all translate the white of the source to the white of the output, and shift all the other colors accordingly.
Relative colorimetric rendering is similar to absolute colorimetric rendering. The only difference is that relative colorimetric scales the white point of the source to the white point of the target. Like absolute colorimetric rendering, it clips out-of-gamut colors to the nearest reproducible hue.
Perceptual Rendering Perceptual rendering attempts to compress the gamut of the source space into the gamut of the target space. Exactly how this is accomplished is left to the discretion of the tool used to build the profile, but typically perceptual rendering desaturates all colors to bring the out-of-gamut colors into the target gamut while more or less maintaining the overall relationship between colors.
Preserving the relationship between colors helps preserve the overall appearance of images. Saturation Rendering The final rendering intent — saturation rendering — maps the saturated primary colors in the source space to the saturated primary colors in the target space, without bothering about differences in hue, saturation, or lightness. Which Should You Use? Conventional wisdom has it that one should use perceptual rendering for natural imagery such as scanned photographs or digital camera captures, and relative colorimetric rendering for vector art such as Illustrator or FreeHand files.
Many photographic images could benefit from a relative colorimetric rendering, rather than a perceptual rendering. Remember, color-management systems know nothing about the content of the image itself; they only know about the gamut of the color space the image inhabits. When you use perceptual rendering, the color management system applies the same gamut compression to all images, even when the image contains no visually or aesthetically significant out-of-gamut colors.
Figure 7 shows an image converted to the same Web press space as used in Figure 6, again using perceptual and relative colorimetric renderings. In this case, the differences are more subtle, but the relative colorimetric rendering preserves more of the saturation in the water at lower left than does the perceptual rendering, without any obvious clipping of saturated colors.
In that case, perceptual rendering would probably produce a better result. Application Notes One of the many wonderful new features of Photoshop 6 is that it allows you to see how the different rendering intents will affect your image, as I described in a past column. Some applications, notably QuarkXPress 4. Almost all output profiles have perceptual rendering set as the default intent. Other applications may use different names for the rendering intents. For example, PageMaker 6. Absolute colorimetric is similar to relative colorimetric except that absolute colorimetric intent does not let the white point change from source white to destination white.
Absolute colorimetric intent creates exactly the colors that were in the original,whenever possible. So, if the original had a yellowish white point, the absolute colorimetric intent would make the reproduction have a yellowish white point as well.
This would mean placing ink in the highlight areas of the reproduction to create a yellowish white in an otherwise clear area of the paper. This intent is used in proofing scenarios where you would like to simulate the output of one printer on a second devise and do a side by side comparison, when measuring accuracy and when printing logo colors.
Something important to remember is that if the paper color of the original matches the paper color of the reproduction then it does not matter which colorimetric intent you use since either would give you the same result.
Saturation Intent: Saturation moves in-gamut colors toward the edge of the destination gamut for maximum saturation and impact. This intent will make an image more colorful by using the full gamut of the destination device. This intent cares not for the genuine representation of color.
You would use this intent for business graphics such as graphs and pie charts, where bright vivid stand out colors are desired. In Summary: If a device is not able to reproduce a color due to gamut limitations, we use rendering Intents to direct how a replacement color should be found.
These descriptions for rendering intent are generalizations. Photographic images are normally processed using perceptual or relative colorimetric intent. This is somewhat analogous to the Relative Colorimetric rendering intent. I realize it's an imperfect analogy. To be a true Rel Col RI, all the wood outside would have to be squished and pressed until it were moved to just within the circumference of the bowling ball.
But let's not get too technical; come on - work with me here The Perceptual rendering intent is analogous to a balloon. Think of a balloon with a pattern of stars on it. When fully inflated, the stars are in a certain position. If some air leaks out, the balloon gets smaller and each of the stars move closer to the center of the balloon. The relationship between each point on the balloon remains the same, but the gamut of the balloon gets compressed. If there were a picture of Mickey Mouse on the balloon, the picture would continue to be recognizable as the balloon got smaller, because the relationship between each point on the surface would be retained as each point evenly moved closer together.
Not so for the bowling ball. If my original glued blocks bore a resemblance to Mickey Mouse when I started, then much of the original would be cut off as I trimmed it down on the lathe. There go the ears! That's what we'd call "clipping of the gamut. Naturally, the choice of these rendering intents depends on one's intent for the image. If it is more important that the relationship between all the colors be retained, then Perceptual is the way to go, while understanding that many of the colors will be desaturated.
If the greater importance is to maintain the most saturation in the colors, then choose Relative Colorimetric and accept some clipping of the out of gamut colors. Red River Paper Inc. Dallas Texas, Prices, specifications, and images are subject to change without notice. Not responsible for typographical or illustrative errors.
Papers with the Archival designtation can take many forms. They can be glossy, matte, canvas, or an artistic product. They are likely to have optical or fluorescent brightening agents OBAs - chemicals that make the paper appear brighter white. Presence of OBAs does not indicate your image will fade faster.
0コメント