# Photoshop Lab Color- P10

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## Photoshop Lab Color- P10

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Photoshop Lab Color- P10: LAB has a reputation for enormous power, yet virtually all reference materials that advocate its use illustrate its capabilities with a single class of image. This chapter introduces the basic LAB correction method and explains why it is so extraordinarily effective. if you happen to have a picture of a canyon.

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## Nội dung Text: Photoshop Lab Color- P10

1. The Universal Interchange Standard 271 out 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 Figure 13.4 The middle row holds the reference colors. describes how far the two colors are from Are the colors at the top closer to the middle ones than the one another. There are several formulas bottom ones are? A machine would think they are each to produce it; this one involves weighting equally far off. the A channel more than the B, a lot of shot from a product of GretagMacbeth, a square roots, and other fancy stuff. But leading vendor of color measurement instru- whatever the formula, a dE of zero is ideal; mentation. It has measured 50 L (30) A 40 B the higher it is the worse the match; and the where the desired value is 50L(40)A30B, and objective of a good interchange between two it wants to know how far different these two devices is to have the average dE be as close greens really are from one another. to zero as possible. Notice ﬁrst that below the color wheel on Opinions on the merit of dE are, shall we the left, the program permits us to express say, varied. On the one hand, certain cali- these colors in xyY or even LUV, if we’d like brationists are so smitten that they make a to avoid LAB. But that’s a side issue. The big Dulcinea out of dE: they believe that she action is underneath the green circle, where embodies perfection and it is only a matter of we ﬁnd one large and three small numbers time before we ﬁnd, attain, and adore her. that are introduced by the lowercase letter d. On the other, one color expert offers the It stands for the Greek delta, which mathe- following succinct opinion: “The original maticians often use to denote the quantity conclusion therefore remains, the CIELAB is of change. The asterisks remind us that the perceptually extremely non-linear and the snooty name is L*a*b*. In comparing the dE* unit of color-difference is totally worth- two colors, the program notes that the L less for all processes and uses in photo- values are identical, so dL*=0. da* and db* graphic imaging.” are both 10. Personally, I am somewhere in the middle, but closer to the second view than the ﬁrst. To Each His Dulcinea The real Dulcinea, like dE, was no thing of “A knight errant who loses his lady,” Don beauty. She herded pigs, a dirty and unpleas- Quijote remarked to Sancho, “is like one who ant task that nevertheless needs to be done. loses the eyes that let him see, or the sun that dE is like that, too. Machines can aid in shines on him, or the food that maintains calibration, and a formula like dE, however him. I have told you this many times before, imperfect, is the only way they can come to a and now I say it again: a knight errant with- decision. We simply have to understand that
2. 272 Chapter 13 sometimes it gives misleading results, and we’ve been discussing—50L(30)A40B on the sometimes we deliberately want to disregard press and 50L(40)A30B when we try it on our it because we get a closer visual match with a printer—demonstrate the deﬁciencies of any worse literal match. such formula, in at least three different ways. Let’s assume that we are trying to make First, humans expect greens to be greener output from a desktop printer match that of a than a machine does. If the desktop printer commercial print shop so that we can make produces a better-looking green than the inexpensive proofs at home. A conventional press does, we may declare that the two idea is to have the print shop produce a match, even though a spectrophotometer variety of patches, which are then measured ﬁnds a big dE. Also, the machine thinks that spectrophotometrically. The desktop printer misses of equal magnitude make for equal spits out the same swatches, which are mea- dE. Not so. If one of the misses is greener sured and compared to the press’s. Adjust- than the desired value, and the other is less ments are then made to try to get dE lower green by an equal amount, humans will by forcing the desktop printer to get closer invariably consider the greener one a closer to the print shop’s results. match to the original. There’s nothing inherently wrong with Second, how far one color channel appears such an approach, but the two greens that 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 Further Reading on Colorspaces patch had measured at 50L(50)A40B, the ma- For those interested in more detailed discussion and chine would compute the same dE, because comparison of the colorspaces described in this there is still a difference of 0L10A10B between chapter, the enclosed CD includes six papers