Photoshop Lab Color- P10 ppt

shot from a product of GretagMacbeth, a leading vendor of color measurement instru- mentation. It has measured 50 L (30) A 40 B where the desired value is 50 L (40) A 30 B , and it wants to know how far different these two greens really are from one another. Notice first that below the color wheel on the left, the program permits us to express these colors in xy Y or even LUV , if we’d like to avoid LAB . But that’s a side issue. The big action is underneath the green circle, where we find one large and three small numbers that are introduced by the lowercase letter d. It stands for the Greek delta, which mathe- maticians often use to denote the quantity of change. The asterisks remind us that the snooty name is L*a*b*. In comparing the two colors, the program notes that the L values are identical, so dL*=0. da* and db* are both 10. To Each His Dulcinea “A knight errant who loses his lady,” Don Quijote remarked to Sancho, “is like one who loses the eyes that let him see, or the sun that shines on him, or the food that maintains him. I have told you this many times before, and now I say it again: a knight errant without a lady is like a tree without leaves, bricks without mortar, and a shadow without the body that cast it.” Right above the three readings we’ve just been discussing in Figure 13.3, much larger type informs us that dE=14.1. That’s the key, the iconic number. Pronounce it “Delta- E ,” and prostrate yourself before it if you believe that machines see color better than you do. Delta- E is an attempt to quantify the three readings on the line below it to create one comprehensive number that describes how far the two colors are from one another. There are several formulas to produce it; this one involves weighting the A channel more than the B ,a lot of square roots, and other fancy stuff. But whatever the formula, a dE of zero is ideal; the higher it is the worse the match; and the objective of a good interchange between two devices is to have the average dE be as close to zero as possible. Opinions on the merit of dE are, shall we say, varied. On the one hand, certain cali- brationists are so smitten that they make a Dulcinea out of dE: they believe that she embodies perfection and it is only a matter of time before we find, attain, and adore her. On the other, one color expert offers the following succinct opinion: “The original conclusion therefore remains, the CIELAB is perceptually extremely non-linear and the dE* unit of color-difference is totally worth- less for all processes and uses in photo- graphic imaging.” Personally, I am somewhere in the middle, but closer to the second view than the first. The real Dulcinea, like dE, was no thing of beauty. She herded pigs, a dirty and unpleas- ant task that nevertheless needs to be done. dE is like that, too. Machines can aid in calibration, and a formula like dE, however imperfect, is the only way they can come to a decision. We simply have to understand that The Universal Interchange Standard 271 Figure 13.4 The middle row holds the reference colors. Are the colors at the top closer to the middle ones than the bottom ones are? A machine would think they are each equally far off. Chapter 13. The Universal Interchange Standard Page 7 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. sometimes it gives misleading results, and sometimes we deliberately want to disregard it because we get a closer visual match with a worse literal match. Let’s assume that we are trying to make output from a desktop printer match that of a commercial print shop so that we can make inexpensive proofs at home. A conventional idea is to have the print shop produce a variety of patches, which are then measured spectrophotometrically. The desktop printer spits out the same swatches, which are measured and compared to the press’s. Adjust- ments are then made to try to get dE lower by forcing the desktop printer to get closer to the print shop’s results. There’s nothing inherently wrong with such an approach, but the two greens that we’ve been discussing—50 L (30) A 40 B on the press and 50 L (40) A 30 B when we try it on our printer—demonstrate the deficiencies of any such formula, in at least three different ways. First, humans expect greens to be greener than a machine does. If the desktop printer produces a better-looking green than the press does, we may declare that the two match, even though a spectrophotometer finds a big dE. Also, the machine thinks that misses of equal magnitude make for equal dE. Not so. If one of the misses is greener than the desired value, and the other is less green by an equal amount, humans will invariably consider the greener one a closer match to the original. Second, how far one color channel appears to be off from the desired value depends on what goes on in the other, a fact lost on a spectrophotometer. If the desktop printer’s patch had measured at 50 L (50) A 40 B , the machine would compute the same dE, because there is still a difference of 0 L 10 A 10 B between that and what’s wanted. However, we would perceive that color as being closer to the original than 50 L (30) A 40 B was, because, while both the A and B were each