by Prof. that and what’s wanted. However, we would Gernot Hoffmann, a color expert at the University of perceive that color as being closer to the Applied Sciences in Emden, Germany. They include a original than 50L(30)A40B was, because, while general introduction to graphics for color science; one paper each on the structure of XYZ and LAB; a study both the A and B were each inaccurate by 10 of the question of gamut generally; and one that is points, both were further away from 0A0 B speciﬁc to the question of CMYK gamut. Finally, there’s than the desired value was. Were they the one paper that has nothing to do with the topic of this same 10 points off, but one channel moved chapter but does bear on Chapters 5 and 11, namely a toward neutrality and the other away from discussion of how gamma can create certain problems it, a spectrophotometer sees less difference when working in RGB. than we do. I suggest starting here because Gernot does a good job If you don’t believe it, check out Figure of reducing technical concepts into somewhat compre- 13.4. The “desired” colors are in the middle hensible language. There is a fair amount of mathe- matics involved, but no calculus. If you don’t row. In the top row, both A and B were moved remember what went on in your 12th-grade algebra 15 points further from zero. In the bottom course, however, you may ﬁnd it heavy sledding. row, one channel went 15 points in one direc- Thanks to Google and its ilk, much more information, tion and one 15 points in the other. Therefore, including the actual formulas for converting between the top and bottom rows both have the same the spaces, is available by typing in search criteria such dE with respect to the middle. (None of the as CIELUV, CIExyY, and so on. original LAB squares were out of the CMYK The CIE is alive, well, and living in Austria. Its URL is gamut, so the conversion for printing didn’t www.cie.co.at. affect the relation of these colors.) Humans would see the bottom row as being much
3. A further from the middle than the top is, partic- ularly 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 B 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 C expensive. Clients often request that the PMS color be emulated with standard inks, no com- PMS 279 PMS 3385 PMS 1655 bination of which can match it. This traditional 56L(2)A(50)B 77L(48)A6B 63L61A75B 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- PMS 1775 PMS 2728 PMS 340 supplied LAB values, converted to CMYK here by the 70L49A11B 33L20A(69)B 51L(73)A13B 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 361 PMS 1787 PMS 2747 62L(57)A52B 58L70A28B 17L20A(57)B
4. 274 Chapter 13 CMYK “equivalents” that Pantone issued for Figure 13.5C is non-competitive in all nine each of its custom inks. Most graphic arts colors. The current separation method, Fig- applications contained a library of these ure 13.5A, has a better match in ﬁve of the “equivalents”; some still do. nine colors, but a serious problem, too. By the turn of the century, the problems In their custom-ink incarnations, the three with this one-size-fits-all approach had reddish colors are all more intense than any- become apparent. The same CMYK values thing you see here. The pink is lighter and produce different results in different settings. purer. The color that prints as orange in the A newspaper, for example, would get darker, upper right is more like an angry pink, and muddier colors than this book, which is the red at bottom center should simply be printed on much higher-quality paper. Fur- redder. I rate Figure 13.5A as the closest thermore, desktop printers that didn’t want match in all three. a CMYK ﬁle at all, but rather an RGB one, It also wins in two of the three greens, but were starting to make a dent in the market. not the one at middle right. Photoshop 5 did a Pantone responded by issuing real equiva- better job, because the machine-generated lents to its inks—using LAB. They were intro- method fell victim to the same problem that duced in Photoshop 7, in 2002. In previous made all its blues purple, and made this versions, specifying a PMS color in Photo- green too yellow. shop’s Color Picker got us the Pantone- Ah, that problem with the blues. If a PMS supplied CMYK value, plus a LAB value of blue is a key color, converting with the v.2 dubious origin. Since then, we get only the SWOP proﬁle is likely to get the job rejected. new Pantone LAB value. All CMYK and RGB The blues—and that’s what they originally values are computed from it, using whatever are, blues, not purples—are a disaster in our current color settings are. Figure 13.5A. In the center swatch ( PMS Today’s method is an improvement, but 2728), the cyan ink is 16 points higher than no magic elixir. When a color can’t be the magenta in Figure 13.5A, but 30 points matched, it can’t be matched. higher in Figure 13.5B. Take a look at emulations of nine PMS col- The atrocity occurred, I suspect, when ors, all of which are out of the CMYK gamut. software decided that Figure 13.5A’s “blue” Figure 13.5A uses the post-2002 LAB values, had a lower dE with respect to the Pantone converted to CMYK using