inaccurate by 10 points, both were further away from 0 A 0 B than the desired value was. Were they the same 10 points off, but one channel moved toward neutrality and the other away from it, a spectrophotometer sees less difference than we do. If you don’t believe it, check out Figure 13.4. The “desired” colors are in the middle row. In the top row, both A and B were moved 15 points further from zero. In the bottom row, one channel went 15 points in one direc- tion and one 15 points in the other. Therefore, the top and bottom rows both have the same dE with respect to the middle. (None of the original LAB squares were out of the CMYK gamut, so the conversion for printing didn’t affect the relation of these colors.) Humans would see the bottom row as being much 272 Chapter 13 Further Reading on Colorspaces For those interested in more detailed discussion and comparison of the colorspaces described in this chapter, the enclosed CD includes six papers by Prof. Gernot Hoffmann, a color expert at the University of Applied Sciences in Emden, Germany. They include a general introduction to graphics for color science; one paper each on the structure of XYZ and LAB ; a study of the question of gamut generally; and one that is specific to the question of CMYK gamut. Finally, there’s one paper that has nothing to do with the topic of this chapter but does bear on Chapters 5 and 11, namely a discussion of how gamma can create certain problems when working in RGB . I suggest starting here because Gernot does a good job of reducing technical concepts into somewhat compre- hensible language. There is a fair amount of mathe- matics involved, but no calculus. If you don’t remember what went on in your 12th-grade algebra course, however, you may find it heavy sledding. Thanks to Google and its ilk, much more information, including the actual formulas for converting between the spaces, is available by typing in search criteria such as CIELUV , CIE xy Y , and so on. The CIE is alive, well, and living in Austria. Its URL is www.cie.co.at. Chapter 13. The Universal Interchange Standard Page 8 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. further from the middle than the top is, particularly in the context of a real image. And third, the age of a spectrophotometer doesn’t affect its color perception. The age of a human does. The measured green is more yellow than the desired one. A younger person is likely to find this fact more objectionable than an older one would. As we age, our corneas become yellower, lessening our sensi- tivity to that color. If you’re over 40, you defi- nitely are seeing less difference between these two greens than you would have in your youth. Matching Unmatchable Pantone Colors If it sounds like the machine may be right and humanity wrong, ask yourself who decides whether the match is a good one. If you want your monitor, or your desktop printer, to predict what output will look like on some other device, are you going to accept something that a machine says matches, when your own eyes tell you it doesn’t? Furthermore, many conversions are better done by ignoring numerical matching al- together. The prime example, and one that features LAB in a big way, is the handling of Pantone Matching System colors. Authentic PMS colors are created by mixing special inks. Such custom-mixed inks can achieve certain colors, particularly pastels and blues, that aren’t otherwise available on any current output device. Unfortunately, printing with an extra ink is expensive. Clients often request that the PMS color be emulated with standard inks, no com- bination of which can match it. This traditional problem has a relatively modern solution. The traditional workaround was a set of Figure 13.5 Three attempts to emulate PMS colors in CMYK . Top, Photoshop 7 and later versions use Pantone- supplied LAB values, converted to CMYK here by the default settings of Photoshop 6 and later. Middle, the same LAB values converted to CMYK using Photoshop 5’s default. Bottom, CMYK values were inserted directly using Pantone-supplied tables (Photoshop 6 and earlier). PMS 279 56 L (2) A (50) B PMS 3385 77 L (48) A 6 B PMS 1655 63 L 61 A 75 B PMS 1775 70 L 49 A 11 B PMS 2728 33 L 20 A (69) B PMS 340 51 L (73) A 13 B PMS 361 62 L (57) A 52 B PMS 1787 58 L 70 A 28 B PMS 2747 17 L 20 A (57) B A B C Chapter 13. The Universal Interchange Standard Page 9 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. CMYK “equivalents” that Pantone issued for each of its custom inks. Most graphic arts applications contained a library of these “equivalents”; some still do. By the turn of the century, the problems with this one-size-fits-all approach had become apparent. The same CMYK values produce different results in different settings. A newspaper, for example, would get darker, muddier colors than this book, which is printed on much higher-quality paper. Fur- thermore, desktop printers that didn’t want a CMYK file at all, but rather an RGB one, were starting to make a dent in the market. Pantone responded by issuing real equivalents to its inks—using LAB . They were introduced in Photoshop 7, in 2002. In previous versions, specifying a PMS color in Photo- shop’s Color Picker got us the Pantone- supplied CMYK value, plus a LAB value of dubious origin. Since then, we get only the new Pantone LAB value. All CMYK and RGB values are computed from it, using whatever our current color settings are. Today’s method is an improvement, but no magic elixir. When a color can’t be matched, it can’t be matched. Take a look at emulations of nine PMS colors, all of which are out of the CMYK gamut. Figure 13.5A uses the post-2002 LAB values, converted to CMYK using the default separation setting of Photoshop 6 and later: the U.S. Web Coated ( SWOP ) v.2 profile, which was derived from machine measurements of actual printed samples. Figure 13.5B uses the same values, but the default separation method of Photoshop 5, which was put together by human observation and tweaking. And Figure 13.5C was never separated at all. The CMYK values were inserted directly, using the Pantone-supplied numbers of Photoshop 6 and earlier. Unless you have access to a Pantone swatch book, you won’t know which methods worked best. I’ve got one, and in my opinion, Figure 13.5C is non-competitive in all nine colors. The current separation method, Fig- ure 13.5A, has a better match in five of the nine colors, but a serious problem, too. In their custom-ink incarnations, the three reddish colors are all more intense than anything you see here. The pink is lighter and purer. The color that prints as orange in the upper right is more like an angry pink, and the red at bottom center should simply be redder. I rate Figure 13.5A as the closest match in all three. It also wins in two of the three greens, but not the one at middle right. Photoshop 5 did a better job, because the machine-generated method fell victim to the same problem that made all its blues purple, and made this green too yellow. Ah, that problem with the blues. If a PMS blue is a key color, converting with the v.2 SWOP profile is likely to get the job rejected. The blues—and that’s what they originally are, blues, not purples—are a disaster in Figure 13.5A. In the center swatch ( PMS 2728), the cyan ink is 16 points higher than the magenta in Figure 13.5A, but 30 points higher in Figure 13.5B. The atrocity occurred, I suspect, when software decided that Figure 13.5A’s “blue” had a lower dE with respect to the Pantone original than Figure 13.5B’s does. And why not? Any sensible dE algorithm is going to give much greater weight to fidelity to the A value than that of the B . The desired numbers are 33 L 20 A (69) B . That (69) B is out of the question in CMYK . Photoshop claims that 33 L 20 A (53) B is achievable. A machine will think that that’s the best we can do, since it tries to get as close as it can to all three LAB numbers. Lowering the 20 A would, in its small mind, increase dE and therefore be undesirable. Human graphic artists instinctively know that the A must nevertheless be lowered, because blues that are overly cyan are apt to 274 Chapter 13 Chapter 13. The Universal Interchange Standard Page 10 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. be much more acceptable to a client than blues that are too purple. So dE goes out the window, and the artist is true to the spirit of the original blue, rather than to its numbers. Maintaining the Distinction The problems of converting out of LAB offer instructive points about converting out of anything else. LAB ’s problems are more severe because it is capable of constructing colors that are wildly out of the gamut of the next space. However, the solutions are entirely applicable to less onerous conversions. RGB to CMYK is the case that most people think of, but it isn’t the only one. Some original LAB values of Figure 13.5 were out of the RGB gamut as well as the CMYK . Consider what happens when we approach the edge of the gamut. Yellows are a strong point for CMYK and a weak one for RGB , so they’re a good example. A value of 94 L 0 A 90 B is fairly extreme. It converts to 0 C 4 M 81 Y , or 255 R 235 G 21 B . (The LAB “yellow” is actually slightly orange.) Raise the original value to 91 B , and one point of yellow is added in CMYK . The RGB value also rids itself of some of the conta- minating blue, dropping to 255 R 235 G 10 B . Raise it to 92 B ,and CMYK adds another point of yellow. The RGB hits 255 R 235 G 0 B . It can’t get any less blue than that, so when we raise the stakes to 93 B , RGB has no way to call the bet. As we continue to increase the B , CMYK continues to add yellow, for as long as it can. By the time we get to 94 L 0 A 99 B , it’s 0 C 4 M 100 Y , and now there’s no more yellow ink to add. The RGB “equivalent,” meanwhile, is still stuck at 255 R 235 G 0 B . Increase the B still further, and nothing happens. Even when we max out at 94 L 0 A 127 B , the “equivalents” don’t change in either RGB or CMYK . Therefore, at least 36 yellows that are all different in LAB will convert to the same color in RGB . At least 29 will convert to the same color in CMYK . Sure, it’s possible to preserve all these distinctions. But nobody with any more rational- ity than Don Quijote would try it. Even 90 B denotes an extremely intense yellow. To save room for 37 even more vivid flavors of yellow would be