the default separa- original than Figure 13.5B’s does. And why tion setting of Photoshop 6 and later: the U.S. not? Any sensible dE algorithm is going to Web Coated (SWOP) v.2 proﬁle, which was give much greater weight to ﬁdelity to the A derived from machine measurements of value than that of the B. The desired numbers actual printed samples. Figure 13.5B uses are 33 L 20 A (69) B . That (69) B is out of the the same values, but the default separation question in CMYK. method of Photoshop 5, which was put to- Photoshop claims that 33 L 20 A (53) B is gether by human observation and tweaking. achievable. A machine will think that that’s And Figure 13.5C was never separated at the best we can do, since it tries to get as close all. The CMYK values were inserted directly, as it can to all three LAB numbers. Lowering using the Pantone-supplied numbers of the 20A would, in its small mind, increase dE Photoshop 6 and earlier. and therefore be undesirable. Unless you have access to a Pantone Human graphic artists instinctively know swatch book, you won’t know which methods that the A must nevertheless be lowered, worked best. I’ve got one, and in my opinion, because blues that are overly cyan are apt to
5. The Universal Interchange Standard 275 be much more acceptable to a client than blues that are too purple. So dE goes A 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 be- cause 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. B 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 ap- proach 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 94L0A90B is fairly extreme. It con- verts to 0C 4 M 81 Y, or 255 R235G 21 B. (The LAB “yellow” is actually slightly orange.) Raise the original value to 91B, and one Figure 13.6 All colors in the LAB original that produced these point of yellow is added in CMYK. The RGB images were within the CMYK gamut. The top version was sepa- rated using the default settings of Photoshop CS2, which employ value also rids itself of some of the conta- Relative Colorimetric rendering intent. The bottom image’s colors minating blue, dropping to 255R235G10B. are more muted because Perceptual intent, which was previously Raise it to 92B, and CMYK adds another the default, was used to separate into CMYK. point of yellow. The RGB hits 255R235G0B. It can’t get any less blue than that, so when Therefore, at least 36 yellows that are all we raise the stakes to 93B, RGB has no way to different in LAB will convert to the same color call the bet. in RGB. At least 29 will convert to the same As we continue to increase the B, CMYK color in CMYK. continues to add yellow, for as long as it can. Sure, it’s possible to preserve all these dis- By the time we get to 94L0A99B, it’s 0C4M100Y, tinctions. But nobody with any more rational- and now there’s no more yellow ink to add. ity than Don Quijote would try it. Even 90B The RGB “equivalent,” meanwhile, is still denotes an extremely intense yellow. To save stuck at 255R235G0B. room for 37 even more vivid ﬂavors of yellow Increase the B still further, and nothing would be outlandish. Every yellow in the happens. Even when we ma x out at image would have to be drastically toned 94L0A127B, the “equivalents” don’t change in down so that these hypothetical brighter either RGB or CMYK. yellows could be distinguished from them.
6. 276 Chapter 13 But what if a picture comes along that This principle may seem obvious when the somehow requires that such distinctions offending colors are so clearly out of gamut as be maintained? Couldn’t such a picture these are, but it didn’t seem that way in the exist, and if so, how can we possibly handle late 1990s, when new capabilities were being it if all the yellows smoosh together during engineered into Photoshop’s separation algo- conversion? rithm. The theory then was that colors that If such pictures exist, they’re rare. And if were barely in gamut should be intentionally the distinctions would be obliterated during toned down, so that any out-of-gamut inter- the conversion, then if we need them we have lopers would seem brilliant by comparison. to act while still in LAB. This was called perceptual rendering, and And that, it turns out, is the generalized effective with Photoshop 6, it became the solution to how to treat out-of-gamut colors default way of doing things. during any kind of conversion. Namely, forget In 2005, the error was corrected in Photo- them. Just match everything else, and let the shop CS2. Figure 13.6A converts the LAB ﬁle weird colors worry about themselves. Unless, using today’s defaults. These colors are all of course, distinguishing the weird colors fairly bright but all were originally within from the rest is a priority. Then, attack them the CMYK gamut. Therefore, the perceptual before making the conversion. 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 RGBs have “white points” that are theoretically not white in CMYK. RelCol remaps them to 0C 0 M 0 Y ; AbsCol may turn them blue.) The perceptual rendering in- tent, 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 Figure 13.7 This image was prepared for prominent use in an advertising campaign.