outlandish. Every yellow in the image would have to be drastically toned down so that these hypothetical brighter yellows could be distinguished from them. The Universal Interchange Standard 275 Figure 13.6 All colors in the LAB original that produced these images were within the CMYK gamut. The top version was separated using the default settings of Photoshop CS 2, which employ Relative Colorimetric rendering intent. The bottom image’s colors are more muted because Perceptual intent, which was previously the default, was used to separate into CMYK . A B Chapter 13. The Universal Interchange Standard Page 11 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. But what if a picture comes along that somehow requires that such distinctions be maintained? Couldn’t such a picture exist, and if so, how can we possibly handle it if all the yellows smoosh together during conversion? If such pictures exist, they’re rare. And if the distinctions would be obliterated during the conversion, then if we need them we have to act while still in LAB . And that, it turns out, is the generalized solution to how to treat out-of-gamut colors during any kind of conversion. Namely, forget them. Just match everything else, and let the weird colors worry about themselves. Unless, of course, distinguishing the weird colors from the rest is a priority. Then, attack them before making the conversion. This principle may seem obvious when the offending colors are so clearly out of gamut as these are, but it didn’t seem that way in the late 1990s, when new capabilities were being engineered into Photoshop’s separation algorithm. The theory then was that colors that were barely in gamut should be intentionally toned down, so that any out-of-gamut inter- lopers would seem brilliant by comparison. This was called perceptual rendering, and effective with Photoshop 6, it became the default way of doing things. In 2005, the error was corrected in Photo- shop CS 2. Figure 13.6A converts the LAB file using today’s defaults. These colors are all fairly bright but all were originally within the CMYK gamut. Therefore, the perceptual method used in Figure 13.6B toned them down, thinking to accommodate any brighter colors that might show up. Rendering intent is set in Color Settings and can be overridden in Edit: Convert to Profile. The current default, Relative Colori- metric, takes the simple view that all matchable colors should be matched and whatever happens to unmatchable ones is our problem. (An alternative, Absolute Colori- metric, should be avoided. Many RGB shave “white points” that are theoretically not white in CMYK . RelCol remaps them to 0 C 0 M 0 Y ; AbsCol may turn them blue.) The perceptual rendering intent, in any case, is too mild to be of any real use. It also is unavail- able for conversions into RGB . In the previous example, it would have increased the yellowness 276 Chapter 13 Figure 13.7 This image was prepared for prominent use in an advertising campaign. Chapter 13. The Universal Interchange Standard Page 12 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. more slowly, maxing out at 104 B rather than the 99 B of RelCol. When we deliberately tone down areas that we could have matched if we had wanted to, usually we want to tone them down a lot more than the perceptual intent does. LAB is the universal interchange standard. We should use it to try to match what we can, in most cases. There are also times when we can use LAB as an insurance policy against the possibility of a bad conversion. The Knight of the Unambiguous Transfer “Señor,” inquired the goatherd, “who is this man, who dresses in such a way and carries on in such a fashion?” “Who could it possibly be,” replied the barber, “but the celebrated Don Quijote de la Mancha, cham- pion of the weak, redresser of injury, righter of wrongs, the shelter and refuge of damsels, the horror of giants and the victor in battle?” “That sounds to me,” mused the goatherd, “like what you read in books about knights- errant, who do all the things that your grace is telling me this man does, but as far as I’m con- cerned either your grace is jok- ing, or this gentleman has holes in every corner of his brain.” While I was writing this book during a break in one of my classes, one of the students, a professional photographer, requested help in my capacity as redresser of injury and righter of wrongs, said wrong being that his job was in jeopardy. He had just prepared Figure 13.7, full of rich browns and dark reds, for a very prestigious placement for his most important client. The printed results had been quite unsatisfactory, he explained—all muddy and lifeless. I opened his RGB file—noting an alert as I did so—and said that it looked fine to me. “Wait until you see what the printer did to it,” he replied. But before he could bring me a printed sample, I said, “Let me guess. It looked a lot like this, right?” And I produced Figure 13.8 on the screen. Bingo. This sad story has been repeated hundreds of times over the past several years, sometimes with pictures as important as this one is, sometimes not, always intensely frustrat- ing, not just to the victims, but to all rational The Universal Interchange Standard 277 Figure 13.8 When the image appeared in print, it looked like this, the result of misinterpretation of the original RGB file. Chapter 13. The Universal Interchange Standard Page 13 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. observers of the graphic arts marketplace, who are tired of watching it happen. The executive summary of what happened is that the photographer handed off an RGB file tagged as Adobe RGB .The printer did not honor it, and assumed that the file was s RGB , wrecking the job. I’m suggesting that sometimes the photographer should hand off an LAB file, which cannot be misinterpreted. Inasmuch as repeated episodes like this demonstrate that even many professionals have difficulty grasping the topic, we will take the scenic route through that last paragraph. In defining a colorspace, everything is a matter of interpretation. For example, when LAB neophytes first see an L channel in isola- tion, they are often surprised that it appears lighter than a grayscale conversion of the document would be. This happens because the L is interpreted as being darker than it appears, for purposes of making a screen pre- view of the color image, or for converting into CMYK or RGB . You could create a different kind of LAB in which the L didn’t behave this way, but you couldn’t use it in Photoshop. In RGB , though, we can use variants that are darker than others, or more colorful. Beginning with Photoshop 5 in 1998, users were encouraged to choose their own definitions of RGB , rather than having a single imposed standard. Its supporters trumpeted this concept as solving more of the world’s problems than Don Quijote ever claimed to even want to. But there was one major unforeseen drawback. Once we choose our own RGB , we needn’t worry about the topic again—provided we never send to or receive files from anyone else. We’ve told Photoshop what “ RGB ” means, so our own work will be interpreted correctly. Somebody else’s work may not be, if they’ve defined RGB differently and our copy of Photoshop doesn’t know it. That’s why, before starting discussion of numbers in Chapter 2, I told you what my own color settings are. If you didn’t change your own settings to match mine, then certain of the numbers I talked about would vary somewhat in your system. That would not be a tragedy. It would be a tragedy if you took one of my RGB files and output it in a professional context without taking account of how I had defined RGB . Fortunately, there’s little chance of that, since I would sooner open a cage full of lions than I would put RGB files in the hands of strangers. The original concept was that each user embeds a tag into each RGB file, identifying what kind of RGB is in play. The tag is recognized by the next user’s system, the file is handled properly, and the knight rides Rocinante off into the sunset. In practice, this works well among those who know what they’re doing. Experienced users throw tagged RGB back and forth all the time without a hitch. Unfortunately, the world at large, and service providers especially, are protagonists in a different picaresque novel. Tagging an RGB file and giving it to a stranger on the assumption that the tag will be honored is a lot like walking into a busy intersection on the assumption that the traffic will stop, except the odds are not nearly as good. To Run Where the Brave Dare Not Go The question of which RGB to use is beyond the scope of this book. But how colorful the definition is makes a difference to the current discussion. The more colorful, the higher the probability that we will run into gamut problems such as the ones in Figure 13.5, where the RGB file calls for colors that can’t be achieved on output. But if the definition isn’t colorful enough, it may not contain colors that you might need. It would be best if the RGB ’s gamut matched the gamut of the output device exactly, but that can’t happen for a variety of technical reasons. The most prominent of the less colorful 278 Chapter 13 Chapter 13. The Universal Interchange Standard Page 14 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. RGB s is s RGB , a definition promoted by Microsoft and Hewlett-Packard in the late 1990s with considerable commercial success. For those who prefer a wider-gamut colorspace, the usual choice is Adobe RGB ,al- though some use even more colorful spaces. Adobe RGB has become somewhat of a standard among professional photographers. Most consumer devices now prefer or require s RGB . Most service providers now assume that any kind of RGB file coming in is an s RGB one. A stalemate has been reached. Adoption of s RGB as a consumer standard has grown so much since 1998 that there is now no chance of dislodging it. Furthermore, vendors are less likely now even to consider the possibility that an incoming RGB file is something else. Yet a substantial group hates s RGB so much that they will never agree to use it. Adobe RGB is quite workable, as is passing an Adobe RGB file to me or anyone else that you know for a fact won’t misinterpret it. Throwing an Adobe RGB file out to strangers or to the world at large, tagged or not, is asking for Figure 13.8 to become your problem instead of my