7. The Universal Interchange Standard 277 more slowly, maxing out at 104 B rather jeopardy. He had just prepared Figure 13.7, than the 99B of RelCol. When we deliberately full of rich browns and dark reds, for a very tone down areas that we could have matched prestigious placement for his most important if we had wanted to, usually we want to tone client. The printed results had been quite them down a lot more than the perceptual unsatisfactory, he explained—all muddy intent does. and lifeless. LAB is the universal interchange standard. I opened his RGB ﬁle—noting an alert as We should use it to try to match what we I did so—and said that it looked ﬁne to me. can, in most cases. There are also times when “Wait until you see what the printer did to we can use L A B as an insurance policy it,” he replied. But before he could bring me against the possibility of a bad conversion. a printed sample, I said, “Let me guess. It looked a lot like this, right?” And I produced The Knight of the Unambiguous Transfer Figure 13.8 on the screen. Bingo. This sad story has been repeated hundreds “Señor,” inquired the goatherd, “who is this of times over the past several years, some- man, who dresses in such a way and carries times with pictures as important as this one on in such a fashion?” is, sometimes not, always intensely frustrat- “Who could it possibly be,” replied the ing, not just to the victims, but to all rational 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 dur ing a break in one of my classes, one of the students, a pro- fessional photographer, requested help in my capacity as redresser of injury and righter of wrongs, said wrong being that his job was in Figure 13.8 When the image appeared in print, it looked like this, the result of misinterpretation of the original RGB ﬁle.
8. 278 Chapter 13 observers of the graphic arts marketplace, own color settings are. If you didn’t change who are tired of watching it happen. your own settings to match mine, then The executive summary of what happened certain of the numbers I talked about would is that the photographer handed off an RGB vary somewhat in your system. That would ﬁle tagged as Adobe RGB. The printer did not not be a tragedy. It would be a tragedy if you honor it, and assumed that the ﬁle was sRGB, took one of my RGB ﬁles and output it in a wrecking the job. I’m suggesting that some- professional context without taking account times the photographer should hand off an of how I had deﬁned RGB. Fortunately, there’s LAB ﬁle, which cannot be misinterpreted. little chance of that, since I would sooner Inasmuch as repeated episodes like this open a cage full of lions than I would put RGB demonstrate that even many professionals ﬁles in the hands of strangers. have difﬁculty grasping the topic, we will take The original concept was that each user the scenic route through that last paragraph. embeds a tag into each RGB ﬁle, identifying In deﬁning a colorspace, everything is a what kind of RGB is in play. The tag is matter of interpretation. For example, when recognized by the next user’s system, the ﬁle LAB neophytes ﬁrst see an L channel in isola- is handled properly, and the knight rides tion, they are often surprised that it appears Rocinante off into the sunset. lighter than a grayscale conversion of the In practice, this works well among those document would be. This happens because who know what they’re doing. Experienced the L is interpreted as being darker than it ap- users throw tagged RGB back and forth all pears, for purposes of making a screen pre- the time without a hitch. Unfortunately, view of the color image, or for converting into the world at large, and service providers CMYK or RGB. You could create a different especially, are protagonists in a different kind of LAB in which the L didn’t behave this picaresque novel. Tagging an RGB ﬁle and way, but you couldn’t use it in Photoshop. giving it to a stranger on the assumption that In RGB, though, we can use variants that the tag will be honored is a lot like walking are darker than others, or more colorful. into a busy intersection on the assumption Beginning with Photoshop 5 in 1998, users that the trafﬁc will stop, except the odds are were encouraged to choose their own deﬁni- not nearly as good. tions of RGB, rather than having a single imposed standard. Its supporters trumpeted To Run Where the Brave Dare Not Go this concept as solving more of the world’s The question of which RGB to use is beyond problems than Don Quijote ever claimed the scope of this book. But how colorful the to even want to. But there was one major deﬁnition is makes a difference to the current unforeseen drawback. discussion. The more colorful, the higher the Once we choose our own RGB, we needn’t probability that we will run into gamut prob- worry about the topic again—provided we lems such as the ones in Figure 13.5, where never send to or receive ﬁles from anyone the RGB file calls for colors that can’t be else. We’ve told Photoshop what “ RGB ” achieved on output. But if the deﬁnition isn’t means, so our own work will be interpreted colorful enough, it may not contain colors correctly. Somebody else’s work may not be, if that you might need. It would be best if they’ve deﬁned RGB differently and our copy the RGB’s gamut matched the gamut of the of Photoshop doesn’t know it. output device exactly, but that can’t happen That’s why, before starting discussion of for a variety of technical reasons. numbers in Chapter 2, I told you what my The most prominent of the less colorful