student’s. For the record, let’s analyze how much color got lost in this critical image. I’ve measured four areas on the bearded elf, fore- ground left, and will give equivalencies in both LAB and CMYK . • His purple cap measures 117 R 43 G 63 B . When properly treated as Adobe RGB ,it converts to 32 L 43 A 10 B , or 32 C 94 M 64 Y 29 K . When misinterpreted as s RGB , the values are 29 L 35 A 5 B and a dismal 39 C 89 M 58 Y 36 K . • A light part of the brown jacket is 16 5 R 115 G 80 B . As Adobe RGB ,that’s 55 L 24 A 33 B and 24 C 48 M 75 Y 8 K . As s RGB , 53 L 17 A 28 B and 31 C 55 M 73 Y 12 K . • The grass between his legs, 79 R 86 G 31 B . As Adobe RGB , 34 L (11) A 32 B and 64 C 46 M 98 Y 39 K . As s RGB , 35 L (9) A 29 B and 62 C 47 M 96 Y 39 K . • His cheek, 235 R 197 G 171 B ; Adobe RGB 84 L 16 A 21 B and 1 C 25 M 30 Y ; s RGB 82 L 11 A 18 B and 7 C 23 M 31 Y . These readings confirm what our eyes tell us. The misinterpretation has turned an attractive mix of subtle colors into something dishrag dull. Some people actually enjoy it when this happens. It gives them an opportunity to vent. They can denounce all printers as stu- pid. They can call up the CSR and scream. They can demand that the job be rerun and threaten to sic their lawyer on them. All this can be psychologically rewarding, and dis- tract attention from others of life’s worries. If you would just as soon have the job done correctly without all the histrionics, and you would otherwise be sending out an Adobe RGB file to a stranger, the options are: • You can call up the people you expect to handle your work and find out how they intend to behave when confronted with a tagged file. Problem: you may not know who they are. • You can convert your own file to s RGB before sending it out. This is a more attractive option than it used to be. The chances of s RGB being misinterpreted as something else have gotten less as s RGB has become more entrenched, but they still aren’t zero. • If the file is going to a commercial printer, you can convert it to CMYK yourself. That was why this particular photographer was in my class. He had, understandably, decided that Figure 13.7 was the very last RGB file of his that any commercial printer was ever going to encounter. If you want to try something similar, though, you’d best have confi- dence in your CMYK skills. • Finally, the one nearly foolproof method. There are many RGB s, but there’s only one LAB . Convert your file before handing it over, and it can’t be misinterpreted. That’s why Photoshop uses it internally for most of its computations. Getting an LAB file forces the next person to convert to his own RGB or his The Universal Interchange Standard 279 Chapter 13. The Universal Interchange Standard Page 15 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. own CMYK , eliminating all ambiguity. The worst that can happen is that you get a call in the middle of the night asking what to do with this crazy messed-up file. To that, I say that anyone who can’t figure out what to do with an LAB file certainly can’t be trusted to figure out what to do with tagged RGB . Of Children and Colorspaces This chapter opened with a problem that could be seen as either one of Photoshop technique or one of calibration. The user had two CMYK spot channels and couldn’t figure out how to convert them into “real” channels, something that Photoshop doesn’t make easy. Coming up with a solution requires a knowledge both of Photoshop and of why LAB is used for interchange. We first think about how to accomplish the task while staying in CMYK (it can be done, but it’s a nuisance). Then, we consider whether LAB can expedite the process, which it can. Hence, the answer: re-create the channels in LAB , just the way Photoshop does when it converts one colorspace to another. More such hybrid uses of LAB will no doubt suggest themselves as the colorspace becomes more mainstream. Our next chapter, for example, is mostly RGB ,but it’s heavily LAB -flavored; the two exist side by side. Other image-processing products have tried to promote the use of LAB , and it isn’t a recent phenomenon. Figure 13.9, a double- page advertisement for scanning software, appeared in 1996. The thrust is that LAB is superior to CMYK or RGB , which it is in a lot of ways. Note the claim on the right side: “The human eye uses CIELAB .” T h e r e’s t r u t h in that, too. The company was so smitten by the LAB connection that it changed the name of its software from LinoColor to VisuaLab. And if you think that “Everything you need to know about color spaces” is a bit of overkill, you should know that the children appear next to the crayons because the theme of the advertising campaign was “Color Is Child’s Play.” Personally, I would like to lock the person who thought that one up in a room with a copy of Chapters 14 and 15, and see if 280 Chapter 13 Figure 13.9 This two-page advertisement, trumpeting the virtues of LAB , ran in 1996. Chapter 13. The Universal Interchange Standard Page 16 Return to Table of Contents Chapter 13. The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Powerful Colorspace By DAN MARGULIS ISBN: 0321356780 Publisher: Peachpit Press Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Print Publication Date: 2005/08/08 User number: 910771 Copyright 2007, Safari Books Online, LLC. This PDF is exclusively for your use in accordance with the Safari Terms of Service. No part of it may be reproduced or transmitted in any form by any means without the prior written permission for reprints and excerpts from the publisher. Redistribution or other use that violates the fair use priviledge under U.S. copyright laws (see 17 USC107) or that otherwise violates the Safari Terms of Service is strictly prohibited. [...]... software’s only real connection with LAB was that it converted its raw scans internally to a form of LAB before saving them, just as drum scanners have always con verted their RGB data to CMYK before saving LinoColor and VisuaLab soft- Figure 13.10 Curvemeister, a Windows-only Photoshop plug-in, offers an LAB interface for RGB files, including the ability to use LAB curves while in RGB ware offered... advantage is that an LAB file is completely unambiguous; it is the same from one Photoshop user to another Once we leave Photoshop, however, that is no longer true There are at least half a dozen variant versions of LAB floating around, one of them in this software It’s called LAB (LH), and is a smaller-gamut version of the LAB we as HSB (Hue, Saturation, Brightness; someknow and love Granted that Linotype-Hell’s... information is available in LAB form The curves, neatly laid out, can be applied as if the file were LAB, CMYK, or even HSB We can even generate LAB- style imaginary colors RGB is the strongest blending space of the four, as we’re about to see in the next chapter But it’s generally the weakest for curves Note that both of the products we’ve just discussed have features that are regrettably missing from Photoshop. .. features that are regrettably missing from Photoshop It would be nice if The Bottom Line Photoshop permits an infinite number of definitions of RGB and CMYK, but only one of LAB Its status as the one colorspace within Photoshop that is entirely unambiguous gives LAB a unique role in information interchange Also, LAB has a stronger direct relation with how humans perceive color than CMYK and RGB do This... analog in the Photoshop world But in Spanish, the structure is quite correct And in Portuguese, Italian, and French, too, which were of course much easier to adjust to once I figured out that Spanish and Oklahoman are similar Returning to color, the grammar of LAB is also useful even when speaking a different language Many LAB techniques work almost unaltered in HSB, which is as close to LAB as Italian... This chapter looks for places where RGB has such an advantage that it would pay to take the extra time to do a contrast-only or color-only version there rather than use LAB As this could be a book-length topic by itself, Chapter 14 Once for Color, Once for Contrast Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Powerful... the RGB curves shown in Figure 14.2 Center, an alternate attempt to add contrast in LAB by steepening the L curve Bottom, a combined version, in which the L channel of the center version is replaced by one from a copy of the top version that was converted to LAB Chapter 14 Once for Color, Once for Contrast Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Prepared for NV Moorthy,... in LAB anyway And the reason we’re in LAB is because of how it defines color, not contrast The AB channel structure is so different from that of RGB that each has strengths that the other doesn’t duplicate The spectacular cases — the ones where we drive colors apart to create massive color variations, or where we completely change the color of objects as we did in Chapter 10— work better in LAB But LAB. .. and are thus easier to attack have about the same H value, but each rates a with curves, Blending Options, and the like higher S than the one before it LAB is a relative of the colorspace known Chapter 13 The Universal Interchange Standard Photoshop Lab Color: The Canyon Conundrum: And Other Adventures in The Most Prepared for NV Moorthy, Safari ID: moorthy@ceybank.com Powerful Colorspace By DAN MARGULIS... the work in RGB No problem—start with two fresh RGB copies of the image Convert one to LAB and do what was done in Chapter 12 Return to the RGB copy and do it the way that you think creates extra detail When finished, convert it to LAB, and use its L channel to replace the one in the other file Presto: color done in LAB, detail created in RGB The power of this method derives from being able to work in . PMS colors in CMYK . Top, Photoshop 7 and later versions use Pantone- supplied LAB values, converted to CMYK here by the default settings of Photoshop 6 and later. Middle, the same LAB values converted. in RGB , but much information is available in LAB form. The curves, neatly laid out, can be applied as if the file were LAB , CMYK ,or even HSB . We can even generate LAB -style imaginary colors Bottom Line Photoshop permits an infinite number of definitions of RGB and CMYK , but only one of LAB . Its status as the one colorspace within Photoshop that is entirely unambiguous gives LAB a unique

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